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NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 4

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 4

What is a lubricant?

A lubricant is defined as a petroleum-derived product used to control and reduce the friction and wear of moving machinery parts (e.g., in engines and turbines). The main purpose of lubricants  is to help protect and prolong the lifetime of the equipment.

Machinery and lubricants go hand in hand, as shown here.
These goals are accomplished in the following ways:

Lubrication by reducing friction and wear. The lubricant forms a film between the mechanical moving parts of the equipment. In this way the metal-to-metal contact and, thus, the wear is reduced.

Cooling by acting as a heat sink. This causes the heat to dissipate away from critical parts of the equipment so that deformation due to increased temperature is prevented.

Protection by building a film. This film is unaffected by oxygen or corrosive substances and therefore prevents metal damage and oxidation (rust) and therefore also prevents wear.

Types of lubricants

For the most part, lubricants consist of oils to which additives and other chemical substances are added. There are two common types of lubricants which are based on the origin of the oil:

1. Lubricants based on mineral oils (Figure 1a) are the most commonly used type. They are comprised of petroleum products (base stock) to which synthetic additives are added. These types of lubricants are used in applications where there are no high temperature requirements. Typical areas where mineral oil-based lubricants are used include: engines, hydraulics, gears, and bearings.
2. Lubricants based on synthetic oils (Figure 1b) are artificially developed substitutes for mineral oils. They are less common and more expensive. Synthetic oils are specifically developed to create lubricants with superior properties to mineral oils. For example, heat-resistant synthetic oils are used in high performance machinery operating at high temperatures.
a)
b)
Figure 1. Difference in the molecular structure found in lubricants: a) mineral oil and b) synthetic oil.
In the following table, different lubricant types with sub-classes are listed.
Table 1. Different lubricating oil types.

Lubricant type

Sub-classes

Automotive oil Engine oil

Gear oil

Transmission fluids

Industrial oil Hydraulic oil

Turbine oil

Greases
Metal working fluids Forming fluids

Cutting fluids

The physical properties of a lubricant (such as viscosity and density) mostly depend on the oil base stock, whereas the additives fine-tune the chemical properties, e.g., the acid number or base number. For each application, the oil is typically formulated to meet the physical and chemical properties required by the customer. Therefore, various types of oil exist (Table 1).

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of lubricants

Near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, available solutions for lubricants are discussed which have been developed according the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation).

Did you miss the first parts in this series about NIRS as a QC tool for the petrochemical industry? Find them all below!

Read our previous blog posts to learn more about NIRS as a secondary technique.

Applications and parameters for lubricant analysis with NIRS

The main NIRS application for lubricants is to easily monitor the oil condition, i.e., checking if the oil is still of suitable quality for proper lubrication of the equipment. Reducing unnecessary oil changes means significant cost savings. On the other hand, changing the oil too infrequently can result in possible damage of the equipment, leading to costly repairs. Therefore optimizing the usage of the lubricating oil is very important.

The following parameters can be correlated between NIRS and the values from a primary method: kinematic viscosity, viscosity index, color, density, water content, TAN (total acid number), and TBN (total base number). A large set of samples provided by several different companies was used to develop working NIRS models of these parameters, including hydraulic oil, gear oil, and others. In some cases, it was not clear for what application the lubricant was used, so the exact identity of the oil was unknown.

The most relevant application notes for NIRS analysis of lubricants are listed below in Table 2.

Table 2. Metrohm’s NIRS solutions for lubricants including application details and benefits.
Parameter Reference method Norm NIRS Application Notes NIRS benefits
Acid number Titration ASTM D664 AN-NIR-071

AN-NIR-041

All parameters are measured simultaneously within a minute, without requiring any sample preparation or chemical reagents.
Kinematic viscosity at 40 °C Viscosimeter ASTM D445
Kinematic viscosity at 100 °C Viscosimeter ASTM D445
Viscosity index Calculation ASTM D2270
Color number Colorimeter ASTM D1500
Moisture content Karl Fischer titration ASTM D6304
Base number Titration ASTM D2896
Density Density meter ASTM D4052

 

Solutions by means of starter models—expedite and simplify quality control of lubricants

Lubricants keep our modern lives running smoothly. During use, the oil needs to be monitored to check if it still of good enough quality or whether it needs to be exchanged.

The data obtained here indicate that lubricants vary per application and per supplier. This means that there is still not sufficient information for each oil type and subtype to prepare a model robust enough to transform into a pre-calibration. However, if a partner provides the samples, a feasibility study can quickly indicate if the NIR spectra are able to be correlated to the primary method values.

Typically, several key parameters such as the acid and base numbers (AN and BN), viscosity, moisture content, color, and density are determined in the laboratory by various chemical and physical methods. These methods not only incur high running costs, they are also quite time consuming to perform.

NIRS on the other hand requires neither chemicals nor sample preparation, and provides results in less than a minute. This spectroscopic technique is also easy enough to be used by non-chemists. Furthermore, multiple chemical and physical parameters can be determined simultaneously. The combined benefits of this technology make NIRS the ideal solution for many daily QA/QC measurements or ad-hoc atline analysis.

Application example: starter model for lubricants with the NIRS DS2500 Liquid Analyzer

For lubricant analysis, determination of the acid number (ASTM D664), viscosity (ASTM D445), moisture content (ASTM D6304), and color number (ASTM D1500) require the use of multiple analytical technologies and, in part, large volumes of chemicals. The time to result can therefore be quite a long and costly process.

In this example, different lubricant samples were measured with a Metrohm NIRS DS2500 Liquid Analyzer in transmission mode over the full wavelength range (400–2500 nm). The built-in temperature controlled sample chamber was set to 40 °C to provide a stable sample environment. For convenience reasons, disposable vials with a pathlength of 8 mm were used, which made a cleaning procedure obsolete.

Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Analyzers in our free brochure.
Figure 2. Quality control of lubricants as performed by the Metrohm NIRS DS2500 Liquid Analyzer.
The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for the determination of key lubricant parameters (such as those in Table 2). The quality of the prediction models was evaluated using correlation diagrams, which display the correlation between the Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 3).
Figure 3. Correlation plots and figures of merit (FOM) for different parameters measured in lubricants.
This solution demonstrates that NIR spectroscopy is excellently suited for the analysis of multiple parameters in lubricants in less than one minute without sample preparation or using any chemical reagents.

In case a large series of samples must be analyzed, there is also the possibility to measure lubricant samples in a fully automated way, as detailed in our free Application Note below.

Here, the samples were measured in transmission mode over the full wavelength range (400–2500 nm) using a NIRS XDS RapidLiquid Analyzer in combination with an 815 Robotic USB Sample Processor, which can carry a total of 141 samples (Figure 4).
Figure 4. Metrohm NIRS XDS RapidLiquid Analyzer equipped with a with 5.0 mm flow cell (left) and the 815 Sample Processor (right).

Summary

Near-infrared spectroscopy is very well suited for lubricant analysis. Available starter models are developed and validated in accordance with the ASTM guidelines. Positive aspects of using NIRS as an alternative technology to primary methods are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

Future installments in this series

This blog article was dedicated to the topic of lubricants and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. The final installment will be dedicated to:

 

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
Side reactions in Karl Fischer titration

Side reactions in Karl Fischer titration

Many chemists that utilize Karl Fischer titration are nervous about the presence of side reactions because they know that the water determination in their samples can only be specific without any side reactions. Other KFT users do not know what the possible side reactions are and therefore may obtain incorrect results.

What are side reactions?

These are reactions with substances in the sample that:

  • interfere with the stoichiometry of the KF reaction
  • change the pH value of the KF reagent
  • either produce or use up water themselves
  • oxidize on the anode of the generator electrode
  • reduce on the cathode of the generator electrode
  • react with the ingredients of the KF reagent

Recognizing side reactions

One of the worst things that can happen with KFT is not knowing that a side reaction is falsifying your results. Below are some characteristic signs of side reactions.

Titration time and titration curve

Some indications of side reactions include longer titration times compared to the titration of a water standard, slow endpoint detection, and a higher drift value after the titration finishes than at the titration start. Comparing the titration curves of the sample and a water standard with a similar water quantity makes it easier to evaluate the situation. Just plot a graph of the volume against time (or µg water against time, in the case of coulometry). If the graph exhibits a curve that increases steadily as illustrated in Figure 1 (in orange), this can indicate a side reaction.

Figure 1. Side reactions can often be identified from checking the titration time and the titration curve, as shown in this graph.
Linearity

If you notice that the water content depends on the sample weight or the titrant consumption (µg water for coulometry), then you can check the slope of a regression line after plotting water content against titrant consumption (µg water).

Ideally, the slope (b) should be 0. Significantly positive or negative values can indicate a side reaction, as shown in Figure 2.

Figure 2. If the slope of the regression line for the water content / titrant consumption value pairs deviates significantly from 0, this indicates a side reaction.
Spiking

If the water recovery value found after spiking the samples is not within 100 ± 3%, this can indicate a side reaction. Depending on the type and speed of the side reaction, the recovery may be too high or too low. For example, samples which contain DMSO (dimethyl sulfoxide) change the stoichiometry of the Karl Fischer reaction and therefore result in false low readings.

Please note that a recovery rate of almost 100% does not guarantee the absence of a side reaction. Side reactions that take place very rapidly will not be detected, since the side reaction is already complete when the spiking process begins. A spiking procedure is described in detail in chapter 2.5.12 of the European Pharmacopoeia.

Preliminary tests

The oxidation of iodide or reduction of iodine leads to incorrect results.

How can you check whether your sample is undergoing a side reaction with iodine or iodide? A simple preliminary test can clarify the situation. Dissolve the sample in a weakly acidic (alcoholic) solution and then add some drops of iodine or potassium iodide solution. Based on the coloring (a discoloration of iodine or the formation of brown iodine), a side reaction can be detected.

Evaluating redox potentials

Comparing the redox potentials of the redox pairs of sample substances with the redox potential of iodine/iodide can be helpful to assess whether an undesired redox reaction may occur.

If the standard potential is higher than that of iodine/iodide, as in the case of e.g., chlorine, the oxidation of the iodide may result in false low readings.

If it is lower (e.g., lead), the reduction of the iodine may result in values that are too high.

Element changing oxidation state oxidized form + x e → reduced form Standard electrode potential E°
Cl Cl2 + 2e ⇌ 2 Cl +1.36 V
I I2 + 2e ⇌ 2 I +0.54 V
Pb Pb2+ + 2e ⇌ Pb -0.13 V

Avoiding side reactions

Most side reactions can be suppressed by taking suitable measures, such as those listed here.

  • For ketones and substances that react with the methanol present in the KF reagent: Use methanol-free reagents.
  • For samples that lower the pH range of the KF reagent: Add buffer solution for acids or a stoichiometric excess of imidazole.
  • For samples that increase the pH value (e.g., aminic bases): Add buffer solution for bases or a stoichiometric excess of salicylic acid / benzoic acid.
  • High drift after titration: Postdrift correction may help. This is done by stopping the titration at a defined time and recording the additional consumption over several minutes. This allows the calculation of the drift after the titration. This postdrift is then used to correct the water quantity found.
  • Samples that reduce iodine: Subtract the iodine consumption of the reductant in the sample from the overall iodine consumption of the sample.
  • Samples that oxidize iodide: Reduce the oxidant, e.g., Cl2, in advance with an excess of SO2, for example, by treating the sample with the solvent of a two-component reagent.
  • General: Carry out the titration in a thermostatically controlled cell connected to a circulation thermostat at, e.g., -20 °C in order to slow down the side reaction. Note that the titration parameters should be adjusted to the low temperatures.
  • General: Extract the water with the KF oven method if the interfering components are thermally stable at oven temperature.
  • General: Mask or eliminate the interfering component, e.g., by adding N-ethylmaleimide in the case of thiols.
Find out more about the Karl Fischer oven method in our blog article.

Summary

Side reactions can negatively influence and falsify your results. Recognizing and avoiding side reactions in KF titration is therefore crucial for the most accurate determinations.

For more information, check out our blog series about frequently asked questions in Karl Fischer titration.

Download our free monograph:

Water determination by Karl Fischer Titration
Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.
Green hydrogen generation: A cross-disciplinary challenge rooted in electrochemistry

Green hydrogen generation: A cross-disciplinary challenge rooted in electrochemistry

Green hydrogen, produced from water electrolysis using renewable energy sources, is being explored as a strategy to reduce the dependence on fossil fuels and decarbonize chemical processes. From an environmental standpoint, this approach is extremely attractive given that mild conditions are used during electrolysis and there are no greenhouse gases produced when using the hydrogen in a fuel cell.

However, the economics of electrolysis and fuel cell systems for energy conversion relies heavily on the costs of electricity and of metals like nickel, platinum, iridium, and titanium. Electrolyzer operating expenses must be minimized for green hydrogen to become an economically viable option. The electricity input contributes heavily to cost. Thus, decreasing the cost of renewable energy is a necessary step. Solar panels becoming more efficient and affordable within the past decades is cause for optimism in this regard [1], but there is much more that can be done to increase the success of green hydrogen. More efficient electrolyzers could make better use of the input electricity and the development of cheaper and more durable components can reduce both the capital and operational costs.

Check out our other blog articles about green hydrogen and decarbonization of chemical processes below!

Cross-disciplinary interest in green hydrogen

Electrolyzers are primarily electrochemical devices with electrocatalysts responsible for water splitting (Figure 1). The scientific challenges related to optimizing electrolyzers are attracting the attention of researchers that are not traditionally trained in electrochemistry. The search for efficient HER (Hydrogen Evolution Reaction) and OER (Oxygen Evolution Reaction) electrocatalysts also piques the interest of inorganic chemists and physicists. Development of better membranes calls for expertise in organic and polymer chemistry. Optimization of catalyst inks and their interaction with substrates requires the know-how of a materials scientist. Heat and mass flow management within the fuel cell stack and balance of plant are engineering endeavors. Clearly, the ongoing development of green hydrogen technologies has encouraged the collaboration of scientists and engineers across many disciplines. The result is an influx of creativity and insight, as well as development of exciting new materials and techniques.

Figure 1. Diagram of the electrolysis of water (water splitting) with respective half reactions at the cathode and anode in alkaline and acidic media.

Back to basics

Working in an unfamiliar domain means there is a need for quickly getting up to speed with best practices and learning a new scientific vocabulary. For many institutions, education on electrochemical principles and laboratory skills was not a key focus area until recent years.

In some cases, the deficiency of fundamental electrochemical training has led to inconsistencies in the reporting of important performance indicators. The electrochemical community has taken note of this and called for a more rigorous approach. As a result, experts have stepped up and provided practical guidance for quantifying and reporting in this domain.

When investigating electrocatalyst materials it is necessary to have benchmarks and well-defined performance indicators. In 2013, a comprehensive benchmarking protocol for evaluating and reporting figures of merit for OER electrocatalysts was published.

This JACS article [2] provides practical advice on how to interpret the catalyst surface in terms of roughness and geometric surface area and how to perform and analyze measurements for valid comparisons of electrocatalytic performance.
A common source of confusion and inconsistency in electrochemical measurements is the use of various reference electrodes (RE). Electrocatalytic activity is judged by the overpotential needed for a specified production rate (i.e., the current density for the HER or OER process, Figure 1). A three-electrode setup is needed to measure the potential, and the RE is crucial for situating this potential on a relative scale, allowing comparison of measurements carried out by different groups and in various conditions.

Find out more about reference electrodes and their usage in our free Application Note.

A 2020 Viewpoint article in ACS Energy Letters [3] provides a detailed explanation of how to report the overpotential of an electrocatalyst, focusing on commonly used reference electrodes like Hg/HgO, Hg/Hg2Cl2 (SCE), and Ag/AgCl.
The reversible hydrogen electrode (RHE) is another commonly used RE that is extremely well-suited for HER and OER studies. A recent ACS Catalysis article [4] explains why the RHE is the ideal reference electrode for electrolysis research and explains how to prepare and work with an RHE. By convention, all standard redox potentials are reported versus the standard hydrogen electrode (SHE). The RHE is a pH-dependent extension of the SHE and refers to the reduction of a proton under non-standard conditions as described by the Nernst equation.
Electrolyzers operate under both acidic and alkaline conditions, thus, the HER and OER are studied across the pH scale (Figure 1). The RHE is suitable for use at any pH and it shares the same dependency on pH as the HER and OER.

A common ground to stand on

Finding common language and understanding between these different fields is vital. This JOC synopsis article [5] clarifies electrochemical concepts for organic chemists. The article is highly visual, providing schematics that link concepts like free energy, redox potential, and overpotential. Equilibrium thermodynamics helps to provide a common point of reference that all chemists can relate to.

Thermodynamic analysis is often applied to quantify the energy efficiency of electrolysis cells and stacks. A recent review article in the Journal of Power Sources [6] highlights diverging definitions for the energy efficiency coefficient from academic and industrial literature. The article provides derivations in various conditions and reminds readers that both electricity and heat must be accounted for in the analysis.

Summary

The articles highlighted in this blog post represent just a small fraction of the many resources available for building a common understanding and better collaboration among all researchers working on the improvement of green hydrogen technologies. When the COVID pandemic shut down laboratory work and travel for many people, the research community carried on with enthusiasm.

Online seminars and working groups held openly and without cost have brought scientists together across disciplines and from around the world. For example, the Electrochemical Online Colloquium was started in 2021. This ongoing series of lectures addresses essential topics in electrochemistry by providing educational content alongside the personal perspective of expert speakers.

The electrochemical community is acutely aware of the importance of transitioning to sustainable and climate-safe energy and chemical processes. Energy storage and conversion through green hydrogen is a promising strategy that requires scientific advancement to thrive. Thankfully, researchers from across many disciplines are bringing their skills and creativity to this topic while the electrochemical community continues to drive collaborative efforts and share their core knowledge.
To find out more about associated topics, download these free Application Notes from Metrohm Autolab.

References

[1] Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5°C Climate Goal; International Renewable Energy Agency: Abu Dhabi, 2020.

[2] McCrory, C. C. L.; Jung, S.; Peters, J. C.; et al. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135 (45), 16977–16987. doi:10.1021/ja407115p

[3] Niu, S.; Li, S.; Du, Y.; et al. How to Reliably Report the Overpotential of an Electrocatalyst. ACS Energy Lett. 2020, 5 (4), 1083–1087. doi:10.1021/acsenergylett.0c00321

[4] Jerkiewicz, G. Standard and Reversible Hydrogen Electrodes: Theory, Design, Operation, and Applications. ACS Catal. 2020, 10 (15), 8409–8417. doi:10.1021/acscatal.0c02046

[5] Nutting, J. E.; Gerken, J. B.; Stamoulis, A. G.; et al. “How Should I Think about Voltage? What Is Overpotential?”: Establishing an Organic Chemistry Intuition for Electrochemistry. J. Org. Chem. 202186 (22), 15875–15885. doi:10.1021/acs.joc.1c01520

[6] Lamy, C.; Millet, P. A Critical Review on the Definitions Used to Calculate the Energy Efficiency Coefficients of Water Electrolysis Cells Working under near Ambient Temperature Conditions. J. Power Sources 2020, 447, 227350. doi:10.1016/j.jpowsour.2019.227350

Post written by members of the Metrohm Autolab group, Utrecht, The Netherlands.
NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 3

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 3

What is Pyrolysis gasoline or «Pygas»?

Pyrolysis gasoline, also known as Pygas, is a byproduct of naphtha cracking during the production of ethylene and propylene. Pyrolysis gasoline is an easily flammable, colorless liquid with high aromatic contents and represents a mixture of light hydrocarbons (Figure 1). It is a high octane number mixture which contains aromatics, olefins, and paraffins ranging from C5 to C12.

Figure 1. Pyrolysis gasoline (or Pygas) shown here is nearly colorless, but extremely flammable.
Because of its high octane number, Pygas has a high potential for blending in various end-user products.  In addition, pyrolysis gasoline can be used as a component separator for benzene, toluene, and xylene. For this purpose, it is used as a component separation additive.

Pygas contains some undesired conjugated diolefins that when present in high quantities makes them unsuitable as a motor fuel. These conjugated diolefins are highly reactive to polymerization and can plug the downstream refining processes causing unwanted shutdowns and high costs for remediation. These compounds also affect the stability of commercial gasoline. Therefore, the conjugated diolefins content must be controlled.

The content of conjugated diolefins is indirectly measured as the «maleic anhydride value» (MAV), or as the «diene value» (DV). This parameter is usually determined by the Diels-Alder wet chemical method (UOP 326). Furthermore, the determination of Bromine Number (ASTM D1159) in pygas is useful as it indicates the degree of aliphatic unsaturation. These determination methods require several hours and must be analyzed by highly trained analysts. In contrast to using primary methods, near-infrared spectroscopy (NIRS) is a cost-efficient and fast alternative solution for the determination of MAV or DV and Bromine Number in pyrolysis gasoline.

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of pygas

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, an available solution for the determination of maleic anhydride value (MAV) or diene value (DV) and Bromine number are outlined which have been developed according to the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation).

Did you miss the first parts in this series about NIRS as a QC tool for the petrochemical industry? Find them all below!

Read our previous blog posts to learn more about NIRS as a secondary technique.

Analysis of Diene Value (DV) and Bromine Number (BN) in pygas with the DS2500 Liquid Analyzer

Historically, NIRS analysis of the diene value and Bromine Number in pygas has been considered to be complicated due to the presence of other non-conjugated dienes as well as alkenes that have similar molecular functional groups. In addition, the majority of the samples are a complex mixture of aromatics and alkanes that varies with process conditions in the ethylene production process, as well with as the different feedstocks used to produce ethylene (e.g., alkanes, naphtha, or gas oil). Also, DV is not reliant on one specific conjugated diolefin, but over a dozen different compounds including cyclopentadiene (a ring structure) and straight-chain diolefins with different chain lengths and side chains. As explained earlier, the diene value is usually determined by the Diels-Alder wet chemical method (UOP 326). Bromine Number (BN) is determined by electrochemical titration at 5 °C (ASTM D1159).

Now, spectroscopic analysis of these parameters in such a complicated system is made successful through a combination of stable NIRS measurements with the DS2500 Liquid Analyzer, and the Partial Least-Squares (PLS) modelling capabilities in the Vision Air complete software package.

Learn more about the Metrohm NIRS DS2500 Liquid Analyzer and Vision Air software here!
Results from NIRS analysis are obtained very rapidly, with no sample preparation required aside from the temperature equilibration of the sample prior to scanning. This makes it possible to monitor and control the process, which is simply not possible using other methods. NIRS measurements do not require highly trained analysts—disposable glass vials are the only things needed for the analysis!

Metrohm offers a related application note for the proper use of NIRS for pyrolysis gas analysis (Table 1).

Table 1. Metrohm’s NIRS solutions for pygas including application details and benefits.
Parameter Reference method   Norm NIRS Application Notes  NIRS benefits
Maleic Anhydride Value (MAV) or Diene Value (DV) Reflux / Hydrolysis / Titration UOP 326 AN-NIR-024 MAV or DV measured within one minute, without requiring sample preparation or use of any chemicals.  A major difference compared to 6–7 hours when using the primary reference methods.
Bromine Number (BN) Cooling / Titration ASTM D1159 AN-NIR-094 Bromine Number measured within one minute without needing chemical reagents or sample preparation.
Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Analyzers in our free brochure.

Application example: determination of DV and BN in pygas using the NIRS DS2500 Liquid Analyzer

The diene value and Bromine Number are key parameters for the quality control of pygas.  According to UOP 326, the maleic anhydride is refluxed with the sample in boiling toluene for three hours. Any unreacted maleic anhydride is hydrolyzed to maleic acid, extracted from the reaction mixture, and then titrated with sodium hydroxide. This wet chemical method requires several hours to perform by highly trained analysts. 

For the determination of the Bromine Number according to ASTM D1159, the sample must be cooled down below 5 °C to minimize side reactions like oxidation or substitution.

In contrast to primary methods, near-infrared spectroscopy (NIRS) is a cost-efficient and fast analytical solution for the determination of DV and BN in pyrolysis gasoline.

Figure 2. Quality control of pygas as performed by the Metrohm NIRS DS2500 Liquid Analyzer.
The obtained Vis-NIR spectra (Figure 2) were used to create a prediction model for the determination of DV and BN. The quality of the prediction model was evaluated using a correlation diagram, which displays the correlation between the Vis-NIR prediction and primary method value. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 3).
Figure 3. Correlation plots and figures of merit (FOM) for DV and Bromine Number in pygas.
This solution demonstrates that NIR spectroscopy is excellently suited for the analysis of diene value or maleic anhydride value in pygas in less than one minute without sample preparation or using any chemical reagents. In comparison to the wet chemical method in UOP 326, the time to result is a major advantage of using NIRS since a single measurement is performed within one minute instead of taking 6–7 hours with the primary method. Additionally, Bromine Number is easy to measure with NIRS without requiring any chemicals or sample preparation (such as cooling) as mentioned in ASTM D1159.

Want to learn more? Download our free Application Notes.

Summary

Near-infrared spectroscopy is an excellent choice for measuring MV / DV and BN in pygas. A simple feasibility study using your own pyrolysis gasoline samples will quickly indicate if the NIR spectra will be able to be correlated to the primary method values.  Positive aspects of using NIRS as an alternative technology are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

Future installments in this series

This blog article was dedicated to the topic of pyrolysis gasoline (pygas) and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. Future installments will be dedicated to other important applications in this industry. These topics will include:

 

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
The evolution of handheld 785 nm Raman spectroscopy: Raman extraction from fluorescence interference

The evolution of handheld 785 nm Raman spectroscopy: Raman extraction from fluorescence interference

MIRA DS (Metrohm Instant Raman Analyzer) is a handheld Raman system that identifies materials using 785 nm laser excitation. The advantages of using 785 nm Raman are well understood. Excitation with shorter wavelengths produces strong Raman scattering with short acquisition times. This results in a high signal-to-noise ratio and provides excellent spectral resolution with lower power draw. These are just some of the reasons that handheld Raman has become so popular over the last two decades.

The sensitivity of Raman at 785 nm also means that lower laser powers can be used. Lower laser powers help to protect sensitive samples from burning or ignition. The silicon detectors used at shorter wavelengths do not need to be cooled, further extending battery lifetimes. The net result is that 785 nm systems can be very small and still provide fast and accurate material identification for long hours in the field.

Learn more about how MIRA became mobile in our previous blog post.
However, while this is considered the «sweet spot» for both a strong signal and fluorescence mitigation among possible wavelengths, approximately 10% of Raman active materials fluoresce under interrogation with 785 nm Raman systems [1]. For example, Gum Arabic is a widely used filler and binding agent. When sampled with 785 nm systems, its fluorescence overwhelms the Raman signal (more on this subject later). Similarly, cutting agents (e.g., sucrose found in street drugs) fluoresce and can prevent positive identification of the target substance. Dyes can be problematic in the analysis of tablets, foodstuffs, art, and plastics as well. Often, weak Raman features can still be observed in fluorescent materials with 785 nm interrogation, but fluorescence mitigation is crucial for library matching.

Previous recommendations to overcome fluorescence

When fluorescence is an issue, 1064 nm laser excitation is often recommended. The tradeoffs include higher laser power, increased sample heating, longer interrogation times, and low Raman scattering efficiency. Often, this means larger instruments with shorter battery lifetimes. Instruments from some manufacturers require longer acquisition times that slow down sampling and can potentially damage the sample.

Is there a better way?

In a word, yes. SSE (Sequentially Shifted Excitation) can be used to remove fluorescent contributions to a Raman spectrum by using a laser that shifts the excitation wavelength as a function of the laser temperature. The result is a very large «handheld» system with a shoulder strap and a high price tag, partly due to the expensive laser used. Aside from the bulk and the cost, another issue with these systems is that the constant temperature cycling of the laser causes the system’s battery to have a short lifetime.

A Metrohm solution

Metrohm Raman has designed a fluorescence rejection system based on its compact MIRA DS package using an IPS single-mode 785 nm laser. The system is capable of producing excellent spectral resolution and flat baseline data with low laser power, short acquisition times, and all of the other excellent functionalities that users have come to expect from MIRA DS.

This fluorescence rejection system is built upon a MIRA DS platform, preserving all of its unique capabilities:

MIRA XTR DS

MIRA XTR DS is the evolution of Raman spectroscopy. It combines the smaller size, higher resolution, and lower power consumption of a 785 nm Raman instrument with patent-pending advanced algorithms to eXTRact Raman data, even from spectra that have strong fluorescence!
Figure 1. Comparison of Raman spectra of Gum Arabic powder measured by 1064 nm, 785 nm (MIRA DS), and XTR® (MIRA XTR DS).
Figure 1 contains Raman spectra from a fluorescent material, Gum Arabic powder, with traditional 785 nm and 1064 nm laser excitation, in addition to MIRA XTR DS. The improvement in resolution with XTR is obvious. Notice the very flat (uncorrected) baseline in the XTR spectrum on the bottom. This is crucial for library matching with a Pearson correlation, where the dot product between spectra and non-zero baselines contribute strongly to the correlation.

Learn more about MIRA XTR DS on our website.

Applications for MIRA XTR DS include Sensitive Site Exploitation / Intelligence Surveillance Reconnaissance (SSE/ISR) of clandestine labs and determination of synthetic routes to illicit products. MIRA XTR DS is designed for real world scenarios like the analysis of methamphetamine lab residues and identification of narcotics in street drug samples. This includes ID of narcotics, despite cutting agents that fluoresce and fail analysis at 785 nm. ORS™ combined with fluorescence rejection means that MIRA XTR DS can also delicately interrogate sensitive materials like colored explosive compounds.

Download our free White Paper below to find out more about the capabilities of MIRA XTR DS.

Classic applications improved with MIRA XTR DS

Lidocaine [2] is a local anesthetic that can also be used to cut cocaine because it enhances the immediate numbing sensation that many cocaine users associate with a high quality product. Since cocaine is typically present at only ~30% in street samples, its signal can be occluded by other components in the mixture. However, positive identification of common cutting agents like lidocaine can lead to further investigation of a suspect sample.

Traditionally, lidocaine was an issue for 785 nm Raman systems, as its fluorescence prevented both positive identification of lidocaine and detection of cocaine. MIRA XTR DS produces an excellent, fluorescence-free, resolved spectrum of lidocaine (Figure 2).

Figure 2. Comparison of Raman spectra of lidocaine hydrochloride measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
Diphenhydramine is another example of a common OTC drug that, when detected, may suggest darker dealings. It can be abused on its own, but it is also a potential precursor in the synthesis of methamphetamine. Diphenhydramine exhibits some fluorescence when interrogated with 785 nm Raman (Figure 3), but it is also typically present in mixtures with inert ingredients that fluoresce. For this type of analysis, SERS can be used to detect trace amounts of a substance. This is an excellent showcase for MIRA XTR DS, because it can perform both 785 nm Raman and SERS tests, while most 1064 nm systems currently on the market cannot be used for SERS analysis.
Figure 3. Left: MIRA XTR DS used for no-contact testing. Right: Comparison of Raman spectra of Diphenhydramine measured by 1064 nm SERS, 785 nm SERS (MIRA DS), and XTR SERS (MIRA XTR DS).
What’s the difference between Raman and SERS? Read our blog article to find out!

But MIRA XTR DS can do more!

With fluorescence mitigation, 785 nm Raman can be used more generally for material identification and chemical analyses.

Microcrystalline Cellulose

Microcrystalline cellulose (MCC) is another inert excipient that is commonly used in food production and the pharmaceutical industry. When interrogated with 785 nm Raman, its fluorescence can overwhelm the Raman signal and prevent identification and mixture matching (Figure 4).

Figure 4. Comparison of Raman spectra of MCC measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
Ketchup

Measurement of analytes in ketchup is a particularly interesting application, as it is a highly colored, complex mixture. With 785 nm testing, it shows fluorescence—with 1064 nm testing, it burns. But XTR analysis carries the added benefit of signal enhancement, returning a spectrum that clearly indicates the presence of trace lycopene in ketchup—the chemical that contributes its red color (Figure 5).

Figure 5. Comparison of Raman spectra of ketchup measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
Another important application demonstrates how MIRA XTR DS can distinguish imitation honey from the pure, unadulterated form in the pursuit of fraudulent food products, and that it shows promise for quantitative analysis. MIRA XTR DS extracts Raman spectra from materials that typically show fluorescence with 785 nm excitation, this time with sufficient resolution to detect different ratios of mixtures (Figure 6).
Figure 6. Comparison of Raman spectra of pure honey (left) and imitation honey (center) measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS). Right: Determination of the ratio of different mixtures of pure honey with adulterants using MIRA XTR DS. (Click image to enlarge.)

A powerful laboratory in the palm of your hand

Historically, Raman users dealt with fluorescence by using instruments with a 1064 nm laser. MIRA XTR DS combines the smaller size, higher resolution, and lower power consumption of a 785 nm laser with revolutionary machine learning to eXTRact Raman from fluorescent samples. The benefits are considerable!
  • Low power 785 nm laser interrogates sensitive samples without risk of ignition or burning.
  • Compact, pocket-sized design enables true single-handed operation of the device
  • The low power consumption means longer battery life for extended field use

MIRA XTR DS: all the best of handheld Raman with virtually unlimited applications.

Find out more about MIRA XTR DS

Download free white papers and learn more on our website.

References

[1] Christesen, S. D.; Guicheteau, J. A.; Curtiss, J. M.; Fountain, A. W. Handheld Dual-Wavelength Raman Instrument for the Detection of Chemical Agents and Explosives. Opt. Eng. 2016, 55 (7), 074103. DOI:10.1117/1.OE.55.7.074103

[2] Barat, S. A.; Abdel-Rahman, M. S. Cocaine and Lidocaine in Combination Are Synergistic Convulsants. Brain Res. 1996, 742 (1), 157–162. DOI:10.1016/S0006-8993(96)01004-9

Post written by Dr. Melissa Gelwicks, Technical Writer at Metrohm Raman, Laramie, Wyoming (USA).
Cyclic voltammetry (CV) – the essential analytical technique for catalyst research

Cyclic voltammetry (CV) – the essential analytical technique for catalyst research

Cyclic voltammetry (CV) is the backbone of most electrochemical research and is an essential electrochemical technique that allows researchers to explore candidate catalysts in greater depth. When coupled with modeling, a systematic goal-focused protocol will supply a range of data that will inform the user of more novel techniques and complex setups. This disciplined approach will save time in the long run, and is especially helpful to those who may have limited access to electrochemical instrumentation in a busy laboratory.
This article provides an overview of possible research goals when using CV along with relevant examples from scientific literature with the approach in action.
Electrocatalysis (ECAT) is defined as the catalysis of an electrode reaction. The electrocatalytic effect leads to an increase of the standard rate constant of the electrode reaction—resulting in a higher current density, or to a decrease in overpotential when other rate limiting steps are involved. The study of an electrocatalytic process requires characterization of the mechanism and kinetics of the electrode reaction. Forced convection methods can offer the advantage of reducing the contributions from mass-transport and providing direct access to the kinetic and mechanistic information.

In the last decade, a greater understanding of critical electrochemical transformations has been established, particularly those that involve water, hydrogen, and oxygen [1]. The expansion of our understanding in this realm was only possible because of the use of critical electrochemical techniques. This has allowed researchers to not only explore a wider variety of catalysts, but explore them in greater detail.

To respond to the potential volume of exploration which may discover more cost-effective and renewable materials that are not at the stage of critical depletion, a systematic approach to analytical research is required.

As always, new techniques are constantly being developed, but the gold standard technique of exploration of catalysts with cyclic voltammetry (CV) is still recommended as the starting point for researchers.
Instrumentation for CV analysis of catalysts from Metrohm Autolab.

Experimental Goals and Procedure Selection

To effectively explore a candidate catalyst, it is important to consider what the experimental goal is and then choose the procedure accordingly. Examples of possible goals are listed in the following sections along with suggested procedures and/or techniques.

Exploring a new system

Determine the (E) stability window of the electrolyte [2]

Method: Perform CV measurement in broad voltage (E) window, using an inert electrode (e.g. glassy carbon) and investigate the general redox behavior of the electrocatalyst material.

 

Investigate the general redox behavior of the electrocatalyst material [2]

Method: Perform CV measurement in a broad voltage (E) window, using a well understood electrolyte and new electrocatalyst.

Determine the electrode surface area for quantitative comparisons [3–5]

Method: Various methods that are material dependent: using a well-defined surface reaction (e.g. stripping or oxide formation) or analysis of electrochemical double layer capacitance (Cdl).

 

Investigate the stability of the electrocatalyst [6, 7]

Method: Perform repetitive CV measurements over several hundred cycles or during several days.

Probing a specific electrochemical reaction

Determine if a reaction is reversible (fast electron transfer kinetics), quasi reversible (slow kinetics), or irreversible (governed by other factors) [8, 9

Method: Perform CV measurements at various scan rate values, then examine dependencies for the peak position (Epeak) and peak height (Ipeak) on the scan rate.

Determine the apparent activation energy of the reaction [10]

Method: Perform CV measurements at various temperatures, then analyze electrochemical Arrhenius plots of log j vs. 1/T.

Combining CV with additional techniques to confirm results and deepen understanding

Determine the molecular structure of products or intermediates at a specific instance of the reaction [9–12]

Method: Perform CV measurement with in-situ spectroscopic determination (spectroelectrochemistry via UV/Vis/NIR or Raman spectroscopy).

Investigate material deposited or removed from the electrode surface during the electrochemical measurement [13]

Method: Measure the mass change at the electrode surface during a CV measurement using electrochemical quartz crystal microbalance (EQCM).

Investigate products and short-lived intermediates via their electrochemical response [14, 15]

Method: Perform bipotentiostat (two working electrodes) measurements in a ring/disk configuration (RRDE).

From action to reactions in the literature

This paper from the Nissan Fuel cell research center (NFCRC) summarizes the analytical approach for reduction of Pt loading in fuel cell catalyst layers (CL) [7]. Using a combined experimental and theoretical approach, they clearly outline the important properties required to measure experimentally or model to reach their goal of reducing the amount of Pt used in the CL. 

Focal parameters for exploration:

 

1. Catalyst Microstructure

Research goal: Determine the electrode surface

Using microscope images combined with the Cdl (double layer capacitance) and ionomer coverage, the researchers were able to analyze and quantify their catalyst layer. They used CV to determine ionomer coverage over the carbon by comparing Cdl values (wet versus dry).

 

2. Transport Properties

Research goal: Investigate material deposited or removed from the electrode surface during the electrochemical measurement

Additional research investigating the electrode surface was performed with CV. Using a rotating disk electrode, the researchers were able to determine the gas transport resistance by measuring the ORR (oxygen reduction reaction). CV also allowed the determination of the Pt roughness factor.

 

3. I-V performance

Research goal: Use CV I-V to calculate the fuel cell performance

I-V performance is a typical measurement for the overall performance of the fuel cell. A potentiostat is needed to measure the actual I-V curve in order to determine the Pt loading so that the I-V performance can be interpreted and compared among various samples.

    This paper illustrates the value of systematic exploration of catalysts with CV to give a comprehensive overview of attributes, structure, and reactions before moving on to more complex setups.

    Your initial investigations with CV may not provide all of the answers at first glance, but you can then move on to more complex setups and experiments with complete insight.

    Curious about electrochemistry?

    Metrohm has you covered.
    References

    [1] Seh Z. W.; Kibsgaard J.; Dickens C. F.; et al. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, 6321. doi:10.1126/science.aad4998

    [2] Kübler, P.; Sundermeyer, J. Ferrocenyl-Phosphonium Ionic Liquids – Synthesis, Characterisation and Electrochemistry. Dalton Trans. 2014, 43 (9), 3750–3766. doi:10.1039/C3DT53402B

    [3] Biegler, T.; Rand, D. A. J.; Woods, R. Limiting Oxygen Coverage on Platinized Platinum; Relevance to Determination of Real Platinum Area by Hydrogen Adsorption. J. Electroanal. Chem. Interfacial Electrochem. 1971, 29 (2), 269–277. doi:10.1016/S0022-0728(71)80089-X

    [4] Trasatti, S.; Petrii, O. A. Real Surface Area Measurements. Int. Union Pure Appl. Chem. 1991, 63 (5), 711–734. doi:10.1351/pac199163050711

    [5] Kinkead, B.; van Drunen, J.; Paul, M. T. Y.; et al. Platinum Ordered Porous Electrodes: Developing a Platform for Fundamental Electrochemical Characterization. Electrocatalysis 2013, 4 (3), 179–186. doi:10.1007/s12678-013-0145-2

    [6] Pilapil, B. K.; van Drunen, J.; Makonnen, Y.; et al. Ordered Porous Electrodes by Design: Toward Enhancing the Effective Utilization of Platinum in Electrocatalysis. Adv. Funct. Mater. 2017, 27 (36), 1703171. doi:10.1002/adfm.201703171

    [7] Xing, L.; Hossain, M. A.; Tian, M.; et al. Platinum Electro-Dissolution in Acidic Media upon Potential Cycling. Electrocatalysis 2014, 5 (1), 96–112. doi:10.1007/s12678-013-0167-9

    [8] Rountree, E. S.; McCarthy, B. D.; Eisenhart, T. T.; et al. Evaluation of Homogeneous Electrocatalysts by Cyclic Voltammetry. Inorg. Chem. 2014, 53 (19), 9983–10002. doi:10.1021/ic500658x

    [9] Sokolov, S.; Sepunaru, L.; Compton, R. Taking Cues from Nature: Hemoglobin Catalysed Oxygen Reduction. Appl. Mater. Today 2017, 7, 82–90. doi:10.1016/j.apmt.2017.01.005

    [10] Barbosa, A. F. B.; Oliveira, V. L.; van Drunen, J.; et al. Ethanol Electro-Oxidation Reaction Using a Polycrystalline Nickel Electrode in Alkaline Media: Temperature Influence and Reaction Mechanism. J. Electroanal. Chem. 2015, 746, 31–38. doi:10.1016/j.jelechem.2015.03.024

    [11] Hernández, C. L.; González García M. B.; Santos , D. H.; et al. Aqueous UV-VIS Spectroelectrochemical Study of the Voltammetric Reduction of Graphene Oxide on Screen Printed Carbon Electrodes. doi:10.1016/j.elecom.2016.01.017

    [12] Görlin, M.; de Araújo, J. F.; Schmies, H.; et al. Tracking Catalyst Redox States and Reaction Dynamics in Ni-Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts: The Role of Catalyst Support and Electrolyte PH. J Am Chem Soc 2017, 139 (5), 2070–2082. doi:10.1021/jacs.6b12250

    [13]  Lee, C-L.; Huang, K-L.; Tsai, Y-L.; et al. A Comparison of Alloyed and Dealloyed Silver/Palladium/Platinum Nanoframes as Electrocatalysts in Oxygen Reduction Reaction. Electrochem. Commun. 2013, 280–285. doi:10.1016/j.elecom.2013.07.020

    [14] Vos, J. G.; Koper, M. T. M. Measurement of Competition between Oxygen Evolution and Chlorine Evolution Using Rotating Ring-Disk Electrode Voltammetry. J. Electroanal. Chem. 2018, 819, 260–268. doi:10.1016/j.jelechem.2017.10.058

    [15] Kocha, S. S.; Shinozaki, K.; Zack, J. W.; et al. Best Practices and Testing Protocols for Benchmarking ORR Activities of Fuel Cell Electrocatalysts Using Rotating Disk Electrode. Electrocatalysis 2017, 8 (4), 366–374. doi:10.1007/s12678-017-0378-6

    Post written by members of the Metrohm Autolab group, Utrecht, The Netherlands.