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NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 1

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 1

Introduction to the petrochemical and refining industry

Oil and gas for fuel are produced in nearly every corner of the globe, from small private wells generating around 100 barrels a day, to the large bore wells producing upwards of 40 times that volume. Despite this great variation in size, many parts of the refining process are quite similar.

Chemicals derived from petroleum or natural gas, so-called «petrochemicals», are an essential part of the contemporary chemical industry. The field of petrochemistry became increasingly popular around the early 1940’s during the second world war. At that time there was a growing demand for synthetic products which was a great driving force for the development of petrochemical products.

Oil refining aims to provide a defined range of products according to agreed specifications. Simple refineries use a distillation column (Figure 1) to separate crude oil into different fractions based on their chemical properties, and the relative quantities are directly dependent on the crude oil used. Therefore, it is necessary to obtain a range of crudes that can be blended into a suitable feedstock to produce the required quantity and quality of end products.

The basic products from fractional distillation are shown in Figure 1.

Wallace Carothers, inventor of polyamide.
Figure 1. Illustration of a fractionating distillation column used for the purposes of refining crude oil into several desirable end products.

Near-infrared (NIR) spectroscopy is a technique that is particularly suited for making quality control of these end products more efficient and cost-effective for manufacturers. Furthermore, NIRS is recognized and accepted by ASTM as an alternative method to other techniques. Dedicated ASTM methods for method development, method validation, and results validation are presented later in this article.

Read on for a short overview on NIR spectroscopy followed by application examples for the petrochemical and refinery industry to learn how petrochemical producers and refineries alike can benefit from NIRS.

NIR technology: a brief overview

The interaction between light and matter is a well-known process. Light used in spectroscopic methods is typically not described by the applied energy, but in many cases by the wavelength or in wavenumbers.

A NIR spectrometer such as the Metrohm NIRS DS2500 Petro Analyzer measures this light-matter interaction to generate spectra such as those displayed in Figure 2. NIRS is especially sensitive to the presence of certain functional groups like -CH, -NH, -OH, and -SH. Therefore, NIR spectroscopy is an ideal method to quantify different QC parameters like water content (moisture), cetane index, RON/MON (research and motor octane numbers), flash point, and cold filter plugging point (CFPP), just to name a few. Furthermore, the interaction is also dependent upon the matrix of the sample itself, which also allows the detection of physical and rheological parameters like density and viscosity.

Figure 2. Diesel spectra resulting from the interaction of NIR light with the respective samples.

All of this information is contained in a single spectrum, making this method suitable for quick multiparameter analysis. Liquid samples such as oils are secured within an appropriate container or vial (Figure 3), then placed as-is on the smart vial holder.

Figure 3. Liquid sample placement for NIR spectra measurement on the smart vial holder from Metrohm.

The measuring mode is referred to as «transmission», generally an appropriate procedure for analyzing liquids. For transmission measurement (Figure 4), the NIR light will travel through the sample while being absorbed. Unabsorbed NIR light passes to the detector. In less than 60 seconds the measurement is completed and the results are displayed.

Figure 4. A. Measurements of liquids are typically done with disposable vials. B. The NIRS measurement mode is known as transmission, where light travels through the sample while being absorbed (from left to right in the illustration).

The procedure to obtain NIR spectra already highlights two major advantages of NIR spectroscopy compared to other analytical techniques: simplicity regarding sample measurement, and speed:

  • Fast technique with results in less than a minute.
  • No sample preparation required – measure samples as-is.
  • Low cost per sample – no chemicals or solvents needed.
  • Environmentally friendly technique – no waste generated.
  • Non-destructive – precious samples can be reused after analysis.
  • Easy to operate – inexperienced users are immediately successful.

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

Where can NIRS be used in the refining process?

The refining process can be divided into three different segments:

  • Upstream
  • Midstream
  • Downstream

Upstream describes the process of converting crude oil into intermediate products. Refineries are usually very large complexes with several hazardous explosive areas. Therefore, operators are reluctant to transport samples from the different processes to the laboratory. Even the process of obtaining samples for analysis at external QC laboratories is laborious and can require significant paperwork and certified transport services. For obvious reasons, in most cases inline measurements are preferred. These types of measurements are typically done by process NIRS analyzers.

Read more about the difference between atline, online, and inline analyses in our blog post.

Curious about NIRS analyzers for inline process measurements, even in explosive areas? Visit our website to learn more.

Midstream, shown here in Figure 5, offers many more opportunities for the Metrohm DS2500 Petro Analyzer to assist in quality control.

Figure 5. Flowchart of how crude oil becomes gasoline at the local gas station, and where NIRS can perform quality checks during the process.

Fuel is constantly checked for quality when it is received as well as supplied, and in addition to this many terminals also test fuel quality prior to offloading the trucks. The total time for receiving and offloading fuel into a storage tank is approximately 30 minutes, so a fast analysis technique like NIRS is very advantageous.

Downstream at fuel depots and gas stations, the regulatory agencies require measurement of many of the same quality parameters as in the production of gasoline and diesel, and this can also be accomplished with NIRS. There is a significant advantage if the analysis can be done on-site using fresh samples and without the hassle of needing to transport them to testing laboratories.

Mobile NIRS fuel testing using the Metrohm NIRS XDS RapidLiquid Analyzer (XDS-RLA) has been successfully implemented in a number of countries where they enjoy the benefits of having instantaneous on-site results for gasoline and diesel testing. The calibrations developed on the XDS-RLA are easily transferrable to the DS2500 Petro Analyzer. The DS2500 Petro Analyzer does not require trained analysts, and the calibrations do not require constant maintenance, making this an ideal way to monitor different fuels at service stations and more.

Figure 6. Examples of mobile fuel testing with the Metrohm DS2500 Petro Analyzer.

Learn more about the possibilities of petrochemical analysis with Metrohm DS2500 Analyzers in our free brochure.

NIRS as an ASTM compliant tool for QC

Method development

ASTM E1655: Standard Practices for Infrared Multivariate Quantitative Analysis

«These practices cover a guide for the multivariate calibration of infrared spectrometers used in determining the physical or chemical characteristics of materials. These practices are applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 to 2500 nm) through the mid infrared (MIR) spectral region (roughly 4000 to 400 cm-1).»

Multivariate analysis of petroleum products

ASTM D8321: Standard Practice for Development and Validation of Multivariate Analyses for Use in Predicting Properties of Petroleum Products, Liquid Fuels, and Lubricants based on Spectroscopic Measurements

«This practice covers a guide for the multivariate calibration of infrared (IR) spectrophotometers and Raman spectrometers used in determining the physical, chemical, and performance properties of petroleum products, liquid fuels including biofuels, and lubricants. This practice is applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 nm to 2500 nm) through the mid infrared (MIR) spectral region (roughly 4000 cm-1 to 40  cm-1).»

Method validation

ASTM D6122: Standard Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared Spectrophotometer, and Raman Spectrometer Based Analyzer Systems

«This practice covers requirements for the validation of measurements made by laboratory, field, or process (online or at-line) infrared (near- or mid-infrared analyzers, or both), and Raman analyzers, used in the calculation of physical, chemical, or quality parameters (that is, properties) of liquid petroleum products and fuels.»

Results validation

ASTM D8340: Standard Practice for Performance-Based Qualification of Spectroscopic Analyzer Systems

«This practice covers requirements for establishing performance-based qualification of vibrational spectroscopic analyzer systems intended to be used to predict the test result of a material that would be produced by a Primary Test Method (PTM) if the same material is tested by the PTM.»

Typical NIRS applications and parameters for the petrochemical and refinery industry

Petrochemicals are subject to standardized test methods to determine their chemical, physical, and tribological properties. Laboratory testing is an indispensable part of both research and development and quality control in the production of petrochemicals. The following test parameters are typically important to measure in the petrochemical and refinery industry (Table 1).

Table 1. Examples for use of NIRS for selected petrochemical QC parameters.
Specific Gravity (API) Gravity meter ASTM D298

AN-NIR-022

AN-NIR-024

AN-NIR-025

AN-NIR-041

AN-NIR-053

AN-NIR-071

AN-NIR-075

AN-NIR-080

AN-NIR-086

AN-PAN-1052

Boiling Point Distillation ASTM D2887
Cold Filter Plugging Point (CFPP) Standardized filter device ASTM D6371
Pour Point Pour Point analyzer ASTM D97
Cloud Point Cloud Point analyzer ASTM D2500
Flash Point Flash Point tester ASTM D93
Viscosity Viscometer ASTM D445
Color Colorimeter ASTM D1500
Density Densimeter ASTM D792
Fatty Acid Methyl Ester (FAME) FTIR ASTM D7806
Reid Vapor Pressure RVP analyzer ASTM D323
PIANO (Paraffins, Isoparaffins, Aromatics, Naphthenes, Olefins) Gas chromatograph ASTM D6729
Octane Number (RON/MON) CFR Engine ASTM D2699

ASTM D2700

Cetane Number CFR Engine ASTM D613
Diene value / MAV index Titration UOP 327-17
Parameter Conventional method ASTM method Relevant NIRS Application Notes

Future installments in this series

This article is a general overview of the use of NIR spectroscopy as the ideal QC tool for the petrochemical / refinery industry. Future installments will be dedicated to the most important applications and will include much more detailed information. Don’t miss our next blogs on the topics of:

 

  • Gasoline / Diesel / Jet fuel
  • Pyrolysis gasoline (Pygas)
  • Lubricants
  • ASTM Norms

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.

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

Nonaqueous acid-base titrations are widely used in several industries, including the petrochemical  and pharmaceutical sectors. Whether you are determining the acid or base number (AN or BN) in oils or fats, titrating substances that are insoluble in water, or quantifying products with different strengths of acidity or alkalinity separately, nonaqueous acid-base titration is the method of choice.

If you already have some experience performing nonaqueous acid-base titrations, you may remember that there are several challenges to overcome in comparison to aqueous acid-base titrations.

In this blog post, I would like to cover some of the most typical issues that could pop up during nonaqueous acid-base titrations and discuss how to best avoid them. An important point to note is that there is no single solution regarding how to perform any nonaqueous acid-base titration correctly. The right procedure depends highly on the solvent and titrant used.

What is a nonaqueous acid-base titration?

Before discussing nonaqueous titrations, first let’s talk a little bit about aqueous acid-base titrations.

Here, a sample is dissolved in water, and depending of the nature of the sample (whether it is acidic or basic) a titration is performed either using aqueous base or aqueous acid as titrant. For indication, a glass pH electrode is used.

However, sometimes due to the nature of the sample, aqueous titration is not possible. Nonaqueous acid-base titration is used when:

  • the substance of interest is not soluble in water
  • samples are fats or oils
  • components of mixtures of acids or bases have to be determined separately by titration

In these cases, a suitable organic solvent is used to dissolve the sample instead of water. The solvent:

  • should dissolve the sample and not react with it
  • permits the determination of components in a mixture
  • if possible, should not be toxic

The solvents that are most often used include ethanol, methanol, isopropanol, toluene, and glacial acetic acid (or a mixture of these). Titrants are not prepared with water but rather in solvent. Frequently used nonaqueous basic titrants are potassium hydroxide in isopropyl alcohol or sodium hydroxide in ethanol, and a common nonaqueous acidic titrant is perchloric acid in glacial acetic acid.

Due to the nature of nonaqueous solvents, they are normally poor conductors and do not buffer well. This makes indication a bit challenging because the electrode must be suitable for such sample types. Therefore, Metrohm offers the Solvotrode which is developed specifically for nonaqueous titrations.

This pH electrode offers the following advantages over a standard pH electrode:

  • Large membrane surface and a small membrane resistance for accurate reading, also in poorly buffered solutions
  • A flexible ground-joint diaphragm which can easily be cleaned even when contaminated with oily or sticky samples, additionally it offers a symmetrical outflow for outstanding reproducibility
  • The electrode is shielded and is therefore less sensitive to electrostatic interferences
  • It can be used with any nonaqueous electrolyte such as lithium chloride in ethanol

In the following sections I will discuss the most common mistakes when performing potentiometric nonaqueous acid-base titrations and how you can avoid them.

Electrostatic effects

The influence of electrostatic effects during analysis is normally negligible. However, maybe you have once seen a curve like the one below which looks relatively normal until suddenly a spike occurs.

Figure 1. Titration curve with a spike which might have occurred from an electrostatic interference.

This is then an indication of an electrostatic effect. However, where does it come from and how can we overcome this?

Electrostatic charge can be generated from many sources, such as friction. For example, while walking across a surface you will generate an electrostatic charge which will be stored in your body. You have probably touched the doorknob after walking across a carpeted space in your socks and obtained a small electric shock—this is the discharge of built up electrostatic charge. If we now assume that you are electrostatically charged and then you approach an electrode that is currently measuring (in use), this will result in a spike (Figure 1). Therefore, it is essential to make sure that you are either properly discharged or that you do not approach the electrode during measurement. You can avoid this issue by wearing the appropriate clothes. ESD (electrostatic discharge) clothes and shoes are mostly recommended when performing nonaqueous titrations.

Blocked diaphragm

A blocked diaphragm is another point which occurs more regularly during nonaqueous titrations. Due to the oily and sticky sample, you might have seen that the electrode diaphragm is clogged and cannot be opened anymore. What should you do then?

In most cases, you can place the electrode in a beaker of warm water overnight. This treatment often helps to loosen the diaphragm. To completely prevent the diaphragm from clogging, a Solvotrode with easyClean technology should be used. With this electrode, electrolyte is released by pressing the head ensuring that the diaphragm is not blocked.

Choice of electrolyte and storage solution

We recommend two types of electrolyte for nonaqueous titrations.

For titrations with alkaline titrants: tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene glycol

For titrations with acidic titrants: lithium chloride c(LiCl) = 2 mol/L in ethanol

Please make sure to store the electrode in the same electrolyte with which it is filled.

Checking the electrode according to ASTM D664

To check whether the Solvotrode is still in good working condition, perform a test according to ASTM D664 using aqueous buffer solutions of pH 4 and 7. The procedure is as follows:

  • Measure the potential of buffer pH 4.0 while stirring and note the value after 1 minute
  • Remove the electrode and rinse it well with deionized water
  • Measure the potential of buffer pH 7.0 while stirring and note the value after 1 minute
  • Calculate the mV difference between the reading of buffers 4.0 and 7.0
  • The difference must be larger than 162 mV (20–25 °C) to indicate an electrode in good shape

If the measured potential difference is less than 162 mV, the electrode requires maintenance. Lift the flexible sleeve of the ground-joint diaphragm to let some electrolyte flow out. Repeat the measurement according to the steps above. If the value is still less than 162 mV, clean the electrode or replace it.

Proper rinsing and cleaning

Proper rinsing is essential if you want to obtain reliable results. Otherwise, the curve might flatten and the equivalence points are no longer recognizable. Figure 2 illustrates this phenomenon well.

Figure 2. Different determinations according to ASTM D664. With time, the start potential of the curves shifts which indicates an unsuitable cleaning procedure.

The sample is the same, however, you see that the equivalence point and starting potential begin to shift and the curves become flatter. This indicates an improper cleaning procedure between measurements. The corresponding electrode is shown in Figure 3.

Figure 3. Appearance of the electrode used in Figure 2 after five measurements.

This electrode was certainly not cleaned properly! Anyone who performs a nonaqueous titration must consider which solvent might best dissolve the residue—this is not an issue that other analysts can easily solve due to the nature of each individual sample. However, do not ignore an electrode with such an appearance.

Conditioning the glass membrane correctly

As you may remember from our previous blog post about pH measurement, it is essential that the hydration layer of the glass membrane stays intact. Nonaqueous solvents dehydrate the glass membrane rather quickly. A change in the hydration layer can have an impact on the measured potential, therefore it is important that the hydration layer is always in the same state before starting a titration to achieve the most reproducible results.

Proper electrode immersion depth

This can be established with a conditioning step of the glass membrane to rebuild the hydration layer. However, if the solvent is able to remove the hydration layer faster than it takes to perform a titration, this can lead to ghost equivalence points. Therefore, the electrode should be completely dehydrated and kept like this for all further titrations.

 

Polar solvents (e.g., ethanol, acetone, isopropyl alcohol, or mixtures with toluene)

Water-free solvents (e.g., dimethylformamide, acetonitrile, acetic anhydride, or mixtures of these)

Preparation of electrode

Store only the pH membrane (not the diaphragm) in deionized water overnight to build up a proper hydration layer.

Lift the flexible sleeve to allow some electrolyte to flow out.

Dehydrate the pH membrane by placing only the pH membrane (not the diaphragm) in the solvent you will use afterwards for titration.

Lift the flexible sleeve to allow some electrolyte to flow out.

Conditioning of glass membrane Place the pH membrane (bulb only) into deionized water for 1 minute. Place the pH membrane (bulb only) into the corresponding solvent for 1 minute.
Rinsing procedure Rinse electrode with 50–70% ethanol. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with 50–70% ethanol. Rinse electrode with glacial acetic acid. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with glacial acetic acid.
Remarks Make sure to always keep the bulb of the electrode in deionized water for the same time duration, otherwise the thickness of the hydrated layer (and therefore the response) may vary. Avoid any contact of the electrode with water as this can induce a reaction with the solvent causing ghost equivalence points and irreproducible results.

Maintenance of burets

It is not only the electrode that needs some special attention when performing nonaqueous titrations, but also the electrical buret. Some special maintenance is required since alkaline nonaqueous titrants are especially aggressive and they tend to crystallize, therefore leakage of the buret is likely.

The buret must be maintained on a regular basis according to the manufacturer’s instructions. Metrohm recommends the following procedure:

  • For shorter titration breaks, it is recommended to refill the cylinder with titrant (especially with OMNIS)
  • Clean the buret with deionized water at the end of the day
  • Lubricate the cylinder unit on the centering tube and on the cylinder disc

Also check the corresponding manual of the buret. The most important points are mentioned there which will lead to a longer working life of the buret.

Thermometric titration as an alternative

One alternative to using potentiometric nonaqueous acid-base titration is thermometric titration (TET), depending on the sample and analyte to be measured. Thermometric titration monitors the endothermic or exothermic reaction of a sample with the titrant using a very sensitive thermistor.

The benefit of TET over potentiometric titration is clearly the maintenance-free sensor which does not require any conditioning nor electrolyte refilling. More information about thermometric titration can be found in our previous blog posts below.

Summary

Hopefully this article has provided you with information about the main problems encountered during nonaqueous titrations. First, make sure that all electrostatic influences are eliminated. This will save a significant amount of troubleshooting. Then prepare and treat your electrode correctly before, during, and after titration. Make sure to condition the electrode right before your first measurement!

Of special importance here is the solvent you plan to use. If it is a polar solvent, the electrode should be conditioned in deionized water. If nonpolar solvents like acetic anhydride are used, the electrode should be dehydrated first. Between measurements, the electrode should be cleaned with a suitable solvent and the diaphragm should be opened on occasion.

Last but not least, take care of your buret. Maintain it regularly and replace it whenever necessary. With this advice, performing nonaqueous titrations should be a breeze!

For more information

about nonaqueous titrations, download our monograph:

Nonaqueous titration of acids and bases with potentiometric endpoint indication

Post written by Iris Kalkman (Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland) and Dr. Sabrina Gschwind (Head of R&D at Metroglas, Affoltern, Switzerland).

ASTM D6304: Easier determination of moisture in petroleum products

ASTM D6304: Easier determination of moisture in petroleum products

Water in petroleum products, such as lubricating oils, jet fuel, or other similar products can have deleterious effects. Moisture is often associated with corrosion and engine wear. Knowing the water content of petroleum products can prevent damage to costly infrastructure and ensure safer operations.

ASTM D6304 «Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration» is a standard that is often cited for moisture determination in the specifications of various petroleum products. It has been recently updated (January 2021) and now offers three procedures for accurate moisture determination.

The direct sample injection into the titration cell (Procedure A) is recommended for low viscosity samples without expected interferences. An oven (Procedure B) or water evaporator accessory (Procedure C) can be used to analyze samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

In this blog post I want to introduce these three procedures, and then discuss when it is appropriate to use each of them.

Determining the moisture content in petroleum products doesn’t have to be messy. Visit our website to learn more about the new automated measurement capabilities allowed with ASTM D6304.

A coulometric Karl Fischer Titrator such as the 851 Titrando from Metrohm is the basis for all three procedures of ASTM D6304.

Direct injection (Procedure A)

The direct sample injection into the titration cell is recommended for low viscosity samples without expected interferences. An aliquot of known mass or volume is injected into the conditioned titration cell of a coulometric Karl Fischer apparatus, where it is titrated automatically, and the results calculated.

Method D6304 permits the use of coulometric generator electrodes with and without diaphragm. We recommend the use of the generator electrode with diaphragm, due to the low water content of the samples.

Not all petroleum products are soluble in Karl Fischer reagent and phase separation can occur when using Procedure A. If phase separation occurs, the reagents need to be replaced. The number of samples which can be analyzed without phase separation depends on the volume and type of sample, the volume of reagent, and the sample solubility in the reagent.

The generator electrode with diaphragm is recommended for water determination according to ASTM D6304 Procedure A.

However, for these kinds of samples, Procedures B or C are often the better solution. The same is the case if your sample contains interfering substances.

For more information about ASTM D6304 Procedure A, download our free Application Bulletin (AB-209). For more tips and tricks about how to improve your Karl Fischer titration, have a look at our blog series: «Frequently asked questions in Karl Fischer titration».

Water extraction using an oven (Procedure B)

An oven (Procedure B) can be used to analyze samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

For the analysis, a representative sample is weighed into a glass vial, which is sealed immediately. The vial is then heated in an oven to extract any water. The vaporized water is carried into the conditioned Karl Fischer titration cell by means of a dry carrier gas where it is titrated.

Schematic drawing of the Karl Fischer oven method.

The ideal temperature used for the evaporation depends on the sample. The 874 Oven Sample Processor can perform a temperature gradient test to determine the optimal temperature for removing water without degrading the sample.

To learn more about the oven method, its working principle and its advantages, check out our blog post: «Oven method for sample preparation in Karl Fischer titration».

Watch our LabCast video below to see the working principle and advantages of using Procedure B.

For more information about using the KF oven method for ASTM D6304 Procedure B, download our free Application Bulletin (AB-209) or free Application Note (AN-K-070).

Just want the highlights? Have a look at our short flyer about how ASTM D6304 has become much easier!

Water extraction using an evaporator (Procedure C)

Instead of using an oven, Procedure C explains how a water evaporator can be used for the water extraction of samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

In this procedure, an aliquot of sample is transferred into a heated chamber containing a suitable solvent (most often, toluene). The temperature of the heated chamber depends on the solvent used. The water vaporizes along with the solvent in an azeotrope distillation. The azeotrope is then transferred into the conditioned Karl Fischer titration cell via a dry non-reactive carrier gas. 

Schematic drawing of the evaporator method.

If you wish to read more about the three procedures and their advantages and disadvantages, download our White Paper: «Moisture in petroleum products according to ASTM D6304».

When to use which procedure

Procedure A is mainly suited for liquid samples with a low viscosity, such as diesel fuel, jet fuel, or aromatics. A low viscosity is required in order to be able to add the sample easily into the Karl Fischer titration cell. Furthermore, the samples require a good solubility in Karl Fischer reagent. Otherwise phase separation will occur, which requires the replacement of the Karl Fischer reagents. While the reagent exchange can be automated, time is still required until the reagents reach dryness again.

Even if samples are soluble in Karl Fischer reagents, there might still be issues with using Procedure A due to the sample matrix creating side reactions and thus false results. In this case Procedure B or C are the better option.

Procedure B is suitable for all kinds of samples, regardless of their viscosity or matrix composition. It is only the evaporated water that is transferred into the titration cell, leaving the sample as well as interfering matrix components remaining in the sealed vial, which can be simply disposed of after the analysis. For this reason, the reagent exchange frequency is greatly reduced, saving costs, as less reagent is required. Depending on the workload in your lab, it is even possible to fully automate the analysis including reagent exchange using an automated Karl Fischer oven.

The 874 Karl Fischer Oven Processor with an 851 Titrando for a fully automated analysis according to ASTM D6304 Procedure B.

Procedure C, like Procedure B, is suitable for all kinds of samples, regardless of their viscosity or matrix constitution. It is only the evaporated water in an azeotrope with the solvent that is transferred into the titration cell. The sample, as well as interfering matrix components, remain in the evaporation chamber. However, it is necessary to manually empty and refill the evaporation chamber from time to time, which is time consuming, as the chamber needs to cool down before the content can be exchanged. Furthermore, walk-away automation is not possible with this method.

For a more detailed comparison of the various factors for each procedure, download our free White Paper: «Moisture in petroleum products according to ASTM D6304».

Visit our website

Save time with the new automated measurement capabilities allowed with ASTM D6304

Post written by Lucia Meier, Technical Editor at Metrohm International Headquarters, Herisau, Switzerland.

Fast determination of acid and base number by thermometric titration

Fast determination of acid and base number by thermometric titration

Acid number (AN) and base number (BN) are critical parameters in the quality control of petroleum products as they are often stipulated by product specifications. Traditionally both parameters can be determined by potentiometric or photometric titration according to various standards such as ASTM D664 (Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration), ASTM D2896 (Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration), or ASTM D974 (Standard Test Method for Acid and Base Number by Color-Indicator Titration). However, there is a rapid and reliable alternative titration method – thermometric titration.

Why determine the acid and base number?

The acid number is an indication for the amount of acids present in petroleum products. Weak acids present in crude oils (e.g. naphthenic acid) can be linked to corrosion of refinery equipment. For petroleum products, aging can lead to the buildup of acids, which increases the risk of corrosion to pipes and holding tanks.

To prevent such an acidic buildup, basic additives are added to refined petroleum products, such as lubricating oil. These basic additives neutralize the weak acids and can prevent corrosion. The amount of basic additives can be characterized using the base number.

What is thermometric titration?

Thermometric titration (TET) is based on the principle of enthalpy change. Each chemical reaction is associated with a change in enthalpy that in turn causes a temperature change. This temperature change during a titration can be measured with a highly sensitive thermistor (Figure 1) in order to determine the endpoint of the titration.

Figure 1. Metrohm’s maintenance-free Thermoprobe used for fast and reliable indication of thermometric titration endpoints.

If you would like to read more about the basic principles of thermometric titration, click below for our previous  blog post «Thermometric titration – the missing piece of the puzzle».

TET: the best choice for AN and BN determination

If you’ve performed a potentiometric titration of the acid and base number, you probably know that not all samples are soluble in the solvent mixture. Even if they are soluble, several cleaning steps (including conditioning of the electrode after each titration) are necessary in order to achieve good reproducibility.

While photometric titration provides an alternative indication method for samples which are not colored, the solubility issue remains. Thermometric titration of the AN according to ASTM D8045 provides the ideal solution to all of these issues.

  • The xylene/IPA (3/1) solution allows better solubility of many samples, especially crude oils
  • Endpoint indication is not affected by colored samples
  • The Thermoprobe requires no conditioning or additional cleaning steps – only a rinse with solvent
  • The Thermoprobe is maintenance-free – no electrolyte refilling necessary, just store it dry

There are even more benefits if compared to the potentiometric titration according to ASTM D664 or ASTM D2896.

 

  • Less solvent used: 30 mL instead of 60 mL or 120 mL saves additional costs and reduces waste
  • Faster titrations: half the time of potentiometric titrations, saving about 2 minutes per analysis
  • Robust sensor: the Thermoprobe is completely maintenance-free and needs no conditioning, further reducing analysis time.

For a comprehensive comparison between the AN determination according to ASTM D8045 (thermometric titration) and ASTM D664 (potentiometric titration), check out Table 1 below. While the titrant and solvent mixtures differ if you perform a base number determination, the values for solvent volume, titration time, electrode conditioning, and sensor maintenance reflect the comparison between thermometric base number determination and potentiometric determination according to ASTM D2896 very well. Discussions for an ASTM standard on thermometric BN determinations are currently ongoing within the respective committee.

Table 1. Comparison between ASTM D664 and ASTM D8045 concerning various parameters.

Since you are titrating faster, using less solvent, and do not have to perform complicated sensor maintenance, you can save quite a bit of money by switching to thermometric titration.

Not convinced yet? Then listen to one of our customers, Thomas Fischer from Oel Check GmbH, Germany, about his positive experiences with Metrohm thermometric titration.

«Thermometric titration has several advantages compared to potentiometric titration. It is much faster and more robust. A typical thermometric titration takes just about 2 minutes. Moreover, the electrode does not need to be regenerated between determinations.»

Thomas Fischer

Laboratory Manager, Oel Check GmBH

Additionally, I suggest downloading our related white paper on this topic: «Avoid corrosion: A new method for TAN determination in crude oil and petroleum products», which contains comparison data between ASTM D664 and ASTM D8045.

How to perform the analysis

During the AN or BN determination, very weak acids or bases (respectively) are titrated, resulting in small enthalpy changes. By using a catalytic endpoint indicator, these weak acids and bases can also be determined by TET.

What is catalyzed endpoint indication?

Endpoint indication becomes difficult for titrations with small enthalpy changes, such as with weak acids or bases. In these situations, a catalytic endpoint indicator is used. The catalytic endpoint indicator undergoes a strongly exothermic or endothermic reaction during the titration. As with an indicator which changes color when all analyte has been titrated, the catalytic endpoint indicator only starts its reaction with the titrant after all analyte has been consumed. In this way, the indication of the endpoint becomes possible.

Figure 2. Thermometric titration system consisting of a 859 Titrotherm fully equipped with a Thermoprobe, titration stand and buret, and the tiamo software for the TAN or TBN determination.

Acid number

An appropriate amount of the sample (depending on the expected AN) is weighed into the titration vessel, then 30 mL solvent mixture (isopropanol:xylene 1:3) and 0.5 g paraformaldehyde are added. After dissolution of the sample, the solution is then titrated with alcoholic KOH to a single exothermic endpoint.

Here, the paraformaldehyde acts as the catalytic endpoint indicator. As soon as there is an excess of KOH available it will de-polymerize in a strongly endothermic reaction, resulting in an exothermic endpoint.

Figure 3. Thermometric titration curve of an acid number determination, resulting in a single, well-defined exothermic endpoint.

 For more detailed information about this application, download our free Application Bulletin AB-427.

Base number

An appropriate amount of the sample (depending on the expected BN) is weighed directly into the titration vessel, then 1 mL isobutyl vinyl ether and 40 mL toluene are added. After dissolution of the sample, the solution is then titrated with HClO4 in glacial acetic acid to a single endothermic endpoint.

In this situation, the isobutyl vinyl ether serves as the catalytic endpoint indicator. When an excess of HClO4 is present, it will polymerize in a strongly exothermic reaction, resulting in an endothermic endpoint.

Figure 4. Thermometric titration curve of a base number determination, resulting in a single, well-defined endothermic endpoint.

For more detailed information about this application, download our free Application Bulletin AB-405.

Summary

Thermometric titration provides a rapid and robust solution for the determination of the acid and base number in comparison to potentiometric or photometric titration. The method solves the issue of sample solubility by using more suitable solvents. Furthermore, less solvent is needed, and the analysis time is reduced. All this results in considerably lower costs per analysis, making it a viable alternative for the acid and base number determination.

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Post written by Lucia Meier, Technical Editor at Metrohm International Headquarters, Herisau, Switzerland.