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Ion-selective electrodes: General tips – Part 1

Ion-selective electrodes: General tips – Part 1

Ions – we encounter these tiny charge carriers constantly. Depending on the concentration of certain anions (negative ions) and cations (positive ions), they can have a significant impact on humans and the environment. Thanks to ongoing quality control in several industries such as food and beverage, the metallurgical industry, and water management, defined limits are neither exceeded nor undercut.
So, how can these small, ubiquitous ions be determined? Mistakenly, I thought at first that ion measurement is only possible by means of more costly analytical methods such as ion chromatography (IC), inductively coupled plasma optical emission spectrometry (ICP-OES), or atomic absorption spectroscopy (AAS). A promising cost-efficient alternative to these techniques is to use so-called ion-selective electrodes (ISE).

Types of ion-selective electrodes

If you want to determine, for example, the fluoride concentration in your toothpaste, the ammonium content of the water in your aquarium, or perhaps how much calcium is really in a fruit juice, then there are many suitable ion-selective electrodes for your application needs.

Membrane material

The very first ion-selective electrode was the pH electrode. However, this article will not discuss pH electrodes—you can find more information in our other posts pertaining to this special ISE.

Aside from the glass membrane used for pH electrodes, there are other membrane materials available for selective measurement of myriad ions. The most widely applied types are listed in Table 1.
Table 1. Various types of ion-selective electrodes.
Membrane material Description Ions Close-up view
Crystal membrane Crystal lattice containing defined gaps for the ion to be measured. Ag+, Cu2+, Pb2+, Br, Cl, CN, F, I, S2-
Polymer membrane Polymer membrane containing a molecule (ionophore) that only binds the ion to be measured. Ca2+, K+, Na+, surfactants, NO3
Glass membrane Framework of silicate glass with interstitial sites for H+ and Na+. Na+, H+
Gas permeable membrane Membrane acts as a permeable barrier through which only specific substances can move across. NH4+
The membrane material can limit the possible matrices in which the ISE can be used. For example, electrodes with a polymer membrane cannot be used to measure ions in organic solvents. For more information on the specific restrictions, check the user manual of your ISE.

Basic theory behind ISEs

Measuring range

Each electrode type has its own specific measuring range (see Table 2). Before beginning any ion measurement, first ensure that the ion-selective electrode is able to measure in the concentration range of the sample.

Table 2. Each ion-selective electrode has its specific measuring range. Note: The given measuring ranges only apply to ion-selective electrodes from Metrohm.
Cation of interest Measuring range
Ag+ 1×10–7 – 1 mol/L
Ca2+ 5×10–7 – 1 mol/L
Cd2+ 1×10–7 – 10–1 mol/L
Cu2+ 1×10–8 – 10–1 mol/L
H+ 1×10–14 – 1 mol/L
K+ 1×10–7 – 1 mol/L
Na+ (Polymer)

Na+ (Glass)

5×10–6 – 1 mol/L

1×10–5 – 1 mol/L

NH4+ 5×10–6 – 10–2 mol/L
Pb2+ 1×10–6 – 10-1 mol/L
Anion of interest Measuring range
Br 1×10–6 – 1 mol/L
Cl 1×10–5 – 1 mol/L
CN 8×10–6 – 10–2 mol/L
F 1×10–6 – sat. mol/L
I 5×10–8 – 1 mol/L
NO3 1×10–6 – 1 mol/L
S2- 1×10–7 – 1 mol/L
However, something more important than the measuring range is the linear range. Figure 1 illustrates a measuring range which also includes a linear range. Within this linear range the Nernst equation applies, and the signal is proportional to the analyte concentration. By performing an ion measurement within the linear range, the most precise and reproducible results will be obtained. Find out more about the Nernst equation in our previous blog post.
Figure 1. The measuring range of an electrode.
Outside of the linear range, the curve becomes flatter, and the potential difference becomes smaller, preventing a reliable measurement by standard addition. Even in this non-linear, flattened range, it is possible to determine the ion concentration by means of direct measurement – provided that your ion-selective electrode is also calibrated for this range.

If the concentration is too low or the sensor is saturated, this situation is considered outside of the measuring range. Potential changes can no longer be determined.

Interfering ions

Compared to a pH electrode with a linear range over 14 decades, the sensitivity of ISEs is limited since interfering ions reduce the linear and measuring ranges (see Figure 1).

There are two different types of interfering ions, both outlined in Table 3.

Table 3. Types of interfering ions and their impact on measurement with ISEs.
Interfering ion Description Impact
Irreversible
  • Binds to the membrane material and reacts with it or
  • Forms complexes and precipitates with the measuring ion
Destruction of the ISE since the irreversible interfering ion reacts with the membrane and is unavailable for further analysis.
Reversible
  • Exhibits cross-sensitivities
Erroneous results since the reversible interfering ion binds to the membrane material and contribute to the signal.
Nowadays, the most important interfering ions for an ISE are known, and information about them is provided by the sensor manufacturer. For the measurement, the impact of interfering ions is considered in a so-called selectivity coefficient, which in turn is used in the Nikolsky equation—an expanded version of the Nernst equation.
Table 4. Each ion to be measured has at least one interfering ion that must be considered when setting up an experiment.
Measuring cation Interfering ion(s)
Ag+ Hg2+, proteins
Ca2+ Na+, Pb2+, Fe2+, Zn2+, Cu2+, Mg2+
Cu2+ Ag+, Hg2+, S2-, Cl, Br, I, Fe3+, Cd2+
K+ Na+, NH4+, Cs+, Li+, H+
Na+ (Polymer)

Na+ (Glass)

SCN, acetate

H+, Li+, K+, Ag+

Pb2+ Ag+, Hg2+, Cu2+, Fe3+, Cd2+
Measuring anion Interfering ion(s)
Br Hg2+, I, S2-, CN, NH4+, S2O32-
Cl Hg2+, Br, I, S2-, CN, NH4+, S2O32-
CN S2-, Ag+ complexing substances, I, Cl, Br
F OH
I Hg2+, S2-, CN, Cl, Br, S2O32-
NO3 Br, NO2, Cl, acetate
S2- Hg2+, proteins
Some examples for the most important interfering ions of ISEs are listed in Table 4. For more information on the theoretical background of pH as well as ion-selective electrodes, download our free monograph below.

Ionic strength adjustment

The measurement depends on the activity of the measuring ion in solution, which in turn depends on the ionic strength. For this reason, ion-selective measurements are always carried out in solutions with approximately the same ionic strength. By addition of ionic strength adjuster (ISA) or total ionic strength adjustment buffers (TISAB), a constant ionic background can be achieved.

ISA and TISAB are chemically inert respective to the measurement, and they contain a relatively high concentration of salt so that the ionic strength of the sample solution can be neglected. Some examples can be found in Table 5. Check the user manual of your ISE to find its ideal ISA or TISAB solution.

Table 5. Recommended ISA and TISAB solutions for ion-selective electrodes and further information about each procedure.
Ion to be measured ISA / TISAB Further information about procedure
Fluoride (F) NaCl / glacial acetic acid / CDTA Application Bulletin AB-082
Potassium (K+) c(NaCl) = 0.1–1 mol/L Application Bulletin AB-134
Sodium (Na+), glass membrane c(Tris(hydroxymethyl)aminoethane) = 1 mol/L Application Bulletin AB-083
Sodium (Na+), polymer membrane c(CaCl2) = 1 mol/L
Ammonium (NH4+) c(NaOH) = 10 mol/L Application Bulletin AB-133

Electrode maintenance and lifetime

Cleaning the ISE
DO:
  • After each measurement or titration, the ISE has to be rinsed thoroughly with distilled water.
DON’T:
  • Never use organic solvents for cleaning. They may attack or irreversibly destroy the polymer membrane ISE or reduce the lifetime of your crystal membrane ISE.
Conditioning the ISE

Conditioning steps must be performed before the first use as well as in between measurements. This step activates the measuring membrane and provides a stable equilibrium of the measuring ion in the membrane. By doing so, an accurate ion measurement is possible. An ion standard solution with a concentration of c(ion) = 0.01 mol/L is recommended as the conditioning solution.

Storing the ISE

An overview of proper storage instructions for your ion-selective electrode is shown in Table 6. For more detailed information, check the ISE manual.

Table 6. Recommended storage conditions for different ISE types.
Membrane material Short storage period Long storage period
Crystal membrane In c(ion) = 0.1 mol/L Dry, with protective cap
Polymer membrane Dry Dry
Polymer membrane, combined In c(ion) = 0.01–0.1 mol/L Dry, with some residual moisture
Glass membrane In c(ion) = 0.1 mol/L In deionized water
Lifetime of an ISE

The lifetime of an ion-selective electrode depends on several influencing parameters including membrane type, sample matrix, and electrode maintenance. Don’t forget to regularly exchange the electrolyte of your combined ISE or – in case of a separate ISE – your reference electrode. Furthermore, never touch the membrane with bare fingers.

In general, the following can be said:

  • Polymer membrane electrodes: Limited lifetime of about half a year since the membrane ages, resulting in a loss of performance.
  • Crystal membrane electrodes: Lifetime of several years – the membrane can be regenerated by polishing using an appropriate polishing material. Watch the video below for more details.

Summary

  • If you decide to perform an ion measurement using an ion-selective electrode, you must consider the measuring range and any interfering ions that may be present in advance.
  • In addition to the membrane type and the sample matrix, the cleaning, storage, and conditioning all have an influence on the lifetime of your ion-selective electrode.
Hungry for more information regarding direct measurement and standard addition? Check out Part 2 (coming soon!) where we will discuss the different determination methods.

Can’t wait? Download our free White Paper: «Overcoming difficulties in ion measurement: Tips for standard addition and direct measurement».

Free White Paper:

Overcoming difficulties in ion measurement: Tips for standard addition and direct measurement
Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.
USP  – simple automated analysis of ultrapure water

USP <645> – simple automated analysis of ultrapure water

H2O – two simple elements, oxygen (O) and hydrogen (H), fuse together to form one of the most important molecules in the world: water. Water is everywhere on Earth and it is vital for our health and survival. It often contains other ions like calcium, magnesium, and chloride which are essential for the human body to function. However, in specific situations, ultrapure water (UPW) is needed to prepare e.g., injections or other solutions used in hospitals. How is the quality of UPW ensured so that it is always suitable for such medical purposes? The answer to this comes from USP <645>. This standard explains how the water quality can be determined and how this analysis must be performed.

Instruments

For this analysis, a measuring device capable of measuring the conductivity and the pH value is required. If a combined device is not available, then using two separate ones is also fine. Then, a pH electrode that is especially suitable for the determination of the pH value of water and a conductivity cell for measurement of low conductivities is necessary. In this case, the Aquatrode plus and the stainless steel conductivity cell are recommended.

The Aquatrode plus responds very quickly in ion-deficient matrices (such as ultrapure water) and thanks to its double junction system, the bridge electrolyte can be chosen freely.
The stainless steel conductivity cell has been specially designed for measurements in samples with low conductivity. With a cell constant of c = 0.1 cm-1, it is ideal for conductivities ranging from 0–300 µS/cm. The built-in temperature sensor makes handling even easier as no additional sensor is needed for the temperature measurement.

USP <645> procedure

Now that the necessary instrumentation has been introduced, it’s time to take a look at the standard procedure itself. At first glance this looks a bit difficult as it is a three-step analysis, or actually a four-step analysis if you count the calibration as well.

Step 1: First, calibrate the pH electrode and the stainless steel conductivity cell (sensor). The pH electrode is calibrated with pH 4 and 7 solutions, whereas the stainless steel sensor is calibrated with a 100 µS/cm standard.

Find the standard solutions you need here:

Step 2: After calibrating the sensors, both the temperature of the water and the conductivity are measured without temperature compensation. If the measured conductivity is lower than the value mentioned in the table of USP <645>, then the requirement for the conductivity is met and the water can be used for medical purposes. If this is not the case, then step 3 must be performed.

Step 3: 100 mL water is transferred to an external titration vessel where its temperature is adjusted to 25 ± 1 °C. The water is stirred vigorously to incorporate carbon dioxide present in the air. If the conductivity does not change by more than 0.1 µS/cm per 5 minutes, the value is noted for further evaluation. If this value is below 2.1 µS/cm, then the water is usable for medical purposes. If not, then proceed with step 4.

Step 4: The solution is tempered to 25 ± 1 °C. Once the temperature is stable, 0.3 mL of saturated KCl solution is added and the pH of the water is measured. The pH value must lie between pH 5 and 7. If this is not the case, the water does not meet the requirements and must be discarded. If the pH is measured between 5 and 7, then the conductivity must additionally be lower as mentioned in the USP <645> table. If this is the case, the analysis passes and the water can be used for medical purposes.

Automation of USP <645>

The analysis can be quite time-consuming and therefore Metrohm has provided a solution to make this process much easier. Our system combines all of these steps into one method, allowing you to perform walk-away automation and focus on more important tasks.

To expand the capabilities of the measuring system, the 856/867 modules can be exchanged with a modular OMNIS Titrator which can also be used for other standard potentiometric titrations.

Take a closer look at our automated analysis solution

Proper electrode immersion depth
  • Aquatrode plus for accurate pH measurement in ion-deficient matrices
  • Stainless steel conductivity cell for low conducting samples
  • Special holder for performing step 2 of the standard procedure
  • Thermostated vessel (for step 2)
  • DIS-Cover lid to prevent the sample from CO2 uptake (before step 3)
  • Rod stirrer to saturate the solution with carbon dioxide (for step 3)

Conclusion

With this complete system, standard analysis of UPW quality according to USP <645> is performed in a fully automatic and reliable manner. At the end of each analysis a clear message is received if the deionized water (UPW) has passed the test or not. Handling is very easy and allows users to check if an analysis passes or not with just a glance.

Download our Application Bulletin

Automatic conductometry in water samples with low electrical conductivity in accordance with USP<645>
Post written by Iris Kalkman (Product Specialist Titration) and Heike Risse (Product Manager Titration – Automation), 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.
Guide to online and inline surface finishing analysis

Guide to online and inline surface finishing analysis

What is surface finishing?

Surface finishing is a series of industrial processes with the main goal to alter the surface of a certain workpiece in order to obtain specific properties. This can be performed chemically, mechanically, or even electronically with the aim of removing, altering, adding or reshaping the material that is being treated.

Industries that use surface finishing techniques

Surface finishing techniques are used by most industries that manufacture industrial parts (e.g., metals, wafers, tools, and more). The use of surface finishing processes has been on the rise globally and is expected to grow further. An article published by Grand View Research (2019) predicted that the market size for metal finishing chemicals is expected to grow to $13.52 billion USD by 2025.

People mostly think about polishing and sanding when surface finishing is brought up, but it is much more than that. Several industries use different processes to treat surfaces with the main objective of obtaining the highest product quality. According to Grand View Research, the top three industries with the biggest market share for metal surface chemicals are automotive and aerospace, semiconductors, and the metal industry (e.g., industrial machinery, construction).

Figure 1 shows that surface finishing is mainly used in the automotive industry. Here, electroplating and electroless plating are the main processes used to protect against corrosion. The electroplating process consists of using electricity to coat a material (e.g. copper) with a thin layer of another material (e.g. nickel). Electroless plating is accomplished with chemical processes that reduce metal cations in a bath and deposit them as an even layer, even on non-conductive surfaces.

Next is the semiconductor industry, which includes the manufacturing and cleaning surface process of electrical and electronic parts as well as silicon wafers. This industry involves plating processes (e.g., electroless plating) as well as chemical cleaning baths. Chemical cleaning baths are used here to remove any contaminants from the wafer surfaces.

Figure 1. Diagram with top five industrial applications that incorporate surface finishing techniques (graphic repurposed from Metal Finishing Chemicals Market Global Forecast to 2021). (Click image to enlarge.)
Finally comes the metal industry, responsible for creating the infrastructure that our modern world depends on. Here, the process of galvanization is used to make metal corrosion- and heat-resistant. Galvanization is an anti-corrosive measure taken with iron and steel (as well as other metals) by applying a protective zinc coating which does not allow oxidation to occur. The zinc also acts as a sacrificial anode which still protects the underlying metal in the event of a scratch in the galvanized surface. Pickling baths are another common surface finishing process for this industry. These acidic baths are used to remove the oxide layer which formed on the surface during the hot strip mill. If the base steel is over-pickled, it can result in pitting of the metal surface, leading to an undesirable rough, blistered coating in the subsequent galvanizing steps and also excessively consumes the pickling acid (e.g. HCl).

Much more than just decorative coatings

Do appearances matter? When talking about products, absolutely! One of the reasons product surfaces are treated is so they have a more pleasant appearance for consumers, but also for more technical reasons that go beyond looks. Since surface finishing processes are used in a broad range of industries, they serve different purposes depending on the uses of the final products.

In the semiconductor industry, any defect on the components (e.g., silicon wafers, microelectronics, printed circuit boards (PCB), etc.) can impact the performance of the final product. Therefore, maintaining the proper concentrations of all components in the chemical cleaning bath ensures a repeatable etching process, which for this purpose means the elimination of surface defects.

Another example includes phosphating baths, which are used to improve corrosion resistance of the product parts used in the automotive and aerospace industry. This process is performed prior to any painting to protect the body structure from environmental factors. Phosphating baths also need to be kept consistent to guarantee the correct (and identical) thickness of the protective layer in each of the products subjected to this process.

Check out our free webinar about how Process Analytical Technology (PAT) brings analytical measurements directly to the process for real-time decision-making, ensuring a high level of control for coating and finishing baths and eliminating unnecessary risk to plant personnel. Learn about real-world case studies and field-tested applications that demonstrate the advantages of optimized bath chemistry and PAT in the surface treatment industry. 

Challenges in surface finishing processes: daily bath maintenance

Like any process, surface finishing has day to day challenges which can be improved upon. Improvement can only come from knowing the bath composition and how it affects the final product. Generally, monitoring the concentration of chemical baths is done via manual sampling and titration in a laboratory on site (in some cases, by a contract lab offsite). While this method works, it can lead to long waiting times from the moment the sample is taken until the final result—therefore the results are no longer representative of the current process conditions. Because of this delay,  bath replenishment can be impaired by over- or under- dosing components, leading to suboptimal bath composition and resulting product quality (Figure 2).

Figure 2. A jagged graph such as this denotes bath quality that suffers from suboptimal conditions. A relatively flat line would suggest a stable bath composition over time, resulting in reproducible high quality surface finishing.
Manual bath analysis and chemical dosage based on old data directly influences the company’s bottom line since the manufacturer loses money either by overusing bath chemicals or producing subpar products. The larger the plating bath volume, the greater the cost of chemicals utilized. Surface finishing baths can be as large as 3500 L (1,000 gallons) or more. Thus, it is extremely important to optimize chemical dosing to reduce unnecessary costs and waste while still providing maximum quality.

If the baths are overdosed, more chemicals are used than necessary which increases overall operational costs. However, if the baths are underdosed based on old data, then the final products may be defective, which results in increased operational costs as well.

Additionally, surface finishing processes involve many hazardous substances. When carrying out any risk assessment, the first resort is the use of personal protective equipment (PPE), and any potential exposure risks should ideally be engineered out of any process.

Automated analysis of the bath components with an online or inline process analyzer completely eliminates the risk of exposure by plant personnel to the hazards associated with the chemicals used, as well as taking care of the sample preconditioning and sampling itself. With a closed loop control, quick measurements are obtained which lead to fast results and response times for optimized process adjustments.

The solution: operate more safely and efficiently with automated process analysis

Process analysis by manual titration typically takes several steps: sample collection, sample preconditioning, volumetric manipulations, calculation, logging and checking results, and finally sending feedback to the process. All of these can be totally eliminated by using online and inline analysis.

The benefits of this are very clear. By limiting the manual handling steps, any risk of exposure to hazardous chemicals is removed. Sampling error, volumetric errors, and end point ambiguity from analyst to analyst are no longer an issue. Furthermore, sampling can be carried out on a timed basis and can be programmed to occur more frequently than possible with manual methods, giving much greater process control.

The analyzer can be used to fully control a process with direct feedback of results for the correct dosage of chemicals to aging baths. Data is automatically recorded and calculated. On-screen plots and signals can warn about deviating process conditions along with alarm outputs to notify operators of bath issues. The user interface is programmed by simple intuitive operation, and can be performed even by non-chemists.

Benefits of online and inline analysis in surface finishing processes:
  • Decrease manual labor – save time and money
  • Safer working environment – avoid contact with hazardous chemicals
  • Faster response time to process changes – better product quality
  • Optimized chemical consumption – less waste, reduced costs
Learn about the differences between inline, online, atline, and offline measurements in our previous blog post.

Metrohm Process Analytics has more than 45 years of experience in process analysis and optimization. The following examples show our expertise with configuring inline and online process analyzers for different surface finishing processes.

Automated monitoring of clean and etch baths

Metal surfaces can have scratches, impurities, and other imperfections which may interfere with further manufacturing processes (e.g., plating or painting). Therefore, clean and etch baths are a key step to obtain clean, polished, and undamaged surfaces.
Figure 3. Trend chart of NH3 and H2O2 concentrations in an SC1 bath. Note the spiking of the baths to maintain their concentrations.
Traditionally, these bath chemicals are measured offline in the lab after taking a sample from the process. However, as mentioned earlier, manual laboratory methods result in long response times in case of process changes (e.g., reaction mixture, moisture levels, …), and the sample preparation can also introduce errors, altering the precision of the analysis. Additionally, it can be quite cumbersome since different operating procedures need to be implemented to analyze multiple parameters including alkalinity, ammonium hydroxide, hydrogen peroxide, and more.
Figure 4. The Metrohm Process Analytics NIRS XDS Process Analyzer is shown here with a diagram of the inline near-infrared spectroscopy (NIRS) system configuration for cleaning bath analysis.
Another example of cleaning baths are mixed acid baths, generally comprised of sulfuric acid, hydrofluoric acid, and nitric acid. Titration only provides the acid value of the sample analyzed; therefore, it is not possible to know how much of a specific acid is present in the baths. However, near infrared spectroscopy (NIRS) is the perfect analytical technique to monitor each acid individually.
Reagent-free NIRS XDS Process Analyzers enable comparison of real-time spectral data from the process to a primary method (e.g. titration) to create a simple, yet indispensable model for process optimization. NIRS is economical and fast, enabling qualitative and quantitative analyses that are noninvasive and nondestructive. Integration of inline spectroscopic techniques allows operators to gain more control over the production process and increase overall safety.

In addition to NIRS process analyzers, Metrohm Process Analytics can design and customize flow-through cells (Figure 5). These clamp on to tubing already present onsite for easy installation with no need to modify the existing setup.

Figure 5. PTFE single fiber clamp-on flow cell from Metrohm Process Analytics.

Automated monitoring of phosphatizing baths

The phosphatizing process produces a hard, electrically non-conducting surface coating that adheres tightly to the underlying metal. This layer protects against corrosion and improves the adhesion of paints and organic finishes to be subsequently applied.

Phosphatization consists of two parts: an etching reaction with phosphoric acid which increases the surface roughness, and a second reaction at the surface between the alkali phosphates and the previously generated metal ions. This coating is quite thin and offers only basic corrosion protection. The addition of metal cations (such as zinc, manganese, and calcium) to the phosphatizing bath results in the formation of very resistant zinc phosphates with a coating thickness between 7–15 times thicker, perfectly suited for outdoor use.

Figure 6. Schematic diagram of the various process stages and baths used in the phosphatizing process. (Click image to enlarge.)
In the cleaning, degreasing, and rinsing baths, and also in the phosphatizing bath itself (Figure 6), the various parameters involved in the process must be kept stable. Conductivity, pH value, free alkalinity, and total alkalinity are among the main parameters that must be determined in the degreasing and rinsing baths. Free and total acids, accelerator, zinc, and fluoride are monitored in phosphatizing baths. The 2060 Process Analyzer from Metrohm Process Analytics (Figure 7) monitors, records, and documents all of these critical parameters at the same time. The combination of different analytical methods within one system as well as the intuitive handling via the well-arranged user interface ensure easy and reliable monitoring of the entire process.

Check out our free related Process Application Note to learn more.

Figure 7. The 2060 Process Analyzer from Metrohm Process Analytics is an ideal solution for online phosphating bath applications.
To sum up, online and inline process analyzers from Metrohm Process Analytics are the ideal solution to automate the analysis of surface finishing processes because of the comprehensive benefits they provide:
  • No manual sampling needed, thus less exposure of personnel to dangerous chemicals
  • Extended bath life by tightening process windows (less chemicals required)
  • Minimize risk of downtime with faster and more precise data
  • Easier compliance with final product requirements by process automation

If you want to learn more about all the applications that we have to offer, download our free application e-book based on 45 years of global installations.

Read what our customers have to say!

We have supported customers even in the most unlikely of places⁠—from the production floor to the desert and even on active ships!
Post written by Andrea Ferreira, Technical Writer at Metrohm Applikon, Schiedam, The Netherlands.
Best practice for electrodes in Karl Fischer titration

Best practice for electrodes in Karl Fischer titration

Have you ever asked yourself why you need an electrode for the endpoint detection in Karl Fischer (KF) titration? Theoretically, the endpoint of a Karl Fischer titration could be determined based on the color change of the reagent. However, if accuracy and reproducibility are important, endpoint detection with a double Pt electrode is a much better choice.

As the indicator electrode detects the endpoint, you can imagine that the results depend highly on the condition of the electrode. In coulometry, an additional electrode (generator electrode) is used to generate the iodine needed for the titration. Both electrode types (i.e. indicator and generator electrode) need to be kept in good shape to guarantee the correct results. It goes without saying that cleaning, storage, maintenance, and checks of the KF electrodes are important factors for success. This blog post takes a closer look at these topics.

Did you catch our series about frequently asked questions in Karl Fischer titration? Find them here!

Cleaning

Indicator electrode

Double  Pt-wire or double Pt-ring electrodes can be easily cleaned with an abrasive cleaning agent like aluminum oxide powder or toothpaste. After cleaning, rinse the electrode well with water and let it dry before mounting it in a titration cell. Check out our video below for more tips and tricks about the proper cleaning procedure for Karl Fischer titration indicator electrodes.

Take special care not to bend the Pt pins of the double Pt-wire electrode. Bending the pins can lead to tiny cracks in the glass body of the electrode. Over time, reagent can flow into the electrode and lead to corrosion (short circuit). If this happens, the electrode is beyond repair and needs replacement. Alternatively, a double Pt-ring electrode can be used instead. Problems with bent pins are then a thing of the past.
Generator electrode
Without diaphragm
Rinse generator electrodes without diaphragms with water, or if the contaminant is not water soluble, then rinse with a suitable organic solvent. If the anode or the cathode of the generator electrode shows discoloration or deposits that cannot be removed with rinsing, the electrode can then be cleaned with concentrated nitric acid (65%). Be aware that nitric acid is a strong oxidizing agent and must be handled carefully according to relevant safety regulations and instructions. Remember to first mount the green protection cap on the connector to avoid corrosion caused by fumes of nitric acid. Afterwards, rinse the electrode with water and finally with methanol.
With diaphragm
To remove salt-like residues, the generator electrode with diaphragm can be rinsed with water. Oily contamination can be rinsed off with an organic solvent (e.g. hexane). Sticky residues on the diaphragm can be removed in the following way: 

  1. Mount the green protection cap on the connector of the electrode.
  2. Place the electrode in an upright position (e.g. in an Erlenmeyer flask) and add a few milliliters of concentrated nitric acid (65%) in the cathode chamber. Let the acid flow through the diaphragm.
  3. Fill the cathode chamber with water and let it flow through the diaphragm to remove the nitric acid. Repeat this step two or three times. A simple way to see whether another rinsing step is required is by performing a quick check of the pH value at the cathode using pH indication paper.
  4. Finally, fill the cathode chamber with methanol and let it flow out.

Now the generator electrode is as good as new and ready for use in a titration cell again.

Maintenance

Except for the generator electrode with diaphragm, KF electrodes are maintenance free. However, the catholyte filled in the generator electrode with diaphragm can decompose over time. To avoid any influence of the decomposition products on the results, exchange the catholyte on a regular basis according to the manufacturer’s recommendations.

Storage

Unlike pH electrodes, KF electrodes do not contain a glass membrane that could potentially dry out. Therefore, no special solution is required in which to store KF electrodes. If you use the electrodes frequently, it is recommended to keep the electrodes mounted in the titration cell and immersed in the KF reagent. Alternatively, all KF electrodes (indicator and generator electrodes) can be stored dry.

What to check for

It is recommended to check the complete titration setup instead of only the electrode(s).

Volumetry

Carry out a threefold titer determination using either a liquid or a solid water standard suitable for volumetry and calculate the mean value of the titer. Then, determine the water content of a water standard (also via triplicate determination). Make sure that you do not use the same standard as for the titer determination but use a different batch of the standard or even a completely different standard. Calculate the water content and compare it to the certified water content of the standard.

If the recovery is determined to be in the range of 97–103%, the titration system (including the electrode) is working fine. Finding values outside this range means that there is something wrong with the titration system or with the determination procedure. Results of the sample analysis would very likely also deviate from the real water content. Therefore, it is important to find the reason for values that are too high or too low. Sometimes the reason for deviations is just an air bubble in the dosing cylinder or due to an exhausted molecular sieve. However, if you do not find the reason, do not hesitate to contact your local Metrohm agency.

Coulometry

Water standards with lower water contents (0.1%) are available to properly check the health of coulometric titration systems. Carry out a water content determination in triplicate with such a standard. Calculate the recovery with the obtained results and the certified water content of the standard.

A recovery value in the range between 97–103% means that everything is fine with the system and that the electrodes work as expected. As with volumetry, in coulometry it is important to find the reason for any deviating recovery values. Make sure that you find and eliminate the problem to obtain correct results for your samples.

What you should avoid

  • Do not use solvents that contain ketones or aldehydes (e.g. denatured ethanol) to clean KF electrodes or any KF accessories.
  • Do not treat KF electrodes in an ultrasonic bath. This might destroy the electrode.
  • For drying, use a maximum temperature of 50 °C. Higher temperatures might damage the electrode.
  • Do not bend the Pt pins of the double Pt-wire electrode.

Summary

As you can see, keeping your KF electrodes in good shape is actually very simple. Regular cleaning helps to avoid erroneous results and ensures that your Karl Fischer electrodes will work for a long time.

Best practice for electrodes in titration

Treat your sensors right!
Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.