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Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

If you perform water analyses on a regular basis, then you know that analyzing different parameters for drinking water can be quite time-consuming, expensive, and it requires significant manual labor. In this article, I’d like to show you an example of wider possibilities in automated sample analysis when it comes to combining different analytical techniques, especially for our drinking water.

Water is the source and basis of all life. It is essential for metabolism and is our most important foodstuff.

As a solvent and transporting agent it carries not only the vital minerals and nutrients, but also, increasingly, harmful pollutants, which accumulate in aquatic or terrestrial organisms.

Within the context of quality control and risk assessment, there is a need in the water laboratory for cost-effective and fast instruments and methods that can deal with the ever more complex spectrum of harmful substances, the increasing throughput of samples, and the decreasing detection limits.

Comprehensive analysis of ionic components in liquid samples such as water involves four analytical techniques:

  • Direct measurement
  • Titration
  • Ion chromatography
  • Voltammetry

Each of these techniques has its own particular strengths. However, applying them one after the other on discrete systems in the laboratory is a rather complex task that takes up significant time.

Back in 1998, Metrohm accepted the challenge of combining different analytical techniques in a single fully automated system, and the first TitrIC system was introduced.

What is TitrIC?

The TitrIC system from Metrohm combines direct measurement, titration, and ion chromatography in a fully automated system.

Direct measurements include temperature, conductivity, and pH. The acid capacity (m and p values) is determined titrimetrically. Major anions and cations are quantified by ion chromatography. Calcium and magnesium, which are used to calculate total hardness, can be determined by titration or ion chromatography.

The results are displayed in a common table, and a shared report is given out at the end of the analysis. All methods in TitrIC utilize the same liquid handling units and a common sample changer.

For more detailed information about the newest TitrIC system, which is available in two predefined packages (TitrIC flex I and TitrIC flex II), take a look at our informative brochure:

Efficient: Titrations and ion chromatography are performed simultaneously with the TitrIC flex system.

Figure 1. Flowchart of TitrIC flex II automated analysis and data acquisition.

How does TitrIC work?

Each water sample analysis is performed fully automated at the push of a button—fill up a sample beaker with the sample, place it on the sample rack, and start the measurement. The liquid handling units transfer the required sample volume (per measurement technique) for reproducible results. TitrIC carries out all the work, and analyzes up to 175 samples in a row without any manual intervention required, no matter what time the measurement series has begun. The high degree of automation reduces costs and increases both productivity and the precision of the analysis.

Figure 2. The Metrohm TitrIC flex II system with OMNIS Sample Robot S and Dis-Cover functionality.

To learn more about how to perform comprehensive water analysis with TitrIC flex II, download our free application note AN-S-387:

Would you like to know more about why automation should be preferred over manual titration? Check out our previous blog post on this topic:

Calculations with TitrIC

With the TitrIC system, not only are sample analyses simplified, but the result calculations are performed automatically. This saves time and most importantly, avoids sources of human error due to erroneously noting the measurement data or performing incorrect calculations.

Selection of calculations which can be automatically performed with TitrIC: 

  • Molar concentrations of all cations
  • Molar concentrations of all anions
  • Ionic balance
  • Total water hardness (Ca & Mg)
  • … and more

Ionic balances provide clarity

The calculation of the ion balance helps to determine the accuracy of your water analysis. The calculations are based on the principle of electro-neutrality, which requires that the sum in eq/L or meq/L of the positive ions (cations) must equal the sum of negative ions (anions) in solution.

TitrIC can deliver all necessary data required to calculate the ion balance out of one sample. Both anions and cations are analyzed by IC, and the carbonate concentration (indicative of the acid capacity of water) is determined by titration.

If the value for the difference in the above equation is almost zero, then this indicates that you have accurately determined the major anions and cations in your sample.

Advantages of a combined system like TitrIC

  • Utmost accuracy: all results come from the same sample beaker

  • Completely automated, leaving analysts more time for other tasks

  • One shared sample changer saves benchtop space and costs

  • Save time with parallel titration and IC analysis

  • Flexibility: use titration, direct measurement, or IC either alone or combined with the other techniques

  • Single database for all results and calculation of the ionic balance, which is only possible with such a combined system, and gives further credibility to the sample results

Even more possibility in sample analysis

TitrIC has been developed especially for automated drinking water analysis but can be adapted to suit any number of analytical requirements in food, electroplating, or pharmaceutical industries. Your application determines the parameters that are of interest.

If the combination of direct measurement, titration, and IC does not suit your needs, perhaps a combination of voltammetry and ion chromatography in a single, fully automatic system might be more fitting. Luckily, there is the VoltIC Professional from Metrohm which fulfills these requirements.

Check out our website to learn more about this system:

As you see, the possibility of combining different analysis techniques is almost endless. Metrohm, as a leading manufacturer of instruments for chemical analysis, is aware of your analytical challenges. For this reason, we offer not only the most advanced instruments, but complete solutions for very specific analytical issues. Get the best out of your daily work in the laboratory!

Discover even more

about combined analytical systems from Metrohm

Post written by Jennifer Lüber, Jr. Product Specialist Titration/TitrIC at Metrohm International Headquarters, Herisau, Switzerland.

Save money by using automated titration systems

Save money by using automated titration systems

Perhaps you read my last blog entry: «Why consider automation – even for simple titrations» and liked the idea of disengaging yourself from the tedious, repetitive, and exhausting manual routine lab work by automating analyses and increasing the accuracy and reproducibility of your results at the same time.

Titration is known to be a bargain analytical method, as a glass buret or even a simple stand-alone titrator are quite inexpensive in comparison with other techniques such as spectroscopy or chromatography. In combination with the short determination time and results based on a known stoichiometry, titration is well-accepted in laboratories worldwide as a primary method.

Nevertheless, the increasing sample throughput in the last decades shows more and more why it is worth it to automate lab analysis. On many occasions, I had discussions with lab managers or purchasing agents who had doubts about buying an automation system for the «cheap» titration technique. These concerns are understandable, especially when adding automation to the titrator can cost the same as the titration installation, or even more. However, consider not only the costs for such an upgrade but also the many benefits.

I will now explain how the usage of a fully automated titration system can result in various savings.

 

Save valuable time

Time savings is one of the biggest benefits when using walk-away automation in the lab.

After preparing the sample and entering the required sample data into the controlling device, the system can run unattended for several hours or even days. During this time, lab technicians can spend their valuable time on samples that cannot be automated, evaluating data, preparing new samples, and documentation or inventing new methods or substances.

Don’t waste money on repetitions

In comparison to only using a stand-alone titrator, the automated system can reduce costs for repetitions. Due to the fact that the procedures have been tested before they are run autonomously, handling errors can be reduced to a minimum during the determination. These are typical tasks such as ensuring the sensor and buret tips are sufficiently covered by the sample solution, using the optimum stirring speed for the sample, as well as applying standardized cleaning procedures in between the analyses.

At first glance, these steps might not seem so important, but these have an impact if each lab technician involved in the analysis performs it in a slightly different manner based on their preference and experience. In the worst case, samples have to be repeated more often to prove the validity of the result. With automation, you no longer have to worry about this issue.

Reduce running expenses

The improved handling procedures mentioned above will reduce the costs for consumables such as electrodes. Thanks to the previously defined cleaning, conditioning, and storage procedures, the sensor lasts much longer. The only thing you have to consider is filling the electrolyte reservoir on a regular basis, which can not yet be automated.

However, if you are only running routine pH measurements, a solution already exists for this: the Ecotrode Gel, which allows you to run continuous measurements without any refilling until the electrolyte is exhausted.

The transparency of the electrolyte gel will show when it is time to change the electrode. Cool, isn’t it? 

Besides the electrodes, reagents and eventual waste disposal are topics that have to be taken into account when discussing the costs per analysis of an analytical device. Unfortunately, there are still several norms and standards using absurd amounts of organic solvents. All of these chemicals need a special treatment for proper disposal, i.e. the more waste solution produced, the higher the costs for its disposal—not to mention the impact on the environment.

In automated systems, the amount of these reagents can be reduced to a minimum as analyses can be carried out in beakers with a smaller diameter and perfectly positioned electrodes. Depending on the application, you can even reduce the cleaning procedure between the analyses to a simple dip in solvent rather than showering the electrode with an excess of solvent.

Fewer accidents thanks to walk-away automation

As an automated titration system runs independently for several hours or even overnight, the direct contact with harmful substances and reagents is already minimized. Modern titration systems do not only take over the analysis, but also guarantee the sample beakers are already pre-cleaned before you remove them from the system to put them in the dishwasher or dispose of them.

Even minimizing exposure to chemicals during the exchange of the reagents is possible if your system is equipped with 3S technology, which makes titrant change simpler than ever. The safer the system, the smaller the risk of exposure to hazardous chemicals while handling during sample analysis or disposal. 

We all know that an accident is one thing, but the administration afterwards can also be a real nightmare. Therefore, it is better to avoid situations prone to causing accidents, as it keeps people healthy and the associated costs down.

Increase your profit

Increasing the sample throughput has a direct impact on your profit. Without automation, more analysts are needed to handle the increasing number of samples, but finding well-educated lab technicians becomes ever more challenging and costly. Furthermore, boring routine titration analysis is not the work that lab staff desires.

Automation – not as expensive as you may have thought

Perhaps you started reading this blog post thinking that automation is too expensive for your lab, especially compared to the investment for a simple stand-alone titrator. However, as shown in this article, several kinds of savings can be achieved using an automated titration system.

Consider your regular expenses for consumables, reagents, and time spent for repeated analyses. How often were the results not available either fast or good enough for the production to continue? Think about discussions on safety in the lab when a colleague was injured. Also, how often was the efficiency and profitability of your lab questioned due to the running costs? Considering this, you will see the return on investment is unbelievably good with automation. The more samples you perform fully automated, the faster the initial investment pays off and creates better financial statements for you.

At Metrohm, we don’t just sell titrators. We provide titration solutionsWe offer systems as sophisticated or as straightforward as you need them. Titrator, accessories, electrodes, sample changers, and software – all from a single source.

Looking for a titrator?

Check out our selection here!

Post written by Heike Risse, PM Titration (Automation) at Metrohm International Headquarters, Herisau, Switzerland.

Photometric complexometric titration

Photometric complexometric titration

…and how to choose the right wavelength for indication

Complexometric titration was discovered in 1945 when Gerold Schwarzenbach observed that aminocarboxylic acids form stable complexes with metal ions, which can change their color by addition of an indicator. From the 1950’s on, this technique gained popularity for the determination of water hardness. Soon it was clear that aside from magnesium and calcium, other metal ions could also be titrated in this way. The use of masking agents and new indicators gave further possibilities to determine not only the whole amount of metal ions present in solution, but also to separate and analyze them. A new titration type was born: complexometric titration.

Dear readers, have you ever performed a complexometric titration? According to my assumption, quite a lot of you will respond with “yes” as it is one of the most frequently used types of titration. However, I assume you probably struggled over the detection of the endpoint and over the titration itself. In contrast to other types of titrations, the boundary conditions such as pH and reaction time play an even bigger role in complexometry since the complex binding constant is very pH dependent and the reaction might be slow. This article presents the most common challenges and how to overcome them when carrying out complexometric titrations.

For a complexometric titration analysis, it is very important to know the qualitative composition of your sample. This determines the indicator, the complexing agent, and the masking agent you need to use.

Due to the length of the article, I have provided an easy legend of the topics so you can click and jump directly to the area that interests you the most.

Complexometric titration and complex-forming constant

Complexometric reactions always consist of a metal ion which reacts with a ligand to form a metal complex. Figure 1 shows an example of such a chemical reaction of a metal ion Mn+ with Ethylene diamine tetra acetic acid (EDTA). EDTA is the most commonly used titrant for complexometric titrations and reacts in a stoichiometric ratio of 1:1. As shown on the right side of Figure 1, EDTA can form six coordinational bonds, in different words: EDTA has a denticity of six. The more coordinational bonds a ligand can form, the more stable is the formed complex.

Figure 1. Example complexation reaction of a metal M with charge n+ with EDTA.

As with most chemical reactions, this type of reaction stays in an equilibrium. Depending on the metal ion used, this equilibrium can shift more to the left (reactants) or on the right (products) of the equation. For a titration, it is mandatory that the equilibrium is on the right side (complex-forming). The equilibrium constant is defined as shown in Equation 1.

Equation 1. Equilibrium constant, where c = concentration of the individual substances.

Equation 1 also illustrates why it is so important to keep the pH value constant. The concentration of hydronium ions influences the complex-forming constant by a factor of the square of its concentration (e.g., if one titrated with H2Na2EDTA). This means if the pH value of the reaction is changed, its complex-forming constant is also changed, which influences your titration.

Generally, the higher the concentration of the complex in comparison to the free metal / Ligand concentration, the higher the Kc and also the log(Kc) value. Some log(Kc) values are shown later on in Table 2 and can give you a hint regarding which titrant is most suitable for your titration.

Complexometric reactions are often conducted as a photometric titration. This means an indicator is added to the solution so that a color change at the endpoint can be observed. 

Color Indicator

As in acid–base titration, the color indicator is a molecule which indicates when the end of titration (the endpoint, or EP) is reached by a change in the solution color. For acid–base titration, the color change is induced by a change of pH, whereas in complexometric titration the color change is induced by the absence/presence of metal ions. Table 1 gives you an overview of different color indicators and the metals which can be determined with them.

Table 1. List of color indicators for different kinds of metal ions.

It is very important to choose the right indicator, especially when analyzing metal mixtures. By choosing an appropriate indicator, a separation of the metal ions can already take place.

As an example, consider a mixture of Zn2+ and Mg2+, which is titrated with EDTA. The log(Kc) value for the zinc ion is 16.5, and 8.8 for the magnesium ion. If we choose to titrate this sample with PAN-indicator then the indicator will selectively bind to the zinc, but not magnesium. As zinc has the higher complex-formation constant, the zinc ion will react first with EDTA, which will lead to a color change, and the endpoint can be detected. In such a case, the separation of the ions is possible. If this is not the case, the choice of a more suitable complexing agent might help you to obtain a separation of metal ions.

Complexing agent

At the beginning of your titration, the metal ions are freely accessible. By addition of the complexing agent (your titrant), the metal ions become bound. The prerequisite for that is a higher complex-formation constant of the metal with the complexing agent than with the indicator. In 95% of cases, this does not pose a problem. Some complexing agents are mentioned in Table 2. In general, ions with higher charges will have a higher complex-formation constant. 

However, what can you do if you are still not able to separate your metal ions sufficiently and determine them individually? The answer to that is: use a masking agent to make the second metal ion “invisible” to the titrant.

Table 2. Complex-formation constants log(Kc) of different complexing agents with various metal ions. The higher the number in the table the higher the binding strength between metal ion and ligand. As an example: aluminum binds stronger to DCTA than to EDTA.

Masking agents

In general, masking agents are substances which have a higher complex-formation constant with the metal ion than the complexing agent. Metal ions which react with the masking agent can no longer be titrated, and therefore the metal ion of interest (which does not react with the masking agent) can be determined separately in the mixture using the complexing agent. Table 3 shows a small selection of common masking agents. There are many more masking agents available which can be used for the separation of metal ions.

Table 3. A selection of different masking agents.

Complexometric titration is still often carried out manually, as the color change is easily visible. However, this leads to several problems. My previous post “Why your titration results aren’t reproducible: The main error sources in manual titration” explains the many challenges of manual titration.

Subjective color perception and different readings lead to systematic errors, which can be prevented by choosing a proper electrode or using an optical sensor, which accurately indicates the color change. This optical sensor changes its signal depending on the amount of light reaching the photodetector. It is usually the easiest choice when switching from manual titration to automated titration, because usually it does not require any changes to your SOP.

Which wavelength is optimal for indication?

Figure 2. The Optrode from Metrohm can detect changes in absorbance at 470, 502, 520, 574, 590, 610, 640, and 660 nm.

If you choose to automate your complexometric titration and indicate the color change with a proper sensor, you should use the Optrode. This sensor offers eight different wavelengths enabling its use with many different indicators.

Perhaps you’re asking yourself “why do I need eight different wavelengths”? The answer is simple. This sensor monitors the absorbance of a certain wavelength in the solution. Each wavelength change is best detected when the light is strongly absorbed by the color of the sample solution, either before or after the endpoint is reached. For example, during a color change from blue to yellow, it is recommended to select the wavelength 574 nm (yellow) for the detection of the color change, as it is the complementary color of blue. For even more accuracy, the optimal wavelength can be chosen by knowing the UV/VIS spectra of the indicator before and after complexation.

Figure 3. Left: spectra of complexed (purple) and uncomplexed (blue) Eriochrome Black T are shown. Right: the difference in absorption of the two spectra is shown.

On the left side of Figure 3 is a graph with the spectra of complexed and uncomplexed Eriochrome Black T. The uncomplexed solution has a blue tint, whereas the complexed one is more violet. On the right, another graph shows the difference of both spectra. According to this graph, the maximum difference in absorption is obtained at a wavelength of 660 nm. Therefore, it is recommended to use this wavelength for the detection of the color change.

For more examples of indicators and their spectra, check out our free monograph “Complexometric (Chelometric) Titrations”.

Challenges when performing complexometric titrations

As mentioned in the introduction, complexometric titrations are a bit more demanding compared to other types of titration.

First, the indicators themselves are normally pH indicators, and most complexation reactions are pH-dependent as well. For example, the titration of iron(III) is performed in acidic conditions, while the complexation of calcium can only take place under alkaline conditions. This leads to the fact that the pH has to be maintained constantly while performing complexometric titrations. Otherwise, the color change might not be visible, indicated incorrectly, or the complexation might not take place.

Second, complexation reactions do not occur immediately, as with e.g. precipitation reactions. The reaction might take some time. As an example, the complexation reaction of aluminum with EDTA can take up to ten minutes to be completed. Therefore it is also important to keep this factor in mind.

Perhaps a back-titration needs to be performed in such a case to increase accuracy and precision. Please have a look at our blog post “What to consider during back-titration” for more information about this topic.

Summary

Complexometric titrations are easy to perform as long as some important points are kept in mind:

  • If more than one type of metal is present in your sample, you might need to consider a masking agent or a more suitable pH range.
  • Reaction duration of your complexation reaction might be long. In this case, a back-titration or titration at elevated temperatures might be a better option.
  • Make sure that you maintain a stable pH during your titration. This can be achieved by addition of an adequate buffer solution.
  • Switching from manual to automated titration will increase accuracy and prevent common systematic errors. When using an optical sensor, make sure that you choose the right wavelength for the detection of the endpoint.

For a general overview of complexometric titration, have a look at Metrohm Application Bulletin AB-101 – Complexometric titrations with the Cu ISE.

For more detailed information

Download our free monograph:

Complexometric (Chelatometric) Titrations

Post written by Iris Kalkman, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.

What to consider during back-titration

What to consider during back-titration

Titrations can be classified in various ways: by their chemical reaction (e.g., acid-base titration or redox titration), the indication method (e.g., potentiometric titration or photometric titration), and last but not least by their titration principle (direct titration or indirect titration). In this article, I want to elaborate on a specific titration principle – the back-titration – which is also called «residual titration». Learn more about when it is used and how you should calculate results when using the back-titration principle.

What is a back-titration?

In contrast to direct titrations, where analyte A directly reacts with titrant T, back-titrations are a subcategory of indirect titrations. Indirect titrations are used when, for example, no suitable sensor is available or the reaction is too slow for a practical direct titration.

During a back-titration, an exact volume of reagent B is added to the analyte A. Reagent B is usually a common titrant itself. The amount of reagent B is chosen in such a way that an excess remains after its interaction with analyte A. This excess is then titrated with titrant T. The amount of analyte A can then be determined from the difference between the added amount of reagent B and the remaining excess of reagent B.

As with any titration, both involved reactions must be quantitative, and stoichiometric factors involved for both reactions must be known.

Figure 1. Reaction principle of a back-titration: Reagent B is added in excess to analyte A. After a defined waiting period which allows for the reaction between A and B, the excess of reagent B is titrated with titrant T.

When are back-titrations used?

Back titrations are mainly used in the following cases:

  • if the analyte is volatile (e.g., NH3) or an insoluble salt (e.g., Li2CO3)
  • if the reaction between analyte A and titrant T is too slow for a practical direct titration
  • if weak acid – weak base reactions are involved
  • when no suitable indication method is available for a direct titration

Typical examples are complexometric titrations, for example aluminum with EDTA. This direct titration is only feasible at elevated temperatures. However, adding EDTA in excess to aluminum and back-titrating the residual EDTA with copper sulfate allows a titration at room temperature. This is not only true for aluminum, but for other metals as well.

Learn which metals can be titrated directly, and for which a back-titration is more feasible in our free monograph on complexometric titration.

Other examples include the saponification value and iodine value for edible fats and oils. For the saponification value, ethanolic KOH is added in excess to the fat or oil. After a determined refluxing time to saponify the oil or fat, the remaining excess is back-titrated with hydrochloric acid. The process is similar for the iodine value, where the remaining excess of iodine chloride (Wijs-solution) is back-titrated with sodium thiosulfate.

For more information on the analysis of edible fats and oils, take a look at our corresponding free Application Bulletin AB-141.

How is a back-titration performed?

A back titration is performed according to the following general principle:

  1. Add reagent B in excess to analyte A.
  2. Allow reagent B to react with analyte A. This might require a certain waiting time or even refluxing (e.g., saponification value).
  3. Titration of remaining excess of reagent B with titrant T.

For the first step, it is important to precisely add the volume of reagent B. Therefore, it is important to use a buret for this addition (Fig. 2).

Figure 2. Example of a Titrator equipped with an additional buret for the addition of reagent B.

Furthermore, it is important that the exact molar amount of reagent B is known. This can be achieved in two ways. The first way is to carry out a blank determination in the same manner as the back-titration of the sample, however, omitting the sample. If reagent B is a common titrant (e.g., EDTA), it is also possible to carry out a standardization of reagent B before the back-titration.

In any case, as standardization of titrant T is required. This then gives us the following two general analysis procedures:

Back-titration with blank
  1. Titer determination of titrant T
  2. Blank determination (back-titration omitting sample)
  3. Back-titration of sample
Back-titration with standardizations
  1. Titer determination of titrant T
  2. Titer determination of reagent B
  3. Back-titration of sample

Be aware: since you are performing a back-titration, the blank volume will be larger than the equivalence point (EP) volume, unlike a blank in a direct titration. This is why the EP volume must be subtracted from the blank or the added volume of reagent B, respectively.

For more information on titrant standardization, please have a look at our blog entry on this topic.

How to calculate the result for a back-titration?

As with direct titrations, to calculate the result of a back-titration it is necessary to know the involved stoichiometric reactions, aside from the exact concentrations and the volumes. Depending on which analysis procedure described above is used, the calculation of the result is slightly different.

For a back-titration with a blank, use the following formula to obtain a result in mass-%:

VBlank:  Volume of the equivalence point from the blank determination in mL

VEP Volume at the equivalence point in mL

cTitrant:  Nominal titrant concentration in mol/L

fTitrant Titer factor of the titrant (unitless)

r:  Stoichiometric ratio (unitless)

MA Molecular weight of analyte A in g/mol

mSample Weight of sample in mg

100:  Conversion factor, to obtain the result in %

The stoichiometric ratio r considers both reactions, analyte A with reagent B and reagent B with titrant T. If the stoichiometric factor is always 1, such as for complexometric back-titrations or the saponification value, then the reaction ratio is also 1. However, if the stoichiometric factor for one reaction is not equal to 1, then the reaction ratio must be determined. The reaction ratio can be determined in the following manner:

 

  1. Reaction equation between A and B
  2. Reaction equation between B and T
  3. Multiplication of the two reaction quotients
Example 1

Reaction ratio: 

Example 2

Reaction ratio: 

Below is an actual example of lithium carbonate, which can be determined by back-titration using sulfuric acid and sodium hydroxide.

The lithium carbonate reacts in a 1:1 ratio with sulfuric acid. To determine the excess sulfuric acid, two moles of sodium hydroxide are required per mole of sulfuric acid, resulting in a 1:2 ratio. This gives a stoichiometric ratio r of 0.5 for this titration.

 For a back-titration with a standardization of reagent B, use the following formula to obtain a result in mass-%:

VB Added volume of the reagent B in mL

cB:  Nominal concentration of reagent B in mol/L

fB:  Titer factor of reagent B (unitless)

VEP:  Volume at the equivalence point in mL

cT:  Nominal concentration of titrant T in mol/L

fT Titer factor of the titrant T (unitless)

sBT Stoichiometric factor between reagent B and titrant T

sAB:  Stoichiometric factor between analyte A and reagent B

MA:  Molecular weight of analyte A in g/mol

mSample:  Weight of sample in mg

100:  Conversion factor, to obtain the result in %

Modern titrators are capable of automatically calculating the results of back-titrations. All information concerning the used variables (e.g., blank value) are stored together with the result for full traceability.

To summarize:

Back-titrations are not so different from regular titrations, and the same general principles apply. The following points are necessary for a back-titration: 

  • Know the stoichiometric reactions between your analyte and reagent B, as well as between reagent B and titrant T.
  • Know the exact concentration of your titrant T.
  • Know the exact concentration of your reagent B, or carry out a blank determination.
  • Use appropriate titration parameters depending on your analysis.

If you want to learn more about how you can improve your titration, have a look at our blog entry “How to transfer manual titration to autotitration”, where you can find practical tips about how to improve your titrations.

If you are unsure how to determine the exact concentration of your titrant T or reagent B by standardization, then take a look at our blog entry “What to consider when standardizing titrant”.

Post written by Lucia Meier, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.

Titer determination in Karl Fischer Titration

Titer determination in Karl Fischer Titration

In a recent post, we have discussed the importance of titer determinations for potentiometric titrations.

Without a titer determination, you will not obtain correct results. The same applies for volumetric Karl Fischer (KF) titrations. In this blog post, I will cover the following topics (click to jump directly to each):

Why should I do titer determinations?

Why is a titer determination necessary? Well, the answer is quite simple. Without knowing the titer of a KF titrant, the water content of the sample cannot be calculated correctly. In Karl Fischer titration, the titer states how many mg of water can be titrated with one mL of titrant. Therefore, the KF titer has the unit «mg/mL».

You might say: “Now, ok, let’s determine the titer. That isn’t too much work and afterwards, I know the titer value and I don’t need to repeat the titer determination.

I agree this would be very nice. However, reality is somewhat different. You must carry out titer determinations on a regular basis. In closed bottles, KF titrants are very stable and the titer does not change appreciably. Once you open the bottle, the KF titrant starts to change significantly. Air will enter the bottle, and considering that 1 L of air contains several milligrams of water, you can imagine that this moisture has an influence on the titer. To prevent moist air from getting into the titrant, the bottle must be either tightly closed after use with the original cap, or should be protected with an absorber tube filled with a molecular sieve (0.3 nm).

Please be aware that temperature changes also have an influence on the titer. A temperature increase of the titrant by 1 °C leads to a titer decrease of approximately 0.1% due to volume expansion. Consider this, in case the temperature in your laboratory fluctuates during the working day.

Do not forget: if your titration system is stopped overnight, the reagent in the tubes and in the cylinder is affected and the titer is no longer equal to the titrant in the bottle. Therefore, I recommend first running a preparation step to flush all tubes before the first titration.

How often should I perform titer determinations?

This question is asked frequently, and unfortunately has no simple answer. In other words, I cannot recommend a single fixed interval for titer determinations. The frequency depends on various factors:

  • the type of reagent (two-component titrants are more stable than single-component titrants)
  • the tightness of the seals between the titration vessel and the titrant bottle
  • how accurate the water content in the sample must be determined

In the beginning, I would recommend performing a titer determination on a daily basis. After a few days, it will become apparent whether the titer remains stable or decreases. Then you can decide to adjust the interval between successive titer determinations.

What equipment do I need for a titer determination?

You need a fully equipped titrator for volumetric KF titration, as well as the KF reagents (titrant and solvent). Another prerequisite for accurate titer determinations is an analytical balance with a minimal resolution of 0.1 mg. Last but not least, you need a standard containing a known amount of water and some tools to add the standard to the titration vessel. These tools are discussed in the next section.

How to carry out a titer determination

Three different water standards are available for titer determinations. There are both liquid and solid standards available from various reagent suppliers. The third possibility is available in every laboratory: distilled water. Below, we will take a closer look at the individual handling of these three standards. For determination of appropriate sample sizes, you can download our free Application Bulletin AB-424, Titer determination in volumetric Karl Fischer titration.

1. Liquid water standard

For the addition of a liquid water standard, you need a syringe and a needle.

There are two possibilities to add liquid standard. One is to inject it with the tip of the needle placed above the reagent level. In this case, aspirate the last drop back into the syringe. Otherwise, it will be dropped off at the septum. The droplet is included in the sample weight, but the water content in the drop is not determined. This will lead to false results.

If the needle is long enough, you can immerse the tip in the reagent during the standard addition. In this case, there is no last droplet to consider, and you can pull the needle out of the titration vessel without any additional aspiration step.

Step-by-step – how to carry out a titer determination:

  1. Open the ampoule containing the standard as recommended by the manufacturer.
  2. Aspirate approximately 1 mL of the standard into the syringe.
  3. Remove the tip of the needle from the liquid and pull the plunger back to the maximum volume. Sway the syringe to rinse it with standard. Then eject the 1 mL of standard into the waste.
  4. Aspirate the remaining content of the ampoule into the needle.
  5. Remove any excess liquid from the outside of the needle with a paper tissue.
  6. Place the needle on a balance, and tare the balance.
  7. Then, start the determination and inject a suitable amount of standard through the septum into the titration vessel. Please take care that the standard is injected into the reagent and not at the electrode or the wall of the titration vessel. This leads to unreproducible results.
  8. After injecting the standard, place the syringe again on the balance.
  9. Enter the sample weight in the software.
2. Solid water standard

It is not possible to add the solid water standard with a syringe. For this, different tools are required. Here, examples are shown of a weighing boat and the Metrohm OMNIS spoon for paste.

Place the weighing boat on the balance, then tare the balance. Weigh in an appropriate amount of the solid standard, and tare the balance again. Start the titration, quickly remove the stopper with septum, add the solid standard and quickly replace the stopper. When adding the standard, take care that no standard sticks to the electrode or the walls of the titration vessel. In case that happens, gently swirl the titration vessel to wash down the standard. After the addition of the standard, place the weighing boat on the balance again and enter the sample weight in the software.

3. Pure water

Pure water can be added to the titration vessel either by weight or by volume.

For a titer determination with pure water, only a few drops are required. Such small volumes can be difficult to add precisely, and results strongly depend on the user. Moreover, addition by weight requires a balance capable of weighing a few milligrams. I personally prefer using water standards, and suggest that you use them as well.

By weight

Fill a small syringe (~1 mL) with water. Due to the very small amounts of pure water added for the titer determination, I recommend using a very thin needle to more accurately add small volumes. After filling the syringe, place it on a balance and tare the balance. Then start the titration, and inject an appropriate amount of water through the septum into the titration vessel. Aspirate the last droplet back into the syringe. Remove the needle, place the syringe on the balance again, and enter the sample weight in the software.

By volume

Fill a microliter syringe with an appropriate volume of water. Make sure there are no air bubbles in the syringe, as they will falsify the result. Begin the titration and inject the syringe contents through the septum into the titration vessel. Enter the added sample size in the software.

Acceptable results

During trainings, I am often asked if the obtained result is acceptable. I recommend carrying out a threefold titer determination. Ideally, the relative standard deviation of those three determinations is smaller than 0.3%.

How long can the reagent be used?

As long as you carry out regular titer determinations, the titer change will be considered in the calculation, and the results will be correct. Just keep in mind: the lower the titer, the larger the volume needed for the determination.

I hope I was able to convince you that titer determination is essential to obtain correct results in volumetric Karl Fischer titration, and that it is not that difficult to perform.

In case you still have unanswered questions, please download Metrohm Application Bulletin AB-424 to get additional information, tips, and tricks on performing titer determination.

Still have questions?

Check out our Application Bulletin: Titer determination in volumetric Karl Fischer titration.

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