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Frequently asked questions in Karl Fischer titration – Part 1

Frequently asked questions in Karl Fischer titration – Part 1

Since I started working at Metrohm more than 15 years ago, I have received many questions about Karl Fischer titration. Some of those questions have been asked repeatedly from several people in different locations around the world. Therefore, I have chosen 20 of the most frequent questions received over the years concerning Karl Fischer equipment and arranged them into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Part 1 will cover instrument preparation and handling, and Part 2 will cover the other two topics.

Summary of questions in the FAQ (click to go directly to each question):

Instrument preparation and handling

1.  How can I check if the electrode is working correctly?

I recommend carrying out a volumetric or coulometric Karl Fischer titration using a certified water standard as sample. In volumetry, you can carry out a threefold titer determination followed by a determination of a different standard. Then, you can calculate the recovery of the water content determination of the standard.

To check a coulometric system, carry out a threefold determination with a certified water standard and calculate the recovery. If the recovery is between 97–103%, this indicated that the system, including the electrode, is working fine.

The color of the working medium is an additional indicator as to whether the indication is working properly.

Pale yellow is perfect, whereas dark yellow or even pale brown suggests indication problems. If this happens, then the indicator electrode should be cleaned.

Check out questions 7 and 8 for tips on the cleaning of the indicator electrode.

2.  How long can an electrode be stored in KF reagent?

Karl Fischer electrodes are made from glass and platinum. Therefore, the KF reagent does not affect the electrode. It can be stored in reagent as long as you want.

3.  Can the molecular sieve be dried and reused, or should it be replaced?

The molecular sieve can of course be dried and reused. I recommend drying it for at least 24 hours at a temperature between 200–300 °C. Afterwards, let it cool down in a desiccator and then transfer it into a glass bottle with an airtight seal for storage. 

4.  How long does conditioning normally take?

Conditioning of a freshly filled titration vessel normally takes around 2–4 minutes for volumetry, depending on the reaction speed (type of reagent), and around 15–30 minutes for coulometry. In combination with an oven, it might take a bit longer to reach a stable drift owing to the constant gas flow. I recommend stabilizing the entire oven system for at least 1 hour before the first titration.

Between single measurements in the same working medium, conditioning takes approximately 1–2 minutes. Take care that the original drift level is reached again.

5.  When conditioning, many bubbles form in the coulometric titration cell with a very high drift, also when using fresh reagent. What could be the reason for this effect?

At the anode, the generator electrode produces iodine from the iodide-containing reagent. The bubbles you see at the cathode are the result of the reduction of H+ ions to hydrogen gas.

After opening the titration cell or after filling it with fresh reagent, the conditioning step removes any moisture brought into the system, avoiding a bias in the water content determination of the sample. Removing the water results in an increased drift level. During conditioning, the aforementioned H2 is generated. The gas bubbles are therefore completely normal and not a cause for concern. Generally, the following rule applies: The more moisture present in the titration vessel, the higher the drift value will be, and the more hydrogen will form.

6.  What is the best frequency to clean the Karl Fischer equipment?

There is no strict rule as to when you should clean the KF equipment. The cleaning intervals strongly depend on the type and the amount of sample added to the titration cell. Poor solubility and contamination of the indicator electrode (deposition layer on its surface) or memory effects due to large amounts of sample can be good reasons for cleaning the equipment.

The drift can be a good indicator as well. In case you observe higher and unstable drift values, I would recommend cleaning the titration cell or at least refilling the working medium.

7.  How do I clean the Karl Fischer equipment?

For a mounted titration vessel, it can be as simple as rinsing with alcohol. For an intense cleaning, the vessel should be removed from the titrator. Water, solvents like methanol, or cleaning agents are fine to clean the KF equipment. Even concentrated nitric acid can be used as an oxidizing agent, e.g. in case of contaminated indicator electrodes or coulometric generator electrodes.

All of these options are fine, but keep in mind that the last cleaning step should always be rinsing with alcohol followed by proper drying in a drying oven or with a hair dryer at max. 50 °C to remove as much adherent water as possible.

You should never use ketones (e.g., acetone) to clean Karl Fischer equipment, as they react with methanol. This reaction releases water. If there are still traces of ketones left in the titration cell after cleaning, they will react with the methanol in the KF reagent and might cause the drift to be too high to start any titration.

8.  Is it also possible to use a cleaning agent like «CIF» or toothpaste to clean the double Pt electrode?

Normally, rinsing with alcoholic solvents and polishing with paper tissue should be enough to clean the indicator electrode. You may also use detergents, toothpaste, or the polishing set offered by Metrohm! Just make sure that you rinse the electrode properly after the cleaning process to remove all traces of your chosen cleaning agent before using the electrode again.

Cleaning instructions can also be found in our video about metal and KF electrode maintenance:

9.  How do I clean a generator electrode with a diaphragm?

After removing the generator electrode from the titration vessel, dispose the catholyte solution, then rinse the electrode with water. Place the generator electrode upright (e.g., in an Erlenmeyer flask) and cover the connector with the protection cap to prevent corrosion. Fill the generator electrode with some milliliters of concentrated nitric acid, and let the acid flow through the diaphragm. Then fill the cathode compartment with water, and again allow the liquid to flow through the diaphragm. Repeat the rinsing step with water several times to make sure that all traces of nitric acid are washed out of the diaphragm.

Please note that the nitric acid treatment can be left out if the level of contamination does not require it.

Finally, pour some methanol into the generator electrode to remove the water. Repeat this step a few times to remove all traces of water. The last step is properly drying the electrode in a drying oven or with a hair dryer at max. 50 °C. After this cleaning procedure, the electrode is as good as new and can be used again for titrations.

Keep on the lookout for our next installment in this two-part series, or subscribe to the blog below so you’re sure not to miss it! In Part 2, I will cover the topics of KF titration troubleshooting and the Karl Fischer oven technique.

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

Best practice for electrodes in titration

Best practice for electrodes in titration

After my earlier blog post on the topic of «Avoiding the most common mistakes in pH measurement», I will now cover the subject of electrodes that are relevant for titration. Here you will not only find out how to select the right electrode for your application – but also how to clean and to maintain it, and most importantly, how you can assure that your electrode is still ok to be used.

The following topics will be covered (click to go directly to the topic):

 

How to select the right sensor

You might wonder what you need to consider when selecting a suitable sensor for your titration, as a huge variety of different sensors exist. The right sensor needs to be selected based on the type of titration that you want to carry out. For a redox titration, you will need a different sensor than for a complexometric titration.

Furthermore, the sensor selection is highly dependent on matrix, the sample volume, or possible interferences. If you are working in non-aqueous media, you must especially consider any electrostatic effects that might arise. Therefore, I recommend working with an electrode that offers an internal electrical shielding.

The sensor has to show a fast response time, and needs to be robust enough for the application, meaning it needs to be resistant to the chemicals used and the applied cleaning procedure.

Table 1. Overview of suggested sensors for various types of titration (click to enlarge).

For further guidelines regarding how to select the right electrode, either consult our online electrode finder or check out our flyer about «Electrodes in titration» which includes practical tips on care and maintenance.

Maintenance and cleaning

Proper cleaning between your titrations is a key factor for obtaining reliable results. The rinsing step has to assure that neither sample nor titrant contaminates the electrode, leading to carry-over and false results. Therefore, between titrations the electrode (as well as buret tip) has to be rinsed with a suitable solvent, such as deionized water, detergent solution, or any other solvent that removes remaining residues. For non-aqueous titrations, it is furthermore important to condition the glass membrane of the electrode in deionized water after each titration.

Furthermore, both the reference and measuring electrode require regular maintenance. For the reference electrode, it is very important that it is filled up to the opening with the correct (and uncontaminated) electrolyte. A daily check of the electrolyte level should be performed, and if necessary, the reference electrolyte should be topped up. Always refill the reference electrolyte level up to the filler opening. This assures a proper electrolyte outflow and reduced contamination of the electrolyte.

Figure 1. Always refill your electrolyte up to the filler opening for the best performance!

In addition to the regular refilling, the electrolyte should be replaced at least on a monthly basis to guarantee a clean electrolyte with the correct concentration (e.g., evaporation of water can increase the concentration of the electrolyte). Usage of old or contaminated electrolyte leads to an undesired change in the measured potential.

Also ensure that the diaphragm is clean, otherwise you might experience a blockage, leading to an unstable potential caused by the missing contact between electrolyte and sample. Figure 2 shows an example of a contaminated diaphragm. Table 2 suggests some possible cleaning agents to remove sticky substances from the diaphragm. After cleaning the diaphragm, always replace the electrolyte.

Figure 2. Close-up view of a dirty diaphragm.
Table 2. Common electrode contaminants and suggested cleaning agents for each situation. Contact your local Metrohm representative for further questions.

The measuring electrode needs a thorough cleaning at least weekly. Uncoated metal ring or ISE electrodes require regular polishing to maintain a quick response. Glass membranes or polymer membranes must not be polished or cleaned with abrasives. If the electrode is used in oily or sticky samples, degreasing or removing proteins might be necessary by using a suitable solvent.

Correct storage of your electrode

Another important point to consider is the right storage for your electrode. Incorrect storage reduces the lifetime of an electrode, and therefore it needs replacement more frequently. Unfortunately, there is not one single storage solution which covers all electrode types. The right storage solution highly depends upon the electrode type.

If it is a separate indicator or only a reference electrode, then it is much easier to determine the correct storage solution, as the perfect solution for only one part must be found. For combined electrodes, the situation is a bit more complicated. Combined electrodes contain a reference electrode and a measuring electrode that each have different preferences. Therefore, sometimes a compromise is necessary. The reference electrode prefers to be stored in reference electrolyte to remain ready to use, whereas an indicator glass membrane prefers deionized water. On the other hand, a metal indicator electrode prefers to be stored dry.

For combined pH electrodes with c(KCl) = 3 mol/L as reference electrolyte, a special storage solution was developed by Metrohm, which maintains the glass membrane as quickly as possible without impairing the performance of the reference system. All other pH electrodes are stored in their respective reference electrolyte (normally indicated on the head of the electrode, see Figure 3).

Figure 3. Different reference electrolytes for different electrode types.

Metal electrodes are also stored differently, depending on the type. Combined metal ring electrodes are stored in reference electrolyte to maintain the diaphragm properly, whereas Titrodes are stored in deionized water, as these electrodes contain a pH glass membrane that needs to be kept hydrated. Always fill the storage vessel of your electrode with approximately 12 mL of storage solution and exchange it regularly as it might be contaminated by sample or cleaning solution.

The table below shows typical storage conditions depending on the type of the electrode. 

Table 3. Storage conditions for various electrode types.

If you are not sure how to store your electrode correctly, check the information in the electrode flyer or on the Metrohm website.

Check your electrode

The easiest way to check the performance of your electrode is to monitor it during a standardized titration (e.g., titer determination) which is performed regularly (e.g., weekly) and where prerequisites such as sample size, concentration of titrant, and volume of added water are always very similar. Otherwise, you can also follow a procedure recommended by Metrohm. To check metal electrodes, you can find a test procedure in application bulletin AB-048, for surfactant electrodes in application bulletin AB-305 and for ion selective electrodes, a check procedure is given in the ISE manual.

As an example, I will explain the test procedure of a silver electrode a bit more in detail. Silver electrodes can easily be checked by a standardized titration using hydrochloric acid (c(HCl) = 0.1 mol/L) as sample, and silver nitrate (c(AgNO3) = 0.1 mol/L) as titrant. Perform a threefold determination with the recommended titration parameters and sample size.

The following parameters are evaluated and compared to optimal values:

  • added volume of titrant at equivalence point (EP)
  • time until equivalence point is reached
  • potential jump (potential difference) between the potential measured at 90% and 110% of the EP volume
Figure 4. Example testing procedure for evaluation of electrode performance.

If the evaluated data cannot meet the specified values, clean the electrode thoroughly and repeat the test. If no improvement is observed, the sensor must be replaced.

Further symptoms can indicate a necessary replacement: sluggish response, unstable or drifting signal, longer duration of titration, smaller potential jumps, and worse shape of the titration curve. In Figure 5 below,  two different titration curves for calcium and magnesium in water are shown using a combined Calcium ISE. In Figure 5, the upper curve is obtained with a new Ca-ISE; the titration is fast and you will obtain 2 equivalence points: one each for calcium and magnesium. In the lower curve, an old electrode was used. The titration takes much longer and the second equivalence point for magnesium cannot be recognized anymore due to the lack of sensitivity of the electrode.

Figure 5. Comparison of the response of a new ISE vs. an aged ISE.

To summarize

  • Select the right indication for your titration type.
  • The quality of the electrode highly influences the quality of your titration results.
  • Proper maintenance and storage can increase the lifetime of the electrode.
  • Check the electrode performance regularly or monitor the titration performance (duration, potential jump) over time to reduce downtime of your instrumentation.

 

If you would like to get some more information and practical tips on electrodes in titration, please check out our white paper on «Basics of potentiometry» or our webinar «Avoid titration mistakes through best practice sensor handling». 

On-demand Metrohm webinar

«Avoid titration mistakes through best practice sensor handling»

Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) 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.

What to consider when standardizing titrant

What to consider when standardizing titrant

If you perform titrations on a regular basis, then you’ve certainly heard about standardization of the titrant. When carrying out a standardization you determine the titer, which is a correction factor for your titrant concentration, as it is normally not exactly the value written on the reagent bottle label. In this blog entry, I want to give you some valuable information about why standardization is important and how to determine the titer.

Please note this blog entry will not deal with the standardization of Karl Fischer titrants.

What is the titer factor?

Titration is an absolute method (or primary method), meaning it is of utmost importance to know the exact concentration of the titrant you are using for your results to be accurate and repeatable by other analysts. This is why you need to carry out a standardization.

Usually the difference between the nominal concentration (e.g., 0.1 mol/L) and the absolute concentration (e.g., 0.0998 mol/L) is given by a dimensionless factor (e.g., 0.0998). The absolute concentration is obtained by multiplying the nominal concentration with this factor, which is usually called «titer». In some cases, it is the absolute concentration which is called «titer».

Over the following sections I will present you the essentials on standardization, regardless if you use the word «titer» for the correction factor or for the absolute concentration.

Why should you standardize your titrant?

Knowing the exact titrant concentration is important for correct titration results. This is especially true for self-made titrants, but this is also an important step for commercially available titrants. Titrants can age over time, and thus their concentrations will change.

For example: alkaline titrants, such as NaOH or KOH, will absorb CO2 from ambient air, or iodine-rich solutions will release iodine. Therefore, standardization will give you more security to obtain the correct results for your titrations. 

What can I do to prevent changes to the titer factor?

This depends on which titrant you use for the analysis. The easiest thing to consider is the bottle you plan to store your titrant inside. Some titrants are light-sensitive, and should be stored in dark brown or opaque glass bottles. Others may react with glass, and are best stored in plastic bottles.

Titrants best stored in brown glass bottles:

  • Iodine (I2)
  • Potassium permanganate (KMnO4)
  • Silver nitrate (AgNO3)

Titrants best stored in plastic bottles:

  • Aqueous bases (e.g., NaOH, KOH)
  • Non-aqueous bases (e.g., TBAOH)

Another preventive measure is the use of absorber or adsorber material filled into a tube which is connected to the ventilation part of your buret. This is especially important for titrants which react with CO2 or water from the air.

Use soda lime to absorb CO2 and a molecular sieve for moisture. Even if your titrant is not sensitive, it is still recommended to fill the tube with cotton, which will prevent the entry of dust into the bottle.

The image shown here (click to enlarge) shows an example of an absorber tube filled with soda lime attached to a buret for NaOH. This will avoid the solution losing strength due to carbon dioxide in the ambient air.

Titrants for which soda lime for CO2 absorption should be used:

  • Aqueous and non-aqueous bases (e.g., NaOH, KOH, TBAOH)
  • Sodium thiosulfate (Na2S2O3)

Titrants for which molecular sieve for moisture adsorption should be used:

  • Perchloric acid (HClO4) in glacial acetic acid

How often should I standardize my titrant?

This question cannot be answered with a general number. Frequency of titrant standardization depends on multiple factors, such as titrant stability, the number of titrations per day/week/month, and the required accuracy for your results.

You should always carry out a standardization when you open a titrant bottle for the first time.

The following table is a guideline which should help you to select the frequency for standardizing your titrants. If you are unsure about the stability of your titrant, carry out frequent standardizations (e.g., daily) over a longer period of time until you are able to establish a standardization frequency based on your obtained titer data. The obtained data will show you how much your titer changes over time, and you can then select a suitable determination frequency. Newer software offers the possibility of monitoring your titer. This will help you as well during this task.

Stable titrants:

  • Aqueous acids (e.g., HCl, H2SO4)
  • EDTA
  • Silver nitrate (AgNO3)
  • Sodium thiosulfate (Na2S2O3)
  • Cationic and anionic surfactants

Unstable titrants:

  • Aqueous and non-aqueous bases (e.g., NaOH, KOH, TBAOH)
  • Non-aqueous acids (e.g., HClO4)
  • Iodine (I2)
  • Potassium permanganate (KMnO4)

How to determine the titer

The titer is determined using a primary standard or an already standardized titrant. In either case, be sure to carry out the standardization at the same temperature as the sample titration, as the temperature influences the density of the titrant. Titrants expand at higher temperatures, and thus their titer factor decreases.

Describing the titer determination for every titrant would be beyond the scope of this blog. I will therefore only describe the titer determination procedure here for both cases – using a primary standard or an already standardized titrant – in a general way. If you want to know more about which primary standard is recommended for which titrant, then check out our corresponding Application Bulletin. 

Download the free Metrohm Application Bulletin here:

If you are using a primary standard, dry it at a suitable temperature for a few hours. Allow it to cool down in a desiccator until the substance reaches room temperature, then weigh out an appropriate amount of dried standard for the titration. The weight of the standard depends on the titrant concentration and on the buret volume. I recommend a standard weight which leads to an equivalence point at approximately 50% of the buret volume. If your weight is less than 100 mg, I recommend to prepare a standard solution with your primary standard, as otherwise the weighing error becomes too large.

After you have weighed out your standard or pipetted your standard solution into a beaker, add enough diluent (solvent or water) to immerse the measuring and reference part of the sensor, and start the titration.

If you are using an already standardized titrant, the procedure is a bit simpler. Don’t forget, this titrant should be freshly standardized with a primary standard. Accurately pipette an appropriate amount of standardized titrant into a titration beaker. Add enough diluent (solvent or water) to immerse the measuring and reference part of the sensor, and start the titration.

Shifting gears: What are primary standards?

Primary standards fulfill several criteria which makes them ideal for the standardization of titrants. Primary standards are of:

  • High purity and stability
  • Low hygroscopy (to minimize weight changes)
  • High molecular weight (to minimize weighing errors)

Additionally, they are traceable to standard reference materials (e.g., NIST traceable).

How to calculate the titer factor

After you’ve finished the titrations for the standardization, now it’s time to calculate the titer factor. Again, the formula for the calculation differs slightly depending on whether you have used a solid, dry primary standard or a standard solution / standardized titrant.

 

For a solid, dry primary standard use the following formula:

mSTD:  Weight of primary standard in mg

MSTD:  Molecular weight of primary standard in g/mol

VEP:  Volume at the equivalence point in mL

cTitrant:  Nominal titrant concentration in mol/L

s:  Stoichiometric factor

For a standard solution / standardized titrant use the following formula:

VSTD:  Volume of standard solution / standardized titrant in mL

cSTD:  Absolute concentration of standard solution / standardized titrant in mol/L

VEP:  Volume at the equivalence point in mL

cTitrant:  Nominal titrant concentration in mol/L

s:  Stoichiometric factor

Modern titrators are capable of automatically calculating the titer factor and saving the result together with other relevant titrant data such as concentration and sample name, further improving the data security in your lab.

To summarize:

Standardization of the titrant is not so difficult, just keep in mind to:

  • Carry out standardization regularly — even for ready-made titrants to improve result accuracy of your results.
  • Use dry primary standards or freshly standardized titrants.
  • Carry out the standardization at the same temperature as the sample titration.

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.

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Practical aspects of modern titration

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

Tips and Tricks for IC Columns

Tips and Tricks for IC Columns

Monitoring and maintaining column performance

One of the basic requirements for ensuring reliable chromatographic analyses is a high-performance separation column. Ion chromatography (IC) users should regularly check the performance of their column. This way, if a drop in performance becomes apparent, steps can be taken in good time to restore or maintain the proper functioning of the column, reducing downtimes in sample throughput. In this blog post, we explain how you can assess column performance, which parameters you should monitor, and which measures you can take to ensure excellent column performance.

 

First-time use of a new separation column

When you use a column for the very first time, we recommend that you check its initial performance. The Certificate of Analysis (CoA), which you receive with every purchase of a Metrohm column, is your source of reference here. Record a chromatogram and use the analysis conditions specified in the CoA: these include flow rate, temperature, eluent (mobile phase), analyte concentration, sample loop size, and suppression.

You can evaluate the column’s performance by comparing some of the result parameters with the values listed in the CoA (e.g. retention time, theoretical plates, asymmetry, resolution, and peak height).

Regular monitoring of column performance

Columns that are already in use should be monitored regularly, too! We recommend carrying out these tests with check standards under the application conditions you normally use, because performance varies depending on the type of analysis and associated analysis conditions as well as the instrumental setup. If a reduction in performance is observed, the requirements of the application are crucial to determine whether it can still be used.

Below, we explain how to determine your column performance based on five performance indicators. You will also find out how you can prevent or rectify a decline in performance.

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Backpressure

Monitor the backpressure: When you use your new column for the first time, save the backpressure value under the analysis conditions of your application as a reference («common variable» in MagIC Net). Then use the user-defined results to monitor the difference between the initial backpressure and the one displayed during the current determination.

If you identify an increase in the backpressure in comparison with the saved initial value, this indicates that particles have been deposited in either the guard column or separation column. If the measured increase is greater than 1 MPa, action must be taken. First, you should check which of the columns is affected (guard vs. separation). If the guard column is contaminated, it should be replaced, as this is its primary function. If the separation column is affected, remove it from the system, turn it around and reinstall, and then rinse it for several hours in this reversed flow direction. If this doesn’t help, we strongly recommend that you consider replacing the column. This will be essential if the maximum permitted backpressure for the column is reached.

Retention time

To track changes in the retention time (which signal a decrease in column performance), the retention time of the last analyte peak is monitored in the chromatogram. Sulfate, for example, is suitable for this, as it usually elutes right at the end of standard anion chromatograms. Here too, work with a common variable in MagIC Net to save the initial value.

Unstable retention times can be caused by carbon dioxide introduced from the ambient air or from air bubbles present in the eluent. Luckily, these problems can be resolved easily (see Table 1).

Table 1. Preventing and correcting performance loss in IC columns (click to enlarge).

The column may also have lost some capacity. This capacity loss can be caused by the presence of high-valency ions which are difficult to remove due to their strong attraction to the stationary phase. The column should then be regenerated in accordance with the column leaflet to remove any contamination. If this doesn’t lead to any improvement, then consider replacing the column depending on the requirements of the application, particularly in the event of progressive capacity loss.

Capacity loss can also occur if the functional groups are permanently detached from the stationary phase. In such a case, the column cannot be regenerated and must be replaced.

Resolution

Monitor the chromatographic resolution by comparing measurements from a predefined check standard with an initial reference value. If the resolution is R > 1.5, the signal is considered baseline-separated (see illustration below). However, in cases involving highly concentrated matrices and for peaks that are more widely spread, the resolution value must be higher to ensure baseline separation.

If a loss of resolution occurs, first make sure that it is not caused by the eluent or the IC system. Once these have been ruled out, it is possible that the adsorptive effect of contaminations in the guard column or separation column may be responsible. A contaminated guard column should be replaced. If the cause of the problem is found to be the separation column, this should be regenerated in accordance with the column leaflet to free it from any organic or inorganic contamination. If the loss of resolution progresses, a column replacement is inevitable.

Theoretical plates

Save the initial number of theoretical plates in MagIC Net as a common variable, as mentioned earlier for other parameters. Usually, the last eluting peak is used – in anion chromatograms, sulfate would yet again prove to be a suitable candidate. Theoretical plates also depend on the analyte concentration. Therefore, it is ideal to monitor this parameter during check standard measurements and not during sample measurements. You can track the development of any changes to the number of theoretical plates via the user-defined results in MagIC Net.

A decrease in the theoretical plates can suggest dead volume in the IC system (see Table 1). A low number of theoretical plates may also be observed if the column has been overloaded by a high salt concentration in the sample matrix, for instance. If the theoretical plates decrease by more than 20%, this indicates that column performance is declining. Depending on the requirements of the application, action may need to be taken.

If the guard column is the reason for the drop in performance, it should be replaced. If the problem is with the separation column, we recommend regenerating the column in accordance with the column leaflet to eliminate any organic or inorganic contamination. If this doesn’t help, you should consider replacing the column, particularly if a trend toward lower theoretical plates is observed.

Asymmetry

Determine the initial asymmetry of the analytes by measuring a predefined check standard under the analysis conditions of your application. Save it as a common variable, then track the user-defined results to observe the development of asymmetry over time. The maximum acceptable values for the asymmetry vary depending on the analyte. For example, calcium and magnesium peaks initially present relatively high asymmetry values.

Asymmetry is defined as the distance from the centerline of the peak to the descending side of the peak (B in the figure below) divided by the distance from the centerline of the peak to the ascending side of the peak (A in the figure below), where both distances are measured at 10% of the peak height. Some pharmacopoeia may use other figures – please check to be sure of the requirements in your country.

AS > 1 means a peak has tailing, and AS < 1 equates to peak fronting. Optimum chromatography is achieved with peak asymmetries as close as possible to 1. As a general rule, column performance is considered in decline when the asymmetry is AS > 2 or AS < 0.5. Depending on the requirements of the application, measures have to be taken in this case in order to improve symmetry and to enable better integration.

The reason for high asymmetry values may be down to the ion chromatograph – due to dead volume, for example. If this is not the case, it is important to find out whether the asymmetry is caused by problems with the guard column or with the separation column. If the guard column causes the asymmetry, it should be replaced. If it is the separation column, it should first be regenerated in accordance with the column leaflet to remove any organic or inorganic contamination. If this doesn’t help, you should consider replacing the column. If a trend toward higher asymmetry values can be observed, replacement is unavoidable.

In summary, there are many ways in which you can estimate the performance of the column and track concrete figures over its lifetime. Proper maintenance can extend the lifetime of the separation column, as well as always using a guard column for extra protection.

Need help choosing the right column for your application?

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For further guidance about IC column maintenance, you can watch our tutorial videos here:

Post written by Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland. Primary research and content contribution done by Stephanie Kappes.