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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.

In Part 2, I cover the topics of KF titration troubleshooting and the Karl Fischer oven technique. You can click directly to the post below!

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.

A History of Chemistry – Part 4

A History of Chemistry – Part 4

This article is the final installment in our four-part series on the history of chemistry. Did you miss the others? Don’t worry – you can find them all here:

The industrialization of electrochemistry

Michael Faraday (1791–1867) had a modest upbringing. He was 14 years old when he began his bookbinding apprenticeship. The young Faraday read a multitude of works that he received for binding and thus educated himself in the sciences as well as in literature and art. A customer at the bookbinding workshop noticed the curious apprentice and mentioned him to his father, who then took Faraday with him to several lectures given by electrochemistry pioneer Humphry Davy. Shortly after, Faraday began working for Davy.

As his assistant, Faraday traveled with Davy across Europe, as they carried out experiments together and met numerous influential scientists. Back in England, Faraday continued training as a chemist and in 1833 became a professor of chemistry. During this time, he investigated the basic laws of electrolysis. These formed the basis of electrochemistry and, in the second half of the century, enabled the development of an electrochemical industry which manufactured products such as chlorine, hydrogen, aluminum, magnesium, sodium, and potassium in its plants located at hydroelectric power stations.

Solvay’s soda ash

The industrial production of soda ash (sodium carbonate) had been possible since the development of the Leblanc process at the end of the 18th century. However, the synthesis required expensive raw materials and produced large amounts of the byproduct hydrogen chloride, which is toxic to the environment in which it is introduced. The produced hydrogen chloride escapes from industrial stacks and kills surrounding vegetation, and is also lethal to aquatic life when added to water.

During the second half of the 19th century, the Belgian Ernest Solvay (1838–1922) occupied himself with the issue. Solvay, who came from an industrialist family, had little formal education but was familiar with chemical procedures thanks to his work at his uncle’s and father’s factories. He developed the process for manufacturing soda ash, which was named after him and only has one byproduct – the harmless calcium chloride (CaCl2).

In 1861, Ernest Solvay and his brother Alfred began soda ash production in their own small factory in Brussels. By continuously adjusting the process, they became increasingly successful and continued to expand. Solvay, who had become very wealthy, became active in furthering scientific research and charitable causes. He also showed his sense of social responsibility in his factories: he established an eight-hour workday, paid holiday leave, a social security system, and a pension for his employees –long before it was legally mandatory.

The majority of the soda ash produced today is still created using the Solvay process.

Would you like to learn more?

Visit our site to read more about the Solvay process and the associated analysis techniques:

The periodic table of elements

There had already been 64 chemical elements had discovered by 1868. However, there was as yet no clear system of regulating which particular atom combinations formed new molecules. Sorting the elements based on their atomic mass had not offered a solution up to this point.

Dmitri Mendeleev (1834–1907) recognized a pattern here: when elements are sorted by their atomic mass, certain elemental properties are periodically repeated – specifically, after every eight elements. Mendeleev therefore retained the arrangement in ascending order of atomic mass, but then also sorted the elements that had the same properties below one another. Whenever properties were repeated after fewer than eight elements, he left open gaps to be filled with elements that had not yet been discovered. Mendeleev arranged the transition elements, which did not fit with his «octet rule», into their own column. This resulted in the first periodic table of elements in 1869.

From aniline to aspirin

Organic chemistry, which now went far beyond the synthesis of artificial urea, had become a significant and rapidly growing industry. The tar dye companies BASF, Bayer, and Hoechst, all of which were founded in the 1860s, grew so rapidly that they were employing thousands of people even before the turn of the century. From the end of the 19th century, the tar dye industry also developed synthetic organic medicinal products. Bayer, for example, patented the byproduct-free synthesis of acetylsalicylic acid in 1898 and marketed the product under the name «Aspirin» from the beginning of the 20th century.

In basic research, chemists began devoting themselves to increasingly complex organic molecules. Emil Fischer (1852–1919) investigated biologically significant molecules such as sugars and amino acids. In 1890, he used glycerin as a basis for synthesizing three sugars: glucose, fructose, and mannose. He later researched proteins. During this period, he discovered new amino acids and shed light on the type of bond which connects them to one another: an amide bond which he gave the name «peptide bond» [1].

First World War: Artificial fertilizer and warfare agents

The use of fertilizer had been common practice throughout agriculture ever since Liebig proved that it would improve yield. The nitrogen needed by plants for growth was added to fertilizers largely in the form of guano. This consists of the weathered excrement of seabirds which forms meter-thick layers over many years, particularly on the beaches of South America, where there are low levels of precipitation. In order to meet the high demand for food – and thus for fertilizer – entire shiploads of guano were being imported to Europe.

However, the import of guano could not keep pace with the rapid growth of population indefinitely, so at the end of the 19th century, researchers began looking for a way to fix nitrogen from the air. The German chemist Fritz Haber (1868–1934) eventually found a solution in 1909 and, with his ammonia synthesis, prevented the famine prophesied in the western world. Unfortunately, this development also enabled Germany’s production of warfare agents during the First World War, as ammonia could be used to create ammonium nitrate, which was then used in ammunition.

Fritz Haber
Carl Bosch

In the Haber-Bosch process, ammonia is produced as a result of a reaction between hydrogen and nitrogen. Fritz Haber achieved synthesis at a high temperature and a high pressure level, and with the aid of a catalyst. Carl Bosch (1874–1940) developed the industrial implementation of the process. For this purpose, he developed specific equipment made of state-of-the-art materials which could withstand both high pressure and temperature levels.

In 1914, the First World War broke out. The nations involved, as well as neutral states, faced blockades in their trade routes and had to become largely self-sufficient. Thanks to governmental structuring and aid, this led to a boom in industrial research across the globe. Numerous reputable scientists were actively involved in the war or supported it, including Fritz Haber, Walther Nernst, and Emil Fischer. In addition to the Haber-Bosch process, the pressure to create innovations that prevailed before and during the war also resulted in the first synthetic rubber as well as mustard gas and the toxic gas phosgene. Chlorine gas, which is produced during ammonia synthesis, was also used as a warfare agent during World War One.

What if . . .

. . . the Haber-Bosch process didn’t exist? Without the nitrogen fertilizer produced using the Haber-Bosch process, there would likely be a lot fewer people on Earth: the population growth of around 1.6 billion in 1900 to nearly 8 billion today would not have been possible without yield improvements brought about by artificial nitrogen fertilizers. Agriculture is still dependent on it today: without this process, the planet would only be able to provide enough food for half the population [2].

Chemistry since WWI

Following the armistice agreement in 1918, the German chemical industry – which had been world-leading until then – lost all of its patents and had to reveal numerous production secrets in order to satisfy the reparation demands of the victorious Allied Powers [3]. The German chemical industry, which had been the world’s largest at the time, had to relinquish its place at the top. Although it experienced another upswing toward the beginning of the Second World War, today’s leading lights in the chemical industry are the USA and France. During the post-war period, polymer chemistry and pharmaceutical chemistry were the fields that saw particular advancement and brought about countless products which are still essentials today. Among these are polymers, including synthetic fibers such as nylon and polyester, and artificially produced vitamins and hormones.

The time around the turn of the 20th century saw rapid advancement in chemistry, both in fundamental research as well as in industry – and to a great extent, it is the relationship between the two which enabled this progress. Numerous processes developed during this time period, including the Haber-Bosch and Solvay processes, have remained the methods of choice in the production of chemicals – in this case ammonia and soda ash, respectively – to this very day. 

References

[1] The Components of Life: From Nucleic Acids to Carbohydrates; 1st ed., Rogers, K., Ed.; Britannica Educational Publishing/Rosen Educational Services: New York, 2011; p 59.

[2] Erisman, J. W., Sutton, M. A., Galloway, J., Klimont, Z., and Winiwarter, W. (2008) Nat. Geosci. 1, 636–639.

[3] Kricheldorf, H. R. Menschen und ihre Materialien: Von der Steinzeit bis heute; 1st ed., Wiley-VCH Verlag & Co. KGaA: Weinheim, 2012; p 111.

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

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.

A History of Chemistry – Part 3

A History of Chemistry – Part 3

This article is the third in our four-part series on the history of chemistry. Missed the first two? Don’t worry – you can read both of them here:

Chemistry and society: An explosive pair

It is the early 19th century, and industrialization in Europe is in full swing. Close collaboration between the chemical industry and research – largely in France to start with, then followed by other European countries – is resulting in rapid advances in both sectors. As the chemical industry grows, chemistry is gaining a higher profile in society. The third and fourth parts of our series on the history of chemistry consider the relationship between chemistry, industry, and society from the 19th century onward.

The chemistry of living organisms

One of the most important chemists of the early 19th century is Jöns Jakob Berzelius (1779–1848). This Swedish scientist improved laboratory techniques and developed methods for elemental analysis. By conducting systematic analyses on a large scale, he determined the molecular formulae of virtually all known inorganic compounds and the atomic masses of the elements that had been discovered at that point. He is also the person we have to thank for element symbols: H for hydrogen, O for oxygen, and so on. The only difference between his notation and what we use today is that Berzelius presented element proportions in molecular formulae as superscript characters rather than the subscript characters we see nowadays (e.g., H2O instead of H2O). 

As well as this, he dealt extensively with the chemistry of organisms, something which he dubbed «organic chemistry». Being a proponent of vitalism, Berzelius was convinced that only living organisms were capable of producing organic substances, claiming that «vital force» was necessary for this process. The findings of one of his apprentices, Friedrich Wöhler, would later put a question mark on this hypothesis.

Organic from inorganic – is it possible?

In 1828, Friedrich Wöhler (1800–1882) was the first person who successfully managed to synthesize an organic compound from inorganic reagents: by heating ammonium cyanate, he was able to create its organic isomer, urea. He thus showed that organic substances can be created in a laboratory and that humans are therefore able to imitate and manipulate nature. More and more organic syntheses were made possible as the 19th century progressed.

Wöhler’s synthesis of urea was revolutionary. Today, urea is produced industrially at a rate of 150 million tons per year. Among other things, it is used for dermatological products and in the polymer industry.

Metrohm offers you solutions for the determination of urea and its contaminants. To find out more, visit our industry page «Chemical» and choose «Basic chemicals»:

Wöhler and Liebig: A fruitful friendship

Wöhler formed a friendship with Justus von Liebig (1803–1873) after they settled a dispute about silver fulminate and silver cyanate in 1825. Both substances share the same molecular formula, but the silver fulminate discovered by Liebig is highly explosive, whereas Wöhler’s silver cyanate is not. They eventually ascertained that the type and number of atoms in a compound alone are not enough to characterize a substance – the arrangement of the atoms must also be considered. As well as a spirit of mutual esteem, Liebig and Wöhler thus discovered isomerism. However, determining the molecular structure was not yet possible at that time.

In 1832, the two researchers worked together to formulate their so-called radical theory, which paved the way for modern organic chemistry. This stated that organic substances are composed of atom groups, which they called radicals. These remain unchanged during chemical reactions and are merely exchanged between the reactants. Although the term «radical» now has a different meaning in chemistry, a very similar principle remains until today: functional groups.

Superphosphate revolutionizes agriculture

Around 1840, Liebig, who had studied at the Sorbonne in Paris under such great names as Gay-Lussac and experienced France’s symbiotic relationship between science and industry, turned his back on fundamental research. Instead, he began studying organic chemistry in physiology and agriculture. He realized that plants extract nutrients needed for growth from the ground – with the exception of carbon dioxide, sourced from the air. From his findings, he deduced practical implications which revolutionized agriculture. Through his work, Liebig was the first to establish the need for fertilizer from a scientific point of view. His research also allowed him to determine which nutrients need to be present in fertilizer. This included simple organic compounds, but also inorganic substances such as salts. Based on this knowledge, Liebig developed the first artificial fertilizer, superphosphate, which led to an enormous increase in agricultural yields.

Liebig’s superphosphate fertilizer is still used today. Thanks to new findings, however, there are now a multitude of fertilizers which can provide the necessary nutrients based on the plants and soil conditions.

You can find several free Metrohm applications for agrochemicals available for download here:

Kekulé: dreamer or fibber?

Liebig’s students carried on his legacy through conducting fundamental chemical research. One such example is August Kekulé (1829–1896), who was inspired by Liebig to study chemistry during his time at the University of Giessen instead of becoming an architect as Kekulé’s family had envisaged. In 1858, Kekulé recognized the ability of carbon atoms to bond directly with one another to form chains. This explained how the few elements found in organic matter could form such a diversity of organic substances. In 1865, Kekulé also published findings on the structure of benzene.

According to his own statements, both of Kekulé’s groundbreaking ideas were inspirations from dreams – but the truth behind this is disputed. Kekulé is regarded as an intellectual who disparaged the culture prevailing among chemists and industrialists at the time: a pragmatic, positivistic way of thinking was spreading – it was a blind sense of empiricism which afforded no room for imagination. Christoph Meinel – a historian of chemical sciences – doubts the truth behind the dream anecdote, first told by Kekulé during a speech at a celebration in his honor. He states: «Kekulé’s ambivalent attitude toward the mentality present during this historical period, known as the ‹Gründerzeit›, and toward the patriarchal views of Berlin society resonates only too clearly in his speech. When Kekulé finishes narrating his vision with the words ‹Let’s learn to dream, gentlemen!›, the irony is very difficult to ignore given the prominent profile of those in attendance, who represented Prussian bureaucracy, Gründerzeit industries, and the elitist universities of the time» [1].

Artificial colors: All thanks to benzene

Regardless of whether Kekulé’s anecdote was based on true events or not, his discovery of the benzene structure and its importance to chemistry cannot be denied. Knowledge of organic and aromatic structures enabled systematic synthesis of the same molecules. The work of chemists was increasingly shifting from the isolation of substances from nature to the synthesis of artificial substances. The colorant industry experienced a boom after the discovery of the benzene structure, as this meant that a multitude of artificial colors could now be produced. Indigo production, for example, became an economically significant industrial process.

Colorant syntheses have not lost relevance since their invention in around 1900 – in fact, quite the opposite case. Colorants have been continuously developed for numerous properties and functions. This makes them inherently more complex than their primitive predecessors.

You can find out which analysis technique you need to monitor colorant properties on our industry page «Chemical» under the menu item «Solvents and colorants»: 

Check out final post in this series to learn about the advancements of chemistry around World War II. Click here to go directly to the next post!

Reference

[1] Sponsel, R. and Rathsmann-Sponsel, I. Kekulés Traum. Über eine typisch-psychoanalytische Entgleisung Alexander Mitscherlichs über den bedeutenden Naturwissenschaftler und Chemiker August Kekulé (1829-1896), Mitschöpfer der Valenz-, Vollender der Strukturtheorie und Entdecker der Bedeutung des Benzolrings. Alternative Analyse und Deutung aus allgemeiner und integrativer psychologisch-psychotherapeutischer Sicht. http://www.sgipt.org/th_schul/pa/kek/pak_kek0.htm (accessed Aug 15, 2016).

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

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.