Karl Fischer Moisture Determination

Transcription

1 KARL FISCHER MOISTURE DETERMINATION 1 Karl Fischer Moisture Determination Related Articles 13 References 13 Katrin Schöffski Sigma Aldrich Laborchemikalien, Seelze, Germany Dieter Strohm Metrohm Ltd, Herisau, Switzerland 1 Introduction 1 2History 2 3 Definitions 3 4 Instrumentation The Titration Stand Volumetric Karl Fischer Titration Coulometric Karl Fischer Titration Thermostatic Titration Cells Karl Fischer Oven Automation of the Volumetric Karl Fischer Titration Oven Sample Processor 6 5 Reagents One-component Reagent Two-component Reagents Auxiliary Products 7 6 Standardization, Calculation and Control of the Results Titer Determination Sample Size and Calculation of the Water Content Control of the Result Interferences 8 7 Sample Preparation Solids Liquids Gases Oven Method 9 8 Important Applications Chemicals Pharmaceutical Products Petroleum Products Plastics Foodstuffs 12 9 International Standard Procedures Comparison with Loss-on-drying 12 Abbreviations and Acronyms 13 The Karl Fischer (KF) titration is a method of determining the water content of solid, liquid and gaseous samples. It is the technique preferred for use in industrial quality control. In principle it involves the oxidation of sulfur dioxide by iodine, in the presence of water, in a buffered solution. An alcohol is used as the preferred solvent. The water is converted stoichiometrically and therefore its quantity is determined indirectly. The end-point (EP) is reached when there is an excess of iodine. It can be indicated visually, photometrically or electrochemically. According to state-of-the-art technology a double platinum electrode determines a voltammetric indication. The KF titration can be carried out either volumetrically or by coulometry. For a volumetric titration the iodine is added by volume to the titration cell containing the sample. In a coulometric titration iodide is oxidized at a platinum electrode and the iodine formed reacts with the water. The amount of current necessary in the generation of the iodine is directly related to the quantity of iodine generated, according to Faraday s first law. In practice, special titrators for the KF titration are available. Preformulated reagents are similarly available ready for use. Water in an extensive number of materials can be determined by KF titration. Chemicals, pharmaceuticals, oils, plastics and foodstuffs are examples of typical sample types. The measurement range spans a few ppm to 100% water. The KF titration is described in ISO 760 and as part of many other international standard procedures. Other methods for the determination of water content include loss-on-drying, IR spectroscopy or azeotropic distillation. 1 INTRODUCTION The water content of various products is an important quality consideration in industrial processes, and, for a raw material, is often taken as a correction factor in the measure of its worth. For example, for oils and cereals, the proportion of water in the gross weight of a delivery is taken into consideration in the price. For intermediate products, such as plastic granulates or tablet powders, the water content influences the mechanical characteristics, e.g. the flow capability in a press. In the final product of a foodstuff the water content must sometimes be determined by law and can also influence the storage stability. For these reasons an exact determination of the water component of the material is necessary. It is necessary to distinguish between moisture and water determination. To be exact, moisture is referred to Encyclopedia of Analytical Chemistry R.A. Meyers (Ed.) Copyright John Wiley & Sons Ltd 14289

2 2 GENERAL ARTICLES as bound water. It can also be used to describe a bound solvent or an evaporated substance. In general, moisture is determined by using an oven. Water, on the other hand, is determined chemically by the KF titration. Both surface water and water bound in crystals can be determined in this way. Water determination according to the KF method is an analytical technique for determining the water content in various matrixes. Chemically, it involves the oxidation of sulfur dioxide by iodine, with the consumption of water, in a buffered solution. Organic solvents are used as the solvent for the sample as well as for the working medium in the titration cell. Methanol, ethanol and higher propylene glycol mixtures are most often used. The KF titration can be carried out both volumetrically and coulometrically. The volumetric titration is carried out with a one- or two-component reagent. When a one-component reagent is used, the buret of the titrator contains all the necessary reagents dissolved in an inert solvent. Methanol or a mixture of alcohols serves as the working medium in the titration cell. This is then pretitrated by the KF reagent in order to remove any traces of water from the cell and working medium. Then the sample is added and its water content determined automatically. In a two-component reagent the reactive components are separated. The so-called titrant contains a preset amount of iodine in a solvent, which can be either methanol or ethanol. The buret of the titrator is then filled with this solution. A base and sulfur dioxide are dissolved in the KF solvent, which is added to the titration cell and pretitrated to dryness. The sample is then added and its water content determined. The one-component reagent has the advantage that it can be used with a wide variety of different working media. The two-component reagent is distinguished by its faster titration rate and greater polarity. Typical water contents for volumetric KF determinations are between 0.5 and 50 mg water. Figure 1 shows a typical KF titration curve of time vs reagent added. A different composition is used for KF coulometry. The reagents contain a solution of iodide rather than iodine, which is then generated from the iodide at a platinum electrode. The iodine then reacts with sulfur dioxide according to the KF chemistry, with the loss of water. The current necessary to generate the iodine is measured. The amount of iodine is determined using Faraday s law and the water content consequently calculated. To carry out a coulometric determination the reagents are added to the titration cell. These solutions must first be pretitrated as in the volumetric method. Any water in the titration cell or dissolved by the reagent from the air is thereby removed. Since coulometry is more sensitive, this pretitration process takes longer than a volumetric titration. The sample is injected, via a septum, into the reagent. Modern titrators calculate the amount of water automatically after injection of the sample. Typical absolute values of water analyzed by coulometry are between 50 and 2000 µg. 2 HISTORY In 1935 the German chemist Karl Fischer was required to determine the water content of liquid sulfur dioxide. Neither loss-on-drying nor distillation were suitable for liquid samples, and he had to find an alternative method. During his investigations he stumbled across the Bunsen reaction, Equation (1), which describes the iodometric determination of sulfate in a buffered, aqueous solution. SO 2 C I 2 C 2H 2 O (H) H 2 SO 4 C 2HI 1 Volume of reagent added (ml) (a) EP (b) Time (min) Figure 1 Typical Karl Fischer titration curve. (a) Reagent adding mode; (b) conditioning mode. According to Equation (1), a determination of the water content should also be possible if the sulfur dioxide is present in excess and the protons that are consequently produced are taken up by a base. Fischer put together his reagent using sulfur dioxide, iodine and pyridine, with methanol as solvent. He formulated Equation (2), 1 SO 2 C I 2 C 2H 2 O Ł Py (H) H 2 SO 4 C 2HI Ł Py where Py represents pyridine. The EP was indicated by the color change from yellow to brown. Using this onecomponent reagent, he carried out the first volumetric KF titrations and determined the water content of sulfur dioxide and various different solvents

3 KARL FISCHER MOISTURE DETERMINATION 3 In 1939 the reaction mechanism was re-examined by an American research group. Smith et al. formulated the two-step reaction, Equation (3), with a different molar relationship of water to iodine of 1 : 1, 2 H 2 O C SO 2 Ł Py C Py Ł I 2 C Py C MeOH (H) 2HI Ł Py C Py Ł SO 3 Py Ł SO 3 C MeOH (H) Py Ł MeSO 3 H 3a 3b where MeOH represents methanol. By 1943 Wernimont and Hopkinson had introduced the electrochemical deadstop EP detection for the KF titration. 3 During the years that followed, there were further improvements in KF reagents, 4 but the next big step came in 1959 when Meyer and Boyd published the coulometric KF titration. 5 Further investigations were published in the 1970s concerning the kinetics of the reaction. In 1974 Cedergren discovered that the reaction rate is dependent on the concentration of iodine, sulfur dioxide and water. 6 Verhoef and Barendrecht found that pyridine functioned only as a base and was not part of the reaction itself. 7 From these observations Scholz formulated Equation (4), the version of the reaction equation that has been accepted since 1984, 8 SO 2 C HO-R C B (H) R-SO 3 C BH C 4a R-SO 3 C I 2 C H 2 O C 2B (H) R-SO 4 C 2I C 2BH C 4b where B is a base. An alkyl sulfite is formed within the reagent: from the alcohol used as solvent and the sulfur dioxide (Equation 4a). Wuensch and Seubert isolated this reaction product in The alkyl sulfite is oxidized by iodine in the presence of water in the second step (Equation 4b). The equations illustrate two essential points. Firstly, in order to guarantee exact stoichiometry, an alcohol must be present in the KF titration. In practice, it is found that approximately 50% methanol or ethanol is necessary in the working medium. Secondly, since the pyridine does not take part directly in the reaction, it should be possible to replace it by other bases. Pyridine has been unwelcome in laboratories since the end of the 1970s due to its unpleasant odor and the fact that it is harmful. Since the 1980s pyridine-free reagents have been the standard. Comprehensive application support and information on the titration of many different samples is available in the form of the monograph the HYDRANAL Manual. 10 Imidazole is used most often as the base in pyridine-free reagents. The reagents manufactured with imidazole as base have a higher ph because imidazole is a stronger Log K ph Figure 2 The relationship between reaction rate, K, and ph. base than pyridine. Since the reaction rate, K, of the KF reaction is dependent on the ph (Figure 2), a faster titration and more stable EP can be achieved. 11 By 1998 further advances in the field of the KF water determination included the development of a coulometric cell without a diaphragm, and reagents free of harmful materials. 12,13 3 DEFINITIONS One-component reagent. A volumetric reagent containing sulfur dioxide, base and iodine in an organic solvent. It is used with working media such as alcohols, or mixtures of organic solvents and alcohols. Two-component reagent. A volumetric reagent combination consisting of a titrant and solvent. The solutions are not mixed prior to use but are used separately, in contrast to the former pyridine-containing KF reagents. Titrant. An alcoholic iodine solution. Solvent. The solution containing the base and sulfur dioxide and which is added to the titration cell. Anodic reagent D anolyte. A reagent consisting of iodide, a base and sulfur dioxide in an alcohol. It is the reagent in which the coulometric KF reaction takes place. Cathodic reagent D catholyte. A reagent consisting of organic salts in an alcohol. It is the reagent in which the coulometric cathode reaction takes place, i.e. the reduction reaction. It counteracts the iodide oxidation and normally involves the reduction of protons to hydrogen. Titer. The water equivalent of the measurement reagent given in mg water per ml reagent. Drift. The blank value in the titration cell prior to the addition of water. Drift is caused by moisture ingress into the cell and by side-reactions which consume iodine. Modern titrators determine and display the drift automatically. The drift is given in different units depending on the instrument supplier: µgmin 1,mgmin 1,µLmin 1 or µgs 1 are in use

4 4 GENERAL ARTICLES 4 INSTRUMENTATION Due to the special qualities of the water content determination by KF titration it is perhaps understandable that specific analytical instruments for this titration have been on offer for many years. As a result of the enormous developments in the field of microprocessor technology, many of these analytical instruments are now driven by microprocessors. In practice, there are two different instruments accepted for the KF water determination: the volumetric KF titrator and the coulometric KF titrator. Both techniques use the EP titration method, and the titration stand consisting of the titration cell and stirrer plays a particularly critical role. 4.1 The Titration Stand The atmospheric moisture present in every laboratory is a common source of error in KF titration. This moisture can penetrate the sample, titration reagent and titration cell, and thereby cause incorrect results. Modern analytical instruments for the KF water determination (such as that in Figure 3) reduce the influence of atmospheric moisture as far as possible by ensuring that the following measures are incorporated into the titration stand. The stand is particularly airtight and has special input ports for the addition of solid, liquid or gas samples. In order to minimize the ingress of atmospheric moisture when changing the reagents, a pump is often integrated into the stand. This pump can suck out used reagent and pump in fresh without having to open the titration cell. During the pumping, it is impossible to avoid air being sucked into the titration cell. Moisture ingress in this way is normally avoided by passing the air over a molecular sieve. No titration stand is absolutely closed to the atmosphere and very small quantities of water can always seep in. Modern KF titrators continually condition the titration cell and determine the quantity of water that has seeped into the cell over a particular period of time. This value is displayed as drift, e.g. as µg H 2 Oor µl of titration reagent required per minute. The drift is continually on display and the user therefore has a constant measure of the condition of the titration stand. The drift value is also measured at the beginning of the titration and it can therefore be taken into consideration in the final result. As with all EP titrations, good mixing of the reagents is an important prerequisite for a fast and exact KF titration. Therefore a magnetic stirrer is built into the titration stand of a modern analytical instrument for KF water determination. 4.2 Volumetric Karl Fischer Titration KF titrators of today are, without exception, fully automatic. The water determination is carried out independently by the press of a button, with the water content calculated and displayed or printed out, according to good laboratory practice (GLP), on an attached printer. To ensure that an automatic titration is possible the volumetric KF titrator has the following essential components Motorized Volumetric Buret with a Titration Stand This buret enables a precise dosing of the titration reagent. To ensure that the hub can accurately add reagent in the microliter range, it is set up with steps Exchange Unit The exchange unit was developed to dose the titration reagent. It consists of a cylinder, buret, tap, buret tip and the stock bottle containing the titration reagent. In order to protect the reagent from ingress of atmospheric moisture, the stock bottle is fitted with a drying tube Indication System Figure 3 A typical, automated volumetric Karl Fischer titrator. Normally a bi-voltammetric indication is used as the indication system for the EP of a volumetric KF titration. For this a double platinum electrode with a constant current (e.g. 50 µa) is used. Whilst iodine is reacting with water in the titration cell, there is no free iodine in the titration solution. A high voltage is necessary to maintain the preset polarization current of the electrode. As soon as all the iodine has reacted with the water, free iodine will remain in the titration solution. This free iodine considerably reduces the voltage, which is necessary to maintain the preset polarization current

5 KARL FISCHER MOISTURE DETERMINATION Controls for the End-point Titration In a KF titration the titration reagent must be added as fast as possible from the buret and exchange unit, and the addition should be stopped exactly at the EP. The control of modern KF titrators adapts itself to the course of the titration curve and thus leads to short titration times and very accurate results Integrated Communications A display and keypad are available to the user in order to input control parameters, sample data for the calculation and the documentation and to be able to read the results. Top-of-the-range KF titrators display a live titration curve in the display, which can give the user useful information about the course of the titration Method of Storage A storage capability is necessary to save preset KF titration methods. In each laboratory at least two methods are necessary: the titer determination and the water content determination. If various different samples are to be analyzed the optimum titration conditions can be saved for each different sample type. Then all that has to be done is to call up the right sample, add the sample and start the determination Interfaces Interfaces for a balance and data system (laboratory information management system (LIMS)) enable the KF titrator to be integrated into the modern analytical laboratory. 4.3 Coulometric Karl Fischer Titration Coulometric KF titrators are also exclusively automatic. The water determination is carried out independently at the press of a button, with the water content calculated and displayed and documented on a printer connected to the titrator. They are differentiated from volumetric titrators by the following aspects. Instead of the motorized buret to dose the iodine solution, coulometers have a generator current circuit such that iodine can be generated electrochemically. For the iodine production there is a generator in the cell rather than the buret tip. In practice there are two types of generator electrode: those with and those without a diaphragm. Wherever possible a generator without a diaphragm is preferred because handling is then easier. Owing to the smaller amounts of water being determined, a more sensitive indication set-up is preferred. The indication principle is, in effect, identical to that in the volumetric KF titration, only here an alternating current is used for the polarization of the double platinum electrode. Coulometric KF titration is a micro method and particularly suitable for the determination of small quantities of water. Owing to the high sensitivity, particular consideration has to be paid to contamination by moisture from external sources. The water content of a wide range of samples, from diverse industrial processes, can be determined by using the KF technique. For this reason, special instruments are available for the preparation of samples that are analyzed regularly. Additionally, automated instruments are available. 4.4 Thermostatic Titration Cells Substances which dissolve slowly in methanol or the KF working medium, cause drifting EPs. The same is true for solids, for example foodstuffs, which give up their water only very slowly. In such cases, titration at C can accelerate the dissolution of the sample or the release of its water. Slow side-reactions sometimes involve iodine and therefore a stable EP is impossible to achieve. It can then be advantageous to carry out the titration at low temperatures, which can reduce the effect of the side-reaction. Special titration cells are available for these techniques. They are constructed with an outer glass jacket through which a liquid can be pumped. When attached to a thermostat or cryostat the titration temperature can be adjusted. 4.5 Karl Fischer Oven Many substances only release their water at high temperatures and are therefore unsuitable for the KF titration. These samples can be heated in an oven to between 100 and 300 C and their water evaporated. The evaporated water is then carried over into the titration cell via a heated, inert stream of gas and titrated either coulometrically or volumetrically. The KF oven can be used for insoluble solids (e.g. plastics and salts), where the water can be released at temperatures above 60 C fairly rapidly, and also for solids and liquids that react with the KF titration reagents (e.g. ascorbic acid, mineral oil). 4.6 Automation of the Volumetric Karl Fischer Titration The aim of many laboratories is to increase the throughput of samples and so improve business efficiency and relieve the personnel of routine work. For this reason, sample exchangers were developed. Samples are weighed into sample beakers, which are then closed with aluminum foil in order to protect them from the ingress of atmospheric moisture. When they are at the titration position they are 14293

6 6 GENERAL ARTICLES displays a very fast fall in the titer value. One-component KF reagents are generally unstable. A slow-side reaction, which involves iodine, takes place within the reagent, and so diminishes the titer. 14 Pyridine-containing reagents often decompose completely within a very few weeks but a modern pyridine-free reagent has a titer decrease of less than 10% per year if stored correctly. Additionally, the ingress of atmospheric water into the reagent can cause the titer value to decrease. Air humidity has a great influence on the stability. A liter of air contains up to 15 mg water and therefore it must be dried before being pumped into the titration cell or bottle of reagent. Figure 4 A typical, automated sample processor. raised and the foil is pierced. A defined volume of solvent is added. Then the desired dissolving time or extraction time elapses before the KF titration is started. Using such equipment, large numbers of samples can be processed fully automatically. Sample exchangers will also work overnight or during the weekend. 4.7 Oven Sample Processor Water determination in plastics, oils, emulsions or salts often cause problems. They can cause contamination of the oven and titration cell, side-reactions, reduced lifetime of the reagent, long analysis times and high cost due to the fact that the possibility of automation is limited. An oven sample processor (Figure 4) has been developed to avoid direct contact between the sample and titration cell and thereby avoid the above-mentioned problems. In addition, the full capacity of the reagent is utilized to the optimum. Both the KF volumetric and coulometric titrator can be set up in coordination with the oven sample processor. The samples are weighed in the small (glass) sample boat, which is then sealed with a septum and placed on the exchanger. When the small sample boat reaches the workstation, the oven sample processor is lowered and the septum is pierced with a needle. At the same time the sample boat is pushed into the aluminum heating block and the heater is switched on. The needle has two openings: an inert gas enters the sample boat via the first one, and leaves through the second. The inert gas then passes into the titration cell along with the evaporated water of the sample. The water is then titrated. 5 REAGENTS The reagent Karl Fischer described has a number of disadvantages. Apart from the strong smell of pyridine, it 5.1 One-component Reagent A pyridine-containing one-component reagent has been described. 15 The titer of these reagents is not given, but must be determined daily since it drops rapidly. Commercially available, pyridine-free reagents are offered with various different concentrations. The water equivalence is often contained in the name of the reagent. Reagents of titer 5, 2 and 1 are the norm. They are used in conjunction with methanol or other special working media. The working media can be varied in order to improve the solubility of the sample or to minimize side-reactions. A chloroform/methanol mixture would be used, for example, to aid the solubility of fats. An ethanolic medium will reduce a side-reaction of the sample with methanol. Aldehydes and ketones react with the components of a usual KF reagent, and therefore special reagents are available for such samples. They have a K in the name, referring to ketone. The water determination is carried out with a one-component reagent of titer 5 in a special, methanol-free working medium. The main components of this working medium are chlorinated or methoxylated alcohols. 5.2 Two-component Reagents Two-component reagents consist of a titrant and a KF solvent. The titrant is available with a titer of 5 or 2. For general use both methanol- and ethanol-based KF solvents are used. Solvents such as chloroform or formamide can be added to the KF solvent to aid sample solubility. There are also special products for nonpolar samples available Coulometric Reagents Reagents for coulometry are in the form of an anolyte and catholyte. The KF reaction takes place in the anode reagent. This contains sulfur dioxide, an organic base and a soluble iodide in an alcoholic solvent. Anode reagents 14294

7 KARL FISCHER MOISTURE DETERMINATION 7 have an A in the name. For general purposes an anolyte that does not contain a chlorinated hydrocarbon is used. For oils, a chloroform-containing anolyte is used; for ketones, a methanol-free anolyte. The reduction takes place in the associated catholyte. The sample does not come into direct contact with the catholyte, and therefore an alcoholic catholyte usually based on methanol can be used for almost all applications. The methanol-free reagent is necessary only for a ketonic sample. Catholytes can be recognized from the C in their name. 5.3 Auxiliary Products In addition to the actual KF reagents, a whole array of auxiliary products is available. Buffers and acids allow the setting of an optimal ph. Karl Fischer standards with a predetermined water content are used to carry out a titer determination and to check the functioning of the instrumentation. 6 STANDARDIZATION, CALCULATION AND CONTROL OF THE RESULTS 6.1 Titer Determination Although a titer is given for volumetric reagents, this must be checked at regular intervals. The titer of a onecomponent reagent falls due to a side-reaction and there is always the possibility of ingress of moisture from the atmosphere, which also reduces its value. Since the titer is actually a measurement of volume, there is also a temperature dependency. The titer can be determined using pure water, salts with a constant water content and liquid water standards. Pure water is injected into the cell using a microliter syringe. The titer is calculated according to Equation (5): titer D water equivalent D amount of water volume of reagent where the amount of water is in mg, and the volume of reagent is in ml. Sodium tartrate-2-hydrate is used as the solid standard. This salt is not hygroscopic, it dissolves in methanol and has a constant water content of 15.66%. The titer is calculated according to Equation (6): titer D water equivalent D sample size ð volume of reagent where the sample size is in mg, and the volume of reagent is in ml. Liquid water standards consist of a non-hygroscopic solvent mixture that contains % water. The standard (1 2 g) is weighed into the cell by 5 6 difference. The calculation for the 1% standard would be as in Equation (7): titer D water equivalent D sample size ð 0.01 volume of reagent where the sample size is in mg, and the volume of reagent is in ml. The titer determination is not only an exact measure of the water equivalent but is also a control of the working conditions. If the titer falls dramatically, this can indicate moisture in the reagent. If the values vary considerably from one another the titration cell may not be firmly closed. A titer determination or any other calibration procedure is unnecessary in the case of coulometry. It is an absolute method in which the amount of iodine is calculated by using Faraday s constant. 6.2 Sample Size and Calculation of the Water Content Sample size depends on the method, water content of the sample and the desired accuracy. Table 1 gives some idea of appropriate sample sizes. 6.3 Control of the Result When validating the KF titration, various sources of error need to be considered. The result can be incorrect due to an error in the titer value, or a deficient reagent or instrument. Additional errors due to a reaction between the sample and reagent are also possible. The whole titration system, i.e. the instrumentation together with the reagents, can be checked using a certified water standard. The water content of the standard is determined during its manufacture by obtaining numerous results at various sample weights. In order to guarantee the KF system, the result obtained with the standard must lie within a predetermined standard deviation limit. Table 2 shows a typical overview of results. This procedure checks both the KF apparatus and reagents together. If the results do not fall within the accepted limits of error then the source of the error must Table 1 Recommended sample sizes 7 Water content Sample weight (g) for Sample weight (g) for of sample (%) volumetric titration coulometric titration

8 8 GENERAL ARTICLES Table 2 Typical results from a reliability test of a KF coulometer Sample Weight of Water Water Water no. water (calculated) (found) content standard (g) (mg) (mg) (mg g 1 ) Water content: Mean of nine determinations, Relative standard deviation, 0.75% Standard deviation control limit, 2% Control requirements fulfilled? Yes. be found and eradicated. Then the procedure is repeated with fresh reagents. The drift and titration curve should be noted. The balance being used must also be checked. If the error persists then the buret of the volumetric titrator can be checked. In order to do this, definite volumes of water are dosed at 20 C from the buret and these volumes are then compared with their exact weight. Further details can be obtained from the manufacturers of the instrumentation. 15 If the source of error remains, the manufacturer should check the instrumentation. Manufacturers of the instruments are able to certify them, and also support the user in many ways in terms of quality management in the analytical laboratory. Practically all manufacturers are ISO 9001 accredited and consider the requirements of the international standards of quality management during the development of their instrumentation. This support involves all phases of instrument procurement and use. Advice involves the setup of the equipment, the necessary solutions to problems, delivery of the instrument, instructions of use, operator training, instrument servicing, and solving customer problems. On request, the manufacturer will help the operator with installation qualification (IQ), the operation qualification (OQ) and performance qualification (PQ). With the integration of GLP tools, for example the monitoring of validation intervals, reagent life and result limits, the operator is efficiently supported by the instrument itself during daily determination of water content by KF titration. 6.4 Interferences If it is to be determined whether or not a sample is interfering with the KF chemistry, the method of subsequent standard addition is recommended. The working medium of the volumetric titration or the anolyte in coulometry is titrated to dryness. Then the sample is added and its water content found. A known amount of water is then added to the titration cell still containing the titrated sample. If the amount of water titrated is 100%, within a predetermined standard deviation control limit, interference with the sample can be eliminated. 7 SAMPLE PREPARATION Due to the wide area of application for the KF titration, there are countless different samples where the water content needs to be determined. It is only possible here to cover the sample preparation and methods for the various different classes of substance rather than individual materials. 7.1 Solids Solids usually have water bound in two different states. It can be adsorbed on the surface of the solid or be in the form of crystallized water or trapped water in the solid. In order to determine the whole water content it is preferable that the sample is fully dissolved. Since many materials do not dissolve in methanol, additional solvents are often added to aid solubility. For lipophilic substances, e.g. fats, a working medium is used which consists of 50% methanol or KF solvent and the remainder is chloroform or a suitable alcohol. To dissolve polar substances the working medium is made up of 50% formamide. Mechanical sample preparation is also possible. For better solubility it is always recommended to reduce the size of large pieces. The sample can be crushed using a pestle and mortar, ground in a laboratory mill, or cut into small pieces. If there is no working medium that dissolves the sample, an extraction titration is sometimes also possible. The sample is ground as fine as possible and suspended in the working medium. The water is then titrated. The hygroscopic solvent extracts the water from the solid. After the EP has been reached it is recommended that a subsequent standard addition is carried out, as described in section 6.4, to ensure that the water is completely extracted. To improve the distribution of the sample, a high-speed stirrer or homogenizer (Ultra-Turrax, IKA Werke, Stauffen, Germany) can be installed inside the titration cell. The sample is dispersed during the titration, which optimizes the conditions for such samples. A further possibility for determining the water content of insoluble samples is external extraction. Here, a known amount of a hygroscopic solvent, typically dried methanol, is added to the sample in a volumetric flask. After a period of stirring, an aliquot is taken and the 14296

9 KARL FISCHER MOISTURE DETERMINATION 9 water content titrated. The result must take into account the blank value of the methanol. When small quantities of water are analyzed, not only the blank value of the solvent but also a blank of the procedure should be determined. For this an identical volumetric flask is taken, filled with the same volume of methanol, stirred under the same conditions for the same length of time and an aliquot taken and titrated to the same volume. Already titrated KF solvent can also be used to extract water from samples. Pretitrated anolyte from coulometry is best suited here due to the very small amounts of water involved. A syringe is rinsed numerous times with the pretitrated anolyte until the syringe is completely free of water. A distinct volume of water-free anolyte is then used as extracting solvent. In this case the blank is always zero. 7.2 Liquids Liquids are ideal samples for the KF titration. They are injected into the titration cell, via a septum, by using a syringe. The cell remains in an ideal water-free condition because ingress of atmospheric water from outside can be minimized. The coulometric cell should, in general, never be opened for sample addition. Therefore, a sample in a liquid or gaseous state is a prerequisite for the KF titration by coulometry. The mass of sample titrated is measured by weighing by difference. Dosing by volume is generally less exact than by weight. Liquids can also present a solubility problem or, put another way, a problem with mixing, if the sample and working medium have very different polarities. In principle, a titration in two phases is possible with sufficient stirring, but, where possible, only one phase should be present. To improve the solubility of fats, oils and other nonpolar samples, chloroform, propanol or other higher alcohols, e.g. hexanol, can be used. Polar samples can be dissolved if formamide is added. 7.3 Gases The water content of gases can also be determined by the KF titration. The gas is directed into the titration cell. The water content of the gas is then transferred to the KF reagent. Sometimes the gas dissolves in the reagent and sometimes it bubbles out. If the bubbles are not too large this does not affect the KF titration. In practice, a fine stream of bubbles is achieved by using a sieve-type tip to the insert tube and therefore good dispersion of the gas. Gases normally contain very small amounts of water and therefore coulometry is the preferred method of analysis. Volumetric titration can also be carried out, although a reagent with a low titer must be used. The amount of gas passed through the titration cell is measured in volume and the water content result obtained is consequently in mg L 1. It must be noted that the water content of gases is not always distributed homogeneously. If the water is adsorbed on the inside of the gas cylinder, higher values are obtained at the start of the determination than when equilibrium has been reached. In order to ensure that there is a homogeneous distribution of water in the gas, a constant flow of the gas should be allowed to bubble through the titration cell for an hour and the drift observed. A titration should only then be carried out when the drift has reached a stable value. 7.4 Oven Method For the water content of samples that react with the KF reagents or that are completely insoluble, the oven method is recommended. This method is described in section IMPORTANT APPLICATIONS The KF reaction is a redox reaction that takes place in an organic solvent. Oxidizing and reducing agents affect the reaction just like substances that react with the working medium. Other interferences take place when a sample is not fully dissolved. Here the water is only released slowly and the titration displays a drifting EP. In order to ensure that the sample fully dissolves, different solvents or physical techniques can be used. The following section gives an overview of the most frequently titrated sample types. A more complete coverage of sample applications is given in the HYDRANAL Guide Chemicals Inorganic Compounds Inorganic salts rarely dissolve in methanol. In order to guarantee the determination of the entire water content, the salt must first be fully dissolved. A mixture of 50% formamide and 50% methanol dissolves some alkali and alkali earth salts. Prior grinding of the sample and warming of the titration cell accelerates solubility. Most inorganic salts do not dissolve in organic solvents. The oven method is then used. The water is evaporated from the sample, carried into the titration cell via an inert gas, and titrated according to the KF method. Many oxides and carbonates react with the KF reagent and a direct titration is not possible. Inorganic salts and bases can also be titrated by KF titration, in principle, although the ph in the titration cell must be considered. The reaction proceeds too slowly if the environment is too acid, and the titrator consequently finds no EP. In an alkaline environment there is a sidereaction that consumes iodine and therefore causes a high 14297

10 10 GENERAL ARTICLES result or a vanishing EP. When titrating acids, a suitable buffer, e.g. one based on imidazole and sulfur dioxide, is recommended and, for basic samples, salicylic or benzoic acid is added to the working medium Organic Compounds Organic compounds are very diverse and often characterized by many different functional groups. Whether the compound causes a side-reaction or not can often be judged prior to titration, by taking into consideration the functional groups present. If the substance is sensitive to oxidation or reduction it could react with iodine or iodide. Active carbonyl groups can react with methanol; iodine and sulfur dioxide can add across double bonds. Figure 5 shows titration curves with and without a side-reaction. Hydrocarbons, halogenated hydrocarbons, alcohols, ethers, esters and other similar substances normally pose no difficulties for KF titration. Propanol or chloroform must sometimes be added to aid the solubility of long-chain compounds. Hydrocarbons that are mainly (a) (b) Volume of reagent (ml) Volume of reagent (ml) ml a ml b Time (min) Figure 5 Titration curves with and without side-reaction. (a) Titration of benzylamine in an unbuffered solution. (b) Titration of benzylamine in a buffered solution. a This is the volume of reagent consumed until the titration is stopped manually; b this is the volume of reagent consumed until the titration is stopped automatically. unsaturated behave inertly in the KF reagent. However, if a multiple bond is present, an addition reaction can take place which consumes iodine. Most carboxylic acids do not have to be neutralized in the same way as mineral acids. However, very strong acids, such as dichloroacetic acid or bromoacetic acid, require the addition of a buffer based on imidazole. Some carboxylic acids form esters with alcohols and therefore need to be titrated very rapidly or require a modified working medium. Working media based on ethanol esterify slower than those based on methanol. Salts of carboxylic acids can be dissolved, often with addition of formamide. Basic organic compounds such as amines require the addition of benzoic or salicylic acid to maintain the optimum ph. Polar organic compounds such as sugars and proteins are titrated volumetrically because they are solids. These substances are dissolved by using a working medium of formamide methanol 1 : 1. Warming the titration cell accelerates the dissolving process. Aldehydes and ketones present particular difficulties for the KF titration. These substances form acetals and ketals with methanol and these reactions release a stoichiometric quantity of water. There are special reagents available for these sample types which contain alcohols that react according to the KF reaction shown in Equation (4) but which do not form acetals and ketals with the sample. Alcohols such as chloroethanol, or sterically hindered compounds such as 2-methoxypropanol, can be used as suitable alcohols. Aldehydes also react according to the bisulfite addition reaction with the KF reagent. Sulfur dioxide reacts with aldehyde groups and this reaction requires water. Therefore with conventional KF reagents a result is obtained which is too low. Special reagents for aldehydes have a reduced sulfur dioxide content to decelerate the bisulfite addition reaction and enable the KF titration to be carried out accurately. Only for very reactive aldehydes, e.g. acetaldehyde, does the side-reaction predominate. An example of substances that are sensitive to oxidation and which react with iodine is the mercaptans. Peroxides are sensitive to reduction and react with iodide to generate iodine. There are many exceptions within each group of compounds and these can behave differently in the KF environment. It is therefore worthwhile carrying out a test titration in each case. A two-step validation is necessary with addition of standard after the titration has been carried out, as described in section 6.4, and the titration curve should be investigated. 8.2 Pharmaceutical Products Different substance groups can be thought of under the term pharmaceuticals: raw materials, intermediates and finished products. Raw materials consist of the active substances, drugs derived from vegetable material, inorganic 14298

11 KARL FISCHER MOISTURE DETERMINATION 11 salts, e.g. sodium phosphate, solvents, e.g. ethanol or polyethylene glycol, and non-active components such as saccharin and isomalt. For these materials there is often a valid, quality control method according to the relevant pharmacopeia. The methods quoted by the pharmacopeia for the determination of water content are KF titration and loss-on-drying Interference with the KF reagents can occur and can be avoided as described in section 8.1. Normally, raw materials are pure substances whose behavior can be determined by looking at their chemical structure. If a reagent being used has a different composition to that described by the relevant pharmacopeia, it has to be validated as described in section 6.4. This validation must be carried out for each individual product being tested. Sometimes loss-on-drying is quoted as the preferred method for water determination. A KF titration can be used instead if a validation is carried out. This validation is necessary since in some cases loss-on-drying and KF titration give results that are not identical. The water content given in the specification of the raw material must also be taken into consideration. If the water content value found by the KF titration is within the specification control limit then this method can be employed. Intermediates are mixtures of active and non-active substances that are not yet in their final state. The water content is an important quality criterion since it affects the physical characteristics of the product, such as its flow ability or its adhesiveness. In comparison to the quality requirements of raw materials, where, as a rule, only a maximum value for water content is quoted, for intermediates there is normally both an upper and lower limit specified. Tablet powders and other similar solids often display solubility problems. External extraction (section 7.1) or the KF oven (section 4.5) must then be used. Similarly, in the finished product, the water content is an important quality criterion because it influences physical properties and storage stability. The water in tablets and capsules can be determined using either the KF oven or by external extraction. Direct titration is rarely possible. Effervescent tablets that release water and carbon dioxide are particularly problematic. A KF titration is not possible in this situation. Liquid medicine can be added direct to the titration cell. Creams and suppositories can also be added to the titration cell and dissolved by using either propanol or chloroform, and warming the cell if necessary. 8.3 Petroleum Products Oils, lubricants and related products generally do not dissolve in the methanolic or ethanolic working medium of KF titration. In addition, they contain very small amounts of water (with the exception of crude oil), and so the determination must be carried out very accurately. Coulometry is recommended when used in conjunction with an anolyte modified specifically for oils. These special reagents contain chloroform or a long-chained alcohol to aid solubility of the oils. Sometimes xylene is added. Crude oils must be homogenized before a sample is taken, because the water is often unevenly dispersed. Homogenization can be carried out either by a highspeed mixer or in an ultrasonic bath. Crude oil contains tar, which can stick to the electrodes. Xylene or toluene should therefore be added to the working medium of a volumetric titration and likewise to the anolyte in coulometry. In this way the tar stays in solution and cannot stick to the electrodes. Refined products such as benzene, diesel and kerosene are added to the specially modified anolytes designed for oils and fats. Disturbances with such products are not expected. Lubricants, insulating oil and motor oils contain additives to improve their performance and increase their active life. Many of these additives cause problems for the KF titration. Antioxidants and mercaptans react with iodine, ketone groups form ketals, and metal oxides create water. Generally, the water content determined for these products by direct titration is far too high. Therefore the oven method (section 7.4) is carried out. The oil is heated at a temperature between 120 and 140 C. Coulometry is absolutely necessary because the water contents are usually in the ppm region. The water content determination of petroleum products is controlled by a large number of ASTM (American Society for Testing and Materials) and ISO standards. 8.4 Plastics The water content of polymers, like pharmaceuticals, determines their physical characteristics and therefore it is a very important criterion. The samples to be analyzed tend to be in the form of granules, fibers or solutions. The water content is often very low and therefore coulometry is the preferred choice of instrumentation. Many of the polymers can be brought into solution by the addition of chloroform or methyl pyrrolidone and can then be titrated directly. Ketones, which are also often used as solvents for polymers, are not recommended for KF titration because they cause side-reactions (see section 8.1). Small amounts of ketone can be titrated when special reagents for ketones are used. A volumetric titration can be tried if the polymer is in the form of a fine powder. The sample is added directly to the methanolic working medium. The methanol extracts the water from the very finely ground polymer without dissolving it