Method for avoiding the interference of formamide with ... - Springer Link

( Springer-Verlag 1997

Fresenius J Anal Chem (1997) 357: 806—811

OR I G I N A L P AP E R

Kornelia Schmitt · Heinz-Dieter Isengard

Method for avoiding the interference of formamide with the Karl Fischer titration

Received: 30 May 1996/Revised: 26 July 1996/Accepted: 30 July 1996

Abstract The Karl Fischer titration is based on a specific chemical reaction. Several measures exist to make all the water of insoluble samples accessible for the chemical reactants. The most efficient are the titration at elevated temperatures, the use of a homogenizer in the titration vessel and the modification of the polarity of the working medium (essentially methanol) by the addition of appropriate solvents like chloroform or formamide. It is known however that formamide interferes with the Karl Fischer reaction and so causes more or less false results. This effect increases with higher temperatures. A method is therefore presented to avoid this interference, even when working at the boiling point of the working medium. It takes advantage of the fact that the side reaction has a practically constant velocity, at least as long as usual titrations last. Thus, a constant additional consumption of Karl Fischer reagent is observed. This can be accounted for by measuring this effect before the start of the determination and by deducting the additional reagent consumption, which is proportional to the duration of the titration, from the totally added volume. With certain modern titrators this can even be carried out automatically. They can continuously measure the so-called drift, the titration rate necessary to keep the titration cell dry, and have the capability to use this drift as stop criterion for the titration. This means that the analysis is terminated when the drift existing before the titration is reached again. The additional consumption of reagent, to be deducted from the total volume, can (automatically) be calculated from the drift rate and the titration time. The proposed procedure allows the use of formamide as additional solvent, even at high temperatures, in order to shorten determination times considerably. It avoids false results due to the interference, which has so far

K. Schmitt · H.-D. Isengard (¥) University of Hohenheim, Institute of Food Technology, Garbenstrasse 25, D-70593 Stuttgart, Germany

prevented its use when exact results were desired and when the duration of the analysis was long.

Introduction Water determination by the Karl Fischer titration The very widely used drying methods to determine the humidity of products do not really measure the water content but the mass loss under the employed conditions. Very strongly bound water may escape detection, while all the substances volatile under the drying conditions — even those produced by and during the process itself — will contribute to the mass loss and, consequently, be determined as water. The true water content can be analyzed by Karl Fischer titration [1—3] which is based on a specific chemical reaction: CH OH#SO #ZPZH`#CH OSO~ 3 2 3 2 ZH`#CH OSO~#I #H O#2Z 3 2 2 2 P3ZH`#CH OSO~#2I~ 3 3 Z is a base that, by neutralizing the methyl esters, leads to practically complete reactions to the right hand side. Nowadays the ‘‘original’’ pyridine has been replaced by more efficient bases like imidazole [4]. As water has to react chemically with the reagents, it has to get in direct contact with them. Therefore, the samples should in the ideal case be soluble in methanol, which is the usual solvent and working medium. Many samples do not fulfil this requirement. In these situations the water of the sample has to reach the working medium by diffusion and extraction. Several measures can be taken to accelerate these processes and provide for a complete water liberation [5, 6], such as the preextraction of the water [7], the use of an internal homogenizer [8], working at elevated temperatures, even at the boiling point [9—11] or modifying the polarity of the working medium by adding further

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solvents to the methanol [4, 12]. In the case of polar samples the addition of formamide is recommended. Formamide is, however, known to accelerate the iodination of imidazole, and thus causes an interference [13]. This additional reagent consumption during titration results in slightly too high values for the found water content. These differences should not be neglected when a high accuracy of the value is demanded. In the following, a method is described to overcome these difficulties, even when working at elevated temperatures when the iodine-consuming side reaction is yet more competitive. End-point criteria Different titration techniques exist. All of them have in common that iodine is added in a solution (volumetric technique) or produced electrochemically (coulometric technique), and is then consumed by the Karl Fischer reaction. The first excess of iodine indicates the end point. The determination of the end point is usually based on an electrochemical effect. Two platinum electrodes are polarized by a constant current, usually in the range of about 50 lA. The voltage necessary to maintain this current is monitored. The first excess of iodine, and thus the presence of the redox system iodine/iodide, results in the corresponding redox reactions at both electrodes. This makes the voltage drop abruptly. If it remains below a certain value, a final voltage of 250 mV is often chosen, for a certain time, the so-called stop delay, the analysis is terminated. This is the bivoltametric technique. The biamperimetric variety is the analogous procedure with a polarizing voltage and a sudden rise of the current at the end point. The stop delay is very important because water may not be immediately free to react. Many samples are not soluble in methanol, and consequently water gets in contact with the working medium only after a certain delay by diffusion and extraction processes. The arrival of such ‘‘delayed water’’ makes the voltage rise again (when working after the bivoltametric principle). The titrator reacts upon this effect with further addition of reagent, until the voltage remains below the chosen end-point value during the desired delay time. A long stop delay is by itself, however, not a satisfactory criterion for a good and complete analysis. It has to be seen in relationship with the minimal reagent volume the titrator can add. If the last added portion is large, this will often lead to a (only momentarily) clearly over-titrated situation. As towards the end of a determination only the last small amounts of water reach the working medium, the voltage will consequently need a considerably long time to reach the critical end-point value again, possibly not within the stop-delay time.

Thus, the analysis would be incomplete. With a more ‘‘sensitive’’ titrator, capable to add little increments of reagents, the danger of premature terminations is smaller. Instead of a fixed stop-delay time, another end-point criterion may be used. To keep the titration cell dry before and between determinations, a slight consumption of reagent is necessary, caused by traces of water intruding from outside and also by water produced in side reactions between the substances in the titration vessel or by iodine-consuming competitive reactions. The amount of this reagent consumption depends on the tightness of the apparatus and on the humidity of the air in the surrounding room and also on the chemicals (and samples) used because of the different reasons given, respectively. This so-called drift can be observed and measured before starting a titration. Assuming constant conditions before and after the analysis of a sample, the drift will be the same after all the water in the sample has been detected. There are titrators in the market that offer the feature of a drift-controlled end point. They measure the drift continuously. The value just before the start of a titration can be set as end-point criterion. It is even possible to measure the determination time and deduct the reagent consumption due to the drift from the total volume. This is particularly interesting, when the duration of the analysis is long or when the drift is considerably high. When this possibility is not available, the drift can be calculated from the reagent volume added to the working medium to keep the cell dry (without sample) during a chosen time.

Experimental Apparatus and reagents The titrator was the KF Titrino' 701 with its standard equipment from Metrohm, Herisau/Switzerland. For titrations at the boiling point specially constructed titration cells [10, 11] were used. The Ultra-Turrax' T 25 with the tool S 25 N-18 G from IKA, Staufen/Germany, served as internal homogenizer [8, 11]. The chemicals were from Riedel-de Hae¨n, Seelze/Germany: Hydranal'Composite 2, Hydranal'-Composite 5, Hydranal'-Titrant 2, Hydranal'-Titrant 5, Hydranal'-Solvent, methanol, chloroform, formamide and sodium tartrate 2-hydrate (Hydranal'-Standard).

Procedure All the experiments were carried out using the bivoltametric technique with a polarizing current of 50 lA and a stop voltage of 250 mV. The drift of the apparatus used was measured under different conditions. For this purpose, fictitious titrations were carried out: after the pre-titration or conditioning of the cell a titration without sample was started with a set extraction time of 30 min and a stop-delay time of 10 s. The drift D in the dimension ll/min was calculated from the reagent volume VVconsumed during the titration time t, and, taking the water equivalent WE into account, as D in .

808 the dimension lg H O/min: 2 D [ll/min]"V [ll]/t[min] V D [lg H O/min]"D [ll/min] · WE [lg H O/ll] . 2 V 2 "V [ll] · WE [lg H O/ll]/t[min] 2 The following titration variations were investigated: ¹emperature. Titrations were carried out at 50 °C and at the boiling point of the working medium. Determinations at room temperature are not included, as they are less problematic concerning interferences. For the experiments at 50 °C the two-component technique was used; the titrating agents were methanolic solutions of iodine with a water equivalent of about 5 mg/ml (Hydranal'-Titrant 5) and of about 2 mg/ml (Hydranal'-Titrant 2), the working medium being a solution of sulphur dioxide and imidazole in methanol (Hydranal'-Solvent). The boiling-point titrations were carried out with one-component reagents with water equivalents of about 5 mg/ml (Hydranal'-Composite 5) and of about 2 mg/ml (Hydranal'-Composite 2) with methanol as working medium. This technique was chosen to avoid a possible evaporation of sulphur dioxide from the boiling working medium. At 50 °C the standard titration cell with a thermostatic jacket was used. For the titrations at the boiling point a specially constructed titration cell, a roundbottomed flask with several necks for the sample input, the electrodes, the tubes for adding and removing the working medium, a thermometer and a reflux condenser, was used [10]. Homogenizer. The investigations were made with and without use of the homogenizer. When it was used at 50 °C, the commercially available special lid of the titration vessel for this purpose was employed, which has an additional hole for the shaft of the homogenizer. For the boiling-point titrations with use of the homogenizer another specially constructed cell (with one neck more than the one mentioned above) was employed [11]. Additional solvents. The drift measurements were carried out using pure methanol respectively Hydranal'-Solvent as working medium as well as with additional chloroform or formamide in different proportions. All the possible combinations of these variations (5 assays each) were investigated. After the drift measurements and the conclusions on how to handle the interference observed with the use of formamide, titrations of sodium tartrate 2-hydrate with its known water content of 15.66$0.05% were carried out (10 assays for each variation) to verify the proposed technique.

Table 1 Drift values determined at 50 °C with and without homogenizer, with and without addition of chloroform or formamide to the working medium (Hydranal'-Solvent) using Hydranal'-Titrant 5 (upper rows) or Hydranal'-Titrant 2 (lower rows) as titrating agents (mean values of 5 assays for each variation)

Titration method (50 °C)

Results and discussion Drift measurements at 50 °C Table 1 shows the results of the drift measurements under different conditions at 50 °C using Hydranal'Titrant 5 and Hydranal'-Titrant 2, respectively, as titrating agents. As the amount of intruding water depends on the tightness of the apparatus, the absolute values are influenced by the way the constituents of the titration cell have been assembled in a specific situation. Therefore, the values of D in Table 1 cannot be compared to . each other in a very strict sense. Interpretations and conclusions are only possible when the numbers differ distinctly. In the case of pure Solvent as working medium the values indicate a very good tightness of the apparatus. This is also true for the addition of chloroform without use of the homogenizer. The values are slightly lower when the homogenizer is used in the mixture with chloroform. The reason might be the fact that, by the dispersion effect, a slight evaporation of chloroform with a consecutive rising of the pressure within the cell is caused which hampers the intrusion of water from outside. The most obvious phenomenon, however, are the clearly higher values of D for the titrations in the . presence of formamide, particularly with the additional use of the homogenizer. They are caused by an interfering reaction [13]. In the case of Titrant 5 as reagent and without the homogenizer the effect seems to be not so striking. But it has to be considered that the given values are the average over the titration time of 30 min. In fact, the interference is not at once obvious, but only after some time, which differs sometimes by several minutes. The reason is that at the beginning of the conditioning (to make and keep the titration cell dry) a slight

Solvent, without homogenizer

Water equivalent of used titrant WE [lg H O/ll] 2 5.007 2.003

Consumption of reagent per min D [ll/min] V 2.000 3.017

Titrated amount of water per min D [lg H O/min] . 2 10.01 6.05

Solvent, with homogenizer 300 s

5.007 2.003

2.267 6.820

11.35 13.66

Solvent : chloroform 2 : 1, without homogenizer

5.001 2.003

2.497 9.067

12.49 18.16

Solvent : chloroform 2 : 1, with homogenizer

5.011 2.003

1.166 2.420

5.84 4.85

Solvent : formamide 5 : 1, without homogenizer

5.023 2.003

4.687 22.593

23.55 45.26

Solvent : formamide 5 : 1, with homogenizer 120 s

5.011 2.003

11.067 24.453

55.46 48.98

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over-titration is usually observed. The amount of this depends on the titrator used, the set parameters and the initial amount of water in the cell. Even if the side reaction starts at once (when formamide is used), this does not result in an immediate reaction of the titrator. It starts the addition of reagent only when the stop voltage is reached again. This is the reason for different mean consumption values for the complete time of 30 min. Once the addition of reagent is started, however, the (additional) titration rate is practically constant. Figure 1 shows a typical titration curve (very high resolution of the reagent volume) for the reagent consumption registered during a fictitious titration without sample in a medium containing formamide. Drift measurements at the boiling point of the working medium Table 2 shows the results of the drift measurements under different conditions at the boiling point of the working medium using Hydranal'-Composite 5 and Hydranal'-Composite 2, respectively, as titrating agents. Due to the prevailing inner pressure during titrations at the boiling point, the amount of intruding water is lower than at 50 °C (Table 1). When formamide is present in the working medium, however, the D values are . significantly higher. The value for a specific case depends, as was described above, on the moment the reagent addition starts. But the found additional reagent consumptions are higher than at lower temperatures, certainly because of an increased reaction speed. Fig. 1 Reagent consumption (Hydranal'-Titrant 5) at 50 °C without use of the homogenizer to keep the working medium (Hydranal'-Solvent: formamide 5: 1) dry (fictitious titration without sample)

Another observation is that the use of the homogenizer seems to favour the effect. Possibly the metal of the tool catalyzes the side reaction. The corresponding values in Table 1 allow, though to a lesser extent, the same supposition. Conclusion for titrations with formamide in the working medium When formamide is used at elevated temperatures to render the working medium more polar, in order to accelerate the water determination, there is a risk of false results. This is the case when the side reaction starts within the determination time. Due to this interference the reagent consumption and the drift are higher. A customary stop-delay time of 10—20 s may be too long to reach an end point. A shorter stop-delay time may be too small for the extraction of all the water in the sample. In any case, the result of such an analysis is not reliable. To account for the interference, the drift should be observed before the start of the analysis. When it has reached a practically constant value (usually only after some minutes), which, of course, is distinctly higher than without the use of formamide, the water determination can be started. The end-point criterion should be the drift, if the titrator offers this feature. Otherwise the necessary stop-delay time has to be calculated from the drift observed and the minimal reagent increment of the titrator. The additional reagent consumption must be deducted from the titration volume, possibly automatically with an appropriate titrator.

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Determination of the known water content of sodium tartrate 2-hydrate Sodium tartrate 2-hydrate (Hydranal'-Standard) has a water content of 15.66$0.05%. It is soluble in methanol, but solubility is improved when formamide is added. Therefore the titrations must be expected to give the correct and complete water content in a short titration time. Titrations were carried out under different conditions with and without consideration of the additional reagent consumption by the drift. Table 3 shows the results for determinations at 50 °C. The standard titration with drift correction gives the correct result. When the additional consumption due to intruding water during the titration is not taken into account, the result is slightly higher, though still acceptable as it lies within the bandwidth of the Table 2 Drift values determined at the boiling point of the working medium, with and without homogenizer, with and without addition of chloroform or formamide to the working medium (methanol) using Hydranal'-Composite 5 (upper rows) or Hydranal'-Composite 2 (lower rows) as titrating agents (mean values of 5 assays for each variation)

Titration method (boiling point) Methanol, without homogenizer (65—66 °C)

theoretical value. This effect is more distinct with the use of formamide in the working medium. When the influence of the drift is neglected, which is more important in this case because of the side reaction, the error becomes significant: the lower limit of the result lies above the upper limit of the theoretical value. When the drift is taken into account, however, the found value is in accordance with the theoretical value. It should be mentioned that the divergence is only very small. But this is due to the very short determination times of below 2 min for this ‘‘easy’’ titration. The error that results for titrations with formamide without accounting for the additional reagent consumption increases with the duration of the analysis and, also, with decreasing water content of the samples. Table 4 contains the results for the determinations at the boiling point. They reveal the same effect, though

Water equivalent of used titrant WE [lg H O/ll] 2 5.148 2.146

Consumption of reagent per min D [ll/min] V 0.226 0.000

Titrated amount of water per min D [lg H O/min] . 2 1.16 0.00

Methanol, with homogenizer 300 s (65—66 °C)

5.155 2.146

0.000 2.233

0.00 4.79

Methanol : chloroform 2 : 1, without homogenizer (62—64 °C)

5.148 2.146

3.263 1.213

16.80 2.60

Methanol : chloroform 2 : 1, with homogenizer (62—64 °C)

determination not possible: chloroform vapours penetrate through the shaft of the homogenizer into the motor and dissolve the oil there!

Methanol : formamide 5 :1, without homogenizer (71—72 °C)

5.148 2.146

5.146 33.787

26.49 72.51

Methanol : formamide 5 : 1, with homogenizer 120 s (71—72 °C)

5.148 2.146

14.734 58.080

75.85 124.64

! Meanwhile a shaft with joints to prevent this effect is available from IKA Table 3 Found water content of sodium tartrate 2-hydrate (theoretical value: 15.66$0.05%) by different determination procedures at 50 °C using Hydranal'-Titrant 5 as titrating reagent (10 assays for each variation) Table 4 Found water content of sodium tartrate 2-hydrate (theoretical value: 15.66$0.05%) by different determination procedures at the boiling point of the working medium using Hydranal'Composite 5 as titrating reagent (10 assays for each variation)

Method (50 °C) Solvent without formamide, Solvent : formamide 5 : 1, Solvent without formamide, Solvent : formamide 5 : 1,

Found water content without drift correction without drift correction with drift correction with drift correction

Method (boiling point) Methanol without formamide, Methanol : formamide 5 : 1, Methanol without formamide, Methanol : formamide 5 : 1,

15.71$0.03% 15.76$0.03% 15.65$0.03% 15.61$0.03%

Found water content without drift correction without drift correction with drift correction with drift correction

15.67$0.03% 15.71$0.03% 15.67$0.03% 15.68$0.03%

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even the result for the titration in the presence of formamide and without considering the higher drift lies within the range of the theoretical value, and is therefore not significantly different. The titration takes only about 1 min, and this short time is not sufficient to cause a higher divergence, in spite of the greater effect of the interfering reaction at the boiling point. This phenomenon was, by the way, the reason to choose sodium tartrate 2-hydrate as sample to demonstrate the influence of the interference of formamide on the titration results. On the one hand the theoretical result must be known, but on the other hand the determination of a given amount of pure water, which would be the most ‘‘logical’’ sample, would not have taken enough time to show a real difference, let alone the problem of the weighing of the sample, which has to be very small and extremely exact in this case (water content 100%). Acknowledgement We thank Riedel-de Hae¨n, Seelze/Germany, for supporting this work by providing the chemicals.

References 1. Fischer K (1935) Angew Chem 48 : 394—396 2. Scholz E (1984) Karl Fischer Titration. Springer, Berlin Heidelberg New York Tokyo 3. Wu¨nsch G, Seubert A (1989) Fresenius Z Anal Chem 334: 22—24 4. Hydranal-Manual — Eugen Scholz Reagents for Karl Fischer Titration (1995). Riedel-de Hae¨n AG, Seelze/Germany 5. Isengard H-D, Nowotny M, Reger H, Zimmermann G (1991) CLB 42 : 315—321 6. Isengard H-D (1995) Trends Food Sci Technol 5 : 155—162 7. Isengard H-D, Nowotny M (1992) Dtsch Lebensm-Rdsch 88 : 246—251 8. Isengard H-D, Nowotny M (1991) Dtsch Lebensm-Rdsch 87 : 176—180 9. Scholz E (1988) Dtsch Lebensm-Rdsch 84 : 80—82 10. Isengard H-D, Striffler U (1992) Fresenius J Anal Chem 342: 287—291 11. Isengard H-D, Schmitt K (1995) Mikrochim Acta 120: 329—337 12. Wieland G (1985) Wasserbestimmung durch Karl-Fischer-Titration. GIT, Darmstadt/Germany 13. Wu¨nsch G, Scho¨ffski K (1990) Anal Chim Acta 239 : 283—290

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