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Determination of water by flow-injection analysis with the karl fischer reagent
Chimica Acta, 114 (1980) 199-208 0 Elsevier Scientific Pub&b.ing Company, Amsterdam -
AMlytica
Printed in The Netherlands
DETERMINATION OF WATER BY FLOW-JXYECTION WITIf3 THE KARL FISCX-IER REAGENT
INGRID K~EvALL,
0vE
liST~ijM
Department of Analytical Chemistry.
and ANDKRS
ANALYSIS
CEDKRGREN*
University of Urn& S-901 87 Umeii (Sweden)
(Received 10th July 1979)
SUMMARY A method for the determination of water in organic solvents by flow-injection analysis (f-i-a.) is described_ The method, which is based on the reaction between water and the Karl Fischer reagent, is capable of 120 determinations per hour. The concentration range 0.01-5% (v/v) of water can be covered by using a single Karl Fischer reagent solution. The results obtained with a specially cbnstructed potentiometric detector showed a relative standard deviation of less than 0.5% (v/v). This value was about 3 times less than that obtained with a spectrophotometric detector. The f.i.a. technique is shown to offer some unique possibilities in minimizing interferences associated with the standard Karl Fischer batch titration method. Measurement of the water content in a wide variety of materials is a problem of universal interest and the determination of water has consequently become one of the commonest procedures in chemical laboratories. A comprehensive review of chemical and physical methods for the determination of water has recently been published [I]. Among the various techniques used, e.g., gaschromatographic, gravimetric, spectroscopic (n.m.r., WV., i.r.), titrimetric (Karl Fischer), the last is probably the most generally used. It has been applied to the determination of water in numerous organic and inorganic matrices including saturated or unsaturated hydrocarbons, alcohols, halides, acids, acid anhydrides, esters, ethers, amines, amides, nitroso- and nitro compounds, sulphides, hydroperoxides and dia.&y1 peroxides. However, many interferences are associated with the Karl Fischer method. Active carbonyl compounds, for example, cause the formation of water through reaction with methanol which is the main constituent of the ordinary Karl Fischer reagent. Other interfering substances which cause a different type of error include mercaptans and certain amines which react with the iodine present in the Karl Fischer reagent. Negative errors will thus ark in determinations of watar in solventS Like peroxy acids, diacyl peroxide, and quinones because of the formation of iodine. In some cases the Karl Fischer reagent has been modified successfully in order to minimize or eliminate these interferences.
There are two additional main drawbacks associated with the standard Karl Fischer method. First, it is time-consuming because of the rather slow rate of reaction between water 2nd the Karl Fischer reagent near the endpoint of the titration. Secondly, the analyst has to handle rather large volumes of toxic reagent which is potentially dangerous. In order to overcome some of these drawbacks, the possibilities of using flow-injection analysis (f.i.a.) were considered. This technique was originally described by Ri?ZiEka and co-workers [ 2-111 and involves the introduction of the sample into an unsegmented carrier stream with subsequent transportation to the detector_ The degree of dispersion of the sample zone can be controlled by varying parameters such as the coil length and the flow-rate. The f.i.a. technique has been shown to be a very flexible and powerful analytical tool, and has been applied to procedures such as extraction, acidbase titrations, dialysis, dilution and multiple determinations. For the determination of water by the Karl Fischer method, it should be possible to develop a completely closed system by exploiting this technique. It should also be possible to take full advantage of one of the other main properties of the f.i.a. method, namely the rapidity of determination. It has recently been shown [12, 133 that the reaction rate in the Karl Fischer determination can be speeded up by more than 100 times either by lowering the concentration of iodide in the reaction mixture [12] or by using a reagent based on a formamide-pyridine mixture 1131 rather than the conventional mixture of methanol and pyridine. The rapid rate of the main reaction obtained by using such reagents can be used to minimize effects from side reactions in a flow-injection system by keeping the reaction time at a minimum. This constitutes an important advantage of the f.i.a. technique compared with ordinary batch titrations. In addition, the f.i.a. technique may offer other unique possibilities for the elimination of interferences. This paper describes a f.i.a. system for the determination of water with a methanolic Karl Fischer (K-F.) reagent, and includes the characterization of two different types of detectors, one based on zerocurrent potentiometry and the other on spectrophotometry. EXPERIMENTAL
Instrumentation
The reagent was driven by a fourchannel peristaltic pump, a Minipuls 2 (Gi_ison, France)_ Silicone rubber tubing was chosen in order to withstand the very reactive K-F. reagent. Several other tubings were investigated but none was found to be sufficiently inert. Spectrophotometric detector. The spectrophotometer, built in this department, consisted of a small grating monochromator, (Model GM 100, Schoeffel Instrument GmbH, Germany), a lens lamp, a u.v.-enhanced photodiode and a flow-through cuvette (Heha Model 178, 31-Q& volme 8 ~1, light path 10 nun).
201 et.
4cm
mlmine1 S=2pl
1.0
Fig. 1. The potentiometric
200 cm
L
W
flow cell. (a) Teflon blocks; (b) O-rings; (c) glass cylinder.
Fig. 2. Flow injection manifold for spectrophotometric or potentiometric determination of water in organic solvents with Karl Fischer reagent. (S) Point of injection (sample loop); (D) detector; (W) waste.
Potentiometric detector. The configuration of the potentiometric detector is shown in Pig. 1. The carrier solution was pumped through a 0.5-mm hole in the lower teflon block, into which a O-l-mm platinum wire indicator electrode was inserted_ The solution then passed down a groove of the declining part of the teflon block to the exit. The indicator electrode was inserted 3 cm into the hole. If the electrode just reached the exit of the bore there was an increase in the sensitivity but the stability and reproducibility were very poor. These drawbacks arise from the fact that the electrode must be very precisely and reproducibly located in the flowing stream. By inserting to a distance of 3 cm, these disadvantages can be almost eliminated. By filling the cell just to a point at which there was only a small liquid film between the exit solution and the residual cell solution, mixing in the cell was kept to a minimum. The other platinum electrode was insulated except for the terminating spiral; since it is exposed to an almost unchanging environment it can function as a pseudo-reference system. The small change in the reagent in the eelI compartment does not significantly affect the measurement. No serious error should result from a systematic drift because the peak height is always related to the base-line. The platinum pseudo-reference electrode was chosen partly because of the inertness of platinum and partly to achieve an air-tight cell with exclusion of water. Potentiometer and recorder_ The signals from the two detectors were monitored either by a lin-log amplifier or by a follower connected to a HP Moseley 680 recorder_ Additionally, an interface was inserted between the amplifiers and the recorder, which allowed the peak maximum to be locked automatically on a meter while the recorder continuously displayed the actual potential output [14]_
202
Reagents and standards All orgtic liquids were of analytical grade. Solvents were dried before
use with molecular sieves (3 or 4 A). The mixtures of water and the organic sobents were standardized coulometrically [15]_ All solutions were calibrated except for the acetone mixtures. Owing to the formation of water by the ketal reaction, a graphical estimation of tbe concentration of water in dried acetone was done amperometrically; it was found to be below 0.01%. This was possible by performing the determination at a high concentration of iodine, so that the rate of the main reaction was sufficiently high to permit a differentiation. The Karl Fischer reagent contained 25.4 g of iodine, 38.4 g of sulphur dioxide and 80 ml of pyridine, diluted to 1 1 with methanol [16, 171.
Manifold and measurement technique The manifold used in the flow-injection system is outlined in Fig. 2. The tubings were made from teflon (0.5 mm id.) and the end-connectors were ordinary chromatographic connectors (A&x). Samples (2 ~1) were injected with a standard liquid chromatography inlet slide valve for low pressure (Cheminert) at point S into the carrier and reagent stream of Karl Fischer solution which was pumped at a flow rate of 1 ml min-‘. After the injection, a 209cm reaction coil sufficed to achieve the desired reaction. The change in concentration of the reagent was then measured either spectrophotometritally at 625 mn or potentiometrically with the platinum electrode system. RESULTS AND DISCUSSION
Kinetics The main Karl Fischer reaction, according to Mitchell [18],
is
CsHSN-I2 + CSHSN-SO2 + CSHSN + Hz0 --f 2CSHSNH+I- + CSHSN-SO3
(1)
which, in the presence of methanol, proceeds further CSHSN-SOJ + CH70H + C,H,NHSO&H,-
(2)
This reaction scheme has been criticized by several investigators. Verhoeff and Barendrecht [12] proposed the existence of a methyl-sulphite complex instead of the pyridine-sulphur dioxide complex. The kinetics of reaction (1) have been studied in two independent reports and it has been shown that the reaction is first order with respect to the concentration of iodine, sulphur dioxide and water [12, 17]_ The value of the rate constant in 0.20 M solution of iodide has been estimated to be about 1 X lo3 I* mol-* s-l [12, 13, 171. This value increases with decreasing concentration of iodide. For example, the value of the rate constant is enhanced by a factor of two when the concentration of iodide is lowered from 0.20 M to 0.15 M as estimated from the diagram given by Verhoeff 1191.
203
In order to estimate the time needed for a complete reaction to take place under conditions appropriate to flow-injection analysis, the following calculation was performed. The iodine concentration of the Karl Fischer reagent used was assumed to be 0.04 M and the concentration of sulphur dioxide to be 0.54 M. Further it was assumed that 2 ~1 of a sample containing 3% of water was introduced and mixed instantaneously with 100 ~1 of the reagent and that no further dilution occurred during transport to the detector. The time required for 99.9% reaction should then be about 2 s. The value of the rate constant used in these calculations, 1 X lo3 l*mol-*s-l, should have been somewhat higher because the concentration of iodide in this example never exceeds 0.18 M. These calculations were complemented by f.i.a. experiments performed under conditions corresponding to the discussed example. In these experiments, which are summarized in Fig. 3, the signal from the spectrophotometric detector was measured as a function of the reaction coil length. It can be seen that the peak height increases with increasing residence time, passes through a maximum, and then decreases again. The rising part of the curve corresponds to a situation where the dispersion is not sufficiently
1
I
I
100
I
300 L
(cm)
.
0u GENERATED
0.075 CONCENTRATION
OF
0.100 IODINE
(M)
3. Dependence of the peak height on the reaction coil length (L). Samples (2 ~1) containing 3% water in ethanol were injected into a flow (1 ml min-‘) of 0.04 M Karl Fischer reagent. Fig.
Fig. 4. The potential of the platinum electrode as a function of iodine concentration generated in a coulometric cell. The starting concentration of iodide was 0.2 M.
204 large to allow iodine to be in excess of water. Consequently, it is difficult to
estimate the shortest tune needed for completion of the main reaction_ Nevertheless, the shape of the curve to the right of the maximum value in Fig. 3 indicates that the main reaction has been completed. The extent of dispersion will thus determine the shape of the latter part of the curve. The potentiometric detector As noted previously [lS], stable redox potential values are obtained with a platinum electrode in Karl Fischer titrations. A platinum electrode was used in a recent investigation of redox potentials of the Ce(III)/Ce(IV) couple [20] by the f.i.a_ technique. In order to characterize the redox properties of a platinum electrode in the configuration used here, values of the electrode potential were determined as a function of the concentration of iodine, which was generated by constant-current coulometry; the results are shown in Fig. 4. The shape of the potentiometric curve indicates the existence of the strong triiodide complex between iodine and iodide. Thus, for low iodine concen?rations the potential-determining factor is probably the logarithm of &--I /II-l 3 while for high concentrations of iodine the logarithm of [In] ‘/[IX-]’ will predominate. The points marked A, B, C and D in Fig. 4 denote the various strengths of the Karl Fischer reagent used in the study represented in Fig. 5. This figure shows the response of the potentiometric detector for the various strengths of Karl Fischer reagent as a function of the
QOOS M 12
150
ZlOO ii
fi
=4
ao1nk.l~
0.053 M 12 0.030 M I*
$
w
Q!io fl
10 _ .
, 2
.
, 4
,
,
6 96H20
.
, 8
Fig. 5. Peak height values obtained
,
, 10
SCAi
with the potentiometric
detector
as a function
of the
percent water in ethanol for various strengths of the Karl Fischer reagent. The total concentration of iodide was 0.2 M in each of the reagents. Fig. 6. Recorder output born the potentiometric detector for the determination of water in acetonitrile. The signals from left to right correspond to 0.10, 0.49, 0.99 and 2.77% of water, respectively.
205
concentration of water in the sample. As can be seen, there are large differences in the shapes of the curves. However, comparison of the curves obtained in the flow-injection experiments with the results shown in Fig. 4 indicates that the shapes of approximately the first 3/4 of each curve fit very well. The disagreement between the last quarters of the curves can be explained by either incomplete main reaction or by slow response of the indicator electrode for low iodine concentrations. As indicated in Fig. 4, the working concentration range as well as the sensitivity can be further increased if higher iodine concentrations than 0.05 M are used provided that the total concentration of iodine is the same. In practice, it is difficult to prepare such a strong reagent because of side reactions and moisture in the chemicals used. One simple way, illustrated in Fig. 4, involves electrical preparation of the reagent. The kinetics of the main reaction should be extremely rapid for very high concentrations of iodine mainly because of the low concentration of iodide present in such a reagent. The precision of the potentiometric measurement is given in Table 1 for a 0.058 M Karl Fischer reagent; the relative standard deviation is less than 0.5% over the whole concentration range. This implies that the stability of the detector is very good; this is also demonstrated in Fig. 6 which shows a recorded scan of some samples iu the concentration range O-l-3% of water. The spectrophotometric detector Figure 7 shows the response of the spectrophotometic detector as a function of the concentration of water in the sample for three different strengths of the Karl Fischer reagent. These results were obtained at 625 run; at this wavelength no significant absorbance is obtained for a spent reagent. It can be seen that the linear range of the calibration curve can be extended but at the expense of decreased sensitivity. The fact that linear curves are obtained over a large concentration range of water indicates that the reaction is almost complete and that Beer’s law is followed. Although it is not clearly seen in Fig. 7, there is a slight deviation from linearity in the first part of the curves. This cannot be explained at present but is currently being investigated. The precision of the measurements is given in Table 1 for a 0.033 M Karl Fischer reagent. The percent standard deviation varies between 0.5 and 1.5% which is about three times that obtained with the potentiometric detector. The stability is illustrated in Fig. 8 which shows a recorded scan of some samples in the concentration range O-l-3% water. Effects of solvents Figure 9 shows the influence of various solvents on the potentiometric and spectophotometic determinations of water, respectively. The shapes of the curves are in good agreement with those shown in Figs. 5 and 7. Owing to the formation of water through the reaction between acetone and methanol, the acetone curve lies above all other curves in both diagrams. The slight deviations between the remaining curves may be due to several factors,
206 TABLE 1 Determination of water in ethanol with the potentiometric and apectrophotometric detecbrs. Reagents: ordinary 0.058 B4 Karl Fischer reagent for the poteutiometric method and ordiuary 0.033 M Karl Fischer reagent (wavelength 625 mu) for the spectrophotometric method. Poteutiometric detector (%I
Peak height (mv)
0.110 0.550 1.06 2.97 4.83
3.36 13.1 21.0 46.1 63.2
I-W
Spectrophotometric
R-s-d. (%I 0.47 0.54 0.41 0.19 co.1
Hz0 (%I
R.s.d. (%I
0.110 0.615 1.06 2.97 4.83
0.014 0.065 0.106 0.306 0.497
1.0 1.5 1.2 0.23 0.50
I
OF’
” 20
” 40
” 60 %
” 8D
’
lo.0
detector
Peak height (abe-
1
H,O
6 min
I
I
Scan -
Fig. 7. Peak height values obtained with the spectrophotometric detector as a function of the percent water in ethanol for various strengths of the Karl Fischer reagent. The total concentration of iodide was 0.2 M in each of the reagents. Fig. 8. Recorder output from the spectrophotometric
determination of water in acetoni-
He. The signala from left to right correspond to 0.10, 0.49, 0.99 and 2.77% of water,
respectively_
but they are surprisingly small in view of the very different properties of the solvents chosen for these experiments. The viscosity, for example, varies between 0.32 and 2.3 centipoise. Further, various contributions from the solvent polarity to the dispersion might be considered as a consequence of the variation in the dielectric constants. The values for acetic acid and acetonitrile
207
Fig. 9. Potentiometic (A) and spectrophotometric (B) calibration cmes for the determination of water in different organic sobents. (*) acetonitrile; (0) propanol; (0) acetone; (0) ethanol; (A) methanol; (0) acetic acid. The strength of the Karl Fischer reagent was 0.06 M for potentiometric measurements and 0.03 M for spectrophotometric measurements (at 625 nm). are 6 and 37, respectively. In addition, adsorption effects between the solvents and the teflon tube may occur, and this might also influence the extent of dispersion. A closer examination of the shapes of the peaks showed that their form is not exactly the same for different, solvents. These effects are
under As
study
at present_
demonstrated in Fig. 9 the determination of water in acetone is easily performed with the f.i.a. technique. Such determinations are troublesome when the conventional Karl Fischer batch titration procedure is used. Similar advantages with the f.i.a. technique were obtained for acetic acid, because there was no noticeable contribution from the water produced in the esterification reaction between this acid and methanol. CONCLUSIONS
This proposed method yields results which are comparable in precision and accuracy with those obtained by using the conventional Ka;l Fischer procedure. The advantages and disadvantages, which are concerned with the present state of the method, can be summarized as follows. One
advantage
is the
rapidity
of
the determination:
the f.i.a.
method
is
least ten times faster than the conventional Karl Fischer method. Safety is improved, because the closed system prevents the analyst from coming in contact with the toxic reagent except during reagent loading. There is less influence from interfering side-reactions compared with the conventional batch titration. Another great advantage is the low cost per determination; about 2000 determinations can be made per like of reagent. Among the disadvantages, the sensitivity is not as good as that obtained with the conventional Karl Fischer method. However, the method described has not been optimized with respect to sensitivity. Simply by increasing the at
208
sample volume, the working range of the method might be extended to 0.001-5% of water. Another disadvantage is the variation in results caused by solvent effects. The origin of this variation is under study and, on the basis of increased knowledge, it should be possible to design an f.i.a. system which can compensate for this. REFERENCES 1 J. Mitchell, Jr. and D. M. Smith, Aquametry Part 1: A Treatise of Methods for the Determination of Water, 2nd edn., Wiley, New York, 1977. 2 J. R%i&a and E. IX Hansen, Anal. Chim. Acta, 78 (1975) 145. Dan. pat. appl. No. 4846174 (1974), U.S. pat. 4,0022.575. 3 J. R%izka and J. W. B. Stewart, Anal. Chim. Acta, 79 (1975) 79. 4 J. W. B. Stewart, J. Ra”&a, H. Bergamin Filho and E. A_ Zagatti, Anal. Chhn. Acta, 81(1976) 371. 5 J. R%i&a, J. W. B. Stewart and E. A_ Zagatto, AnaL Chim. Acta, 81(1976) 387. 6 J. W. B. Stewart and J_ R%%&a, Anal. Chim. Acta, 82 (1976) 137. 7 J_ RG?ziaa and E. H. Hansen, AnaL Chim. Acta, 87 (1976) 353. 8 I RGva E. H. Hansen and E. A_ Zagatto, Anal. Chim. Acta, 88 (1977) 1.
9 E. H. Hansen,J. R&%&a andB. Rietz, Anal. Chim.Acta, 89 (1977) 241.
10 J. R&%&a, E. H. Hansen and H. Mosbaek, Anal. Chim. Acta, 92 (1977) 235. 11 J. Rf.%&a and E. H. Hansen, Acal. Chim. Acta, 99 (1978) 37. 12 J. C. Verhoeff and E. Barendrecht, J. Electroanal. Chem., 71 (1976) 305. 13 A. Cedergren, TaIanta, 25 (1978) 229. 14 E_ Lundberg, AppL Spectrosc., 32 (1978) 276. 15 A. Cedergren, Talanta, 21(1974) 367. 16 A. Cedergren, Talanta, 21(1974) 553. 17 A. Cedergren, TaIanta, 21(1974) 265. 18 J. MitcheR, Jr. and D. M. Smith, Aquametry, Interscience, New York, 1948. 19 J. C. Yerhoeff, Mechanism and Reaction Rate of the Karl-Fischer Titration Reaction, Dissertation, Amsterdam, 1977, p_ 92. 20 B. Karlberg and S. Thelander, Analyst, 103 (1978) 1154_
AMlytica
Printed in The Netherlands
DETERMINATION OF WATER BY FLOW-JXYECTION WITIf3 THE KARL FISCX-IER REAGENT
INGRID K~EvALL,
0vE
liST~ijM
Department of Analytical Chemistry.
and ANDKRS
ANALYSIS
CEDKRGREN*
University of Urn& S-901 87 Umeii (Sweden)
(Received 10th July 1979)
SUMMARY A method for the determination of water in organic solvents by flow-injection analysis (f-i-a.) is described_ The method, which is based on the reaction between water and the Karl Fischer reagent, is capable of 120 determinations per hour. The concentration range 0.01-5% (v/v) of water can be covered by using a single Karl Fischer reagent solution. The results obtained with a specially cbnstructed potentiometric detector showed a relative standard deviation of less than 0.5% (v/v). This value was about 3 times less than that obtained with a spectrophotometric detector. The f.i.a. technique is shown to offer some unique possibilities in minimizing interferences associated with the standard Karl Fischer batch titration method. Measurement of the water content in a wide variety of materials is a problem of universal interest and the determination of water has consequently become one of the commonest procedures in chemical laboratories. A comprehensive review of chemical and physical methods for the determination of water has recently been published [I]. Among the various techniques used, e.g., gaschromatographic, gravimetric, spectroscopic (n.m.r., WV., i.r.), titrimetric (Karl Fischer), the last is probably the most generally used. It has been applied to the determination of water in numerous organic and inorganic matrices including saturated or unsaturated hydrocarbons, alcohols, halides, acids, acid anhydrides, esters, ethers, amines, amides, nitroso- and nitro compounds, sulphides, hydroperoxides and dia.&y1 peroxides. However, many interferences are associated with the Karl Fischer method. Active carbonyl compounds, for example, cause the formation of water through reaction with methanol which is the main constituent of the ordinary Karl Fischer reagent. Other interfering substances which cause a different type of error include mercaptans and certain amines which react with the iodine present in the Karl Fischer reagent. Negative errors will thus ark in determinations of watar in solventS Like peroxy acids, diacyl peroxide, and quinones because of the formation of iodine. In some cases the Karl Fischer reagent has been modified successfully in order to minimize or eliminate these interferences.
There are two additional main drawbacks associated with the standard Karl Fischer method. First, it is time-consuming because of the rather slow rate of reaction between water 2nd the Karl Fischer reagent near the endpoint of the titration. Secondly, the analyst has to handle rather large volumes of toxic reagent which is potentially dangerous. In order to overcome some of these drawbacks, the possibilities of using flow-injection analysis (f.i.a.) were considered. This technique was originally described by Ri?ZiEka and co-workers [ 2-111 and involves the introduction of the sample into an unsegmented carrier stream with subsequent transportation to the detector_ The degree of dispersion of the sample zone can be controlled by varying parameters such as the coil length and the flow-rate. The f.i.a. technique has been shown to be a very flexible and powerful analytical tool, and has been applied to procedures such as extraction, acidbase titrations, dialysis, dilution and multiple determinations. For the determination of water by the Karl Fischer method, it should be possible to develop a completely closed system by exploiting this technique. It should also be possible to take full advantage of one of the other main properties of the f.i.a. method, namely the rapidity of determination. It has recently been shown [12, 133 that the reaction rate in the Karl Fischer determination can be speeded up by more than 100 times either by lowering the concentration of iodide in the reaction mixture [12] or by using a reagent based on a formamide-pyridine mixture 1131 rather than the conventional mixture of methanol and pyridine. The rapid rate of the main reaction obtained by using such reagents can be used to minimize effects from side reactions in a flow-injection system by keeping the reaction time at a minimum. This constitutes an important advantage of the f.i.a. technique compared with ordinary batch titrations. In addition, the f.i.a. technique may offer other unique possibilities for the elimination of interferences. This paper describes a f.i.a. system for the determination of water with a methanolic Karl Fischer (K-F.) reagent, and includes the characterization of two different types of detectors, one based on zerocurrent potentiometry and the other on spectrophotometry. EXPERIMENTAL
Instrumentation
The reagent was driven by a fourchannel peristaltic pump, a Minipuls 2 (Gi_ison, France)_ Silicone rubber tubing was chosen in order to withstand the very reactive K-F. reagent. Several other tubings were investigated but none was found to be sufficiently inert. Spectrophotometric detector. The spectrophotometer, built in this department, consisted of a small grating monochromator, (Model GM 100, Schoeffel Instrument GmbH, Germany), a lens lamp, a u.v.-enhanced photodiode and a flow-through cuvette (Heha Model 178, 31-Q& volme 8 ~1, light path 10 nun).
201 et.
4cm
mlmine1 S=2pl
1.0
Fig. 1. The potentiometric
200 cm
L
W
flow cell. (a) Teflon blocks; (b) O-rings; (c) glass cylinder.
Fig. 2. Flow injection manifold for spectrophotometric or potentiometric determination of water in organic solvents with Karl Fischer reagent. (S) Point of injection (sample loop); (D) detector; (W) waste.
Potentiometric detector. The configuration of the potentiometric detector is shown in Pig. 1. The carrier solution was pumped through a 0.5-mm hole in the lower teflon block, into which a O-l-mm platinum wire indicator electrode was inserted_ The solution then passed down a groove of the declining part of the teflon block to the exit. The indicator electrode was inserted 3 cm into the hole. If the electrode just reached the exit of the bore there was an increase in the sensitivity but the stability and reproducibility were very poor. These drawbacks arise from the fact that the electrode must be very precisely and reproducibly located in the flowing stream. By inserting to a distance of 3 cm, these disadvantages can be almost eliminated. By filling the cell just to a point at which there was only a small liquid film between the exit solution and the residual cell solution, mixing in the cell was kept to a minimum. The other platinum electrode was insulated except for the terminating spiral; since it is exposed to an almost unchanging environment it can function as a pseudo-reference system. The small change in the reagent in the eelI compartment does not significantly affect the measurement. No serious error should result from a systematic drift because the peak height is always related to the base-line. The platinum pseudo-reference electrode was chosen partly because of the inertness of platinum and partly to achieve an air-tight cell with exclusion of water. Potentiometer and recorder_ The signals from the two detectors were monitored either by a lin-log amplifier or by a follower connected to a HP Moseley 680 recorder_ Additionally, an interface was inserted between the amplifiers and the recorder, which allowed the peak maximum to be locked automatically on a meter while the recorder continuously displayed the actual potential output [14]_
202
Reagents and standards All orgtic liquids were of analytical grade. Solvents were dried before
use with molecular sieves (3 or 4 A). The mixtures of water and the organic sobents were standardized coulometrically [15]_ All solutions were calibrated except for the acetone mixtures. Owing to the formation of water by the ketal reaction, a graphical estimation of tbe concentration of water in dried acetone was done amperometrically; it was found to be below 0.01%. This was possible by performing the determination at a high concentration of iodine, so that the rate of the main reaction was sufficiently high to permit a differentiation. The Karl Fischer reagent contained 25.4 g of iodine, 38.4 g of sulphur dioxide and 80 ml of pyridine, diluted to 1 1 with methanol [16, 171.
Manifold and measurement technique The manifold used in the flow-injection system is outlined in Fig. 2. The tubings were made from teflon (0.5 mm id.) and the end-connectors were ordinary chromatographic connectors (A&x). Samples (2 ~1) were injected with a standard liquid chromatography inlet slide valve for low pressure (Cheminert) at point S into the carrier and reagent stream of Karl Fischer solution which was pumped at a flow rate of 1 ml min-‘. After the injection, a 209cm reaction coil sufficed to achieve the desired reaction. The change in concentration of the reagent was then measured either spectrophotometritally at 625 mn or potentiometrically with the platinum electrode system. RESULTS AND DISCUSSION
Kinetics The main Karl Fischer reaction, according to Mitchell [18],
is
CsHSN-I2 + CSHSN-SO2 + CSHSN + Hz0 --f 2CSHSNH+I- + CSHSN-SO3
(1)
which, in the presence of methanol, proceeds further CSHSN-SOJ + CH70H + C,H,NHSO&H,-
(2)
This reaction scheme has been criticized by several investigators. Verhoeff and Barendrecht [12] proposed the existence of a methyl-sulphite complex instead of the pyridine-sulphur dioxide complex. The kinetics of reaction (1) have been studied in two independent reports and it has been shown that the reaction is first order with respect to the concentration of iodine, sulphur dioxide and water [12, 17]_ The value of the rate constant in 0.20 M solution of iodide has been estimated to be about 1 X lo3 I* mol-* s-l [12, 13, 171. This value increases with decreasing concentration of iodide. For example, the value of the rate constant is enhanced by a factor of two when the concentration of iodide is lowered from 0.20 M to 0.15 M as estimated from the diagram given by Verhoeff 1191.
203
In order to estimate the time needed for a complete reaction to take place under conditions appropriate to flow-injection analysis, the following calculation was performed. The iodine concentration of the Karl Fischer reagent used was assumed to be 0.04 M and the concentration of sulphur dioxide to be 0.54 M. Further it was assumed that 2 ~1 of a sample containing 3% of water was introduced and mixed instantaneously with 100 ~1 of the reagent and that no further dilution occurred during transport to the detector. The time required for 99.9% reaction should then be about 2 s. The value of the rate constant used in these calculations, 1 X lo3 l*mol-*s-l, should have been somewhat higher because the concentration of iodide in this example never exceeds 0.18 M. These calculations were complemented by f.i.a. experiments performed under conditions corresponding to the discussed example. In these experiments, which are summarized in Fig. 3, the signal from the spectrophotometric detector was measured as a function of the reaction coil length. It can be seen that the peak height increases with increasing residence time, passes through a maximum, and then decreases again. The rising part of the curve corresponds to a situation where the dispersion is not sufficiently
1
I
I
100
I
300 L
(cm)
.
0u GENERATED
0.075 CONCENTRATION
OF
0.100 IODINE
(M)
3. Dependence of the peak height on the reaction coil length (L). Samples (2 ~1) containing 3% water in ethanol were injected into a flow (1 ml min-‘) of 0.04 M Karl Fischer reagent. Fig.
Fig. 4. The potential of the platinum electrode as a function of iodine concentration generated in a coulometric cell. The starting concentration of iodide was 0.2 M.
204 large to allow iodine to be in excess of water. Consequently, it is difficult to
estimate the shortest tune needed for completion of the main reaction_ Nevertheless, the shape of the curve to the right of the maximum value in Fig. 3 indicates that the main reaction has been completed. The extent of dispersion will thus determine the shape of the latter part of the curve. The potentiometric detector As noted previously [lS], stable redox potential values are obtained with a platinum electrode in Karl Fischer titrations. A platinum electrode was used in a recent investigation of redox potentials of the Ce(III)/Ce(IV) couple [20] by the f.i.a_ technique. In order to characterize the redox properties of a platinum electrode in the configuration used here, values of the electrode potential were determined as a function of the concentration of iodine, which was generated by constant-current coulometry; the results are shown in Fig. 4. The shape of the potentiometric curve indicates the existence of the strong triiodide complex between iodine and iodide. Thus, for low iodine concen?rations the potential-determining factor is probably the logarithm of &--I /II-l 3 while for high concentrations of iodine the logarithm of [In] ‘/[IX-]’ will predominate. The points marked A, B, C and D in Fig. 4 denote the various strengths of the Karl Fischer reagent used in the study represented in Fig. 5. This figure shows the response of the potentiometric detector for the various strengths of Karl Fischer reagent as a function of the
QOOS M 12
150
ZlOO ii
fi
=4
ao1nk.l~
0.053 M 12 0.030 M I*
$
w
Q!io fl
10 _ .
, 2
.
, 4
,
,
6 96H20
.
, 8
Fig. 5. Peak height values obtained
,
, 10
SCAi
with the potentiometric
detector
as a function
of the
percent water in ethanol for various strengths of the Karl Fischer reagent. The total concentration of iodide was 0.2 M in each of the reagents. Fig. 6. Recorder output born the potentiometric detector for the determination of water in acetonitrile. The signals from left to right correspond to 0.10, 0.49, 0.99 and 2.77% of water, respectively.
205
concentration of water in the sample. As can be seen, there are large differences in the shapes of the curves. However, comparison of the curves obtained in the flow-injection experiments with the results shown in Fig. 4 indicates that the shapes of approximately the first 3/4 of each curve fit very well. The disagreement between the last quarters of the curves can be explained by either incomplete main reaction or by slow response of the indicator electrode for low iodine concentrations. As indicated in Fig. 4, the working concentration range as well as the sensitivity can be further increased if higher iodine concentrations than 0.05 M are used provided that the total concentration of iodine is the same. In practice, it is difficult to prepare such a strong reagent because of side reactions and moisture in the chemicals used. One simple way, illustrated in Fig. 4, involves electrical preparation of the reagent. The kinetics of the main reaction should be extremely rapid for very high concentrations of iodine mainly because of the low concentration of iodide present in such a reagent. The precision of the potentiometric measurement is given in Table 1 for a 0.058 M Karl Fischer reagent; the relative standard deviation is less than 0.5% over the whole concentration range. This implies that the stability of the detector is very good; this is also demonstrated in Fig. 6 which shows a recorded scan of some samples iu the concentration range O-l-3% of water. The spectrophotometric detector Figure 7 shows the response of the spectrophotometic detector as a function of the concentration of water in the sample for three different strengths of the Karl Fischer reagent. These results were obtained at 625 run; at this wavelength no significant absorbance is obtained for a spent reagent. It can be seen that the linear range of the calibration curve can be extended but at the expense of decreased sensitivity. The fact that linear curves are obtained over a large concentration range of water indicates that the reaction is almost complete and that Beer’s law is followed. Although it is not clearly seen in Fig. 7, there is a slight deviation from linearity in the first part of the curves. This cannot be explained at present but is currently being investigated. The precision of the measurements is given in Table 1 for a 0.033 M Karl Fischer reagent. The percent standard deviation varies between 0.5 and 1.5% which is about three times that obtained with the potentiometric detector. The stability is illustrated in Fig. 8 which shows a recorded scan of some samples in the concentration range O-l-3% water. Effects of solvents Figure 9 shows the influence of various solvents on the potentiometric and spectophotometic determinations of water, respectively. The shapes of the curves are in good agreement with those shown in Figs. 5 and 7. Owing to the formation of water through the reaction between acetone and methanol, the acetone curve lies above all other curves in both diagrams. The slight deviations between the remaining curves may be due to several factors,
206 TABLE 1 Determination of water in ethanol with the potentiometric and apectrophotometric detecbrs. Reagents: ordinary 0.058 B4 Karl Fischer reagent for the poteutiometric method and ordiuary 0.033 M Karl Fischer reagent (wavelength 625 mu) for the spectrophotometric method. Poteutiometric detector (%I
Peak height (mv)
0.110 0.550 1.06 2.97 4.83
3.36 13.1 21.0 46.1 63.2
I-W
Spectrophotometric
R-s-d. (%I 0.47 0.54 0.41 0.19 co.1
Hz0 (%I
R.s.d. (%I
0.110 0.615 1.06 2.97 4.83
0.014 0.065 0.106 0.306 0.497
1.0 1.5 1.2 0.23 0.50
I
OF’
” 20
” 40
” 60 %
” 8D
’
lo.0
detector
Peak height (abe-
1
H,O
6 min
I
I
Scan -
Fig. 7. Peak height values obtained with the spectrophotometric detector as a function of the percent water in ethanol for various strengths of the Karl Fischer reagent. The total concentration of iodide was 0.2 M in each of the reagents. Fig. 8. Recorder output from the spectrophotometric
determination of water in acetoni-
He. The signala from left to right correspond to 0.10, 0.49, 0.99 and 2.77% of water,
respectively_
but they are surprisingly small in view of the very different properties of the solvents chosen for these experiments. The viscosity, for example, varies between 0.32 and 2.3 centipoise. Further, various contributions from the solvent polarity to the dispersion might be considered as a consequence of the variation in the dielectric constants. The values for acetic acid and acetonitrile
207
Fig. 9. Potentiometic (A) and spectrophotometric (B) calibration cmes for the determination of water in different organic sobents. (*) acetonitrile; (0) propanol; (0) acetone; (0) ethanol; (A) methanol; (0) acetic acid. The strength of the Karl Fischer reagent was 0.06 M for potentiometric measurements and 0.03 M for spectrophotometric measurements (at 625 nm). are 6 and 37, respectively. In addition, adsorption effects between the solvents and the teflon tube may occur, and this might also influence the extent of dispersion. A closer examination of the shapes of the peaks showed that their form is not exactly the same for different, solvents. These effects are
under As
study
at present_
demonstrated in Fig. 9 the determination of water in acetone is easily performed with the f.i.a. technique. Such determinations are troublesome when the conventional Karl Fischer batch titration procedure is used. Similar advantages with the f.i.a. technique were obtained for acetic acid, because there was no noticeable contribution from the water produced in the esterification reaction between this acid and methanol. CONCLUSIONS
This proposed method yields results which are comparable in precision and accuracy with those obtained by using the conventional Ka;l Fischer procedure. The advantages and disadvantages, which are concerned with the present state of the method, can be summarized as follows. One
advantage
is the
rapidity
of
the determination:
the f.i.a.
method
is
least ten times faster than the conventional Karl Fischer method. Safety is improved, because the closed system prevents the analyst from coming in contact with the toxic reagent except during reagent loading. There is less influence from interfering side-reactions compared with the conventional batch titration. Another great advantage is the low cost per determination; about 2000 determinations can be made per like of reagent. Among the disadvantages, the sensitivity is not as good as that obtained with the conventional Karl Fischer method. However, the method described has not been optimized with respect to sensitivity. Simply by increasing the at
208
sample volume, the working range of the method might be extended to 0.001-5% of water. Another disadvantage is the variation in results caused by solvent effects. The origin of this variation is under study and, on the basis of increased knowledge, it should be possible to design an f.i.a. system which can compensate for this. REFERENCES 1 J. Mitchell, Jr. and D. M. Smith, Aquametry Part 1: A Treatise of Methods for the Determination of Water, 2nd edn., Wiley, New York, 1977. 2 J. R%i&a and E. IX Hansen, Anal. Chim. Acta, 78 (1975) 145. Dan. pat. appl. No. 4846174 (1974), U.S. pat. 4,0022.575. 3 J. R%izka and J. W. B. Stewart, Anal. Chim. Acta, 79 (1975) 79. 4 J. W. B. Stewart, J. Ra”&a, H. Bergamin Filho and E. A_ Zagatti, Anal. Chhn. Acta, 81(1976) 371. 5 J. R%i&a, J. W. B. Stewart and E. A_ Zagatto, AnaL Chim. Acta, 81(1976) 387. 6 J. W. B. Stewart and J_ R%%&a, Anal. Chim. Acta, 82 (1976) 137. 7 J_ RG?ziaa and E. H. Hansen, AnaL Chim. Acta, 87 (1976) 353. 8 I RGva E. H. Hansen and E. A_ Zagatto, Anal. Chim. Acta, 88 (1977) 1.
9 E. H. Hansen,J. R&%&a andB. Rietz, Anal. Chim.Acta, 89 (1977) 241.
10 J. R&%&a, E. H. Hansen and H. Mosbaek, Anal. Chim. Acta, 92 (1977) 235. 11 J. Rf.%&a and E. H. Hansen, Acal. Chim. Acta, 99 (1978) 37. 12 J. C. Verhoeff and E. Barendrecht, J. Electroanal. Chem., 71 (1976) 305. 13 A. Cedergren, TaIanta, 25 (1978) 229. 14 E_ Lundberg, AppL Spectrosc., 32 (1978) 276. 15 A. Cedergren, Talanta, 21(1974) 367. 16 A. Cedergren, Talanta, 21(1974) 553. 17 A. Cedergren, TaIanta, 21(1974) 265. 18 J. MitcheR, Jr. and D. M. Smith, Aquametry, Interscience, New York, 1948. 19 J. C. Yerhoeff, Mechanism and Reaction Rate of the Karl-Fischer Titration Reaction, Dissertation, Amsterdam, 1977, p_ 92. 20 B. Karlberg and S. Thelander, Analyst, 103 (1978) 1154_