Microdetermination of Water, Using Rapid Controlled Potential Coulometry in a Karl Fischer System MELVEN R. LINDBECK and HARRY FREUND Department o f Chemistry, Oregon State University, Corvallis, Ore.
b Microgram quantities of water are determined by rapid controlled potential coulometry. Excess iodine is produced and removed coulometrically in a Karl Fischer system. By means of a special cell design, 2 pmoles of iodine in methanol may be reduced in 96 seconds. Tenth-milliliter samples of methanol containing 10 to 74 pg. of water and 18 to 82 pg. of water in dimethyl sulfoxide may be determined with an absolute standard deviation of *0,2 and lt0.3 pg. of water, respectively.
SCE Ref. Electrode
d b
Liquid Level Ag Counter
T
improvement of the Karl Fischer method for water determination has been the subject of many investigations (S), although the use of coulometry has only recently been considered. Constant current coulometric titration methods have been developed ( 5 , 7 , 9) in which electrolytic generation of iodine provides the titrant in a depleted Karl Fischer reagent. The absolute standard deviation is approximately A2 pg. of water. Other niicrodeterminations of water involving the use of the Karl Fischer reagent include a direct (2) and a spectrophotometric method ( 6 ) , which give approximate standard deviations of k 3 and 1 2 fig. of water, respectively. I n controlled potential coulometry for microdeterminations an end point determination is not necessary. The determination is terminated by the completion of the oxidation or reduction, and, assuming 100% current efficiency, the addition of excess titrant (electrons) is not possible. The precision of the method is not limited by the absolute error in reproduction of an end point. .+ disadvantage, I however, IS that controlled potential coulometric determinations usually require from 20 minutes to several hours. This paper applies rapid, controlled potential coulometry to the microdetermination of water in liquid solvents, in which coulometric control of iodine is achieved in a Karl Fischer reagent. HE
EXPERIMENTAL
Apparatus. CELL. T h e three-compartment cell used is illustrated in Figure 1. T h e reference and isolated side cells are connected by fine-
Electrode
Liquid Level
Pt Working Electrode
one inch
II_/I
N2
Figure 1.
Electrolysis cell
porosity frits. The nitrogen inlet frit a t the bottom of the working compartment is of medium porosity. The working compartment is closed to the atmosphere by a Teflon top with a silicone rubber seal. The sample inlet system consists of a silicone rubber septum held in position by a 5/16-in~h stainless steel cap screw with a '/*-inch hole through the center. The nitrogen outlet connection may be removed from the Teflon top for addition and removal of the cell solution. The cell solution is added with a 10-ml. graduated syringe fitted with 0.03-inch-diameter Teflon tubing, and removed with a 0.03-inchdiameter Teflon tubing by means of vacuum. The reference electrode assembly and counterelectrode are sealed into standard taper fittings which can be removed when the side cells are loaded. ELECTRODES. The working electrode is shaped into a tightly wound spiral consisting of 7.6 grams of 52-mesh platinum gauze. The counterelectrode is made of heavy-gauge silver foil. The reference electrode is a saturated calomel electrode. All voltages reported are
us. this electrode. The electrolytic continuity from aqueous to nonaqueous systems is established by an agarpotassium chloride salt bridge and frit, as shown in Figure 1. CONTROLLEDPOTENTIAL COULOMETER. The instrument used was essentially that described by Scott and Peekema ( 8 ) . Its electronic current integrator was continuously monitored by a 50-mv. recorder. CHEMICALS.Pyridine, Baker and Adamson reagent grade. Methanol, Baker and Adamson reagent grade absolute. Dimethyl sulfoxide, Crown Zellerbach Chemical Products Division, Spectro grade 40823. Nitrogen, Matheson Co., Inc., prepurified, passed through Anhydrone drying trains before entering the electrolysis cells. Sodium iodide, Baker and Adamson reagent grade. Sulfur dioxide, Matheson Co., Inc., anhydrous grade, purity 99,99% minimum. Karl Fischer reagent, Matheson Coleman & Bell, stabilized. VOL. 37, NO. 13, DECEMBER 1965
1647
6.0 4.0 2.0
2
0.0
p:
5 -2,. 0 -4.0
0.2
Figure 2. potential
-0.2 -0.4 -0.6 -0.8 -1.0 A P P L I E D P O T E N T I A L , v o l t s v s . SCE
0.0
-1.2
Variation of electrolysis current with applied
0.1 M Nal, 0.1 5 M SO2 solutions in methanol Matheson Coleman & Bell stabilized Karl Fircher reagent 3. 0.1M Nal, 0.15M S02, and 0.6M pyridine solutions in methanol 4. 0.1 M N a l solution in methonol
1. 2.
RESULTS AND DISCUSSION
Iodine Coulometry in Methanol. A I X D U C EB DA C K G R O U CURREKT. ND plot of current us. potential, showing the oxidation of iodide to iodine a t positive potentials us. a saturated calomel electrode and the decomposition of methanol at negative potentials in a 0.1111 S a 1 solution in methanol, is given in Figure 2 , curve 4. The measurements were made in the electrolysis cell described above. The solution was first deaerated with nitrogen, after which the measurements were made in the absence of stirring. The applied potential mas adjusted to give the desired value, and the current recorded after a 10-second interval. Theoretically it should be possible to oxidize iodide to iodine a t a controlled potential to give a quantity of iodine corresponding t o a readout in coulombs, and to reduce that quantity a t a controlled potential to give the same readout. However, this was the case only when the applied potential \vas more positive than -0.08 volt. If the reduction was carried out a t more negative applied potentials, there was an increase in the ratio of number of coulombs required for reduction and for oxidation. This increase in ratio is shown in Figure 3. The increase in the number of coulombs for reduction was more a function of the initial current (initial rate of reduction) than the applied potential. The ratio of coulombs remained a t unity if the initial current was not allowed to exceed 10 ma., regardless of the applied potential. This was demonstrated by adjusting the applied potential from -0.05 to -0.32 volt during the reduction at a rate such that the current never exceeded 10 ma. Another approach was to reduce the nitrogen stir1648
ANALYTICAL CHEMISTRY
ring, so that the reduction a t -0.2 volt gave an initial current below 10 ma. It seems reasonable to suspect that the reduction step causes the deviation of the coulomb ratio from unity, because the system contains 0.1N iodide and secondary oxidation should be insignificant. However, the possibility that the oxidation step caused the deviation was checked by making all reductions a t an applied potential of -0.05 volt and oxidations a t applied potentials between f 0 . 2 volt (with an initial current of 5 ma.) and +0.7 volt (with an initial current of 25 ma.). The ratio of the number of coulombs given by the reduction and oxidation processes did not deviate from unity.
These results indicate that a t higher initial currents a different reduction mechanism for iodine or an induced reduction of some other substance takes place. A localized voltage excursion a t the working electrode was considered unlikely. If this were the case, the Karl Fischer reagent should show a greater change in ratio of reductionoxidation coulombs because the reagent contains substances which are reduced more easily than methanol. However, the Karl Fischer reagent showed no change in the ratio of reduction-oxidation coulombs when subjected to the applied potential variations described for iodine in methanol. There may have been an induced hydrogen reduction a t the higher initial currents, in view of the fact that the 0.1M NaI solution in methanol contained 0.05% water, whereas the Karl Fischer reagent was absolutely water-free. Background currents involving the induced reduction of hydrogen in controlled potential coulometry of aqueous systems have been discussed by hleites and illoros (4). RAPIDELECTROLYSIS. The electrolysis cell (Figure 1) was designed to provide a large ratio of electrode area to solution volume and efficient nitrogen stirring, t o decrease the time necessary for electrolysis. Determinations in which 2.04 pmoles of iodine were produced by oxidation gave an iodine reduction of 2.05 i 0.002 pmoles in 96 seconds. The time of complete reduction was taken as the time a t which the current had decayed to 0.1% of its initial value. The oxidations and reductions rvere made a t controlled potentials of +0.4 and -0.08 volt, respectively, in 5 ml. of 0 . M XaI solution in
INITIAL CURRENT, ma.
8. 5
11. 0
I
13. 5
16.0
I
18. 5 I
21, 0 I
23, 5 I
-0. 1 -0. 2 -0. 3 A P P L I E D R E D U C T I O N P O T E N T I A L , v o l t - v s , SCE
Figure 3. Ratio of coulombs for iodine reduction to iodide oxidation as a function of iodine reduction potential Iodide oxidations maintained at f0.4 containing 0.1 M Nal
volt to give 2 #moles of iodine in 5 mi. of methanol
methanol 1vit.h a nitrogen stirring flow rate of 20 ml. per minute. This flow rate was considered optimum because decrease caused a relat'ively large increase in t,he time of analysis, and a n increase did not' significantly decrease the time. =In increase in the cell solution volume to 10 ml. increased the time of analysis to 180 seconds. This compares closely with times reported by Bard ( 1 ) . No significant amount of iodine was lost from the solut,ion because of nitrogen stirring; when 2 pmoles of iodine in 5 ml. of methanol containing 0.1JI S a 1 were subjected t,o nit.rogen stirring wit8h 20 ml. per minute for 10 minutes before reduction, no change in iodine concentration was observed. The loss of iodine was expected t'o be les:: likely from a Karl Fischer reagent because, in addition to the 0.1JI SaI, the syst,eni also contains pyridine, which forms a n addition compound with iodine. Iodine Coulometry in Karl Fischer Reagents. REAGEYT STABILITY. Current os. potential curves resulting from spent Karl Fischer reagents are shown in Figure 2, curves 2 and 3 . The conditions and procedure for the measurements were the same as those for iodine in methanol. Oxidation a t positive applied potentials corre.Qpondsto the oxidation of iodide to iodine, and the reduction a t negative potent,ials was prohably due to the reduction of pyridinium ion. When 2.04 pmoles of iodine were produced by osidat,ion a t $0.4 volt', the reduction a t -0.2 volt gave 1.98 j=0.004 pmoles recovered. The oxidation and reduct,ion t8imes were 27 and 92 seconds, respect'iv iodine was continuo t.hroughout a series of the same cell solut'ion reagent. Determinations were made after 2.04 pmolea of iodine had remained in the 5 minutes, and the 105s of iodine was found t'o correspond to 0.53 f 0.01 pg, of water per minute. .Ipproximately the same loss was obqerved when the iodine ivas produced in t,he system only just' before the end of the 5-n1inut.e waiting period. This indicated t'hat the iodine loss was continuous even in the absence of iodine and was probably due to both water getting into t'he system and side reactions which continuously produced water or a product that could reduce iodine. Changes in the nitrogen flow rat,e had very little effect on the amount of iodine lost per minute. A further check was made on the possible \vat,er contribution from t'he nitrogen by placing immediately ahead of t,he electrolysis cell a dummy cell which contained 20 ml. of Karl Fischer reagent approximately 0.0131 in iodine wit,h a bubbling path four times t,hat of the
elect,rolysis cell. Loss of iodine per minute decreased very little, indicative t,hat tmhenitrogen was not conbributing a significant amount, of water. Contribut.ion of wat,er from the isolated and reference cell compartments mas eliminat,ed by maint'aining the Karl Fischer reagent. in these cells at a slight iodine concentration. A contribution from oxidizable or reducible species from the cell containing the isolated electrode was unlikely because the silver (I) osidation product was insoluble in the iodide system, and t'he reduction product, was iodide from the slight excess of iodine in the cell solution. The loss of iodine varied considerably with composit.ion and age of the Karl Fischer reagent,. Karl Fischer reagents gave a greater loss of iodine with increasing SO2 concenbrations. The reagent purchased from Mat'heson Coleman and Bell (Figure 2, curve 2) and other stabilized reagent,s containing large ratios of pyridine t'o methanol gave somewhat, smaller losses of iodine per minute, but, great'ly increased the iodine reduction time. This lack of improvement from the stabilized reagents, coupled with their tendency to plug the nitrogen inlet, frit, of the electrolysis cell, discouraged t.he use of these reagents. Many Karl Fischer reagent's wit,h compositions ranging from 0.015 to 0.45.15 in SO, and 0.3 to 9 V in pyridine lvere checked; the most' satisfactory consisted of 0.15111 SO,,0 . 6 X pyridine, and 0.lM Sa1 solut.ions in methanol. The loss of iodine from this reagent was constant' and did not int'erfere with the determinat,ion of wat#er,as iodine loss per minute was t'reated as a blank which was determined before and after each analysis. RATER DETERMINATION.The controlled potential coulometric procedure for determination of \vat,er, employing spent, Karl Fischer reagent, closely resembles t,he Karl Fischer back-titration procedure. I n the back-titration procedure a known escess of iodine is added t'o the sample in the form of Karl Fischer reagent', \Thereas in the cont,rolled potent'ial coulometric procedure a known excess of iodine is produced coulometrically in a spent' Karl Fischer reagent, cont'aining the sample. I n t,he back-titration procedure the excess iodine is determined by tit'rating with a standard Lvater-methanol titrant. I n the controlled potent'ial coulometric procedure the excess iodine i. determined coulometrically. PROCEDURE. 1. Add 5 ml. of Karl Fischer reagent approximately 0.01J5 in iodine and adjust the nitrogen flow rate t o 20 ml. per minute. 2. .ifter several minutes reduce the excess iodine a t -0.2 volt applied potential. 3, Adjust' the applied potential to +0.4 volt and oxidize 2.00 @moles of iodide to iodine.
4. Add the 0.1-ml. sample, which should contain less than 1 @mole of water. Allow one-half minute for complete reaction. 5 . Reduce the excess iodine at -0.2 volt applied potential and subtract this value from the original 2.00 @moles. 6. Debermine the system loss of iodine (blank) by oxidation of iodide and reduction of iodine in the absence of a sample, maintaining the iodine quantity and t'ime as above. Obtain the determination t'ime from the distance that t'he recorder chart paper bravels. Add this loss of iodine to the value found in step 5 . 7. Convert the micromoles of iodine consumed by the water in the sample to micrograms of water on a 1-to-1 mole basis. For samples which contain more than 20 pg. of water, a change in procedure is necessary! in order to maintain a sufficient excess of iodine in step 4 aft,er addit.ion of the sample. This is accomplished by producing a recorded amount of iodine after the sample addition, approximately equal to the iodine consumed by t,he sample. The increase in value for iodine produced is added to the original 2.00 pmolei. The remaining procedure i.: the same. An excess of iodine wa.; necessary in the determination of w t e r in dimethyl sulfoxide, in order to obtain complete recovery. ;Ilso, the reactmiontime in step 4 was increased t,o 3 minutes. The lack of complete recovery was demonstrated by determining consecutive blanks after a water determination. The blank immediately after a water det'ermination, in which the reaction time in st'ep 4 was only one-half minute, was considerably larger than the folloxing blanks. This effect was not, observed for water determinat'ions in methanol. If the electrolysis cell was not used for several days, the time in step 2 was increased by 15 or 20 minutes. If moisture came into contact wit'h the frit,s, t'he absorbed moistmurewas removed very slowly, yielding larger blanks which decreased with time. This problem was eliminated by keeping the frits in contact, 1vit.h Karl Fischer reagent when the cell \vas not in use. In consecutive sample det.erminations the over-all time of determination was kept const'ant', and a blank mas determined before and after the sample. The method was evaluated by determining the water cont'ent of methanol and dimethyl sulfoxide by t,he standard addition technique. The int'ersections at zero water addit.ion agreed with the \vat,er cont,ent. found for the solvents before standard water addit'ions. This demonstrates that the method is absolute, in that 1 niole of iodine consumed corresponds t.o 1 mole of water. Tenth-milliliter samples of met,hanol containing 10 to 74 p g . of wat'er were analyzed j the absolute standard deviation was *0.2 pg. of water. TenthVOL. 37, NO. 13, DECEMBER 1 9 6 5
* 1649
milliliter samples of dimethyl sulfoxide containing from 18 to 82 pg. of water were analyzed; the absolute standard deviation was 1 0 . 3 pg. of water. The number of samples of dimethyl sulfoxide analyzed in the same electrolysis cell solution reagent had a considerable effect on the reaction time necessary in step 4 for 100% recovery. For 3minute reaction times only four 0.1ml. samples could be analyzed per cell solution reagent, This number was increased to eight by increasing the reaction time in step 4 to 5 minutes. This effect was not observed for methanol and from ten to twenty 0.1-ml. samples were analyzed for each cell solution reagent. The dimethyl sulfoxide appears to have a greater affinity for water than methanol. This demonstrates the advantage of a back-titration type of procedure for solvents which release water slowly to the Karl Fischer reagent and shows that with such solvents the ratio of sample volume to reagent volume should be kept as small as possible.
CONCLUSIONS
Other determinations developed on the basis of the Karl Fischer reaction typically show an uncertainty of =t2 pg. of water when tens to hundreds of micrograms of water are determined in 1- to bgram samples. The large sample size accentuates the influence of interfering side reactions given by the solvent and the Karl Fischer reagent. The proposed controlled potential method, employing 0.1-ml. samples, permits water determinations at the 10- to 80pg. level with an absolute standard deviation of k0.2 pg. of water. The small sample size minimizes the effect of undesirable side reactions and is particularly significant when little sample is available. Furthermore, the method is not limited by the sensitivity of a conventional end point detection system. By permitting excess iodine to remain in the system a predetermined time, complete reaction is assured in t.he case of solvents which slowly give up the last traces of water. The development of a rapid electrolysis system allows the
method to be competitive with existing procedures &s far as over-all time of analysis is concerned. LITERATURE CITED
(1) Bard, A. J., ANAL.CHEM.35, 1125 (1963). (2) Bastin, E. L., Siegel, H., Bullock, A. B., Ibid., 31, 467 (1959): (3) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, Vol. 1, Sec. A, p. 69, Interscience, New York, 1961. (4) Meites, L., Moros, S. A., ANAL.CHEM. 31, 23 (1959). (5) Meyer, A. S.,Jr., Boyd, C. M., Ibid., 31, 215 (1959). (6) Otterson, D.A., Ibid., 33, 450 (1961). (7)Pribyl, M., Slovak, Z., Mikrochim. Ichnoanal. Acta, No. 6, 1097 (1964). (8) Scott, F. A,, Peekema, R. M., Proc. 8nd U . N . Intern. Conf. Peaceful Uses At. Energy 28, 583 (1958). (9) Swensen, R. F., Keyworth, D. A., ANAL.CHEM.35, 863 (1963). RECEIVEDfor review July 6, 1965. Accepted September 21, 1965. M. R. L. is indebted to the Shell Companies Foundation, Inc., for financial assistance in the form of the Shell Fellowship in Chemistry.
Continuous Back-Titrations with Direct Readout Application to EDTA Systems W. J. BLAEDEL and R. H. LAESSIG Department o f Chemistry, University of Wisconsin, Madison, Wis.
A continuous automated potentiometric titrator with direct readout has been adapted to the back-titration of metal ions with EDTA. The metals studied were those which are commonly back-titrated because of slow reaction with the chelate. After calibration of the titrator with standard samples, readout of sample concentration is both digital and direct, requiring no computation to obtain the analytical result. The average standard deviation for a typical series of samples was 0,0001 mole per liter with sample concentrations ranging from 0 to 0.035M. The precision of the titrator was obtained for the following titrations at pH 9.7 or 4.8: Zn(ll), Ce(lll), La(”, Pr(lll), Cu(ll), Th(lV), In(lll), Zr(lV), Cr(lll), and AI(II1). For Zr(lV), Cr(lll), and AI(III), 1 minute on stream at 98” C. was used to promote reaction.
A
AUTOMATED continuous titrator with direct readout has been described (2) and applied (3, 4) to (ethylenedinitrilo) tetraacetic acid (EDTA) titrations with mercury-EDTA electrodes, using the potentiometric
N
1650
ANALYTICAL CHEMISTRY
methods developed by Reilley and Schmid (5, 6). Without sacrificing direct readout of sample concentration, these studies introduced the concept of automatic correction for end point error caused by asymmetry in the titration curves. I n the present study, the titrator has been adapted to the continuous EDTA back-titration of metal ions with direct readout, overcoming the disadvantages usually associated with back titrationthat is, the measured addition of two standard solutions and computation of the analytical result. The continuous back-titration technique permits automated determination of metal ions (Al, Cr, Zr, etc.) which react so slowly with EDTA that direct titration is unfeasible. THEORY AND DESIGN OF TITRATOR
I n the titrator, a continuously pumped stream of sample is combined with a buffered excess of reagent EDTA. After a reaction period, an excess of reagent metal ion is added [Zn(II) or Cu(II), whose reaction with EDTA is rapid], consuming the surplus chelate and leaving a small concentration of easily titratable metal ion. The excess
Zn(I1) or Cu(I1) is then titrated with another EDTA stream to a preselected end point potential by the addition of EDTA through a variable speed pump. This system gives rapid response time (1 to 2 seconds) and a linear relationship between titrant EDTA pump rate and sample concentration. The excess reagent EDTA could be titrated with the Zn(I1) or Cu(I1) stream, dispensing with the second titrant EDTA solution, but this procedure would result in a disadvantageous inverse relationship between sample concentration and Zn (11) or Cu(I1) reagent pump speed. When the formation constants of the sample metal ion-EDTA complexes are comparable to or larger than those of Cu (11)-EDTA or Zn (11)-EDTA, the final titration is a simple direct one. The effects of stability constants, buffer concentration, etc., have been investigated by Reilley and Schmid (6, 6) and applied to continuous titration (S,4). Figure 1 is a block diagram of the titrator. I n the chemical system, separate streams containing excess EDTA (0.0131M, 5.74 ml. per minute), buffer (0.2M in base and its conjugated acid, 9.93 ml. per minute), and sample (metal ion being titrated, 0 to 0.035M,
