higher than the expected value of 3.00. This can be explained by the presence of varj ing amounts of dialkylphenol. The presence of the disubstituted material tends to increase the acetate to aromatic proton ratio. I n some cases disubstitution was verified by mass spectrometer analysis. Hoaever, the data on disubstitution, although reasonable, must be accepted provisionally until independent checks become available. DISCUSSION
The integration accuracy of the Varian A-60 proton spectronieter used in this work is usually independent' of sweep sl'eed. However, the very sharp, closely spaced acetate methyl peaks niakes sweep speed during integration an important factor in the accuracy of the peak area measurements. > I t the 50-second sweep rate and, to a lesser extent, a t the 100-second sweep rate on the 500-c.p.s. range, the integral does not agree with planimeter area nieasuremerits, presumably because of timeconstant limitations of the integrating circuit of the spectrometer. This is based on the results which show a pronounced bias in favor of the peak integrated first regardless of the sweep direction. The bias is removed by slow sweep rates, either by lengthening sweep time or shortening the sweep range. A combination giving 2 c.p.s. per second is satisfactory. Peak height ratios agreed very a-ell with planimeter area rat,ios, which is not surprising because the two methyl peaks have equal widths. < I t low concentrations of ortho or para isomer in the presence of high con-
centrations of the other isomer, correct'ions must' be made for peak overlap. This is shown by the data in Table IT.' which gives the results on known mixtures of ortho- and para-sec-butylphenol acetates. Known mixtures were made from weighed amounts of the individual isomers and analyzed by NMR, with and without overlap corrections. Overlap was det'ermined from an expanded scale trace by manually drawing in the absorption in the overlap region. I s is usually t'he case, the overlap without correction gives high results for t,he isomer present a t the lowest coricentration. *it the ly0 range the errors become very large. I n practice, it, is easier to use a nomograph constructed from the dat,a of Table IV than to make the corrections by manual planimeter methods. I n most cases excess acetyl chloride acts as the solvent for viscous samples. Tetrachloroethylene is also very good. Benzene, toluene, and xylenes should not be used because they interfere and because they shift, t,he acetyl chloride absorption upfield far enough to somet'imes overlap the methyl grouii absorptions of the phenol acetates. During the course of this study it was noted that, t,he alkylation of phenol with straight chain olefins resulted in a preference for ortho substitution; whereas, alkylation with branched material gave predominantly para substitution. KO meta isomer is expected; if it is present, it is lumped ait,h the para isomer in this analysis on t,he basis of mefa isomers in Table I. The acetate methyl of ortho-paradisubstituted phenol acetates absorbs
Table IV. Resultsa on Known Mixtures of ortho- and para-sec-butylphenol
Mole "i; ortho isomer found 90 Corrected correction for overlap
Known
10 0 13 f 1 25 0 27 zI= 1 50 0 50 f 1 75 0 73 f 1 90 0 86 i 1 Av. of three analyses
10 f 1 R 25 i 1 5 5 0 i 1 0 i5+15 90 i 1 *?I
in the same place as the ortho-substituted monoalkylphenol acet'ates, based on the results on dimethylphenols. Compounds \ d i large substituents in both ortho positions, as in the case of 2,6-di-tert-butylphenol, cannot be acetylated. 2,6-I)imethylpheno1 acetylates readily. In case of doubt, heating and repeated scanning in the spectrometer helps to ensure complete reaction. ACKNOWLEDGMENT
The authors are indebted to ,J. J. Shook of this labpratory for preparing many of the samples used in this study and for his valuable criticism of the manuscript. LITERATURE CITED
i l l Crutchfield. 11. M.. Irani. R 13.. Yoder, J. T.,' J . A m . hi1 Chemiats' Soc: 41, 129 (1964). ( 2 ) Jackman, I>. If., ',Suclear Magnetic Resonance Spectroscopy," p. 52, Pergamon Press, Xew York, 19.59. ~
Differential Controlled-Potential Coulometry Application to Determination of Chromium G. A. RECHNITZ and K. SRlNlVASAN Department o f Chemistry, University o f Pennsylvania, Philadelphia, Pa
'
b A new electroanalytical technique, which combines the selectivity of controlled-potential coulometry and the potential accuracy of differential methods, is proposed for the precise determination of electroactive materials in the sub-millimolar concentration range. The method is based upon the simultaneous electrolysis of the sample and a standard material in two identical cells, connected in series, under conditions which ensure selective oxidation or reduction of the substance to b e determined. The advantages of this method over direct controlled-potential coulometry are il-
I
lustrated by a study of the reduction of chrornium(V1) in sulfuric acid media. Using commercially available components, relative accuracies of better than 0.1% are easily attained.
D
techniques have been employed with advantage in such fields of analytical chemistry as spectrophotometry (1) and polarography ( 7 ) . One might expect that the combination of the differential approach with an analytical technique of high precision and accuracy such as coulometry could offer significant advantages over existing electroanalytical methods. IFFEHENTIAL
This has recently been demonstrated by the preliminary work of Monk and Goode (6), who employed c( nstant current coulometry i n a differential arrangement for the highly accurate determination of chroniium(V1) with electrogenerated iron(I1). These authors also suggested, but did not explore, the possibility of differential methods in constant' potential coulometry. Since controlled-potential coulometry has the further merit, of ensuring the occurrence of a specific electrode reaction, it was thought worthwhile to test the application of differential methods to controlledpotential coulometry. VOL. 36, NO. 13, DECEMBER 1 9 6 4
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Auxiliary Comportment
Figure 1.
Electrolysis cell
Differential controlled-potential coulometry can be carried out by connecting two identical coulometric cells in series so that the first contains a standard of known concentration and the second contains the sample having a concentration of electroactive material slightly greater than the standard. The electrolysis is then carried out in two stages; first, the exhaustive oxidation or reduction of the standard with potential control a t the working electrode of the standard cell and the sample cell connected in series, followed by further electrolysis of the remaining electroactive material in the sample cell with potential control now a t the working electrode of the sample cell. During the second stage of this process, the electrolysis current is integrated; the number of equivalents so determined are added to those of the standard to yield the total amount of electroactive material in the sample. Whereas in direct controlled-potential coulometry, the current due to the electrolysis of the entire amount of electroactive material in the sample is integrated from beginning to end, in the proposed technique of differential controlledpotential coulometry, the integration of the current is restricted to that fraction of the sample remaining in the sample cell after electrolysis of a known standard in an identical cell. The electrolysis of the standard is, of course, accompanied by the consumption of an equivalent amount of electroactive material in the sample cell. Thus, any error in the integration process applies only to a small fraction of the total electrolysiq and the overall error can be greatly reduced. The proposed method requires the use of identical reference electrodes and the maintenance of similar mass transport conditions in both cells. The latter requirement ensures that during the first stage of the electrolysis when the
2418
ANALYTICAL CHEMiSTRY
potential of the working electrode of the standard cell is being controlled, the working electrode in the sample cell does not assume a potential more cathodic (in a reduction process) than that in the standard cell. If the supply of electroactive material in the sample cell is insufficient to support the electrolysis current imposed by potential control of the working electrode in the standard cell, some other electrode process will take place in the sample cell and result in a positive error for the second stage of the electrolysis. Fortunately, favorable mass transport conditions can be established by keeping the volume of electrolyte in the standard cell slightly larger than that in the sample cell, if the two cells are otherwise identical. As an illustration of the application of the technique of differential controlled-potential coulometry, the determination of chromium by reduction of dichromate a t a platinum cathode is reported in this paper. The electrolytic reduction of dichromate to chromium (111) a t a platinum cathode was reported previously by Merritt, Martin, and Bedi ( 6 ) , who used a constant current electrolysis procedure in a medium containing a large excess of sodium iodide. In reality, the electroactive species in this case is not dichromate but iodine, which is liberated by dichromate oxidation of iodide in the highly acidic medium employed (NHBOJ. It was not thought advisable to employ the iodide mediator in the present work in view of the risk of air oxidation of iodide and possible loss of iodine from solution. More recently, controlled-potential coulometry using a gold cathode has been employed by Harrington, Propst, and Britt (3) for the preparation of a standard chromium(II1) solution with a relative accuracy of 1%. EXPERMENTAL
The two identical electrolysis cells used were open-necked vessels of 6-cm. length of 25-mm. (diameter) tubing flared to 65 mm. at the top (Figure 1). These cells could accommodate standard platinum gauze electrodes (5 cm. X 12.5 mm.) efficiently with little dead volume. The auxiliary and reference compartments separated by fine sintered glass disks were positioned from above (Figure l ) , and the platinum gauze working electrode in each cell was rotated a t 1800 r.p.m. by means of a Sargent synchronous rotator. About 40 ml. of the solution was sufficient to fill the bottom portion of each cell and establish contact with the sintered glass disks of the isolation compartments for the reference electrode and platinum auxiliary electrode. The reference electrode for each cell was a Hg/HgzS04, saturated KzSOa electrode brought into equilibrium with each other before use. Agar bridges containing saturated KzSOd were used to
Figure 2. Schematic diagram of experimental system First stage of electrolysis-solid lines Second (differential) stage-broken lines
provide contact between the reference electrodes and the solution in the reference compartments. A potential of -0.250 volt with respect to the above reference electrode was chosen as the control potential. The background current in the supporting electrolyte (,VH2S04) a t this potential was a few microamperes after preelectrolysis. Occasional pre-treatment of the platinum electrodes by brief electrolysis in dilute sodium iodide solutions was found to be beneficial in maintaining low background currents. A Wenking potentiostat (Model S o . 6431R) was employed to control the potential of the working electrode. The current integration system and its calibration have been previously described ( 2 ) . A11 reagents used were of reagent grade. Procedure. &liter a separate preelectrolysis of the supporting electrolyte in each cell, known volumes of the dichromate solution were delivered to the two cells by means of grade 1 pipets after withdrawing the requisite volume of the supporting electrolyte from each cell to maintain constant volume in each cell. The actual amounts of dichromate added were calculated from pipet calibration d a t a obtained from the weight and density of the solution. Figure 2 gives the schematic of the electrical connections for carrying out the differential controlled-potential coulometric analysis. Cell .I,the standard cell, contains less dichromate than Cell B, the sample cell. During the first stage of electrolysis, the potential of the working electrode in Cell -4 is controlled, and Cell B is connected in series. Electrolysis is carried out, without integration of the current, until the current reaches its background value.' Potential control is then transferred to Cell B, and electrolysis is continued with the integration system in the circuit. The electrolysis is terminated when the background current is once again attained; under the mass transport conditions employed, each electrolysis required about 30 minutes. At the highest concentration employed in the present work (0.3854 mg. of chromium as dichromate in approximately 40 ml. of solution) initial currents of the order of 30 ma. were obtained while the initial currents during the second (differential) stage of electrolysis ranged from 3 ma. to 30 Ma.
RESULTS AND DISCUSSION
Table I suniiiiarizes the results of esperiinrnt> in lvhich the difference in the amounts of dichromate in t,he two wlls is kept essentially constant while the size of the sample is progressively decreased. examination of the data shows that the precision of the deterniination of a given difference in presence of samples of different size is quite satisfactory. -1s expected, the accuracy of the determination decreases with decreasing sample size. The result:, presented in Table TI relate to experiments in which the ratio of the sample size to the difference is varied. I t is clear from the data that an increase of this ratio does improve the accuracy which becomes particularly noteworthy a t ratios of 1OO:l. At the same time, the overall accuracy is dependent on the size of the sample itself, as is evident from the last row of Table 11. To evaluate the performance of the differential method, a comparison with the accuracy obtainahle in direct coulometry for the identical electrolysis is in order. Table 111 gives the results of the direct coulometric determination of chromium as dichromate. comparison of Table I11 and Table I1 clearly reveals that differential controlled-potential coulometry yields an improvement in relative accuracy. [-sing a sample size as small as 0.08 mg. of chromiuni and a 5 per cent differential, the error is leas than 0.8%; while in the direct .method, the determination of a sample twice this size involves twice the error. I t should be pointed out that the direct reduction of dichromate a t a platinum. electrode is not the best example to bring out the advantage of the differential method, in view of the known adsorption of dichromate ions on platinum surfaces ( 4 ) . Chromium is an important constituent of laser crystals. however; and it is hoped that the proposed technique niay be useful for highprecision analysis of such materials. The present work has been restricted to samples containing less than 0.5 mg. of chromium; a t this level it is not practicable (because of volumetric and aeighing limitations) to use differentials smaller than lyo of the total sample.
Table I. Determination of Chromium as Dichromate by Differential Controlled-Potential Coulometry-Constant Differential with Varying Sample Size
Cr in cell A, mg. 0 3448 0 3448 0 3084 0 3084 0 2686 0 2686 0 2307 0 2307 0 1923 0 1923
Cr in cell B, mg 0 3848 0 3848 0 3448 0 3469 0 3084 0 3084 0 2686 0 2686 0 2307 0 2307
Ihfference (added) 0 0400 0 0400 0 0364 0 0385 0 0398 0 0398 0 0379 0 0379 0 0384 0 0384
Table 11. Effect of Ratio Ratio of sample size Cr in Cr in t o difference cell A, mg. cell B, mg. 0 3469 8: 1 0 3084 0 3833 9:l 0 3448 0 3848 0 4233 10: 1 12: 1 0 2307 0 2499 16: 1 0 3084 0 3276 50: 1 0 3848 0 3925 100: 1 0 3848 0 3886 20: 1 0 06781 0 0806
Table 111. Direct Coulometric Determination of Chromium as Dichromate
Cr, mg. (found) 0 3834 0 3854 0 3823 0 3416 0 3423 0 3430 0 3039 0 3042 0 3061 0 1527 0 1520 0 1518
Error, %;o
-0 36
+o -0 -0 -0 -0
-1 -1 -0 -0 -1 -1
Cr in cell 13, mg (found) 0 3841 0 3856 0 3441 0 3479 0 3079 0 3072 0 2677 0 2695 0 2297 0 2284
of Sample Size to the Differential Cr in Difference Difference cell B, mg. (added) (found) (found ) 0 0385 0 0394 0 3479 0 0385 0 0396 0 3844 0 0385 0 0397 0 4245 0 0192 0 0189 0 2496 0 0192 0 0190 0 3274 0 0077 0 0079 0 3928 0 0038 0 00389 0 3887 0 00385 0 00323 0 08000
A \
Cr, mg. (taken) 0 3848 0 3848 0 3848 0 3448 0 3448 0 3448 0 3084 0 3084 0 3084 0 1539 0 1539 0 1539
Difference (found) 0 0392 0 0407 0 0356 0 0394 0 0393 0 0386 0 0370 0 0388 0 0373 0 0361
10 65 93 72 52 46 36 74 78 23 36
Kevertheless, the error a t this level is found to be less than 0.037,. If the sample size mere 5 mg., the same differential amount would have been equal to only one tenth of a per cent of the total sample and would have rebulted in relative errors of a few thousandths of a
Overall error, c y
/c
-0 +0 -0 +0 -0 -0
18 21 20 29
16 39 -0 33 +o 33 - 0 43 -1 0
Overall error,
7L
+0 29 +0 29 +O 28 -0 12 -0 06 +0 07 $0 026 - 0 77
per cent. Differential controlled-potential coulometry could thus lead to very substantial improvements in the relative accuracy, provided the ratio of the total sample to differential can be kept a t 1OO:l or ‘greater with reasonable sample sizes. LITERATURE CITED
(1) Bacon, &4., hlilner, G . W. C., Analyst
81, 456 (1956). (2) Bard. A. J.. Solon. E.. ANAL.CHEM. 34. 1181 11962) (3) Harrington, ’D. E., Propst, R . C., Britt, R . D., Jr., Zbid., p. 1663. (4) Laitinen, H. A., “Chemical Analysis,” p. 335, AIcGraw-Hill, Xew York, 1960. (5) Alerritt, L. L., Jr., Martin, E. L., Jr., Bedi, R. D., AKAL.CHEM. 30,487 (1958). (. 6,) AIonk. It. G.. Guode. G. C.. Talanta 10. 51 (1963). (7) ihalgosky,’H. E., Watling, J., ilnal. Chim. Acta 26, 66 (1962).
RECEIVEDfor revieF August 31, 1964. Accepted October 1, 1964. The financial support of Contract DA-36-034-ORD3696-RD is gratefu!ly acknoFledged.
VOL. 36, NO. 13, DECEMBER 1964
2419
