Citric acid reduces vanadium(V) to vanadium(1V) in a boiling aqueous solution at pH 2 ppm), although applicable to the purified solvent, could not be used on LiC104 salt solutions due to decomposition reactions within the injection port. An alternative method for measuring water in these solutions is differential spectrophotometry, employing the 1.9-p near infrared absorption band. This approach has been applied to a variety of organic and inorganic solvents (5, 6, 7). The limit of detection has been high, -O.l%, and very little information is available on the effects of added salt on the absorption spectra. The present paper discusses the determination of H 2 0at and above the 20 ppm level in propylene carbonate (PC) and in its solutions with LiC104. EXPERIMENTAL
Apparatus. The determination of water in PC was done on the Model 450 Perkin-Elmer spectrophotometer with a tungsten lamp source and a PbS detector. Matched (1) W. Harris, Ph.D. thesis, University of California, Berkeley, Rpt. UCRL 8381, 1958. (2) R. Jasinski, “High Energy Batteries,” Plenum Press, New York, 1967. (3) R. Jasinski, Electrochem. Technol., 6, 28 (1968). (4) R. Jasinski and S. Kirkland, ANAL.CHEM., 39, 1663 (1967). (5) H. Cordes and C . Tait, ibid., 29, 485 (1957). (6) R. Meeker, F. Critchfield, and E. Bishop, ibid., 34, 1511 (1962). (7) D. Chapman and J. Nacey, Analyst (London), 83, 377 (1958).
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ANALYTICAL CHEMISTRY
pairs of Beckman NIR cells with ground glass tops and path lengths of 1 cm were used in the analysis. The normal variations in path length of commercial matched cells prohibited the use of cells with path lengths greater than 1 cm. Reagents. PC (Jefferson Chemical Co.) was first distilled and the LiC104 solutions then dried by passing down a column of powdered Molecular Sieves (Linde 4A). This column approach was found to be much more effective than the batch method described by Meeker, Critchfield, and Bishop (6). Although the gas chromatographic analysis (4) indicated less than 1 ppm HzO after treatment of the solvent itself, approximately 2 ppm HzO was detectable in 0.5M and 1.OM LiCIOa salt solutions after an identical drying treatment (8). Repetitive passes down the column were unsuccessful in further reducing the water content by any substantial amount. The solutions prepared with distilled PC appeared to be otherwise unaffected by the Molecular Sieves. However, solutions prepared with as-received solvent, generally a pale yellow on entering the column, become a turbid pink on leaving the column, indicating some interaction between solvent impurities and the column material. Procedure. Standards were made by direct addition and by dilution over the range 10-1000 ppm HzO. All solutions were prepared and dried in an inert atmosphere and transferred from the dry box immediately before each run. It was apparent from visual inspection of the spectra that an uncontrolled shift in baseline was occurring with the salt solutions. Such shifts were not noted in runs on the pure solvent, so that the problem was not one of instrumentation. Apparently LiC104-PC solutions have a greater affinity for water than does the pure solvent. When the necks of the (8) B. Burrows and S. Kirkland, J. Electrochem. SOC.,in press.
Table I. Absorptivity and Peak Maxima in Salt Solutions LiC104 concentration, M Xlnm P Absorptivity 0.0 0.5 1.0
1.900 1.905 1.910
2.85 1.65 1.44
Table 11. Peak Areas for 50 ppm HzO Relative areas, cm2 LiCIOa concentration, M 0.0 0.5 1.0
-
t
0.50
1.8 1.85
1.9 1.95 MICRONS
2.0. 2
Figure 1. Differential spectrum for 100 ppm H 2 0 in PC
1-cm absorption cells were dry, the baselines were erratic; when the necks of the cells were wet with solution, the spectra were much more reproducible. In effect, the excess solution acts as a seal to atmospheric humidity. When the cells were left out in the atmosphere for an extended length of time (more than 20 minutes), the sample cell would occasionally pick up -10-20 ppm HzO. Because the solutions are generally run within three minutes of removal from the dry box, this slow H 2 0 gain would not automatically affect the results obtained. The results reported below were taken on wet-sealed cells. It is to be expected that some scatter in extinction coefficient will result in the analysis of low water content solutions. Nevertheless, the capabilities of the method should be discernible. RESULTS AND DISCUSSION
Water has an absorption band in the near IR at 1.9 p ; PC has a weak band, at 1.87, slightly overlapping the water band. A differential spectrum for 100 ppm water in PC--i.e., US. dried, distilled solvent-is shown in Figure 1 over the wavelength range of 1.8 to 2.05 p. It is obvious that a good deal of the interference band can be eliminated by this approach. The spectra of dimethyl sulfoxide, dimethylformamide, tetrahydrofuran, and acetonitrile, solvents also of interest in nonaqueous electrolyte batteries, were surveyed in this region. Each of these four solvents has a major interferring absorption band at 1.9 p , which cannot be compensated with commercially available absorption cells. The differential spectrophotometric analysis of HzO in PC was straightforward; another solvent can be added to the list
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of materials (6) amenable to this analytical approach. The pertinent absorptivity is contained in Table I. However, these data are not directly applicable to HzO-PC mixtures which contain substantial amounts of lithium perchlorate. With increasing concentration of the LiC104 salt in PC but equal HzO concentrations, there is a shift in the peak maximum and a decrease in the (Table I) from the values obtained with the pure solvent. This is indicative of an interaction between the HzO and the salt; further evidence for such interactions was also apparent from electrochemical data (8). Presumably, the water exists as hydrated lithium ions. It would be expected, however, in view of the large excess of lithium ions over water molecules (10-3M) that there would be little effect of LiC104 concentration over the range 0.5-1M. This, however, was not the case. The decrease with LiClO4 concentration of the extinction coefficient is accompanied by a real but smaller decrease in total area of the absorption band (Table 11). Unlike the peak height, the area decreases proportionately with LiC104 concentration. To evaluate the reproducibility of this analysis for trace amounts of water, the standard mean deviation was found for repeated runs of the 50 ppm and 500 ppm HzO standards. The SMD for 6 runs in 50 ppm H20-PC with no LiClO, was iO.0001, or -1%. The deviation increased with the 0.5M LiC10,-PC standard to &0.0003 ( 5 % ) for 14 runs. A further increase to =t0.0005 (10%) for the 1 M LiCl0,-PC standard was found for 18 runs. The increase in the SMD with LiC104 samples is apparently a result of the greater susceptibility of the LiC104 solutions in the cells to pick up H20. Repeated runs on 500 ppm H 2 0 in 0.5MLiC104-PC showed a SMD of 10.001 or 2%. The Beer’s law absorbance-concentration plots (10-1000 ppm HzO) were linear for all solutions. The method appears to give reasonable results above 20 ppm. Between 10 and 20 ppm the results are to be considered only as semiquantitative. RECEIVED for review June 10, 1968. Accepted July 18, 1968. The authors wish to acknowledge the financial support of the Air Force Cambridge Research Laboratories, Office of Aerospace Research, Bedford, Mass., under Contract No. F 196 28-68-C-0052.
VOL. 40, NO. 12, OCTOBER 1968
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