NOTES
Rapid Potentiometric Determination of Sodium Polysulfides L. W. Kao, D. A. Aikens, and K. W. Fung Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12 18 1
F. A. Ludwig Scientific Research Laboratory, Ford Motor Company, Dearborn, MI 48 12 1
Chemical methods for analysis of alkali metal polysulfides are either lengthy or require specialized apparatus, and are therefore not well suited for certain applications. Several titrimetric and gravimetric methods capable of good accuracy have been developed (1-3) but are rather time-consuming, and the spectrophotometric method developed by Teder ( 4 ) is rapid, but requires use of special 0.02-cm path length cells which must be frequently calibrated. A simple, rapid method, capable of accuracy of a few percent, was needed in connection with a study of polysulfide electrochemistry which required analysis of a large number of samples of synthetic sodium polysulfides. These materials correspond to the empirical formula NaZS,, where x may be any value from 1.0 to 5.0 so that both stoichiometric compounds and mixtures are encountered. Polysulfides with values of x between 2.5 and 4.8 were of principal interest, and a simple method of determining the value of x for these materials within 2-4% in approximately 15 minutes per sample has been developed. T h e method is based on potentiometric measurement of the sulfide ion concentration in a solution of the sodium polysulfide sample in aqueous alkali. The polysulfide ion, S X 2 - , is formally analogous to a complex in which x - 1 neutral sulfur atom ligands are coordinated to a central sulfide ion and, in a strongly alkaline polysulfide solution, the sulfide ion concentration is controlled by the extent of dissociation of the polysulfide ion. The extent of dissociation in turn is quite sensitive t o the value of x , so that the sulfide ion concentration in a solution of NaZS, of fixed concentration is a direct indication of the value of x .
EXPERIMENTAL Apparatus. An Orion Model 94-16 sulfide ion electrode and a double junction saturated calomel electrode were used with an Orion Model 801 digital meter for potential measurements. Materials. Materials were reagent grade except Na&9H20 (Fisher Scientific Certified) and sulfur (Alfa, 99.999%). Synthetic sodium polysulfides were prepared by reaction of anhydrous sodium sulfide and sulfur as described by Rosen and Tegman ( 5 ) ,and the sodium content and sulfur content of each sample were determined gravimetrically by standard methods. (2). Procedure. A 100 0.5 mg sample of sodium polysulfide, previously crushed in an agate mortar, was weighed into a 50-ml volumetric flask using an analytical balance in a nitrogen atmosphere ( < 5 ppm 0 2 and H20) and the flask was transferred to a glove bag filled with high purity nitrogen. The sample was dissolved in and diluted to volume with deaerated 1M NaOH, the solution was transferred immediately to a beaker, and the potential measured with continuous stirring. Measurements were made in a temperature controlled room at 22 i IoC.
*
RESULTS The potential became stable to within 1 mV in less than 1 minute after immersion of the electrodes and remained constant for several minutes, after which it drifted slowly in the negative direction. The rate of potential change depended strongly on the composition of the particular polysulfide, ranging from approximately 1 mV per minute for 4.8 to less than 0.05 mV per minute for samples with x samples with 3 < x < 4. The influence of the polysulfide composition on the electrode potential for the composition range NazS to Na2S4.8 is illustrated in Figure 1, in which the potential of the sulfide electrode is reported relative to the value in a solution of NaZS. The standard potential of the sulfide electrode varies somewhat from day to day, but this effect can be compensated by standardizing the electrode daily in NazS and determining relative potentials. Values of x in Figure 1 are based on the sodium content of the various polysulfides standards because the sodium analysis appears to be more accurate than the sulfur analysis for these materials. The sulfur content was estimated gravimetrically as Bas04 ( 2 ) after oxidation with bromine, and the results were consistently 1-3% (abs) below the theoretical values. The sodium content was estimated gravimetrically as NaCl and results consistently were within 0.5% (abs) of the theoretical values, with random deviations about the mean. The low results for sulfur are attributed primarily to loss of sulfur in the oxidation step, because the probable errors that can be ascribed to coprecipitation ( 6 ) are much less than the observed bias. The data in Figure 1 were collected over a period of several days, with daily standardization of the sulfide electrode, and each calibration point represents the average of two or three values determined on different days. The repeatability of determining polysulfide composition in this manner was estimated by determining standard deviations for x from replicate measurements a t several values of x in the range of 3 to 4.8. The standard deviation for a single measurement of x is 0.06 unit, and there does not appear to be a significant variation of the standard deviation with composition. As an independent check of the reliability of the method, the method was applied to a sample of NazS3.9; prepared electrolytically by controlled discharge of a high temperature sodium-sulfur cell and assayed by measurement of the equilibrium potential of the high temperature cell (7, 8). The composition found using the present method was Na2S4.01.
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an assumed binary compound is inferred from determination of a single component, the method is sensitive to the presence of impurities. Thus, if a sample of NaZS, contains an impurity such as water, the experimental value of x will be less than the true value, because, in effect, less than the specified amount of NaZS, is weighed out. Potentiometric assay of polysulfides is, therefore, not a substitute for complete analysis, but a technique which can be useful in determining whether a complete analysis is necessary. It should be useful for rapidly assaying the precise composition of polysulfides which are known to be free of impurities and for routine monitoring of polysulfides to detect decomposition or moisture pickup over periods of time.
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LITERATURE CITED I
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Figure 1. Potential of sulfide ion selective electrode as influenced by sodium polysulfide composition
The potentiometric assay should be applicable in general t o soluble metal polysulfides, the metal ion of which does not form strong complexes with sulfide or polysulfide ions. In common with other techniques in which composition of
(1) G. Schwarzenbach and A. Fischer, Heiv. Chim. Acta, 43, 1365 (1960). (2) N . H. Furman, Ed., "Scott's Standard Methods of Chemical Analysis", 2nd ed., Vol. 1, Van Nostrand, NY. 1962, p 1009. (3) P. Ahlgren, Sven. Paperstidn., 70, 730 (1967). (4) A. Teder, Sven. Paperstidn., 70, 197 (1967). (5) E. Rosen and R . Tegman, Acta Chem. Scand., 25,3329 (1971). (6) W. F. Hillebrand, G. E.F . Lundell, J. I. Hoffman and H . A. Bright, "Applied Inorganic Analysis", 2nd ed., Wiley, NY, 1953, pp 716-717. (7) B. Cleaver, A. J. Davies, and M. D. Hames, Nectrochim. Acta, 18, 719 (1973). (8) N. K. Gupta and R. P. Tischer, J. fiectrochem. Soc., 119, 1033 (1972)
RECEIVEDfor review January 16, 1975. Accepted February 11, 1975. Work performed under NSF Contract NSF C-805.
Optically Transparent Carbon Film Electrodes for Infrared Spectroelectrochemistry James S. Mattson and Carroll A. Smith Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33 749
Internal reflection spectroscopy (IRS) has been used in conjunction with electroanalytical techniques to study reactions in the thin region adjacent to the prism-electrolyte interface since about 1966. Initial efforts a t combining these two techniques, employing UV-visible spectroelectrochemistry, were reported by Hansen et al. (1, 2 ) . They ( I , 2) restricted their initial investigations to the system glasstin oxide-solution, although they suggested that metalcoated, infrared-transparent internal reflection elements could be employed to extend spectroelectrochemistry into the infrared region. The first extension to the infrared was reported by Mark and Pons ( 3 ) ,employing a germanium prism as a combination electrode and internal reflection element (IRE). Laser and Ariel ( 4 ) examined the electrochemical oxidation of 1-naphthol a t Pt and Au optically transparent electrodes (OTE), using UV-visible spectroelectrochemistry, followed by examination of the resulting organic film by infrared IRS. Tallant and Evans (5) employed germanium as an OTE in examining the reduction of p - benzoquinone, using dimethyl sulfoxide (DMSO) as the solvent. Trifonov (6-8) used a single-reflection germanium prism to follow the electrochemically initiated polymerization of acrylonitrile a t the germanium surface. Reed (9) and Reed and Yeager (IO),in their study of electromodulation of the germanium space charge region, employed germanium as an OTE, and D20 as their solvent. Tallant and Evans ( 5 ) intended to go on to the logical step of making OTEs by depositing metal films on germa1122
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
nium or KRS-5, but were stopped by the total absorption of infrared energy by 50- to 100-8, metal films (11). Laser and Ariel (12), in an effort to get around the high absorption coefficients of metal films, demonstrated the applicability of a gold wire grid electrode (200, 1000, or 2000 lines per inch (lpi)) wrapped around a germanium IRE. They used the wire grid as the electrode and the germanium as the IRE, monitoring changes in the internal reflectance a t 1510 cm-I, for the oxidation and reduction of p - benzoquinone in DMSO. In an attempt to extend his studies to opaque metals, Rice (13) prepared ultra-thin metal films of gold, palladium, nickel, and aluminum on NaC1, KBr, and sapphire, and observed that films of sub-30-A thicknesses were generally infrared-transparent. I t is known ( 1 4 ) that cold vacuum deposition of gold, palladium, platinum, etc., in amounts equivalent to only a 20- to 30-A thickness, results in islands of metal, rather than a continuous film. These islands grow epitaxially, until they finally begin to coalesce. Such deposits do not conduct a t these thicknesses, and thus behave optically as though they were dielectrics, rather than metals. We have confirmed this observation in our laboratory with ultra-thin films of gold, platinum, aluminum, copper, and iron on germanium, KRS-5, and zinc selenide. The problem of making a thin film which will be representative of the metal, while retaining some infrared transmission, requires careful attention to both the vacuum deposition parameters and the later selection of internal reflection geometry and polarization.
