AIDS FOR ANALYTICAL CHEMISTS A Phosphorodithioate Ester as a Stoichiometric Source of Sulfide, Stable in Water Solution J. S . Blair Department of Chemistry, Stetson University, DeLand, Fla. 32720
DETERMINATION of microgram quantities of sulfide is requisite for workers in diverse fields, as in the study of foods, of atmospheric pollution, or of problems involved in paper manufacture. Whether the method adopted is the reflectometric measurement of a lead sulfide stain, or the colorimetric measurement of methylene blue formed from the sulfide, the most troublesome aspect of the procedure is the calibration of the results in terms of sulfide. Smith, Jenkins, and Cunningworth ( I ) studied both methods, calibrating by use of sodium sulfide solutions standardized iodometrically, then quantitatively diluted to give temporary working standards at the requisite low levels of sulfide. They emphasize the rapid deterioration of such solutions. The standard procedure of the pulp and paper industry (2) specifies the use of sodium thiosulfate solutions, similarly standardized and diluted, and considers these solutions to be adequately stable for only two days. Neither of these substances can qualify as stoichiometric sources of sulfide. Evidence is presented herein to show that pure potassium diisopropylphosphorodithioate is a stoichiometric source of sulfide, which can be used as a primary standard, and which is quantitatively stable for at least ten months in water solution at a concentration equivalent to 10 pg sulfide per milliliter. This stability is enhanced when the pH level is established at about 8.5 by the presence of 0.2% Na2HP04. Quantitative release of H S is effected by boiling for an hour in a strong acid such as 0.18M HzS04or 0.3M H3P04 solution. If no reflectometer is available for use with the lead sulfide stain method, approximate values can be obtained within reliable limits by visual comparison with freshly-prepared standard stains. Scott (3) implies that a standard sulfide stain can be relied upon for valid comparisons over an indefinitely-stated period of time. The writer’s observations do not confirm this. A stain in its original moist state will fade visibly overnight; the stain is best protected by hanging it in a desiccator as soon as possible, and even then it fades within a couple of weeks so as to give uncertain results. Washing the freshlyprepared stain with water has been found to erratically alter the reflectance of the stain by disturbing a surface layer of lead sulfide which is responsible for the specular component of the reflectance. EXPERIMENTAL
Potassium diisopropyldithiophosphate was initially received from the American Cyanamid Company in 1951. It was found necessary to recrystallize from acetone, after filtering out a slight turbidity. W. N. Jensen, collaborating with the writer in 1958 in the laboratory of the American (1) A. P. Smith, D. G. Jenkins, and D. E. Cunningworth, J. Appl. Chem., 11, 317 (1961). (2) TAPPI, “Reducible Sulfur in Pulp and Paperboard,” Technical
Standard T-406 m-60, revised 1960. (3) Wilfred W. Scott, “Standard Methods of Chemical Analysis,”
fourth ed., revised, Van Nostrand, New York, 1925, p 520.
Can Company, prepared a 0.01M solution of the purified salt (formula weight 252.38) and added aliquots to boiling 0.18M HzS04in an apparatus which enabled nitrogen, as a carrier gas, to transfer the liberated H2S through cadmium ammonium chloride solution in a reaction tube of the Peligot type, closable at both ends by stopcocks. After the transfer was complete, an aliquot of a standard iodine solution was added, and the closed reaction vessel was agitated until the precipitated cadmium sulfide was completely dissolved. Excess iodine was determined by thiosulfate titration. Four such experiments indicated a recovery of 101.2 =k 0.8 (average deviation) per cent of the theoretical. Six blank experiments gave an average of 1.6 “per cent recoverable sulfide” as a corrective term, so that the net average sulfide recovery was 99.6% of the stoichiometric value. The salt is not now available, but the Explosives and Mining Chemicals Division of the American Cyanamid Company will supply a crude grade of the corresponding phosphorodithioic acid. To prepare the pure potassium salt, 50.0 grams of the opaque dark greenish liquid acid were added dropwise, with agitation, to 142 ml of 1.19N KOH cooled in ice. This amount of KOH was just sufficient to maintain alkalinity, indicating that the crude acid contained less than 72 of its weight as the phosphorodithioic acid. The system then consisted of a turbid greenish aqueous phase underlaid with a black liquid phase. The aqueous phase was decanted and extracted with 50 ml of toluene. The initial black phase and the toluene extract were both discarded. The aqueous phase was filtered through retentive filter paper, and the clear filtrate evaporated nearly to dryness by a current of warm dry air, followed by drying to a constant weight in a vacuum desiccator over CaC12. The dry material was dissolved in 300 ml of acetone; the solution was filtered and evaporated to about 140 ml by a current of warm dry air. White crystals weighing 20.1 grams were obtained after washing with a small amount of acetone. These crystals were found to yield H,S, as evaluated by the sulfide stain procedure, in satisfactory agreement with the salt previously obtained. A second recrystallization from acetone did not improve the purity. RESULTS
H2Sliberated during an hour from an aliquot of a standard phosphorodithioate solution by boiling 0.3M phosphoric acid was carried by a controlled current of nitrogen from the top of a reflux condenser chamber through lead acetate paper placed transversely to the flow of gas. Immediately thereafter, the reflectance of the stain was measured by use of a Model 610 Photovolt Photoelectric Reflectance Meter, and Search Unit 610-T. The arbitrary 100% reflectance intercept was established by use of an unstained lead acetate paper. The arbitrary zero reflectance was that from Standard Color Chip No. 301 18, obtainable from the General Services Administration, Specifications Activity, Washington, D. C. 20407. Data are presented to compare the stability of two solutions, both stoichiometrically capable of yielding 50 p g of sulfide sulfur from the 5.00 In1 of solution added in each case. One solution contained 0 . 2 z Na2HP04; the other did not. VOL. 41, NO. 11, SEPTEMBER 1969
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Table I. Measured Per Cent Reflectance Blue Green Red As freshly prepared 21.5 i 1 . 0 22.5 i 1 . 3 49.0 f 1 . 0 Unalkalized, after three 26.5 & 2 . 0 28.8 i 2 . 0 54.5 f 2 . 7 months Alkalized, after ten 21.5 i 1 . 8 21.3 i 1 . 4 47.3 f 2 . 3 months ~~~~
Other data are at hand to substantiate this conclusion. Other data show that in more concentrated solutions of the phosphorodithioate, the effect of alkalinization on stability is less marked. It seems plausible that a trace of nitrite in the distilled water used could be an important factor (3). The ratio of nitrite to phosphorodithioate would be higher in the more dilute solutions of the latter. When the solution is alkalized, nitrite is rendered less potent as an oxidant.
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Freshly prepared stains from the two solutions agreed within acceptable limits. The values in Table I represent two values for the unalkalized and two for the alkalized solution as the overall average of the four, expressed with the average deviation. The values for the unalkalized solution after three months and for the alkalized solution after ten months are each the average of four determinations. Though the data illustrate the degree of imprecision inherent in the analytical method, they demonstrate the advantage of adding Na2HP04 when preparing the solutions.
ACKNOWLEDGMENT
The writer is grateful to the American Can Company for the support of the earlier phases of this investigation, and for donating to Stetson University the reflectometer used in the later work. W. N. Jensen, whose contribution to the investigation is in part described above, is now associated with the Milwaukee Department of Health. RECEIVED for review May 17, 1968. Resubmitted May 20, 1969. Accepted June 13,1969.
Precision Assembly for Rotating Ring-Disk Electrodes and Related Techniques R. H. Sonner, B. Miller, and R. E. Visco Bell Telephone Laboratories, Znc. Murray Hill, N. J. THE INTRODUCTION of ring-disk electrochemistry was made in the USSR by Frumkin, Nekrasov, and Levich (1-3). Theory appropriate for this electrode configuration has been given by Levich (4) and significantly extended in a series of papers by Albery and Bruckenstein (5, 6). Descriptions of several specialized rotating assemblies are available (7-11). Inexpensive rotators are available from laboratory supply houses but do not provide for the necessary electrode contacts and mechanical stability. A complete electrode rotator with 9 fixed speeds has been recently offered commercially by Pine Instruments, Grove City, Pa. The rotating electrode system used in our laboratory was developed to satisfy a requirement for a precision multispeed unit. Two bench models have been in use for several years. A third unit has been installed in a controlled atmosphere dry box where it is used for studies in nonaqueous solvents. The modular control system enables the latter unit to be (1) A. N. Frumkin and L. N. Nekrasov, Dok. Akad. Nauk SSSR, 126, 115 (1959). (2) A. N. Frumkin, L. N. Nekrasov, V. Levich, and Ju. Ivanov, J. Electroanal. Chem., 1,84 (1959). (3) . , L. N. Nekrasov and N. P. Berezina, Dok. Akad. Nauk SSSR, 142, 855 (1962). (4) V. G. Levich, “Physicochemical Hydrodynamics,” PrenticeHall, Englewood Cliffs, N. J., 1962, Chapter VI. (5) W. J. Albery and S . Bruckenstein, Trans. Faraday Soc., 62, 1946 (1966). (6) Ibid., p 1920. (7) ~, M. P. Belvanchikov. Yu. V. Pleskov. and B. G. Pominov, Russ. J. PhG. Chem. English Transl., 34, 782 (1960). (8) H. E. Hintermann and E. Suter, Rev. Sei. Instrum. 36, 1610 (1965). (9) J. Wojtowicz and B. E. Conway, J. Electroanal. Chem., 13, 333 (1967). (10) D. C. Johnson, Ph.D. Thesis, U. of Minnesota, Minneapolis, Minn., 1967. (11) D. T. Napp, Ph.D. Thesis, U. of Minnesota, Minneapolis, Minn., 1967. 1498
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operated remotely. A line drawing of the mechanical system is shown in Figure 1 and a photo of the complete system with cell and electrode mounted is shown in Figure 2. The foundation for the unit is a rigid aluminum base and backplate assembly. Suitable openings for signal cables are provided in the base and rear plates. A cell positioning assembly (Velmex, Inc., Holcomb, N. Y . #A15120) is bolted to the backplate. The spindle bracket (K),Figure 1, is mounted on the backplate and provides support for the spindle and the adjustable brush assembly (I). The spindle (H. P. Smith Co., Cheshire, Conn.) is a precision, pre-loaded ball bearing assembly with dimensions of 2.0 inches body diameter and 6 inches overall length. It will accept nylon insulating collets (C) for electrodes of either 0.250 or 0.375 inch 0.d. Total run-out of the electrode tip at 4 inches is less than 0.001 inch. The spindle is driven from the side with a belt and pulley system at either one-half or twice the motor speed. Ball bearing idler pulleys ( H ) adjust belt tension. The servo motor drive system, including tachometergenerator and SCR amplifier is available from Electro Devices, Inc., Paterson, N. J. The system has a speed range of 50 to 5000 rpm with a constant torque capability of 6 inch-ounces. The speed of the motor (F)is adjusted with a 10-turn potentiometer on the motor control station. The regulation of the spindle speed is at least 0.5% and improves at higher speeds. The desired motor speed is maintained automatically by comparing the speed reference signal with the output of the tachometer generator (C) and feeding back the error signal to the SCR speed control amplifier. The exact speed of the spindle is determined with a photoelectric pickoff ( D ) (General Radio, West Concord, Mass. Model 1536-A or equivalent) together with a reflecting strip on the top collet nut. The period of one or ten revolutions is displayed on a digital frequency counter (General Radio Model 1151-AP or equivalent).
