of CuBr with CuSCN, studies were made that showed that CuBr is completely solubilized by the N H 10H-HNO:j treatment while CuSCN remains analytically insoluble. The final pH adjustment prior to the filtration was found to be a critical factor in achieving the separation. Standard mixtures, containing from 50-220 mg KBr and 100 mg NH4SCN, were analyzed by the proposed procedure. The results are shown in Table I (7, 8). The data clearly indicate that this method is equivalent in precision and accuracy to the classical Volhard method ( 9 ) as applied to pure bromide or thiocyanate solutions. Furthermore, only a single titrant is required, minimizing potential sources of error. The relative merits of the sulfide specific ion electrode as an indicator electrode for argentometric titrations have been investigated and reported (Orion Research Inc., Cambridge, (7) IUPAC Information Bulletin No. 26, August 1966. (8) W. J . Youden; “Statistical Methods for Chemists,” Wiley, New Yorh, N. Y . . 1951. (9) E. H. Swii‘t, G. M. Arcand, R. Lutwack, and D. J . Meier, ANAL.CHEV.: 22, 306. 1950.
Table I. Recovery Data for Mixtures of KBr and KSCN Compot~nd KBr KSCN Number of analqses 7 7 Mean value 99.64 99.61 Median value 99.60 99.50 Range 0.70 1.50 Mean deviation 0.25 0.37 Per cent variation 0.25 0.37 Standard deviation 0.27 0.46
Mass). The salt bridge for the calomel reference electrode was used t o prevent chloride contamination and to prevent clogging the fiber junction with insoluble silver salts. The end point in the present procedure was occasioned by a potential change of about 50--60 mV per drop (0.03 mi) of 0.1N AgNO,{. RECEIVED for review May 29, 1968. Accepted August 2, 1968.
Spectrophotometric Determination of Silver(1) in Nonaqueous Solutions by Oxidative Coupling of Aromatic Amines Robert I,. Rebertus and Vaughn Levin Central Research Laboratories, Minnesofa Mining and Munufacfuring Co., St. Puul, Minn. 55101 SPECTROPHOTOMETRIC methods for determining silver(1) ion are usually based upon complexation reactions with ligands such as dithizone ( I ) , p-dimethylaminobenzalrhodanine( I ) , toluene-3,4-dithiol (2), 2-amino-6-methylthio-4-pyrimidine-carboxylic acid (3), or 4,4’-bis(diniethy1amino)thiobenzophenone ( 4 ) . Generally, the reported procedures apply only to aqueous solutions of the silver. In the present study we have demonstrated that oxidation-reduction reactions, similar to the coupling reactions employed in some color photographic processes ( 5 ) , are useful for the determination of silver(1) in nonaqueous solvents. In particular, the four-electron oxidation of equimolar mixtures of 111- and p-phenylenediamine permits the rapid and convenient spectrophotometric determination of silver(1) in the concentration range, 1 to 10 pg/ml, in alcohols, acetone, toluene, or ethyl acetate. EXPERIMENT 4 L
Apparatus. Spectra and absorbance measurements were made with a Beckmm Grating Spectrophotometer, Model DB-G. Matched cells with a 1-cm optical path were used. Reagents and Chemicals. m-Phenylenediamine, p-phenylenediamine, N,N-dimethyl-p-phenylenediamine,and toluene(1) E. P. Przybylowicz and C. W. Zuehlke, in “Treatise on Analytical Chemistry,” Part 11, Vol. 4, I. M. Kolthoff and P. J. Elving, Eds., Interscience, New York, 1966, pp 61-3. (2) J. P. Dux and W. R. Feairheller, ANAL.CHEhi., 33, 445 (1961). (3) 0. K . Chung and C. E. Meloan, ibid., 39, 383 (1967). (4) K . L. Cheng, Mikrochitn. Acta, 1967, 820. ( 5 ) G. F. Duffin, “Photographic Emulsion Chemistry,” The Focal Press, London-New York, 1966, pp 190-4.
2,4-diamine, all practical grade, were obtained from Eastman Kodak Co. Procedure for Methanol and Other Solvents. To a 10-ml volumetric flask equipped with a small reflux condenser are added 5 ml of methanol containing silver(I), 1 nil of 0.1M sodium acetate in methanol, 1 ml of in-phenylenediamine solution (1 gram/liter in methanol), and 1 ml of p-phenylenediamine solution (1 gram/liter in methanol). The solution is boiled for 10 minutes, cooled, diluted to volume, and filtered through a Whatman No. 5 filter paper. The first portion of the filtrate is discarded, after which the optical cell is filled and the spectrum measured. Calibration data are obtained from diluted aliquots of 0.0015M solution of silver nitrate in methanol. I n this procedure, toluene-2,4diamine and N,N-dimethyl-p-phenylenediamine may be substituted for rn-phenylenediamine and p-phenylenediamine, respectively . The procedure for determining silver(1) in isopropanol or acetone is similar to that for methanol except that the periods of time for color development are different. The reaction time for isopropanol is 5 minutes at the boiling point, and that for acetone is 1 minute at 25 “C. Water solutions require a pH 7 buffer and a color development period of 5 minutes at 25 “C. In order to analyze solutions of silver(1) in ethyl acetate, it is necessary to introduce the reagents in methanol and to dilute the ethyl acetate to a final concentration of 50% (v/v) ethyl acetate-methanol. Color development is then complete within 10 minutes. Solutions of silver(1) in toluene are handled by introducing the reagents in isopropanol, diluting with isopropanol and wzter to a final composition of toluene (50 vol %)-isopropanol (49.5 vol %)-water (0.5 vol Z), and allowing 1 minute for color development. VOL. 40, NO. 13, NOVEMBER 1968
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Table I. Spectrophotometric Data for the Product of Oxidative Coupling of m- and p-Phenylenediamine Produced by Silver Nitrate in Various Solvents €dye/4 liter Solvent A,, mp mol-' cm-l ABI~~L Isopropanol 615 8700 0.01 0.03 Methanol 605 6700 Acetone 605 6300 0.01 Water 550 4600 0.03 50% (v/v) Methanolwater 587 5900 0.02 50% (v/v) Ethyl acetatemethanol 605 7000 0.12 Toluene (50 vol %)-isopropanol (49.5 vol %)water (0.5 vol %) 610 6500 0.02
RESULTS AND DISCUSSION
Silver(1) reacts with equimolar mixtures of nz- and p-phenylenediamine to produce a dye which probably is structurally similar to indamine:
1
1
400
H 2 N a
+
H 2 N o N H 2 t 4Ag'
I
I
I
500
I
I
I
600
Wavelength, rnp
Figure 1. Absorption spectrum of the product of oxidative coupling of m- and p-phenylenediamine produced by 10-4M silver nitrate. Solvent: methanol \
"2
Although the dye is not easily isolated without polymerization, the above structure is strongly suggested by the similar reaction of toluene-2,4-diamine and N,N-dimethyl-p-phenylenediamine, which yields Toluylene Blue (6),
"* Photometric titrations confirm the stoichiometry of the reaction given above, and the sorption of the dye on cation exchangers is additional evidence that the structure is correct. The oxidative coupling of in- and p-phenylenediamine with silver(1) can be carried out conveniently in water, alcohols, acetone, and some mixed solvents. The spectrum of the dye in methanol is shown in Figure 1 ; the spectrophotometric data are summarized in Table I. The molar absorptivities of the dye in the various solvents are based upon the stoichiometry of the proposed reaction at a silver(1) concentration of 5 pg/ml. The calibration curves generally display good linearity u p to this concentration. At higher concentrations up to 10 ,ug/ml, slight deviations probably result from side reactions between silver(1) and the individual amines. This effect is minimal for isopropanol. In most of the solvents studied, color stability was excellent for periods up to one hour. However, the absorbances obtained for the acetone solutions began to decrease immediately and therefore were recorded within one minute after the final mixing. In water the absorption maxima and molar absorptivities vary reversibly within certain limits of pH. As the p H is increased from pH 2 to 11, a yellow-blue-yellow color ( 6 ) H. A. Lubs, "The Chemistry of Synthetic Dyes and Pigments," Reinhold, New York, N. Y., 1955, p 237.
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ANALYTICAL CHEMISTRY
change is observed with the maximal values of absorptivity and Xmax occurring at p H 6.5 to 7.0. Acetonitrile, toluene, dimethylformamide, ethyl acetate, and methylethyl ketone are unsatisfactory solvents because of slow or undesirable reactions. However, ethyl acetate and toluene containing silver(1) were incorporated into mixed solvent systems with no difficulty, and therefore it is possible to determine silver salts dissolved in these solvents. The oxidative coupling method is particularly useful for determining the solubilities of slightly soluble silver salts in the various solvents. For example, silver acetate was found to be soluble in methanol to the extent of 0.006, 0.009, and 0.013 gram/100 ml at 0, 23, and 50 "C, respectively. At 23 "C the solubilities of silver stearate in methanol, isopropanol, ethyl acetate, toluene, and acetone are 0.0023, 0.0040,
