ANALYTICAL EDITION
JULY 15,1935
Literature Cited Fischer, H., Wiss. Verdfentlieh. Siemens-Konwn, 4, 158 (1925); 2. angew. Chem., 42, 1025 (1929). ;2) Fischer, H., and Leopoldi, G., Wiss. Verofentlich. SiemensKonzern, 12, 44 (1933). :3) Samuel, B. L., and Shockey, H. H., J. Assoc. 0ficial Agr. Chem., 17, 141 (1934).
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(4) Vorhes, F. A,, Jr., and Clifford, P. A., Ibid., 17, 130 (1934). (5) Wichmann, H. J., and Clifford, P. A., Ibid., 17, 123 (1934). (6) Wichmann, H. J., and T'orhes, F. A., Jr., Ibid., 17, 119 (1934). RECEIVED May 31, 1935. Presented before the Division of Physical and Inorganic Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 t o 14, 1934. Published with the permimion of the director of the Experiment Station as Journal Article 212 (n. 8.).
Oxidation-Reduction Indicators I. Diphenylbenzidine Sulfonic Acid L. A. SARVER AND WM. VON FISCHER School of Chemistry, University of Minnesota, Minneapolis, Minn.
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T HAS already been pointed out ( 2 ) that diphenylbenzidine disulfonic acid is an intermediate product in the oxidation of diphenylamine sulfonic acid to its violet colored holoquinoid form, and that it should possess all the advantages of the latter indicator, yet require a much smaller correction when used with dilute oxidizing solutions. The authors, accordingly, have tried a number of possible means of synthesis; but only the direct sulfonation of diphenylbenzidine gave any promise of success. This reaction proceeds vigorously, and one or more of the poly-sulfonic acids may be easily obtained.
is not necessary for the present purpose. Presumably, several different poly-sulfonic acids might be produced under different experimental conditions. STOCKSOLUTION OF INDICATOR. A 0.1 per cent solution of the sodium diphenylbenzidine sulfonate in water is recommended; this can be kept for several months, but not indefinitely. One drop or even less of this solution is satisfactory for titrations with 0.01 or 0.001 N solutions; 10 drops are preferable when 0.1 N solutions are used.
Prt:paration of Sodium Diphenylbenzidine Sulfonate
As might be expected, diphenylbenzidine sulfonic acid strongly resembles diphenylamine sulfonic acid in most of its properties. When small amounts are oxidized in 0.5 to 1.0 N sulfuric acid solution, the pale yellow becomes first deep yellow, then green, and finally violet. Upon reduction, the color changes are reversed. It works perfectly in the presence of tungstate, and its color development is extremely sensitive to the catalytic effect of traces of ferrous iron, when dichromate is used as the oxidant. In such a case, backtitration with ferrous solution, after treatment with an excess of oxidant, is more reliable; the error involved by even 10 minutes' contact with a reasonable excess of dichromate is negligible. For accurate microanalysis, the indicator corrections should be determined experimentally, but they are approximately equal to the volumes of indicator used, in terms of a 0.001 N solution. ABSORPTION SPECTRA. When diphenylbenzidine sulfonic acid is fully oxidized by dichromate, permanganate, or ceric sulfate, reddish violets are produced which are stable, but which fade after a long time. Bromine yields blues which fade very rapidly. The half-oxidized green form, on the other hand, is extremely stable, and does not precipitate out on standing, as was the case with that resulting from diphenylamine or diphenylbenzidine. Absorption curves were determined with a Keuffel and Esser color analyzer, the violet form giving one similar to that for diphenylamine sulfonic acid violet (a), with a maximum a t 5600 A,, while the green has a minimum a t this point, and absorbs strongly a t both ends of the visible spectrum.
One gram of dry powdered diphenylbenzidine, prepared as described in a recent paper ( I ) , is added little by little t o 10 ml. of 60 per cent fuming sulfuric acid in a shallow dish over a period of about 2 minutes. The mixture heats up, but should not be allowed to become warmer than 40" C . As complete solution as possible is obtained by stirring and mashing up any lumps with a rod; then, after not more than 3 minutes, it is poured onto ice. Difficulty in controlling the reaction makes it undesirable to work with larger quantities, but the products of as many portions as desired may be combined at this point, and diluted t o a volume of 200 ml. for each gram of diphenylbenzidine used. The holution is then heated t o boiling, and the green color reduced t o brown with a stream of sulfur dioxide; the excess of the latter is removed by boiling, and any charred material filtered off. The filtrate is carefully neutralized with sodium carbonate, diluting if an precipitation occurs, and the sodium sulfate rendered insolubye by the addition of four volumes of ethyl alcohol; the sodium diphenylbenzidine sulfonate is appreciably soluble in 80 per cent alcohol, while all but an extremely small amount of the sodium sulfate is precipitated. After filtering, the residue should be redissolved in water, and reprecipitated with alcohol as often as necessary t o secure complete removal of the yellow color; the total volume of the combined filtrates is usually about 5 liters for each gram of diphenylbenzidine used. The alcohol is boiled off rapidly, and the evaporation completed on a steam bath, avoiding long exposure or overheating of the residue, which should be a light yellow. The yield is 3 to 4 grams per gram of diphenylbenzidine used.
Composition of Sulfonated Product Microscopic examination showed no appreciable quantity of sodium sulfate to be present; moreover, the known insolubility of this salt in 80 per cent alcohol precludes such a possibility. Chemical analysis indicated that ten sulfonate groups had been added in most cases; products prepared with 25 per cent fuming sulfuric acid contained four sulfonate groups. Exact information as to the positions of the substituted groups in the molecule would be difficult to obtain, and
Indicator Properties
Literature Cited (1) Sarver, L. A., and Johnson, J. H.,
J. Am. Chem. SOC.,57, 329 (1935). (2) Sarver, L. A., and Kolthoff, I. M., Ibid., 53, 2902 (1931). March 25, 1935. RECEIVED
