A New Indicator for pH 11 to 12 ROD O’CONNOR, WILLIAM ROSENBROOK, Jr., and GARY ANDERSON Department of Chemistry, Montana State College, Bozeman, Mont.
b Observation of the color change produced when an acetone solution of the potassium salt of the p-nitrophenylhydrazone of benzaldehyde was poured into water has led to elucidation of the chemical change involved. Benzaldehyde p-nitrophenylhydrazone exists in the yellow hydrazone form a t p H 11.3 or below, while at higher p H the proton is removed from the nitrogen, producing a red anion. The red color predominates in solutions of p H 1 1.7. Thus, a solution of benzaldehyde p-nitrophenylhydrazone or its potassium salt provides an indicator for the pH range 1 1 to 12.
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of the pnitrophenylhydrazone of benzaldehyde, prepared by the method of Cuisa and Rastelli (S), was investigated as a possible water titrant since acetone solutions of the salt changed from red to yellow on addition of water. However, the reaction of the salt with water was found to be pH dependent, and not irreversible. The salt was then studied as a possible pH indicator, as the intense yellow color a t p H 11 or below contrasted sharply with the red color a t pH 12 and above. Concentrations of the salt necessary for sharp color change were low enough so that the pH of the solutions tested was not altered appreciably by addition of the indicator. HE POTASSIUM SALT
PROCEDURE
A solution of 6 grams per liter of the potassium salt of the p-nitrophenyl-
hydrazone of benzaldehyde in anhydrous methanol was used for the initial studies. One drop of this solution was required for optimum color production in 100 ml. of aqueous buffer solutions, and it produced less than 0.02 of a pH unit change in the reading. The Beckman Model H-2 pH meter was standardized using a pHydrion buffer of p H 8.0 (Micro Essential Laboratories, Brooklyn, N. Y.). Solutions of pH 9, 10, 11, 12, and 13 were prepared with aqueous KOH, and 100-ml. portions of each were tested with 1 drop of the indicator solution. The indicator produced a definite yellow color ,,A,( = 415 mp) a t pH 11 and below, and a definite red color (Amx = 515 mp) a t pH 12 and above. Potassium hydroxide solutions were prepared for the range 11.0 to 12.5, varying by 0.1 pH unit. The yellow color predominated to pH 11.3 and the red color above p H 11.7. The colors remained constant for about 30 minutes, but the red colors of solutions below pH 12.3 gradually faded with time as the hydrazone precipitated. As was expected, a solution of the p-nitrophenylhydrazone of benzaldehyde of equivalent concentration may be substituted for that of the potassium salt with identical results. A methanol solution, 15 grams per liter, of the N-methyl-pnitrophenylhydrazone of benzaldehyde, repared by the method of Cuisa and stelli (S), produced no red color until pH 14.
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DISCUSSION
Apparently the colors obtained in basic solution of the p-nitrophenylhydrazone of benzaldehyde result, pri-
manly, from removal of the hydrogen from nitrogen by base (1, 4). Thus, the hydrazone form (I) is yellow in solution and the anion (11) is red. This production of the red color does not appear to involve reaction of the nitro-group itself with base ( 2 ) . This is shown by the absence of color formation by the N-methyl-p-nitrophenylhydrazone (111) until a thousandfold increase in pH is obtained. The pK. for the conversion of I to 11 has been calculated from the pH data to be a p proximately 11.5. H
CHa
LITERATURE CITED
(1) Bohlmann, F., Chem. Ber. 84, 490 (1951). (2) Bost, R. W., Nicholson, F., IND. ENQ.CHEM.,ANAL.ED. 7, 190 (1935). (3) Cuisa, R., Rastelli, G., Garz. Chim. Ital. 52, 11, 125 (1922). (4) Ragno, M., Ibid., 75, 193 (1945).
RECEIVED for review April 10, 1961. Accepted May 10, 1961.
Analysis of Products from the Electrolytic Oxidation of Acetate Ion in Acetonitrile SIR: As reported by Geske (8), the oxidation of acetate ion at platinum electrodes in acetonitrile solution occurs a t electrode potentials low enough so that no significant oxidation of the solvent interferes. This fact was discovered independently in these laboratories in 1960 and is being used to advantage in B chronopotentiometric study of the kinetics of the electron1282
ANALYTICAL CHEMISTRY
transfer process in the electro-oddation of carboxylate salts. To establish the principal reaction taking place and to make certain that acetonitrile serves as an inert solvent, an anode product analysis was undertaken for the electrolysis of a solution of tetrabutylmnmonium acetate in acetonitrile with tetraethylammonium perchlorate as the supporting electro-
lyte. Because of the more general interest of these analytical results, they are presented in this preliminary report. A discussion of the electrochemical kinetics of the oxidation of carboxylate salts will be presented later. EXPERIMENTAL
Reagents. The tetrabutylammonium acetate used was in the form of
