To demonstrate the applicability of cyclic derivative voltammetry to the measurement of rate constants for rapid coupled chemical reactions, the oxidation of ascorbic acid followed by a rapid hydration reaction was studied. This system has been studied at a variety of pH's (13). The conditions chosen for study here were a p l l of 7.2 in phosphate buffer at ambient room temperature. The rate constant for the succeeding chemical reaction was obtained from the measured ratio of the derivative anodic and cathodic peaks obtained at a scan (13) S . P. Perone and W. J. Kretlow, ANAL.CHEM.,38, 1760
(1966).
rate of 413 volts/second. The value measured for this rate constant was 1.3 X l o 3second-'. This is in good agrec ment with values obtained previously with conventional readout (13). RECEIVED for review April 29, 1966. Accepted December 28, 1966. Work presented in part at the 152nd National Meeting, ACS, New York, September 1966. Work supported in part by the National Science Foundation, Grant No, GP6131. Partial support was also received from Public Health Service Grant No. CA-07773 from the National Cancer Institute.
Acid-Base Indicator Constants in Acetonitrile I. M. Kolthoff, M. K , Chantooni, Jr., and Sadhana Bhowmik School of Chemistry, Uniaersity of Minnesota, Minneapolis, Minn. 55455 The indicator constisnts PK'HI in acetonitrile (AN) of sulfonated para-substituted phenylazonaphthol, neutral red (one PK'BI of 6.0 and the other of 15.6 corresponding to color ch(anges from blue to red and red to yellow, respectively) tropeolin 00, methyl orange, methyl red, azo violet, p-naphtholbenzein (one pK'HI of 6.8 and the other Df 23.4) and phenolphthalein have been determined spectrophotometrically at 2 5 O C. Table Ill lists spectrophotometric characteristics and pKdH1 of all the indiicators studied so far in AN, the group covering a ~ K " Hrange I from 2 to 30. Limitations for paH determination of indicators of various charge types are discussed. The dissociation constants of diethylbarbituric acid and phenol have been determined, pK%\ values are 23.4 and 27.2 and KfHA2- values of 4 X 101 and 1.1 X LO4, respectively, at 2 5 O C.
In the past 20 years probably more than a thousand papers on acid-base titrations in nonaqueous solvents have been published. The best and most complete source of information on the progress in this field is the Biennial Reviews of ANALYTICAL CHEMISTR~'. Most of the work is being done potentiometrically, usually with the glass electrode as pH electrode. Other electrodes like antimony, platinum, and others have been fount1 useful in many instances. In considerations of the acic strength it always has been tacitly assumed that the change of the glass electrode potential with paH corresponds to the theoretical value. Interesting semiquantitative information has been published on the relation between the half-neutralization potential (HNP) in several aprotic solvents and pKdHIin water. However, quantitative information on acid-base equilibria and determination of paH in aprotic solvents in still lacking. In many publications, indicators have been recommended for acid-base titrations. When the constant(s) determining the equilibrium of the system titrated iind the pKdm values are known, the most suitable indicators can be selected for acid-base titrations. Also paH can be determined spectrophotometrically when pKdrrIis known. In the last several years studies have been carried out in this laboratory on acid-base equilibria and dissociation constants of acids in acetonitrile (AN). It has been found ( I ) that the (1) I. M. Kolthoff and M .K. Chantooni, Jr., J . Am. Chem. SOC., 87, 4428 (1965).
glass electrode in this solvent responds reversibly to paH. Dissociation constants of a large number of acids, including several indicators have been obtained from conductometric, potentiometric, and spectrophotometric measurements. In the present paper the constants of several indicators, not yet studied previously have been determined. In mixtures of weak acids and their tetraalkylammonium salts, the paH of which were measured with the glass electrode, the ratio of
[I1I denoting the alkaline form and HI the acid form of the
[HI]' indicator, was determined spectrophotometrically. A plot
Dlfi
was then made of log __ cs. paH, f denoting the activity [HIlfHI coefficient. The ionic strength was always kept small and the limiting Debye-Huckel expression: - log f = 1.512*
