Parker et al. ( 9) have also determined the pKa of BCG in DMF, and report a value that is approximately 1.2 units lower than obtained in the present studies. These workers (9) give pKa values for acetic acid, benzoic acid (Table I), and p-nitrophenol (Table II) that are 1 to 2 units below the values obtained by other workers. However, Parker ( 14) has recently revised the value for p-nitrophenol in DMF to ca. 12.2, and has indicated that this revision will change the pKa values reported by Parker et al. ( 9) for other compounds.Salicylic Acid. Absorption measurements of BCG were made in salicylic acid-sodium salicylate buffers. The isosbestic point remained at 504 nm. Results are listed in Table VI. The linear plot of pcH against log [(A-Aa)/(Ab-A)] has least-squares slope and pcH intercept of 1.1 and 8.86, respectively.Juillard and Mallet (13) give 8.23 as the value of the pKa for salicylic acid in DMF. These workers used measurements based on picric acid, for which widely-differing pKa values have been reported (Table II). The concentration of picric acid in the salicylate buffers is not stated. Petrov and Umanskii (11) have shown that the pKa of picric acid in DMF is extremely concentration dependent.

The separation of copper(II) from eadmium(II) is usually a trouble spot in the scheme of qualitative analysis. Since copper(II) interferes with the identification of cadmium (II), most procedures suggest the use of some complexing agent. The cyanide method is potentially hazardous; the separation of copper(ll) by tartrate is seldom satisfactorily complete to allow a clear-cut identification of cadmium(II) as the yellow CdS.The procedure here suggested is based on the ob-406 JOURNAL OF CHEMICAL EDUCATION

A separation of arsenic as As(III) has been developed based upon the observation that As(III) is extracted selectively from hydrochloric acid solution of the common cations encountered in the usual qualitative analysis scheme.

Sir : In a recent publication, Abbott and Goliat (1) have shown that the polarographic reduction of 1,4-benzoquinone (quinone) can be used for the indirect determination of certain acids.In this study, the procedure of Abbott and Goliat was adapted, with minor modifications, to the analysis of boron in glass and colemanite. The boron content of these materials is usually determined by methods which utilize the titration of the mannitol-boric acid complex with standard base; however, the preliminary separation of boron from the hydrolyzable sample ions is always required in order to obtain sharp and accurate end points (8).The prior separation of boron is omitted in the procedure outlined in this paper. Support for by-passing this step is provided by the results of numerous preliminary tests and the analysis of both synthetic and standard samples.These results indicate that under the conditions and techniques of the proposed method, low concentrations of hydrolyzable ions in the polarographic solution do not adversely affect the boron evaluation. By circumventing this step much time is saved, and a possible source of error is avoided.About 20 minutes is required for a single determination.