Coulometric Titration of Tetraphenylborate Ion with Electrogenerated Silver Ion Application to Determination of Potassium, Rubidium, and Cesium Gaston J. Patriarchel and James J. Lingane Department of Chemistry, Harvard University, Cambridge, Mass.
The coulometric titration of tetraphenylborate ion with electro-generated silver ion in aqueous acetone media has been investigated. In neutral media containing 35 to 50 volume % acetone, and 0.4M sodium nitrate as supporting electrolyte, the coulometric titration is stoichiometrically exact. It serves accurately to finish determinations of small quantities (0.3 to 10 mg) of potassium, rubidium, and cesium following precipitation as the tetraphenylborates, provided the washed precipitates are dissolved in acetone while still wet. When wet potassium tetraphenylborate i s dried, it undergoes hydrolytic decomposition, which decreases its tetraphenylborate content by several per cent.
PRECIPITATION WITH TETRAPHENYLBORATE ion has become a familiar method for the determination of potassium, rubidium, cesium, and a few other metals, and also certain amine bases and alkaloids. The field has been reviewed thoroughly by Flaschka and Barnard (1)and Renault (2). When small quantities are to be determined, a titrimetric, rather than gravimetric, finish is most convenient. Among, the several titrimetric methods that have been proposed for the determination of tetraphenylborate ion, titration with silver ion, using various means of end point detection, is most common. The washed precipitates are dissolved in a watermiscible organic solvent (usually acetone), and the solution is then diluted with water, prior to titration. However, doubt has been cast on the stoichiometry of the precipitation of silver tetraphenylborate. Crane (3) concluded that the titration of tetraphenylborate ion with a standard silver nitrate solution (in an acetic acid-sodium acetate buffer in 50 acetone with potentiometric equivalence point detection) was in error by about - 5 % relative, and therefore he recommended empirical standardization of the silver nitrate titrant solution against potassium tetraphenylborate. Crane (3) implied that this large negative error was caused by a significant solubility of the silver tetraphenylborate in the titration medium, whereas Kirsten et a/. (4) concluded Permanent address, Institut de Pharmacie, UniversitC Libre de Bruxelles, 50, Ave. F. D. Roosevelt, Bruxelles 5, Belgium.
(1) H. Flaschka and J. Barnard, Jr., “Advances in Analytical Chemistry and Instrumentation,” Vol. 1, Interscience, New York, 1960. (2) J. Renault, in “Mise au Point de Chimie Analytique Pure et AppliauCe et d’Analyse Bromatologique,” 66me SCrie, J. A. - . . Ga&i&, Paris, 1958. (3) F. E. Crane, Jr.. Anal. Clzim. Acta, 16, 370 (1957). . .
