Dissociation constants and pH-titration curves at constant ionic strength from electrometric titrations in cells without liquid junction: titrations of formic acid and acetic acid

Abstract

An improved method for obtaining the titration curves of monobasic acids is outlined. The sample, 0.005 mole of the sodium salt of the weak acid, is dissolver! in 100 ml of a 0.05-m solution of sodium chloride and titrated electrometrically with an acid-salt mixture in a hydrogen-silver-chloride cell without liquid junction. The acid-salt mixture has the composition: nitric acid, 0.1 m; pot assium nitrate, 0.05 m; sodium chloride, 0.05 m. The titration therefor e is performed in a. medium of constant chloride concentration and of practically unchanging ionic-str ength (1'=0.1). The calculations of pH values and of dissociation constants from the emf values are outlined. The tit ration curves and dissociation constants of form ic acid and of acetic acid at 25 0 C were obtained by this method. The pK values (negative logarit hms of the d issociation constants) were found to be 3.742 and 4. 754, respectively. CONTENTS Page I. Tntrod uction __ _____ ~ __ _______. ______ __ ______ ____ ________________ 347 II. Discussion of the titrat ion metbod __ __ ___ ______ _______ ______ ______ _ 348 1. Ti t;at~on. clU,:es a t constant ionic strength from cells without ltqUld JunctlOlL-_-__ _-__ __ ____ ____ _____ __ _____ ____ __ _ 349 2. Dissociation constant s ____ _____ ___ ____________ ______ ______ _ 350 III. Dissociation constants of formic and acet ic acids at 25° C _____ ___ ____ 352 1. M aterials __ ____ __ __ __. ____ ___ ____ __ __ ___ _______ ___ _____ ___ 352 2. Proccdure ___________________ ________________ _____________ 353 3. Resul ts and calculations __ _________ __________ _______ __ _____ 355 IV. R efer ences _____ ______ ______ _______ ________ ___________ ________ ___ 359