Acidic dissociation constant of ammonium ion at 0 to 50 C, and the base strength of ammonia

Abstract

The acidic dissociation of many acids has been studied in detail, but weak bases have genera lly been neglected because of t he experimental difficulties encountered in the investigation of some of their buffer solutions by electromotiye-force methods. These difficulties may arise from volatility of the free base or the existence of extraneous electrode reactions such as the formation of ammine complexes at the s ilver-silver-chloride electrode. In th is study of ammonia, special saturators were used to prevent, insofar as possible, t he removal of ammonia from the solutions by the hydrogen gas. A co rrection was applied for the amou nt of diam-mine silver complex formed at the sil ver-silver-ch loride electrode, a nd t his silver ion was prevented by mechanical means from reaching the platinum electrodes. With these precautions, the acidic dissociation constant of ammonium ion at 0° to 50° C was determined f rom electromotive-force measurements on 19 b uffer solutions containing equal molalities of ammonia and ammonium chloride. The changes of free e nergy, heat content , entropy, and heat capacity that accompany the dissociation processes in the standard state were derived from the temperatu re coefficients of the acidic dissociation constant of ammonium ion and of the basic dissociation constant of ammonia. The activity coefficient o[ ammonium chloride in equ imolal buffer solutions at 25° C was calcu lated.