First dissociation constant of phosphoric acid from 0-degrees-C to 60-degrees-C; Limitations of the electromotive force method for moderately strong acids

Abstract

An accurate det.ermination by t he electromotive-force method of the dissoc iat ion constant of an acid as strong as t he first acidi c group of phospho ric acid req uires t hat the hydrogen-ion concen t rat ion be estab lished wi t h greater accuracy than is usually attainable. The un cer tain quantity is t he activ ity coe ffi cient of hydrochloric acid in the particular mixture unde r st udy. It is shown that the calculation of this coefficient by t he Debye- Hi.ickel eq uation , which afford s a n adequate estimate of the small hydroge n-ion co rrection in buffer solut ions composed of weak acids, fa il s to y ield the desired accuracy for mixtures in which the acid is dissocia ted to an apprec iable extent. In order to determine t he common logari thm of t he first di ssociation co nstan t of phosphoric acid wi t h an acc uracy be lieved to be ± 0.005 uni t, the hydrogen-ion concentra tion of some of t he buffer mixtlll'es stud ied was kept at low levels by use of a la rge excess of potassium dihydrogen phos phate. Other solu tions contain ed formic acid in stead of hyd rochlo ric acid , thereby lowerin g the hyd rogen-ion concentration st ill further. In addition , pa r t icular attention was g iven to a cho ice of reason- able values fo r t he activity coefficient used to estim ate the hyd rogen- ion cOl'J'ection . By t hese means the first dissociation constant, Ie , of phosphoric acid was calculated from emf measurements of 7] phosphate buffer sol ut ions, together ",ith supplementary data from the lit e rature foJ' 18 add itional solu tio n . The result from 0° to 60° C is g iven by - log [(1 = 79; ;31 __ 1.5535 + 0.013486 T, \\'here T is th e temperature in °IC The changes of heat content, e ntl'Op.v, a lld heat capac it y foJ' thc dissociation of 1 mole of aqueous phospho ri c acid in t he standa rd state were fOlilid to be - 7,650 j , - 66.8 j deg- I , and - 154 j c1 eg-l , respective ly, at 25° C. r