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 electro motive-force m ethod of t he di ssoc iat ion constant of an acid as strong as t he first ac idi c g roup of phospho ric acid r eq uires t hat the hydrogen-ion concen t rat ion be estab li s hed wi t h greater accuracy than is usually attainable. The un cer tain quantity is t he activ ity coe ffi cie nt of hydrochloric acid in the particular mixture und e r st ud y. It is shown th at the calculation of this coefficient by t he Debye-Hi.ickel eq uat ion , which afford s a n adequate estimate of the sma ll hydroge n-ion co rrection in buffer solut io ns com posed of weak acids, fa il s to y ield the desired accuracy for mixtures in whi ch the acid is dissocia ted to an a pprec iable exte nt. In order to determine t he common logari thm of t he first di ssociation co ns tan t of phos phoric acid wi t h an acc uracy be lie ved to be 0 .005 uni t, the hydrogen-io n concentra tion of some of t he buffer mixtlll'es s tud ied was kept at lo w levels by use of a la rge excess of potassium dihydrogen phos phate. Ot her so lu tions contain ed formic acid in stead of hyd roc hlo ric acid , the reby lowerin g the hyd rogen-ion concentration st ill further. In addit ion , p a r t icu lar attention was g ive n to a cho ice of reaso nable valu es fo r t he activity coefficient used to es t im ate t he hyd roge n-io n cOl'J'ection .