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 DebyeHi.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 .By t hese m eans t he first di ssociatio n con stant, Ie , of phosphoric acid was calculated from e mf measure ments of 7] phosphate buffer sol ut io ns, togeth er ",ith supplementary data from th e lit e rature foJ' 18 add itional so lu tio n . The result from 0° to 60° C is g iven by -log [(1 = 79 ; ; 31 __ 1.5535 + 0.013486 T , \\'h ere T is th e temperat ure in °IC Th e changes of heat content, e ntl'Op.v , a lld heat capac it y foJ' thc di ssociat ion of 1 mo le of aqueous phospho ri c acid in t he sta nd a rd state were fOlilid to be -7,650 j, -66.8 j deg-I , and -154 j c1 eg-l , res pective ly , at 25° C. Introdu ctionExperience has proved Lhat Lhe electromo tiveforce m ethod is well sui ted to the accura te determination of the dissociation constants of many weak acids and bases in aqueous solu tion . However, it is not gen erally r ecogniz ed that tbe accuracy obtainable by this method diminishes with increasing strength of the acid or base. I'Yh en approximately 10 percent or more of a weak acid is dissociated in solution, th e uncertain ty in establi shing th e tru e concen trations of the ionic and molecular species becom es so large that an exact determination of the dissociation con stant is no 'longer possible.Som e years ago th e author undertook to m easure the electromo tive force of a series of cells of the type Pt; H 2 (g, 1 atm) , KH2P04 (xm), H CI (m) AgCI; Ag (Cell 1) where m is molality, in an attemp t Lo extend the investigation of N ims [l jl Lo oLh er temperatures and buffer ratios. It was found to b e impossible to obtain an accura te value for th e first dissociation constant of phosphoric acid by analysis of t h e data for mixtur...

Deuterium isotope effect on the dissociation of weak acids in water and deuterium oxide

“…As pKI=2.148 in ordinary water [36], IlpK = 0.214, 0.125, or 0.234, depending on which of these assumptions about the activity coefficient terms is chosen.…”

Formation and growth of clusters in conventional and new kinds of simulated body fluids

“…It has been reported that phosphate ions form ion pairs with Ca 2+ as shown in the reactions (11) and (12), 29 where phosphate ions are involved in the equilibrium reactions (8) to (10). 30,31 When HPO 2− 4 was excluded from r-SBF (p-r-SBF), the actual Ca 2+ concentration in the fluid increased. The increased Ca 2+ concentration accelerates the nucleation rate of calcite by increasing the ionic activity product of the fluid for calcite.…”

“…Derivative r-SBFs excluding either HCO The −log (IP) for hydroxyapatite 24 and calcite 30 are 117.0 and 8.6, respectively. Superscript a, b, and c indicate supersaturation, saturation and undersaturation, respectively, with respect to the corresponding phases.…”

Effect of Citric Acid Clearance on the Saturation with Respect to Hydroxyapatite in Saliva

“…A and B were set to 0.5242 and 0.3318, respectively [Bates, 1964], Values used for a, were 9 for IP , 6 for Ca2' and 4 for P 0 43" [Kielland, 1937]. The dissociation constants employed for phosphoric acid were 6.22 • 10 ' [Bates, 1951], 6.58 ■ 10 s Aeree, 1945 and6.61 • 10 13 [Bjcrrum andUnmack, 1929]. The degree of saturation was expressed as the difference between the logarithms o f the ion product and the solubility product constant for hydroxyapatite.…”