Constants of slight soluble acids. Determination by Coulometry

The flowing of constant direct current allows planning a coulometric method to determine solubility and acidity constants of slight soluble acids. Coulometry and electromotive force measurements are applied to obtain such parameters for eight bile acids in solutions without formation of micellar aggregates. This method can be applied to little soluble acids without complicated elaboration of the experimental data. This communication is applying it to cholic (HCol, (3α,7α,12α- trihydroxycholanic), glycocholic (HGC), deoxycholic (HDC, 3α,12α- dihydroxycholanic), glycodeoxycholic (HGDC), chenodeoxycholic (HCDC, 3α-7α dihydroxycholanic), ursodeoxycholic (HUDC, 3α,7β-dihydroxycholanic), lythocholic (HLC, 3α-hydroxycholanic) and dehydrocholic (HDHC, 3α,7α,12α-trihyketholanic) acids. Solubility and acid constants are determined at 25°C and in constant ionic medium 1.00, 0.50 and 0.15 mol dm-3 NaCl. The values determined in 0.15 mol dm-3 NaCl are directly applicable (in physiologic conditions). Also the others have thermodynamic values. The method of the ionic medium [1], minimizing the variation of the reagent activity coefficients in spite of the change of their concentration, allows to substitute activities with concentrations. To determinate solubility, s, an excess of each acid is added to an aqueous solution of the selected ionic medium stirring until equilibrium was reached. The saturated solutions are filtered (Solutions S’) and potentiometrically titrated by means of the following galvanic cell: R.E. /Solution S’ / G.E. (I) The titration of solution S’ is carried out coulometrically. The equivalence point was appreciated applying a modification to the method proposed by Gran [2]. The equilibrium: HA ⇔ H+ + A-, is defined by the constant: k = cH cA /cHA (1). The free concentration cHA in equilibrium with the corresponding solid is constant (i.e. HA solubility, s). Eq. (1) can be written: cHA k = K’ = cH cA (2), where K’ is constant. Electromotive force (e.m.f.) measurements of the galvanic cell: R.E. /solution S/ G.E., (II) provides cH. The solution was gradually alkalinised generating constant current by coulometry. The e.m.f. measurement and the electricity quantity allow to calculate K’of eq. (2) for each addition of current. Combining K’ values with solubility, s, acid constants k for each HA are calculated. [1] G. Biedermann and L.G. Sillèn, Arkiv Kemi, 5, 1953, 425. [2] G. Gran, Analyst, 77, 1952, 661.