Determination of Standard pH Values for Potassium Hydrogenphthalate Reference Buffer Solutions in 10, 20, 50 64 and 84.2 wt per cent Methanol/Water Mixed Sovents at Temperatures from 283.15 to 313.15 K

Standard pH0 values for 0.05 mol kg-1 potassium phthalate reference buffer solutions in 10, 20, 50, 64 and 84.2 wt % methanol/water solvent mixtures have been obtained from cells without transport, in the temperature range 283.15-313.15 K. The consistency of results has been analysed by a new method of multilinear regression of the quantity p(aHγCl) as a function of both temperature and solution composition. The pH° values found can be reproduced within ± 0.01 by the equation pH0 = 4.00 + 4.38x - 5.02x2 + 4.23x3 + 0.13z - 0.91xz, where x = mole fraction of methanol in the solvent mixture, z = (T - θ)/θ and θ = 298.15 K. Subsidiary values of the first ionization constant of o-phthalic acid in the above solvent mixtures have also been obtained by the buffered cell method.