Ion-pair formation by some alkali, alkaline-earth, transition-metal, and lanthanide acetates in water at 25" has been studied by addition of inorganic salts to sodium acetate-acetic acid solutions. The corresponding pHs were obtained to f0.002 unit by using the pK of acetic acid to derive the standard potential of the cell before adding the salts. The answers are expressed as dissociation constants at zero ionic strength.CELLS such as ours,(A = anion, M = cation, Y = chloride, nitrate, or perchlorate) , have been used extensively to derive the ion-pair constants of MA, MA,, etc. The general procedure of using standard buffers to calibrate the pH meter, then measuring the pHs of fixed mixtures of m2 and m3 to which varying amounts of m, are added, may err in that the electrodes must be washed free from buffer before they are applied to the test solutions. This was avoided by obtaining the potential E from m1 + m2 alone and the pH calculated from (Stockholm conventions 1)(1) log K(HA) = -pH + log [A-] + l og! , log [HA] * I n this Paper, concentrations denoted m, and those in the Tables, are molarities. A represents acetate ion.

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H2-AgCl, glass-calomel and glass-AgC1 electrode cells have been tested to estimate the dissociation constants at zero ionic strength of several ytterbium ion-pairs. The cells were designed so stock solutions could be added and the standard electrode potentials evaluated from the dissociation constants of the ion-pairing acids. Intercomparisons indicate that the uncertainties in the primary dissociation constants of YbAc2+, YbMal+ and YbSO: are about f5 %.