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Alkali-metal and calcium(ti) complexes of monocarboxylate A -(acetate and salicylate), dicarboxylate A2-(malonate, maleate, succinate, malate, tartrate, phthalate, and oxydiacetate), or amino acid HA (glycine or L-histidine) ligands have been studied potentiometrically, using a glasssaturated calomel electrode, at different temperatures and ionic strengths. The monocarboxylate ligands form [MA] and the dicarboxylates [MA] and [ M (HA)] species (charges omitted) with both alkali metals and calcium. Glycine and L-histidine form [MA] + and [M(HA)]*+ {and [M(H2A)I3+ for L-histidine} complexes with Ca*+, whilst alkali metals form only [M(HA)]+ with glycine. Some interesting regularities in the formation constants are pointed out. From the dependence on temperature of formation constants, values of AH* and AS* have been determined. The function log fl = f ( 1 ) has been carefully studied in the range 0.02 < / < 1 mol dm-3. The reliability of a new model for the dependence on ionic strength of formation constants (when dealing with weak complexes it is in practice impossible to use the constant ionic medium method) is widely discussed. Two methods of calculation are described and the more general method has been checked by simulated curves.Recently, an interesting lecture by Bjerrum ' was based on the question: 'How far is it possible to determine small stability constants in aqueous solution?' This question arises from the uncertainties in determining activity coefficients when working with high and varying concentrations. In the last five years some of us have been concerned with the study of weak alkali 2-9 and alkaline-earth ' O-' metal complexes of low molecular weight * Acetate and suainate: experimental data and calculations from refs. 4, 12, 14, and 15; Na' and Caz+ complex formation constants were checked by means of four titration curves per system: previously reported values were fully confirmed. Malate, tartrate, oxydiacetate, Lhistidinate, and salicylate: new experimental data and calculations (salicylic acid was considered as monoprotic, pH < 4). Malonate, maleate, and phthalate: experimental data from refs. 4, 12, and 13; calculations repeated.

The hydrolysis of (CH 3 ) 2 Sn 2+ was studied potentiometrically, in different aqueous media (sodium and tetramethylammonium chlorides, sodium nitrate, sodium perchlorate, and sodium sulfate), in a wide range of ionic strength, at t ) 25°C. Least squares calculations are consistent with the formation of the species [M(OH)] + , [M(OH) 2 ] 0 , [M(OH) 3 ] -, [M 2 (OH) 2 ] 2+ , and [M 2 (OH) 3 ] + , with M ) (CH 3 ) 2 Sn 2+ . The dependence on ionic strength for different salt solutions was taken into account by using a Debye-Hü ckel type equation. Medium effects were explained by considering the formation of the chloride and sulfate complexes, [(CH 3 ) 2 SnCl] + , [(CH 3 ) 2 SnCl 2 ] 0 , [(CH 3 ) 2 Sn(OH)Cl] 0 , [(CH 3 ) 2 Sn(OH) 2 Cl] -, [(CH 3 ) 2 Sn(SO 4 )] 0 , [(CH 3 ) 2 Sn(SO 4 ) 2 ] 2-, [(CH 3 ) 2 Sn(OH)(SO 4 )] -, and [(CH 3 ) 2 Sn(OH) 2 (SO 4 )] 2-. Thermodynamic hydrolysis constants of (CH 3 ) 2 Sn 2+ and formation constants of the complex species with chloride and sulfate ions are reported.