In connection with a potentiometric investigation of the complex formation between itaconate and various lanthanide ions an accurate determination of the dissociation constants of itaconic acid, under the conditions of ionic strength (I = 1 in NaC104) and temperature (25Β°C) used in the complex formation, was required. By employing this appropriate ionic medium it is possible to control the activity coefficients of the various species participating in the reaction, and the various liquid junction potentials a.
The dissociation constants of itaconic acid have already been investigated at different ionic strengths or temperatures 2-4, but none of these determinations refers to the authors' conditions. In the present paper a potentiometric investigation and calculation procedure of the dissociation constants of itaconic acid are discussed and constants reported over a certain concentration range of itaconate in water.
CALCULATION PROCEDURE
Several methods are reported in the literature for calculating the dissociation constants of dibasic acids 5-9. The treatment of the specific data depends upon the ratio of the dissociation constants. With a favourable ratio (> 1000) the acid is generally treated as a mixture of monobasic acids. In all other cases, such as for itaconic acid, graphical treatment of the equations is necessary. As this is a laborious and time-consuming work, especially for polybasic acids, and as computers have become more readily available, Dunning and Martin 9 have reported a general computational method for mono-and polybasic acids and mixtures of acids, based upon the least squares method. Thus it becomes possible to calculate dissociation constants without making simplifying assumptions, and with full consideration of all available data.
In a later paper, Thun et al. 1Β° reported a computer version of Fronaeus' graphical integration method for calculating stability constants of complexes. The author indicated that there had already been some attempts to calculate the dissociation constants of dibasic acids with the aid of this new computer method, which gave entirely good agreement with other methods. Moreover, in this kind of problem, this method seems to enable individual constants to be calculated more rapidly than with the least squares ,treatment, without losing the advantages of the latter. If one considers the undissociated acid form H2L of a dibasic acid as a complex of the anion L 2 -, acting as central group, and protons, acting as ligands, the following relationships