5872
WILBUR S I M O N AND LEROYEYRING
Vol. 76
the iron(II1) precipitated from solution, either preventing the appearance of a wave or causing the wave to be too small for analysis. At lower pH values, where no precipitation occurred, many of the waves followed so closely after the anodic 3.0 2.53 1.59 0.80 1.27 mercury dissolution wave that residual current 2.0 1.98 1.40 1.03 1.45 corrections could not be made accurately. I n 1.5 1.64 1.28 1.14 1.17 the cases where the reversibility test was applied, 1.0 1 ,3*5 1.16 1.25 1.45 the slopes of the plots of Ed.=.vs. 1?g (id - i) 0.70 1.25 1.12 1.36 1.52 i showed considerable variation, ranging from 0.05 .40 1,lO 1.05 1.42 1.49 to 0.085. Double waves, indicating the presence .lo" 1.04 1.02 1.44 1.47 of two complexes in sluggish equilibrium, were ob.05" 1.04 1.02 1.44 1.47 served in a few cases. Because of the failure t o establish with certainty the reversibility of the rea Ionic strength adjusted to 0.5 with XaC10,. ductions occurring in succinate solutions, detailed In the present case, for example, the diffusion presentation and interpretation of the data are not current is more than 15% lower in 1 M potassium warranted a t this time. However, the data obtained, such as they are, indicate the dissociation malonate solutions than in 0.1 M solutions. Succinate Solutions.-Previous experimen- constant for the iron(II1) complex to be on the and that of the iron(I1) complex to t e r ~ agree ~ ~ that , ~ any ~ ~complexes ~ ~ formed be- order of tween iron and succinate ions are much weaker be about unity. This latter value is of a similar than those involving oxalate or malonate ions. order of magnitude as that reported by Franke. It is apparent that the succinato ('?-ring) comFrankeli reports the dissociation constant of the disuccinatoferrate(I1) ion t o be approximately 13 plexes are considerably weaker than the malonato on the basis of solubility studies. Hydroxo- (6-ring) and oxalato (?&ring)complexes. Furthersuccinato complexes have been claimed to exist on more, succinic acid is a rather weak acid, so that in and also on the acidic solutions below pH 4 virtually no C4H404-the basis of preparative studies11$21 ions are present in solution. At higher pH values basis of breaks in potentiometric titration Polarographic studies were attempted with a t which the CaH404-- concentration becomes solutions ranging from PH 5.5 to 9.5 and from 0.1 appreciable, the large and weakly bound succinate to 4.0 111 potassium succinate. Of the more than groups tend to be replaced by OH- ions causing thirty solutions studied, the reversibility test could partial or complete hydrolysis of the iron (111). be applied to less than half the resulting polaro- This situation explains why several worker^'^,^^ grams. At the higher PH values all or most of have reported no noticeable complex formation (20) I,. Michaelis and E. Friedheim, J . B i d rhem., 91,343 (1931). occurring between iron and succinate ions. TABLE IV
I
