Mixtures of citric acid (H3Ci) and Fe(NO3)3 with variable molar ratios (H3Ci/Fe(NO3)3 from 0 to 2.55) were titrated with NaOH. The change in pH and the occurring inflection points were registered with a potentiograph (Fig. 1). For each inflection point (I.P.) the existing H3Ci/Fe(NO3)3 molar ratio was plotted against the NaOH/Fe(NO3)3 molar ratio which existed at the corresponding inflection point (at most three I.P. for each titration curve). The coefficients ej, fj and gj for each dehydrated
Fe-Ci-complex [Fefj(OH)gj-3ejCicj](gj-3fj)⁻ with j = 1, 2, . . . ,
could be calculated from the points of intersection (Pj) in the x-y-diagram (Fig. 2). Thus, the following three Fe-Ci-complexes were calculated:
From the 1st I.P. [Fe10(OH)18Ci5]3- with its maximum at pH ≃ 4.3.
From the 2nd I.P. [Fe5(OH)9Ci4]6- with its maximum at pH ≃ 8.0.
From the 3rd I.P. [Fe4(OH)11Ci]2- with its maximum at pH ≃ 10.1.
The range of their existence as functions of H3Ci/Fe(NO3)3 ratios (Fig. 3 and 4) and pH (Fig. 4) is discussed.