Manganese oxidation products were precipitated in an aerated open-aqueous system where a continuous influx of mixed Mn²⁺ and Cd²⁺ solution was supplied and pH was maintained with an automated pH-stat adding dilute NaOH. X-ray diffraction and electron diffraction identified the solids produced as mixtures of Cd₂Mn₃⁴⁺O₈, Mn²⁺₂Mn⁴⁺₃O₈, MnO₂ (ramsdellite), and CdCO₃. Mean oxidation numbers of the total precipitated Mn as great as 3.6 were reached during titrations. During subsequent aging in solution, oxidation numbers between 3.8 and 3.9 were reached in some precipitates in less than 40 days. Conditional oxidation rate constants calculated from a crystal-growth equation applied to titration data showed the overall precipitation rate, without considering manganese oxidation state in the precipitate, was increased by a factor of ~4 to ~7 when the mole ratio (Cd/Mn + Cd) of cadmium in the feed solution was 0.40 compared with rate constants for hausmannite (Mn²⁺Mn₂³⁺O₄ precipitation under similar conditions but without accessory metals. Kinetic experiments were made to test effects of various Cd/Mn + Cd mole ratios and rates of addition of the feed solution, different temperatures from 5.0 to 35°C, and pH from 8.0 to 9.0. Oxidation rates were slower when the Cd mole ratio was less than 0.40. The rate increased by a factor of ~10 when pH was raised one-half unit. The effect of temperature on the rate constants was also substantial, but the meaning of this is uncertain because the rate of formation of Mn⁴⁺ oxide in the absence of Cd or other accessory metals was too slow to be measurable in titration experiments.

The increased rate of Mn⁴⁺ oxide formation in the presence of Cd²⁺ can be ascribed to the formation of a labile adsorbed intermediate, CdMn₂O_{4 Int}, an analog of hausmannite, formed on precipitate surfaces at the beginning of the oxidation process. The increased lability of this structure, resulting from coordination-chemical behavior of Cd₂₊ during the titration, causes a rapid second-stage rearrangement and facilitates disproportionation of the Mn³⁺ ions. The Mn²⁺ ions thus released provide a positive feedback mechanism that couples the two steps of the conversion of Mn²⁺ to Mn⁴⁺ more closely than is possible when other metal ions besides manganese are not present. During aging of precipitates in contact with solutions, proportions of Cd₂Mn₃O₈ and MnO₂ increased at the expense of other precipitate components.