Determination of the point-of-zero, charge of manganese oxides with different methods including an improved salt titration method

Manganese (Mn) oxides are important components in soils and sediments. Points-of-zero charge (PZC) of three synthetic Mn oxides (birnessite, cryptomelane, and todorokite) were determined by using three classical techniques (potentiometric titration or PT, rapid PT or R-PT, and salt titration or ST) and a modified salt titration method with a prolonged equilibration time (ST method with a prolonged equilibration time [PST]). The same methods have been applied to goethite, which was used as a reference material. The PZC values of goethite obtained by PT and R-PT methods were both 7.95, and those by the ST and PST method were 8.16 and 8.30, respectively, for birnessite cryptomelane, and todorokite. The PT method yielded PZC of 1.18, 1.98, and 3.98, and the RPT method yielded 1.60, 2.11, and 3.47, respectively, for birnessite, cryptomelane, and todorokite. In contrast to goethite, there was no PZC found with the ST method, even when the types and concentrations of the added electrolytes changed. However, when after KCl addition the equilibration time was prolonged 28 h, the PZC of birnessite, cryptomelane, and todorokite could be found and were 0.97, 1.74, and 3.39, respectively. The fact that the normal ST method failed for the Mn oxides is due to their low PZC, because at this low pH value, the oxides may start to dissolve. Compared with PT and RPT methods, the PST method is reliable, simple, and convenient. The PST approach seems also suitable for other similar colloid systems.