Effects of selected cations and anions on pK‐0.5pCa values for a Mn oxide system

We studied the relative influences of Mn 2+ , Al 3+ NO 3 − and SO 4 2− on the K‐Ca relationship, as expressed by a pK‐0.5pCa value, in a Mn oxide suspension system using a double indicator electrode technique. With an increase in the quantity of solution Mn 2+ and Al 3+ in the system, Mn 2+ ions showed a stronger effect than Al 3+ on increasing pK‐0.5pCa values, though this was not the case when the amount of the cations added was low. The stronger effect of Mn 2+ on the K‐Ca relationship was attributed to stronger specific adsorption of the Mn 2+ ion on the Mn oxide as compared with Al 3+ . When K + and Ca 2+ were added after the addition of Mn 2+ and Al 3+ , activities of K and Ca in equilibrium solution were higher than those when K + and Ca 2+ were added first, indicating that the exchange reaction of the cations on the Mn oxide was not fully reversible. Compared to NO 3 − added systems, the pK‐0.5pCa values for SO 4 2− added systems were lower when the initial K/Ca molar ratios were 0.5 and 2. This effect of SO 4 2− on the pK‐0.5pCa values was also related to the quantity of Ca ions in the system since no significant difference in pK‐0.5pCa values was observed between the SO 4 2− and NO 3 − added systems when the K/Ca molar ratio was 8. The present results revealed that both cationic and anionic environments had a profound influence on the adsorption of K and Ca and, thus, the pK‐0.5pCa values in a Mn oxide system. The findings of this study help attain a better understanding of the chemistry of K in soils, particularly those with variable charge.