pH of cell sap is maintained by

The pH of cell sap is maintained by Calcium (Ca). It is related to differences in cation and anion absorption. The relative proportion of calcium ions to calcium also varies, with organic acids, such as malate and citrate, chelating calcium ions in xylem sap.
Cell sap is a fluid found in the vacuoles (small cavities) of the living cell. It contains variable amounts of food and waste materials, inorganic salts and nitrogenous compounds. The pH of cell sap is around 5.5 thus these are acidic.