3. Phosphate standard solution : Potassium dihydrogen phosphate, KH2PO4 (relative molecular mass 136.09), is dried in an oven at 110 ″c, then placed in a desiccator. Exactly lar mass 136.09 mg are dissolved in pure water to which has been added 0.2 ml of sulphuric acid (reagent 1) and made up to 100 ml. Stored cold in a glass bottle the solution is stable for months. This standard stock solution contains 10 mmol/L phosphate.