Potassium persulphate solution : Dilute 5 mL of sulphuric acid (4.5 mol/L : reagent 1 of the phosphate determination in section) with water to 100 mL . Add and dissolve 5 g of potassium peroxodisulphate, K2S2O8. Store this solution at room temperature in a polyethylene bottle protected from direct sunlight. the reagent is stable for about one week.