Reagents used for spectrophotometric method is methyl orange solution with
concentration 5, 10 and 15 mg/L using distilled water as a solvent. Reagents used in
volumetric method are potassium iodide solution with concentration 15, 20 and 25 g/L. The
potassium iodide were freshly made from potassium iodide crystal with phosphate buffer pH
7 that consisted of 895 mL Na2HPO4 25,127 g/L and 1 L KH2PO4 13,508 g/L.