From your 1.00 x 10-3 M phosphate solution, prepare 100 mL each of five standard solutions: 2.00 x 10-5, 5.00 x 10-5, 1.00 x 10-4, 2.00 x 10-4, and 5.00 x 10-4 M. These will serve as your analytical standards, which means that their phosphate concentrations must be precisely known.