1. Measure 0.25M Na2S2O3 for amount of 5cm3 into 50 cm3 beaker. Then place the beaker on cross-mark white paper
2. Add 2M HCI for amount of 5 cm3 into the beaker containing sodium thiosulfate in step 1. And then start to time counting. Completely stir the solution until cross-mark on white paper could not be seen and then stop timer immediately. Record the consumed time.