Fill the large tube to the top solid line with pool water from a depth of 18 inches. Add one drop of solution No. 4 and mix by gently swirling. This solution is sodium thiosulfate, a chlorine neutralizer. Add five drops of Solution 2, a phenol red indicator, and mix by gently swirling. Compare the color with the pH color standards on the plastic tester. Warning: don’t perform this test if the chlorine residual is above 3.0.