Phenolphthalein is a clear liquid that is commonly used to determine the pH level of soap and test for excess lye. Phenolphthalein will react by turning pink or fuschia in the presence of excess lye, with the degree of pink indicating how alkaline your soap is. If a drop of solution applied to the middle of a soap cut turns deep pink or fuchsia, the soap should not be used on the skin, but could be used as a house or laundry soap. If the solution remains clear or turns the lightest shade of transparent pink, the soap should be safe for the skin. Phenolphthalein can also be used to test liquid soap and cream soap.