The solution to be tested must be at a pH of 0.5 in order for this test to be accurate. This pH measure can be accomplished using concentrated Hydrochloric acid (muriatic acid) to acidify the test solution, and measured with the pH 0-1.5 test strip.
Hydrochloric acid should be available at a local hardware store or pool/spa store, as well as at any chemical supply distributors, such as Fisher Scientific or VWR.