Let’s say you’re testing the free chlorine content of 1% bleach, which has a pH of 11.8. If the pH is reduced to 8, for example, will the colorimetric response of the Extra High Level Chlorine test strip for this 1% bleach change, too? Yes, it does appear that pH will influence the result.

• A 1% (10,000 ppm) hypochlorite standard was prepared from ACS hypochlorite (11.75%). The pH of the 1% solution was 11.5. This was tested with the 0-10,000 Extra High Level Chlorine test strip. The color matched the 10,000 color block very well.

• The pH of the 1% standard was lowered by adding 9 drops of concentrated HCL. The pH was 8.05. The test strip read 7,500 ppm; lower than the non-adjusted solution.

• The pH 8.05, 1% solution was diluted 1:1 with water. Theoretically, this should be 5,000 ppm. When tested with the strips, it read much closer to the 2,500 ppm color block. The pH of the diluted solution was 8.10.

• A 0.1% (1,000 ppm) standard was prepared from the original 1% standard. The pH was 10.3. The test strip results matched the 1,000 ppm color very well.

• The pH of the 1,000 ppm standard was adjusted lower by adding one drop of concentrated HCl. The pH was 8.1. The test strip read lower than 1,000 ppm.

In summary, it appears that lowering the pH to about 8 will depress the chlorine test strip result by about one color block.