The reaction of the Fluoride test strip is very dependent upon the acidity level of the solution to be tested. Before testing, the solution has to be acidified to a pH ≤ 0.5. For acidification, we recommend hydrochloric acid (muriatic acid), preferably concentrated (1-2-3 M) depending on the rough presumption of the fluoride concentration in the test sample because in the process of acidification, the solution is always more or less diluted with added HCl (dilution has to be noted and the result is then multiplied with the dilution factor).
For example, 1M HCl has pH about 0.1
0.1M HCl has pH about 1.09
If a high concentration of fluoride is predicted in the sample to be tested, then 1M HCl can be used for acidification, added to the sample in the ratio, 2 parts of 1M HCl and 1 part sample, meaning the sample is 3 times diluted and the result must be multiplied by 3. For example, if the result is read by comparing the test pad with the color chart at about 10ppm, then the actual result is 10 x 3 = 30ppm.
If a lower concentration of fluoride is predicted in the sample to be tested, then more concentrated HCl should be used to avoid a higher dilution than necessary. The final pH of the sample before testing should be between 0 and 0.5. After acidifying the solution, the test strips are easy to use. Simply dip the strip into the pH adjusted solution for one second, remove, and compare the strip to the included color chart after 10 seconds.
Hydrochloric Acid (muriatic acid) should be available at a local hardware or pool/spa store, as well as at any chemical supply distributors, such as Fisher Scientific or VWR.