The pH of water is usually tested by matching reagent colors against a colormetric standard. The reagent generally used for swimming pool water is phenol red, which has a pH range of 6.8 to 8.4 and a corresponding color range of yellow to red. Other reagents that are occasionally used for water pH analysis include bromothymol blue, with a pH sensitivity range of 6.0 to 7.6 and a color range of yellow to blue; and cresol red, with a pH sensitivity range of 7.2 to 8.8 and a color range of yellow to red. Knowing the pH of pool water is essential for properly controlling all the water chemistry parameters. Test pH at least daily, or 3 times a day when the disinfectant residual is checked. Confirm that the pH is within the desired 7.2-7.6 range. Take water samples from the pool for testing the pH, not from a pipe tap or in the equipment room. The pH can be measured either colormetrically or with an electric metering device. The colormetric method is the preferred method of analysis. Sodium thiosulfate is added to the sample to neutralize any chlorine-based residual, then a colored indicator solution—phenol red—is added. Use ONLY the reagent supplied by the manufacturer for testing purposes since the standards are calibrated for use with a specific test kit and may give inaccurate readings if used with another kit from a different company.
Testing pool water for pH, chlorine residual, total alkalinity, and calcium hardness is an important part of pool maintenance. Chemical reagents are added to a water sample that gives an indication of the water’s condition.
To Colormetrically Test the pH of the Water
1. Fill the viewing tube or cell of the test kit with pool water to the correct level marked on the tube. Some test kits require that a “comparator tube” also be filled for analysis.
2. Using the dropper provided, add sodium thiosulfate (chlorine neutralizer) into the sample to remove any residual chlorine. It is recommended to add one drop for every part per million of disinfectant level in the pool. For example, if the free available chlorine residual is 3.0 ppm, use 3 drops of sodium thiosulfate to neutralize the solution. Neutralizing the chlorine prevents the disinfectant from interfering with the phenol red indicator reaction. This is especially important when the disinfectant is bromine or the chlorine residual is excessively high.
3. Add a measured amount of the indicator solution to the sample. Usually 5 drops of indicator is used. Mix this solution by swirling the tube of sample water with the top or stopper on the cell. Do not contaminate the sample by placing your finger over the cell.
4. Compare the color of the sample test water to the standard on the test kit. Do not attempt to interpolate closer than midway between two standards or interpolate to a number of the scale. According to your state’s Public Swimming Pools Act, swimming pool water probably should be maintained between 7.2 to 8.0. However, a pH range of 7.2 to 7.6 is more practical from a management standpoint.