Next comes water balance, and the two important tests for it are total alkalinity (TA) and pH. Service professionals address TA first because it is the buffer for pH. Primarily, it acts as a buffer to prevent pH from decreasing while also slowing its increase. The TA test is a titrimetric test which uses differing reagents. An initial reagent is added, which appears as a green colour, then a titrant reagent is added one drop at a time until the colour changes to red. The drops used are counted as a calibration number, usually up to 10, to determine the results.
To obtain a true carbonate alkalinity reading, any amount of cyanuric acid (CYA) must be determined and considered as one-third of the TA reading. For example, a CYA reading of 60 ppm should be divided by three. The resulting number is then subtracted from the TA test, and the result is the true carbonate alkalinity. For example, if a TA test results in 120 ppm and the CYA comes out as 60 ppm, then one-third of 60 ppm is 20 ppm. Subtracting the TA of 120 ppm minus CYA 20 ppm determines a carbonate alkalinity of 100 ppm.
A pH test is a measurement of the presence or lack of hydrogen in water. Hydrogen lowers the pH, and it comes from the addition of muriatic acid—also known as hydrochloric acid (HCl). It is important to note the “H,” which stands for hydrogen. Every time acid is added, hydrogen is increasing, which means the pH is decreasing. Lack of hydrogen causes the pH to increase.
One way to decrease hydrogen is by aerating the water. Forcing air into the pool water causes carbon dioxide (CO2) to off gas and leave rapidly. When this happens, more hydrogen is consumed, and the pH goes up.
The most common pH test is performed by using phenol red reagent and comparing the colour to a standard from 8.0, down to 7.0. This is a fairly accurate test; however, it is impossible to determine if pH is below 7.0 or above 8.0. Further, false readings can occur if chlorine levels are greater than 10 ppm, and there may be a false high pH purple colour. Some technicians add one drop of sodium thiosulfate (Na2S2O3) if the chlorine test high. A digital pH tester may prove to be a better investment as it will measure exact pH readings and colour comparison is not necessary.
