There are three concerns that can affect an operator’s analysis of pH when testing recreational water.
The first concern, which is often a common error, is using the proper light when comparing the water sample to the colour standards on the comparator apparatus. If natural light is not used, it will change the operator’s colour interpretation of the result.
The second concern occurs when extremes in pH of the sample cause the colour to be yellow or red. When using phenol red, any pH of 6.8 and below will be the same colour, yellow. Whether the water sample registers a pH of 6.8 or 4.8, the colour range for acidic solutions ends at yellow. The same occurs when red is generated, the sample could have a pH of 8.4 or 9.4; there is no additional colour change after the pH increases above 8.4.
The third concern is a true interference caused by high levels of chlorine reacting with the phenol red in the sample to form chlorphenol red, resulting in a purple colour. With this colour, operators may falsely assume the pH is high and take drastic measures to lower this level. Therefore, it is essential to recognize this interference and eliminate the possibility of high chlorine levels effecting test results. With high chlorine levels in water samples the purple colouration will appear darker than the red at the top of the pH scale. Chlorphenol red actually turns purple when the pH is above 6.6.
Total alkalinity (TA)
The alkalinity of water is its capacity to resist changes in pH when limited amounts of acid or base are added. Sodium bicarbonate is used to create a buffer to keep the pH of the water between recommended levels.
Total alkalinity is tested by the titration of sulfuric acid in a sample of water using a dual acid-base indicator, bromocresol green and methyl red. When this mixed indicator is added to the water sample, assuming some bicarbonate alkalinity is present, the sample will turn green. As the sulfuric acid titrant is added the pH of the sample decreases and the colour will change from green to red. When the sample turns pink, the endpoint has been achieved. The amount of the titrant used will give the buffering capacity in parts per million (ppm).
A closer look reveals this alkalinity test is a simple pH test. It is the amount of acid needed to decrease the pH of the sample to 4.5 where total alkalinity is zero causing the mixed pH indicator to turn red from its original green.
That said, there are a couple of interferences that should be discussed. First (as with the effects of high chlorine levels on phenol red in the testing of pH) high chlorine levels will also affect an alkalinity test.
Remember, the alkalinity indicator is a mixture of bromocresol green and methyl red. If chlorine levels are too high, the methyl red will become bleached and leave behind the bromocresol green. Bromocresol green is blue in the base form and yellow in the acid form. Simply stated, the starting point of the titration will be blue with an endpoint of yellow.
The second interference occurs in recreational water using cyanuric acid. Cyanuric acid is used in outdoor swimming facilities to reduce decomposition of chlorine by ultraviolet radiation. However, as cyanuric acid is a weak acid, it contributes to total alkalinity or the buffering capacity of the sample being tested. This addition increases the measured alkalinity giving a false reading. A correction factor of 33 per cent of the measured cyanuric acid must be subtracted from the measured alkalinity to reflect an accurate alkalinity reading. The 0.33 factor is used for recreational water in the recommended pH range. Cyanuric acid’s influence on alkalinity changes, however, if the pH of the water is outside the recommended range—with a factor of 0.11 at a pH of 6.5, to a factor of 0.36 at a pH of 8.0.