By Michael Lowry
Author’s note: This past year (2018) in Ontario, the Health Protection and Promotion Act, regulation 565 for public pools, was revised. In this revision, the reliance on automated sensing devices for pH and free chlorine testing has put into question the importance of standard methods and current best practices of water analysis. In reading the revised regulations, some could interpret that manual testing for pH and free chlorine is only required once every 24-hour period, as the operator can record the display on the automatic sensing device. Now, the discussion about the reliability of automation is not mentioned in this article, what is discussed is the relevance of operators understanding the water chemistry and the testing of the parameters that make recreational water safe.
Most individuals in the pool and hot tub industry are familiar with water testing procedures used to maintain a properly balanced pool, but human error and interferences, as well as methods appealing to convenience can quickly turn a once simple test into a guessing game.
This article will look deeper into the mechanisms of testing water quality parameters and help to anticipate and remove interferences and errors when conducting water tests. By reviewing the procedures and principles behind water quality tests in the 21st edition of Standard Methods, pool and hot tub professionals can better understand the results of each test. The water parameters under review will include pH, total alkalinity, calcium hardness, cyanuric acid, and free and total chlorine.
pH: The influential parameter
In examining the testing procedures of recreational water, it is only natural to start with pH. Water is both an acid and a base because it ionizes to form hydrogen and hydroxide ions in solution. The term pH is used to quantify how acidic or basic a solution is. The scale ranges from one, being very acidic, to 14, which is very basic.
A pH meter is the most common way to measure this parameter in recreational water. Meters use specially designed electrodes to measure voltage, which is generated between the electrodes depending on pH levels. Results are displayed on the meter, which is calibrated directly in potentials of hydrogen.
Although not as precise as a meter, acid-base indicators are also used to measure pH by way of water-soluble organic dyes. These indicators are also dependent on pH levels, and are comprised of a weak acid that produces one colour in its acidic form and another in its basic form. Colour change occurs when the hydrogen ion (attached to the indicator molecule) transforms the molecule’s structure whereby its light absorption characteristics are different than the indicator molecule without the hydrogen ion.
For example, if there are more hydroxyl ions in the sample than hydrogen ions, causing the sample to be basic, the hydrogen ion from the indicator will be stripped. This leaves the device’s indicator molecule in a form that reveals the basic indicator.
When the pH of recreational water is tested, the assumption is the water is between a certain range, specifically 6.8 to 8.4. As there are different acid-base indicators used for different pH ranges, the indicator used for the purposes of recreational water is phenol red. The colour range when using phenol red is a graduated scale from yellow, when the sample is acidic, to red when the sample is basic.