March 29, 2019
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.
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.
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.
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.
In maintaining properly balanced water, calcium hardness is one factor that should not be ignored. Water will become aggressive if calcium hardness is too low. As a result, water in this aggressive state will try to equalize itself by drawing calcium from the shell of the pool, leading to concrete pitting. If calcium hardness is too high, calcium will start to precipitate out as scale in the form of calcium carbonate.
Nearly 100 per cent of total hardness includes both magnesium and calcium hardness. The amount of magnesium in the water has no effect on a pool’s concrete shell. Therefore, the calcium hardness test is specific to calcium and is measured in ppm of calcium carbonate.
To perform this test, sodium hydroxide is first added as a buffer to increase the pH of the water sample to 10. This removes the interference of any magnesium in the sample and produces a sharper endpoint. If too much buffer is added, the calcium may precipitate as calcium carbonate and lead to a false negative result. A small amount of organic dye is then added to the buffered sample as the indicator, which changes the colour to light red.
Ethylenediaminetetraacetic acid (EDTA) is used as the titrant in this test to form a chelated soluble complex with the calcium as the EDTA binds this up. When all the calcium has been complexed, the sample will turn blue from light red. The number of drops or amount of EDTA used to achieve the endpoint is converted to ppm of calcium carbonate.
The most common interference when conducting this test involves the presence of metal ions in the sample. Metal ions interfere by causing a fading or indistinguishable endpoint. This can be rectified by adding a few drops of EDTA titrant to the sample before the addition of the buffer and indicator. The amount of EDTA added before the test should then be included in the overall amount of titrant needed to achieve the endpoint. Another solution to a fading endpoint problem is to dilute the sample by half with deionized water and then multiply the end amount of EDTA used by two.
As another cautionary measure, the titration should be conducted at, or close to, room temperature. As the temperature of the sample approaches freezing, colour change becomes very slow; inversely, if the sample is hot, the indicator can decompose.
As mentioned earlier, cyanuric acid is used in outdoor swimming facilities to reduce the degradation of chlorine by ultraviolet light. The determination of pH is done by electronic or colourmetric means and total alkalinity and calcium hardness is determined by titration. Cyanuric acid uses a turbidimetric method whereby the reagent melamine reacts with the cyanuric acid to form a precipitate known as melamine cyanurate. This insoluble compound clouds the sample proportionately to the amount of cyanuric acid present.
Turbidity tests, in general, are notoriously subjective and are highly dependent upon the observer. Consider testing a standardized solution to calibrate the observer’s subjectivity.
In determining chlorine in recreational water, the differentiation of free chlorine and total chlorine is needed. Free chlorine is the sum of hypochlorous acid and the hypochlorite ion. With pH control, hypochlorous acid, the sanitation workhorse of chlorine, can be maximized in normal swimming pool conditions. Combined chlorine is a group of compounds formed by the reaction of free chlorine with inorganic nitrogen waste and other organic contaminants. Chloramines have little disinfecting capabilities relative to free chlorine and are known to contribute to health issues with bathers. Total chlorine is the sum of both free and combined chlorine.
Due to these facts, it is necessary to know the difference between free and total chlorine. Orthotolidine (OTO), which only reacts with total chlorine, is not used in the testing of commercial recreational water. The methods used are N, N-diethyl-p-phenylenediamine (DPD) and ferrous ammonium sulphate-N, N-diethyl-p-phenylenediamine (FAS-DPD), where one is a colourmetric test and the latter a titrimetric test.
The DPD method is the colourimetric version of the chlorine test, whereby free chlorine reacts with this indicator. When this indicator is added to the sample and free chlorine is present, a reaction occurs producing the colour red. The intensity of the colour is then compared to standardized colours to determine the amount of free chlorine. Using the same sample, potassium iodide is then added; if combined chlorine is present the intensity of the red will increase. This result is then compared to the standardized colours to determine total chlorine. A subtraction calculation is used to determine combined chlorine.
In the FAS-DPD test, DPD is used as the indicator, while FAS is used as the titrant to determine free and combined chlorine. When the DPD indicator is added to the sample—similar to the method above—the sample will turn red if free chlorine is present. The DPD indicator in this test is typically a powder and the sample size is smaller, but the same principle exists as the colourimetric DPD test.
FAS is used as a reducing agent and as the titrant is added, the original red will disappear. The endpoint occurs when the red has completely disappeared. Based on the amount of titrant added, the amount of free chlorine is determined. The subsequent potassium iodide is added, and again, if combined chlorine is present, the sample will turn red. Titrate this sample with FAS and the amount needed to eliminate the red is the endpoint, which can be recorded as combined chlorine.
High chlorine levels can also interfere with this test by bleaching the indicator and preventing red from developing. Diluting the sample with deionized water will help eliminate this interference. Be sure the end result is multiplied by the factor of the dilution.
If potassium monopersulfate is used as an oxidizer, the presence of the non-chlorine oxidizer, which reacts with potassium iodide, will produce a more intense red. This will lead to a false positive reading when testing for total chlorine using the DPD method and false positive reading when testing for combined chlorine using the FAS-DPD method.
Many test kit manufacturers have developed an amine-masking agent, which allows for proper chlorine determination in the presence of non-chlorine oxidizers.
By understanding the principles behind water quality tests, pool and spa professionals can recognize and eliminate interferences normally blamed on the reagents. The theory found in Standard Methods is an invaluable resource allowing the chemistry surrounding testing procedures to be easily understood.
Michael Lowry is an instructor with, and promoter of, the Lowry School of Pool & Spa Chemistry. His 25 years of experience range from servicing pools to selling commercial, industrial, and residential pool chemicals. In 2012, Lowry was recognized by the National Swimming Pool Foundation (NSPF) for exceptional performance at the group’s annual instructor meeting earning two distinguished instructor awards—one for the highest increase in certified pool operator (CPO) certifications in 2011 over 2010, and the second for the highest number of certifications outside the U.S. He can be reached via e-mail at firstname.lastname@example.org.
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