Honing your pool-water testing skills
The pH is among the most important of parameters to test after the disinfectant concentration. It affects both bather comfort as well as the strength of the sanitizer in the water. It plays a strong role in whether the water may be scale forming or corrosive.
According to the Model Aquatic Health Code, for commercial aquatic venues, the pH should be tested prior to opening the pool to the public. For venues using a manual disinfectant feed system, the pH should also be tested every two hours that the pool is open. For venues using an automatic disinfectant feed system, the interval required is every four hours while open.
For residential pools, the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends testing the pH daily. The service tech should test the pH at each visit.
The pH may be tested colorimetrically, with test strips, or with an electronic meter.
In a colorimetric pH test, a pH indicator is added to the sample. It is an organic dye that changes color based on the acidity of the water. There are many types of indicator dyes that exhibit color changes over a variety of pH differences.
For swimming pools, the most common indicator dye for measuring pH is phenol red. This dye is chosen because it undergoes a reaction at pH levels relevant in the pool environment (6.8-8.2). At a lower pH, the dye exists in a form that makes it appear yellow in color. At a higher pH, the dye converts to a different form, which makes it appear purple. At a neutral pH, the dye exists in equilibrium between the two forms, and it appears orange.
It should be noted that high chlorine or bromine concentrations can interfere with colorimetric pH results. High concentrations of chlorine or bromine can turn the color to purple. If this happens, an accurate pH reading cannot be obtained. The addition of one to three drops of a chlorine inhibitor, such as sodium thiosulfate, can eliminate the interference. Most phenol red reagents have an inhibitor that will neutralize up to 10 ppm chlorine and 20 ppm bromine. However, some of these inhibitors can cause a small shift in the pH. Combination inhibitors avoid this pH shift, so it is best to use pH reagents with a combination chlorine/bromine inhibitor that minimizes pH shifts. If possible, it is best to wait for the chlorine or bromine levels to drop before testing the pH.
Most phenol red pH indicator reagents have a pH of 7.5 – 7.6. This can result in an interference when the alkalinity is very low (50 ppm or less), because low alkalinity will allow small changes in the pH of the water sample. Addition of the phenol red indicator causes the pH of the sample to shift to the pH of the phenol red indicator.
If a test indicates the alkalinity is very low and the pH is close to ideal, the pH may be wrong.
One other thing to watch out for is when the test result is at the top or bottom of the pH range where the indicator no longer changes color with variation in pH. The resulting test color at pH 8.4 is the same as the color at pH 9.0 or 9.5. The test color at pH 6.8 is the same as the color at pH 6.0 or 5.5. So the results can be over or under range when at the top or bottom of the range, respectively, without any indication of how far over or under range.
Also, if test strips are being used, be sure to use strips designed for pool and spa testing. Multi-pad pH strips for laboratory use will not work in low buffer pool and spa water. The correct reading could take up to 10 minutes with this type of pH test strip.
Electronic meters are also available for pH testing. They have an electrode that gives a digital readout when they are submerged in a water sample. pH meters are not affected by chlorine, but it is important to calibrate pH meters regularly to ensure accuracy. This is done with a pH buffer. pH probes should also be serviced and cleaned regularly in accordance with the manufacturer’s direction.
Total alkalinity is a measure of the alkaline materials that are dissolved in the water, which affects the water’s capacity to resist fluctuations in pH.
Maintaining the total alkalinity depends on keeping the concentration of calcium carbonate, CaCO3, within the recommended range.
Total alkalinity should be maintained between a minimum of 60 ppm and a maximum of 180 ppm as CaCO3. Ideally, where electrolytic chlorine generators, calcium hypochlorite, lithium hypochlorite and sodium hypochlorite are used, total alkalinity should be maintained between 80 and 100 ppm as CaCO3, because these sanitizers cause the pH to rise. Where sodium dichlor, trichlor, chlorine gas and bromine are used, the ideal range is between 100 and 120 ppm as CaCO3, because these sanitizers will cause the pH to drift downwards.
When the alkalinity is too low, the pH may be seen to seesaw from one extreme to the other. That’s because water with low total alkalinity has little buffering capacity, and the pH of this water can easily be changed by adding acidic or basic chemicals. Low total alkalinity can cause corrosion as well as bather discomfort.
On the other hand, high total alkalinity can lead to scale and cloudy water. High total alkalinity can also make it difficult to change the pH.
According to the Model Aquatic Health Code, for commercial aquatic venues, alkalinity should be tested weekly.
For residential pools, the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends that the service tech should test the alkalinity at each visit.
There are two types of alkalinity tests, titration and colorimetric. The more accurate test is the titration. However, high chlorine and bromine levels can cause the endpoint color of the titration to be yellow, or colorless, instead of pink. Addition of a chlorine inhibitor, such as sodium thiosulfate, before the titration will neutralize the chlorine or bromine and give the correct endpoint color.
Many algaecides contain quats (quaternary ammonium cations) or polyquats. High levels of these can cause low readings in colorimetric alkalinity tests. Inhibitors are added to minimize this interference but over dosing with these algaecides can cause errors in alkalinity readings. Very high levels of biguanide have also been known to cause a similar interference.
The alkalinity titration measures total alkalinity, which mainly includes carbonate, bicarbonate, cyanurates, and borates. In pools, only alkalinity due to carbonate and bicarbonate is important for balanced pool water and the saturation index. This is why it is critical to subtract out contributions from cyanuric acid and borates if used.
Cyanuric Acid Correction
Cyanuric acid makes up part of total alkalinity, a fact that is frequently overlooked. This must be accounted for in the ultimate analysis. That is because the relevant part of total alkalinity is the carbonate alkalinity — the part of the alkalinity that causes either scale or corrosion.
If the cyanurate portion of the total alkalinity is not subtracted out, the pool operator may falsely believe that the alkalinity is within an acceptable range when actually the alkalinity is too low.
Smart phone apps and/or websites that do not require a cyanuric acid
Table 1. Cyanuric Acid Correction Factors as a function of pH.
measurement input are not accounting for this contribution to the total alkalinity.
In this case, after the total alkalinity has been determined, calculate the carbonate alkalinity by subtracting out the cyanurate portion. The cyanuric acid level is not subtracted directly from the total alkalinity reading. Instead, a percentage is subtracted. The cyanuric acid reading is first multiplied by a correction factor and then subtracted from the total alkalinity reading. That multiplier is pH dependent. See accompanying table above. Borate Correction
Borates, which are also sometimes used in swimming pools, have a similar effect.
Smart phone apps and/or websites that do not require a borate measurement input are not accounting for this contribution to the total alkalinity.
The borate correction factor is pH dependent. The borate correction factors may be found in the accompanying table. To obtain the carbonate alkalinity from the measured total alkalinity in the presence of borates, the following procedure can be used.
1. Select a borate factor based on the measured pH: F(B) 2. Calculate the carbonate alkalinity from the measured total alkalinity, the measured borate concentration (B), and the selected borate factor (F(B))
Alkalinity corrected = Alkalinity total – (B x F(B))
For example, if the measured total alkalinity is 100 ppm, the pH is 7.6, and the measured borate concentration is 50 ppm, then: Alkalinity corrected = 100 – (50 x 0.17) Alkalinity corrected = 92 ppm
The absolute most important item to test in any recreational aquatic venue — be it a pool, spa, or splash pad — is the sanitizer level. Recreational aquatic venues that do not maintain a sanitizer residual at all times pose a serious health risk, which can result in sickness and even death. So if you are testing nothing else, do test the sanitizer level at every visit.
The current Model Aquatic Health Code requires that aquatic venues not using cyanuric acid must maintain a minimum free available chlorine concentration (FAC) of 1.0 ppm. Those using cyanuric acid must maintain a minimum FAC concentration of 2.0 ppm. Spas must maintain a minimum FAC concentration of 3.0 ppm. The maximum allowed FAC when bathers are present is 10.0 ppm.
For aquatic venues using bromine, the minimum bromine concentrations must be maintained at all times in all areas as follows:
• All aquatic venues: 3.0 ppm.
• Spas: 4.00 ppm.
•Maximum Bromine Concentrations must not exceed 8.0 ppm at any time the aquatic venue is open to bathers.
For commercial aquatic venues, sanitizer levels must be tested prior to opening the pool to the public. For venues using a manual disinfectant feed system, the sanitizer must also be tested every two hours that the pool is open. For venues using an automatic disinfectant feed system, the interval required is every four hours while open.
For residential pools, the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends testing sanitizers daily. The service tech should test the sanitizer at each visit.
Chlorine may be tested in a variety of ways, which include colorimetric, titrimetric, and electronic testing. Colorimetric testing includes such using methods as using OTO, DPD, and test strips and comparing the color of the sample to a reference. Titrimetric testing for chlorine is generally the FAS-DPD method. Electronic testing, such as ORP, provides an idea of the oxidative strength of the water, but not how much sanitizer is actually in the water.
The most common interference in chlorine and bromine colorimetric testing with DPD is bleaching due to high levels of chlorine or bromine. What happens is that excess chlorine or bromine reacts with pink DPD to form a colorless DPD compound. The bleaching results in lower readings compared with the true concentration of chlorine or bromine. To minimize this bleaching reaction, an excess of DPD reagent is needed. Bleaching starts to interfere between 5 and 10 ppm and becomes significant above 10 ppm chlorine. With bromine, the interference starts above 10 ppm and is significant above 20 ppm. If bleaching is suspected, add extra DPD, reduce the sample size, or do a dilution.
Combined chlorine will interfere in the DPD test for free chlorine if the reading is not taken within 30 seconds. This will result in high readings. If the free chlorine reading cannot be taken immediately, Steadifac can be added to freeze the free chlorine reading. Steadifac is 0.25% thioacetamide. Steadifac is also useful in preventing high readings from oxidized manganese. Another thing that can cause combined chlorine to show up in the free chlorine is residue from DPD3 reagent. This reagent speeds the reaction of DPD with combined chlorine. Any residue of DPD3 on testing equipment can cause a problem in testing for free chlorine. So after testing for total chlorine, immediately wash test equipment.
Potassium monopersulfate (MPS or potassium peroxymonosulfate) is a non-chlorine oxidizer that will interfere in the total chlorine DPD test. Treating recreational water with MPS to remove bather waste can result in false high combined chlorine or total chlorine readings. To prevent this interference:
• Test free chlorine before adding MPS.
• After adding MPS, wait at least 12 hours before testing free chlorine.
• Use a test kit specified for MPS-treated water.
There are two important reasons to know a pool’s cyanuric acid levels. The first is that cyanuric acid moderates chlorine’s strength: more chlorine may be necessary to perform the same operations at high cyanuric acid levels. The second is that cyanuric acid is unfortunately measured in the total alkalinity test and must be subtracted out from the test result to get the necessary alkalinity information.
Cyanuric acid is most commonly tested with a turbidity test. This test uses a chemical reaction where the water sample becomes cloudy in proportion to the original concentration of cyanuric acid.
The latest Model Aquatic Health Code requires testing cyanuric acid on a monthly basis for commercial aquatic venues.
For residential pools, the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends that the service tech test cyanuric acid monthly.
When testing for cyanuric acid with a melamine-based turbidity test, the most significant interference is water temperature. High temperatures, above 90 °F, can result in readings as much as 15 ppm low. Low temperatures, below 60 °F, can result in readings that are 15 ppm high. The ideal temperature is about 75 °F.
A problem with cyanuric acid test strips is that they are sensitive to the pH of the water. For the best result, adjust the water pH to the ideal range, 7.4 to 7.6, before testing for cyanuric acid.
Calcium hardness a measure of the calcium concentration of the water, expressed as ppm calcium carbonate. It affects both the clarity of the water, as well as whether the water will be scale forming or corrosive to surfaces.
The ideal range for calcium is recommended to be within 150 to 250 ppm, although the acceptable range extends to 1,000 ppm.
Experts recommend a lower level (100 to 800 ppm) for spas because hot water promotes scale.
The latest Model Aquatic Health Code and the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends testing calcium hardness on a monthly basis for both commercial and residential aquatic venues.
Calcium hardness is usually tested by titration.
A buffer and indicator reagent are added to a sample of water, and the solution is swirled to mix. The solution will turn red in the presence of calcium compounds. Next, a calcium hardness reagent (EDTA) is added, and every drop is counted as the solution is swirled. When the solution turns blue, the endpoint has been reached. The number of drops is then multiplied by an equivalence factor and recorded in parts per million calcium carbonate.
A high level of metals, such as copper or iron, is the most common interference in calcium hardness titrations. Normally the endpoint of a hardness titration is a color change from red to blue. If the color change is red to purple, a high level of copper is usu- ally the cause. The addition of metal sequestering agent to the pool or spa can minimize this problem. Many manufacturers provide directions for avoiding such metal interference.
Metals in water exist in three major forms: insoluble (metal oxides and hydroxides), free, and complexed. Test methods used to determine the concentration of metals in pool water may not measure insoluble metals and, depending upon whether the method measures free or complexed dissolved metals, may exhibit interferences from chelators and sequestering agents.
Iron (Fe) Test Methods: Most total iron test methods require the reduction of iron to its ferrous form. Because a reducing agent is used in these tests, typical concentrations of chelators and sequestering agents may not interfere with these total iron tests. Elevated levels of these chemicals in the pool or spa water, however, may interfere with the iron test, resulting in an inaccurate lower value. Additionally, depending upon the reducing agent used, insoluble suspended iron such as rust may or may not be reduced to the ferrous form of iron and be measured. Consult the manufacturer’s test instructions to determine if insoluble iron is measured by the test method.
Copper (Cu) Test Methods: Most copper test methods measure free unsequestered copper. Typical concentrations of chelators or sequestering agents interfere with these tests, resulting in an inaccurate lower value for total copper. To determine the concentration of total dissolved copper (both free and sequestered), the use of a total copper test procedure is required. Consult the manufacturer’s test instructions to determine if the test method used measures free copper or total copper.
Colorimeters are becoming more common for running water tests. The main advantage of colorimeters is that they give an objective measurement of water tests. They also allow color-blind people to make accurate measurements. According to the National Eye Institute, as many as 8 percent of men and 0.5 percent of women with Northern European ancestry have the common form of red-green color blindness.
Colorimeters work by measuring the passage of light through a sample. Anything that interferes with the passage of light through the sample will interfere in the test. This includes scratches, dirt, stains, fingerprints, and water droplets on tubes or in the reaction chamber. Always clean the reaction tube or cell and rinse with distilled or deionized water immediately after use. Follow manufacturer’s instructions for maintenance.
Portable electrochemical sensors and probes can also be subject to interferences. There are few interferences in pool water for pH probe measurements, other than dirty or poorly maintained probes. The interferences for pH testing with reagents mentioned above can be avoided with a pH probe. Calibrate probes on a regular basis with calibration solutions as recommended by the manufacturer. Rinse probes with distilled or deionized water before and after use. Most pH probes need to be stored in a storage solution or moist environment.
Salt and TDS meters are really conductivity meters with special calibrations. They measure any conductive ions in the water, including sodium, chloride, calcium, magnesium, sulfate and others. The interferences for salt meters are other ions and dirty and poorly maintained probes. Rinse these probes with distilled or deionized water after use and store dry. Do not allow fingerprints or other residues to remain on the probe.
The main interference for ORP probes is a change in pH. Another interference may be elevated cyanuric acid levels. As the pH drops, the ORP reading can increase, and as pH increases, the ORP reading can drop. Dirty probes are also a problem. Rinse probes with distilled or deionized water and store in a moist environment. In all cases, follow manufacturer’s instructions for cleaning and maintenance.