While it is not necessary to calculate a saturation index to maintain water chemistry, many service techs still use it to know if the water is balanced.
Many of today’s service techs use target values instead, which they determine through frequent testing.
Chlorine can be tested in a variety of ways, which include colorimetric, titrimetric, and electronic testing. Calorimetric 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.
Today, most pool service technicians use some form of DPD testing (N,Ndiethyl- p-phenylenediamine) to determine chlorine content.
In the first step of a DPD colorimetric test, DPD 1 and DPD 2 (which are sometime combined, depending on the manufacturer) are added to a specified volume of water. This includes both the DPD reagent as well as a phosphate buffer to bring the sample to a desired pH. It reacts with the free chlorine in the sample to produce a pink to magenta color, depending on the concentration of free chlorine. Comparing the produced color with a reference standard gives the free chlorine concentration.
In the second step, DPD 3 is then added to the same sample cell. DPD 3 is a form of potassium iodide, which further reacts with combined chlorine to produce a further color change. Comparing the produced color with a reference standard gives the total chlorine concentration.
The free chlorine can then be subtracted from the total chlorine to get the combined chlorine content.
FAS-DPD titration testing does not use a color comparator and is the more accurate of the standard chlorine tests. It is a count-the-drops test that has higher resolution than the standard DPD colorimetry test, where it can measure free and combined chlorine as low as 0.2 ppm and as high as 20 ppm..
In this test, DPD is again added to a fixed volume of sample, which produces a pink color in the presence of free chlorine. Next ferrous ammonium sulfate (FAS) is added drop by drop while counting the drops and swirling which turns the solution clear. The number of drops to reach clear is multiplied by a factor that corresponds to the free chlorine.
In the second step, an iodide is added to the sample which reacts with the combined chlorine to again produce a pink color. FAS is then added drop by drop while counting the drops and swirling which turns the solution clear. The number of drops to reach clear is multiplied by a factor that corresponds to the combined chlorine.
After the disinfectant concentration, pH is among the most important of parameters to test. It plays a role in whether the water may be scale forming or corrosive. 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 the pH levels that are 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, appearing orange.
Test strips are also available that exhibit color changes that correspond to pH values.
High chlorine levels (over 10) can discolor pH test strips or the indicator dye in water samples. Chlorine can react with phenol red to make a dark purple solution at pH values above 6.7. It’s easy to mistake this for elevated pH’s. To avoid this error, the sample can be pretreated with 1-3 drops of sodium thiosulfate solution to remove the chlorine. This may not be effective if the chlorine is exceptionally high. Alternately, use an indicator dye that already contains an inhibitor. Be sure to read manufacturer instructions.
Note that phenol red is limited to testing from between 6.8 to 8.2. If the sample appears to be at either of the extreme colors, the pH may be higher or lower than the color suggests.
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. It is recommended to maintain total alkalinity 80 and 140 ppm, depending on what type of sanitizer is being used. 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 at all.
Total alkalinity should be measured every week. The most common test uses an indicator dye called bromocresolgreen- methyl red. To conduct test, a number of drops of indicator dye are added to a fixed volume of sample. The sample is then titrated with a counted number of drops of an acid until the endpoint is reached, which is signified by the dramatic color change from green to grey to red.
Many kits require a chlorine neutralizer to avoid chlorine’s interference with the test. This is done as a first step, when a drop or two is added to prevent chlorine or bromine to bleach out the color of the indicator dye.
Some total alkalinity indicators have a built-in chlorine neutralizer so it is important to read manufacturer directions.
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.
Popularized by the online forum Troublefreepools and others, many pool service professionals are increasingly using borates as pH buffers and to promote additional water clarity. Like cyanuric acid, if used, a pH dependent fraction of the borates used must be subtracted out of the total alkalinity to determine whether the alkalinity is within the acceptable range
It is important to know a pool’s cyanuric acid levels for two big reasons. The first is that CYAmoderates chlorine’s strength; more chlorine may be necessary to perform the same operations at high CYA levels. The second is that CYA 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.
This test is notoriously inaccurate. It is influenced by a variety of variable including operator error, time, lighting conditions, temperature, and concentration.
It works best if cyanuric acid is within normal ranges or dilution will be necessary. It is also best for the water to be at an ideal temperature of 75 °F.
A reagent (melamine) is added to a sample of pool water. When a one to one mixture of melamine reacts with cyanuric acid, melamine cyanurate is forms, a crystalline complex that precipitates out of solution, making the solution cloudy..
The cyanuric acid concentration is then measured as a function of the turbidity of the resulting solution – this measurement varies per manufacturer.
In some tests, the solution is then transferred to a secondary test cell with a black dot on the bottom of the tube. Viewing the black dot, the solution is continuously added until the dot just disappears from view, and the cyanuric acid is recorded as parts per million.
Low cyanuric acid concentrations will give fine particles, making a hazy solution, while high cyanuric acid concentrations will produce larger particles.
This test is temperature dependant, so it should be attempted at temperatures of about 75 °F. Low temperatures, below 60 °F can result in a false high reading as much as 15 ppm. Meanwhile, high temperatures, above 90 °F can give false low results
If borates are used, it will be necessary to test for them about once a month. Over time, water loss from backwashing and splash outs – but not evaporation – will lower the borate levels.
Borate positively contributes to water clarity and a silky feel, and they also buffer the pH from rising. They do this best at a concentration of about 50 ppm. When the borate level goes lower than about 30, pool operators may decide to add more.
Test strips are available from a variety of manufacturers to get a read on borate levels.
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 or not 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) for spas because hot water promotes scale.
Testing calcium hardness should be conducted monthly. It 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.
Sometimes, the solution will turn purple rather than blue at the endpoint. This is due to metal ion interference. The metal, usually copper from copper based algaecides, causes this purple endpoint. Manufacturers recommend re-doing the test and adding the hardness reagent to the test prior to adding the first reagent.
Total dissolved solids
Salt and total dissolved solids are both tested using conductivity meters, which measure the conductivity of the water from conductive ions such as sodium, chloride, calcium, magnesium, sulfate and others.
Water is more or less conductive because of the dissolved ions in the water. Ions that conduct electricity are called electrolytes. For example, salt is an electrolyte. TDS and salt meters measure the conductivity of water due to these electrolytes. Not all dissolved solids are electrolytes and therefore do not register on a TDS meter. For this reason, the TDS meter is really only an estimate of the true TDS. The true TDS is higher.
By the same token, not all electrolytes are in salt water chlorine generated pools are sodium chloride, and salt readings may test higher than they should.
Conductivity meters can give inaccurate results if they are not clean. Don’t allow fingerprints or other residues to remain on the probe. Clean and store the probe according to manufacturer’s directions.
While not an aspect of water balance, high levels of phosphates can certainly contribute to algae, which is something paying customers will not tolerate from a pool service technician. Especially at high cyanuric acid levels, phosphates allow algae to grow because they provide an essential nutrient. This can occur even when the chlorine is in what might be considered an acceptable range. That’s why it’s a good idea to keep phosphates as low as possible, and definitely under 500 ppb.
Test strip and powder packets are available to get an idea of phosphate levels that are simple to use. With the powder packet, just add the reagent to a fixed volume of water and invert the tube for a minute before comparing the color to a chart provided.
With the test strips, the strip is bent and placed in the lid of a vial of water. The tube is inverted five times and the color compared a provided color chart.