Don’t Get into Hot Water When Testing

Testing Tips

Tom Seechuk

The chemistry of hot water systems – for example, spas – is similar to that of pools, but there are some differences that should be considered. The bather load is greater, reactions occur faster, and the environment is suitable for bacteria growth. All these factors warrant the use of higher sanitizer concentrations in spas. Higher chlorine or bromine concentrations can lead to some difficulties with testing.

Halogen Testing

Diethyl-p-phenylene diamine (DPD) is used for halogen testing. This is available as a tablet, liquid, or powder. While many test comparators can read up to 10 ppm of chlorine or bromine, some bleaching of DPD can occur at concentrations of 6-8 ppm. One answer to possible bleaching is to dilute the sample and multiply the resulting reading (if the dilution is 1:1, multiply by 2; if the dilution is 1:3, multiply by 4, etc). Another answer is to double the amount of DPD added.

The DPD test depends on there being enough DPD available to react with all the chlorine. Most test kits base this on a 5 to 10 mL sample. The volume of water is not critical in this test. One can always use less water than the test requires, as long as there is enough water to compare with the color standards. This not only saves time (not having to adjust the volume to read exactly on the line), but also increases the ratio of DPD to chlorine, thus preventing possible bleaching at higher concentrations.

There are two liquids used in the DPD-free chlorine test. The indicator has to be acidic to be stable. To offset this acidity, the first reagent is alkaline and is a phosphate buffer. Sometimes calcium in the water reacts with this to form calcium phosphate – a precipitate – which makes the sample cloudy. Usually, this cloudiness disappears upon addition of the second reagent. There is also a slight bleaching of DPD when it is added to a water sample. The first drop of the liquid indicator and the outside of a DPD tablet are slightly bleached upon contact with the sample. This amounts to a loss of only about 0.1 ppm. For these reasons, the best way to add DPD reagents to a test is by adding them to the sample vial before adding the sample. When using liquids, add these to an empty vial, and then add the sample; when using tablets, dissolve the tablet in a small amount of sample, and then add the rest of the sample.

For many years, the standard recommended maximum concentration in pools of combined chlorine has been 0.2 ppm (0.5 ppm for public spas). At higher concentrations, comparators lose definition. The standards usually read 2.0, 2.5, 3.0, 4.0, 6.0 ppm, etc. At higher chlorine concentrations, this makes reading 0.2 ppm almost impossible. To solve this, there is a titration kit that uses Ferrous Ammonium Sulfate (FAS). The methodology allows the sample to be overloaded with DPD powdered indicator – which prevents bleaching – and each drop of titrant is usually equal to 0.2 ppm, allowing one to determine combined chlorine within +/- 0.2 ppm, even at higher concentrations. One can also test bromine with some versions.

Monopersulfate (MPS) is used in hot water systems to convert bromide to bromine and as a shock product. Since its reaction with DPD is much like combined chlorine, it will appear in the test as combined chlorine. If someone shocks to eliminate combined chlorine, the test will show it is still present. There is a reagent available from many test kit manufacturers that can be added after the free chlorine test to eliminate this interference.

pH Testing

Halogens interfere with pH indicators. Phenol red reacts with chlorine to form chlorophenol red. Phenol red reacts with bromine to form bromphenol blue. These are completely different indicators, but can appear similar to an 8.4 pH reading in a comparator.

To prevent halogen interferences, inhibitors are usually used in pH testing. Some kits provide a bottle of sodium thiosulfate, which is added before the phenol red. Some phenol red indicators have a built-in halogen inhibitor. The dose of thiosulfate is usually one to three drops, but this is based on testing water with lower halogen concentrations, such as in pools. The higher halogen concentrations in hot water systems require the addition of more thiosulfate. This causes a problem because the reaction of halogens and thiosulfate can actually change the pH. The same thing can occur with phenol red containing built-in halogen inhibitors. There are several inhibitors used with phenol red, and the effect on pH is different with each. Since some of these are proprietary and the effect is based on alkalinity and halogen concentration, it is difficult to predict how the pH will change. This is the probable reason why test results may be different when using pH kits from different manufacturers.

The inhibitors used in phenol red can degrade with time. Usually, a fresh bottle of reagent can neutralize 15-20 ppm of halogen. After six months, depending on climate and storage, this can decrease to below 10 ppm.

Since there are some difficulties testing pH in water containing higher halogen concentrations, many operators are using pH meters. These are becoming more affordable and easier to use, and they aren’t affected by halogens. However, they require daily calibration with pH buffers, as well as proper care and storage of the electrode.

Some erosion-fed chlorine and bromine products decrease pH. Since higher halogen concentrations and lower water volume are the norm for hot water systems, this decrease can become a problem if not monitored and if the acid is not neutralized. All pH indicators are limited to a specific pH range. The range of phenol red is 6.8 to 8.4. If the reading is below 6.8, the test will still read as 6.8. Many operators are not aware of this and think that a 6.8 reading might require only a small adjustment to get to the 7.2 to 7.8 range. However, the pH might be far below 6.8. Some test kits have acid-demand reagents and base-demand reagents. These allow the operator to determine how much chemical is needed to adjust pH up or down. If a pH test reads 6.8, a base-demand test should be used to see if the reading is actually close to 6.8 or if it is far below 6.8. The same is true with an 8.4 reading, which would require an acid demand test.

Total Alkalinity Testing

High halogen can interfere with the total alkalinity test. The total alkalinity indicator is a mixture of bromcresol green and methyl red indicators. Together, when added to a water sample, they produce a green-blue color that changes to a pink color at the endpoint. Halogens can bleach the methyl red, making the endpoint yellow. To prevent this, as with pH, either thiosulfate is added before the indicator, or the indicator contains a built-in neutralizer. If the endpoint is yellow, repeat the test and add more thiosulfate or indicator. This will neutralize the interference without affecting test results.

Cyanuric Acid Testing

Some outdoor facilities use cyanuric acid, a chlorine stabilizer used to prevent UV degradation of chlorine. Cyanuric acid concentration is usually regulated by local health codes. The test for this is based on the reaction of a chemical with the cyanuric acid to form a cloudy solution. This solution is added to a calibrated tube until a dot in the bottom of the tube disappears. The value of the cyanuric acid is then read from the side of the tube. For best results, the test should be done at a temperature between 70°F and 80°F. If the water is hotter than this, the reading will be low. Lowering the temperature of the test water may be difficult in the field, but it will make a difference.

Hopefully, some of the above will help keep everyone out of hot water!

Tom Seechuk

Tom Seechuk has been a market manager at LaMotte Company for 18 years. He has been in the testing business for 27 years.