How to Solve the Most Common Problems Affecting your Water Tests

How to Solve the Most Common Problems Affecting your Water Tests

Managing wrong levels of pool chemicals during water treatment for your pool or hot tub can produce readings that at times can be throw you wildly off course compared to usual recordings. An overdose of chlorine or bromine, for example, can significantly impact pH and total alkalinity, while many other items can affect both the drop-test and color-matching methods. With the costs of care and physical integrity of your pool or spa at stake, it’s imperative that interferences throughout water tests be taken care of carefully. This article will cover some of the common interferences you might encounter in your next testing treatment.

pH test

When the level of sanitizer is is more than 15 part per million (ppm), the organic dye used in the indicator may be significantly affected, resulting in a purple sample, instead of the normal yellow to red range.


Solution: Remove all the water and rinse the bottle thoroughly. Once you obtain a new sample, add one drop of thiosulfate to neutralize any excess sanitizer in the sample (be sure to add only one drop, as any additional drops may change the entire pH of your sample). Add 5 drops of phenol red indicator, mix the sample and match the corresponding color.

Color matching

DPD reagents are chemicals used to react with water when testing oxidizer concentrations. When chlorine or bromine levels are greater than 10 - 15 ppm, DPD reagents can be partially or totally bleached out, which can trick you into thinking there is no chlorine or bromine in the pool.


Solution: Upon testing, if there is no color development and you are positive there is present chlorine and bromine, dilute the sample and retest to determine the correct concentration. You’ll want to cut your sample size in half - for example, if the required sample size is 10mL, take it down to 5mL, then add 5mL of water with zero sanitizer (like bottled water). As you follow the same the testing instructions, compare the resulting color with the chemistry chart. When ou identify the color on the chart that matches your water sample, you’ll find a value alongside it. The correct reading you’ll need is double the value the color represents.


Another possible problem is that a small amount of DPD could be traced inside the test cell. Even a small amount can make the total chlorine value read less than the free chlorine value.

Solution: Thoroughly rinse your test cells after each use.

Calcium hardness (CH) test

Metal ions from algaecides, pipes or the water itself can dramatically affect the results for CH tests. Usually, traces of copper, iron and/or manganese are the culprit, as the presence of these metals can often show themselves once the sample turns purple or stays red instead of the blue end point.


Solution: First, obtain a new sample and add 5 or 6 drops of titrant before adding the buffer and indicator. Follow the testing procedure normally, then add the initial 5 or 6 drops of the titrant to the total drop count. Multiply the result by the drop equivalence to obtain the correct reading.

Total Alkalinity test

With the addition of total alkalinity reagents, you’ll obtain the most accurate results when the water turns green to red. If it turns blue to yellow, this indicates an excess amount of chlorine oxidizer is present in the sample.


Solution: Obtain a new sample. Start by adding four to five drops, instead of the instructed number of one or two drops. This will remove excess halogen without compromising the sample, allowing you to acheive the right color spectrum (green to red).

Monopersulfate Readings

Monopersulfate (non-chlorine oxidizer) can alter both total and combined chlorine readings. This occurs due to a quick reaction to DPD No. 3, which results in an abnormally large number (a false-high. This also applies to the color-matching method.


Solution: To remove any monopersulfate, use a deox reagent, then attempt another test.

Iron/Copper test

High levels of copper can impede on iron test readings and vice versa.


Solution: If you receive questionable readings, dilute the sample, and retest to determine the concentration of these metals. You can even use a digital reader calibrated to detect copper and iron individually.

Proper water tests require an adequate understanding of the mechanical and chemical aspects of the water systems. If any odd chemical readings appear to throw you off, you can follow the information above and determine exactly what’s going on in the pool in order to bring water back into balance. If you want to ensure you, your pool or your hot tub continues to stay healthy, keep your it balanced with pHin. It constantly monitors your water and tells your smartphone what you need to do to keep the water in your pool and your hot tub healthy. Use it with your own chemicals for flexibility or get our single-dose, pre-measured chemicals delivered to your door. If you need someone to service your equipment, Pool Service on Demand connects you to local, qualified pool techs.

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