07/04/2022 | pH | 8 MINUTE READ

Does Chlorine Raise or Lower PH?

Chlorine measurements are regularly used during industrial processes to make sure that the water quality has reached a certain standard and that any chlorine disinfection techniques have been effective. When free chlorine is present in drinking water, this indicates that the water can avoid becoming recontaminated in storage and that a high amount of chlorine has been added to the water to get rid of any viruses and bacteria that are present.

Suppose you measure chlorine levels in drinking water and find that some chlorine remains. In that case, it’s likely that there aren’t any disease-causing organisms in the water, which increases the likelihood that the water is potable. When performing free chlorine measurements, pH is among the most important elements of the measurement. The pH levels in the water must come in at around 6.5-7.5 for the free chlorine measurement to be accurate when telling you how much chlorine is present in the water.


Free chlorine testing can be performed with several different tools or pieces of equipment. If chlorine testing is being done for industrial processes, these facilities will use free chlorine analyzer systems that can provide precise measurements on a large-scale basis. The types of instruments that can be used for measuring free chlorine include amperometric analyzers and colorimetric tests.

When looking at colorimetric tests, you can use either a manual testing method or an automated colorimetric test. While colorimetric tests provide users with accurate measurements, they aren’t ideal for real-time measurements. Amperometric analyzers contain chlorine sensors that immediately take pH levels into account. The following guide offers a comprehensive look at chlorine and its effect on pH levels.

What Are Chlorine Measurements Used For?

Measuring free chlorine is mainly done for two reasons, which include dosage testing and monitoring chlorination compliance for different types of projects. Dosage testing is a common treatment technique that’s used to identify the amount of chlorine that needs to be added to maintain proper free chlorine levels in drinking water. When using this method to adjust chlorine levels, it’s possible for drinking water to be stored for anywhere from 4-24 hours.

The goal of dosage testing differs completely from the objective that piped water treatment systems focus on. The purpose of a piped treatment system is to provide proper disinfection at the numerous endpoints throughout the system, which are usually water taps.

Free chlorine levels must be around 0.5 mg/L to ensure that the overall quality of water is maintained throughout the distribution network. However, these chlorine levels aren’t enough when storing the water in your home. In this situation, dosage testing can be helpful to identify how much chlorine should be added to the water before it’s stored.

When performing dosage testing, it’s recommended that the free chlorine content in water is around 2.0 mg/L or less after 30 minutes of adding sodium hypochlorite, which ensures that the water doesn’t have a bad odor or taste. Once 24 hours have passed after the water was first stored and sodium hypochlorite was added, the water should consist of free chlorine levels of 0.2 mg/L or higher.

Keep in mind that the Safe Water System Program maintains methodology that is considerably different than the WHO guidelines, the latter of which require free chlorine levels of 0.5 mg/L or higher. The SWS recommends that free chlorine levels are regularly tested in all households to determine how much chlorine is found in the drinking water.


Why Does PH Matter in a Free Chlorine Measurement?

Once you add chlorine to water, a hypochlorite ion and hypochlorous acid are formed. When combined, these substances create free chlorine. The hypochlorous acid in this solution is the chemical that eliminates any pathogens and bacteria that are currently present in the water. Pathogens can cause diseases and illnesses to spread, which is why it’s important for drinking water treatment facilities to rid water of any pathogens.

Keep in mind that amperometric measurements are only able to detect how much hypochlorous acid is in the water. This acid will be reduced with the measuring electrode, which effectively tells the sensor what the concentration of free chlorine is.

The system’s pH will determine what types of pathogens or bacteria are in the water and what the concentration of these contaminants are. When the water contains pH levels that are higher than 8.0, it’s likely that there’s isn’t much hypochlorous acid in the solution, which makes it more difficult for amperometric sensors to identify free chlorine.

In the event that the pH readings come back at around 7.0, this is the preferred reading for a free chlorine amperometric measurement. It’s also important that substantial pH fluctuations don’t occur. In this situation, it’s possible that the accuracy of the measurement would be at risk. When pH levels are higher than 8.5, obtaining an accurate measurement will be challenging. For accurate pH readings, it’s highly recommended that you use a combination or differential pH sensor.

What Is the Process of Free Chlorine Testing?

There are a range of different tools that you can use when you want to perform free chlorine testing, the primary of which include pool test kits, digital colorimeters, and color-wheel test kits. Each method works by identifying how much chlorine is in the water via a color change. The intensity of this color allows the individual measuring chlorine concentration to determine what the chlorine levels are and if they match the intended concentration.

Manual Free Chlorine Testing

When looking specifically at pool test kits, these use a type of liquid chemical OTO that’s able to create a color alteration to yellow if the water contains total chlorine. Once the tube has been filled with water, all you need to do is place around 1-5 drops of this chemical solution into the water, after which a color change should occur. Pool test kits can be found in many retail stores as well as online stores like Amazon.com. Keep in mind that free chlorine can’t be measured with a pool test kit.

The main advantages of using a pool test kit include affordability and an easy-to-use interface. On the other hand, these test kits can create inaccurate readings over time and don’t provide quantitative results. While the affordability of these test kits is beneficial, the low cost means that there’s a lack of standardization and calibration with these kits.

Color-wheel test kits use tablet chemical DPDs or powders to create a color change. The water will change to pink when chlorine is present. The intensity of the color determines how much chlorine is in the water. In most cases, color wheels are less expensive than their digital meter counterparts as a result of their simplistic design. A color wheel is used to match a total or free chlorine reading with a color. These test kits provide a chlorine range of 0-3.5 mg/L.

This type of measurement is beneficial if you’d like to use a low cost tool that’s able to provide relatively accurate readings if used correctly. On the other hand, color-wheel test kits have a lack of standardization and calibration when compared to digital meters. There’s also a higher possibility of user error.


Digital Free Chlorine Testing

Digital colorimeters are considered to be the most precise tools for measuring free or total chlorine concentrations in water. Colorimeters can use a powder or DPD tablet to create a color change. The vial of water is then placed into a meter to identify how intense the color change is. These meters can provide readings that extend from 0-4 mg/L.

The main benefits associated with digital colorimeters include quick results and precise readings. However, there are a couple of drawbacks to using digital colorimeters, which include a high cost compared to other options as well as the need to calibrate the sensor with specific standards.

Choosing the right type of measurement to use when calculating chlorine concentration depends on numerous factors, which include everything from the cost and accuracy of the method to the total number of samples that need to be tested. Keep in mind that the option you use to test chlorine levels depends on how you will use the data. If you’re currently set to perform dosage testing for a large-scale project, it’s highly recommended that you opt for a digital colorimeter to obtain precise readings. Accuracy is necessary to ensure proper dosage.

In the event that you are going to use the measurement to determine if homeowners are properly using chlorine, you shouldn’t need anything more than a pool test kit. This kit will give you a good idea if total chlorine is present in the water. If the main goal of your project is to identify if the right amount of chlorine is being added to the water, a color-wheel kit may suffice and will provide you with more accurate readings than a pool test kit.



Measuring chlorine levels will help you determine what the current quality of the water is and if additional treatment must be done before consuming the water or using it for industrial processes. When chlorine levels are too high, the water’s pH will start to decrease, which can eventually lead to corrosion. However, liquid chlorine has a pH of 13 and can cause the water’s pH to increase. Now that you understand how chlorine affects water, you should be able to use this chemical to disinfect water and remove any harmful pathogens.

Posted by Sensorex on July 4, 2022

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