Chlorine is a widely used sanitizer, added to public water systems and recreational water sources to kill harmful microorganisms and prevent the spread of waterborne diseases. Regular testing is necessary to ensure the water remains safe for consumption or recreation and to prevent equipment damage from overly corrosive conditions. Monitoring chlorine levels provides a direct measure of the water’s ability to defend against contaminants. Maintaining the appropriate concentration protects public health and preserves the quality of the water environment.
Understanding Chlorine Types Measured
When testing water, it is important to distinguish between three primary forms of chlorine, as they indicate different aspects of water quality. Free Available Chlorine (FAC) is the active, unspent portion of chlorine, existing primarily as hypochlorous acid and hypochlorite ions. This is the most important measurement to determine if the water has sufficient sanitizing power at a given moment.
Combined Chlorine, also known as chloramines, forms when free chlorine reacts with organic compounds and nitrogen-containing substances like sweat, urine, and cosmetics. These chloramines are byproducts of the disinfection process and are significantly less effective as sanitizers than free chlorine. High levels of combined chlorine are responsible for the unpleasant, strong “chlorine” smell, as well as eye and skin irritation.
Total Chlorine is simply the sum of the Free Available Chlorine and the Combined Chlorine. Measuring both free and total chlorine allows for the calculation of the combined chlorine level by subtraction. If total chlorine is significantly higher than free chlorine, it indicates a buildup of ineffective chloramines in the water.
Comparing Available Testing Methods
The general public typically uses three main methods to determine chlorine concentration, each offering a different balance of convenience, accuracy, and cost.
Test strips are the most convenient and least expensive option, consisting of small pads treated with chemical reagents. The strip is dipped into the water sample, and the resulting color change is compared against a color chart printed on the container. While fast, test strips are the least precise method, with measurement errors often ranging from 5.1% to over 40%.
Liquid reagent test kits, often employing the DPD (N,N-Diethyl-p-phenylenediamine) method, offer a higher level of accuracy and are commonly used by pool and spa owners. A specific number of liquid drops or a tablet of the DPD reagent is added to a measured water sample, causing it to turn a shade of pink proportional to the chlorine concentration. The user then visually matches this color against a graded color standard, which provides readings for both free and total chlorine. Accuracy is higher than strips, but results can still be subjective due to ambient lighting and the user’s color perception.
Digital colorimeters, or photometers, represent the most precise and most expensive option for home and commercial use. These electronic devices also use the DPD chemical reaction to induce a color change in the water sample. Instead of a visual comparison, the meter shines a light through the treated sample and measures the exact intensity of the color change, converting it into a direct digital reading in parts per million (PPM). This eliminates human error in color matching and provides the most reliable data for making adjustments to water chemistry.
Standard Procedure for Accurate Testing
Following a standardized collection and analysis procedure is necessary for obtaining a reliable result, regardless of the testing method chosen. Begin by collecting the water sample in a clean, dedicated container, ensuring it is free of soap residue or contaminants that could interfere with the chemical reaction. For tap water, let the cold water run for at least one to two minutes before collecting the sample to ensure it is fresh.
When testing recreational water like a pool, collect the sample from elbow-depth, away from return lines, skimmers, or jets to get a representative measure of the main body of water. If using a liquid reagent or a colorimeter, be precise with the amount of water and the number of drops or powder added, as instructed by the manufacturer. Gentle mixing is necessary to dissolve the reagent, but avoid vigorous shaking, which can introduce air bubbles that interfere with the final reading.
Timing the test reaction is important, especially for DPD tests, which typically require a waiting period of 10 seconds up to five minutes for the color to fully stabilize before reading. For visual tests, always read the result under consistent, natural lighting, and hold the color standard against a white background to minimize distortion. Monitor the expiration dates of chemical reagents and store the test kit components in a cool, dark location to preserve their integrity.
Interpreting Results and Safe Concentration Levels
Chlorine levels are typically expressed in parts per million (PPM), representing the concentration of the chemical in the water. For drinking water, the U.S. Environmental Protection Agency (EPA) requires a measurable residual of disinfectant be maintained throughout the distribution system. The maximum residual disinfectant level (MRDL) is set at 4.0 PPM of total chlorine, but most systems maintain a free chlorine residual between 0.2 and 0.5 PPM at the farthest points of the network to ensure ongoing protection.
In recreational swimming environments, the ideal range for Free Available Chlorine is generally higher to account for the continuous introduction of contaminants from swimmers. For pools, a free chlorine concentration of 1.0 to 3.0 PPM is commonly recommended for effective sanitation. If the pool uses a stabilizer like cyanuric acid, the minimum free chlorine level is often raised to at least 2.0 PPM to maintain adequate disinfection power.
A reading that is too low indicates the water is vulnerable to microbial growth and is not properly sanitized, posing a health risk. Conversely, a reading that is too high, especially above the EPA’s 4.0 PPM threshold for drinking water, can cause undesirable taste, odor, and irritation to the eyes and skin. In pools, the combined chlorine level should ideally be less than 0.5 PPM; a higher reading signals a buildup of irritating and ineffective chloramines that must be addressed.