Chlorine is widely recognized for its function in water sanitation, primarily serving as a disinfectant to ensure public health. It effectively destroys harmful microorganisms, preventing waterborne disease outbreaks. Chlorination leaves a residual compound in the water distribution system, which continues to prevent microbial regrowth as the water travels to the consumer. While formal testing kits are the standard for accurate measurement, non-kit methods offer a quick, preliminary, qualitative assessment of chlorine presence when confirming treatment or investigating a strong odor.
Qualitative Sensory Methods
The most immediate way to assess chlorine is through the use of human senses, specifically smell and taste. Chlorine has a sharp, pungent odor often described as bleach-like, which becomes noticeable when the concentration is high enough. To conduct a smell test, collect a water sample in a clean, wide-mouthed glass and gently swirl it to allow volatile compounds to release. Bring the glass close to the nose and inhale cautiously, noting any chemical or bleach-like scent.
A strong odor might indicate a higher concentration of chlorine or the presence of chloramines. Chloramines are combined forms of chlorine that result from the reaction with ammonia or organic compounds. These chloramines often produce a more noticeable “chlorine” smell than free chlorine alone. Increasing the water temperature, such as by running hot water, will intensify the odor because chlorine volatilizes more easily at higher temperatures.
The taste test provides another qualitative indicator. Chlorine can impart a chemical, medicinal, or metallic flavor to the water, especially at elevated levels. Taking a small sip and noting the intensity of this flavor can help gauge the presence of residual chlorine. If the taste is overwhelmingly strong, it suggests a concentration high enough to cause irritation, and further consumption should be avoided until the level dissipates.
Utilizing Simple Chemical Indicators
Moving beyond sensory detection, a simple chemical reaction using common household materials can provide a more objective, though still rough, indication of chlorine presence. This method relies on the principle of the starch-iodide reaction, which is highly sensitive to oxidizers like chlorine. Chlorine acts as an oxidizing agent, converting iodide ions into elemental iodine.
To perform this test, you need starch, typically cornstarch, and an iodine solution, such as iodine tincture or povidone-iodine (Betadine). Prepare a simple starch solution by dissolving about one teaspoon of cornstarch in cold water, then stirring this into a half-cup of boiling water until a translucent, slightly thickened mixture forms. Allow this starch indicator solution to cool completely before use.
Next, take a small, clean glass of the water sample you wish to test. Add a few drops of the iodine solution, which will turn the water a light yellow or amber color. The presence of chlorine in the water sample should cause the initial yellow color to fade or disappear, as the chlorine reacts with and consumes the iodine.
Finally, add a few drops of the prepared starch indicator solution to the glass. If chlorine was present, the resulting elemental iodine will immediately form a deep blue-black or dark purple complex with the starch. The appearance of this distinct dark color provides a visual confirmation of the presence of an oxidizing agent, which in municipal water is highly likely to be residual chlorine.
Assessing the Reliability of Non-Kit Testing
Non-kit methods, whether sensory or based on simple chemical indicators, are purely qualitative. They can only confirm the presence or absence of chlorine; they cannot determine the precise concentration in parts per million (PPM), which is necessary for regulatory compliance or safety monitoring. The sensitivity of the starch-iodide test is generally poor for the low levels of chlorine found in potable water, often only being reliable above 5 to 10 PPM. This is significantly higher than the typical safe residual range of 0.2 to 4.0 PPM.
Several factors can interfere with the results, leading to false positives or negatives. Simple sensory perception is subjective and highly variable among individuals, and the odor can be masked by other contaminants. For the starch-iodide test, other common oxidizers found in water, such as peroxides or high levels of oxidized manganese, can also trigger the blue-black color change, leading to a false indication of chlorine.
For applications requiring accurate and precise measurements, such as monitoring swimming pool chemistry or verifying drinking water safety, professional colorimetric test kits or laboratory analysis are required. Specialized methods, like the DPD (N,N-diethyl-p-phenylenediamine) method, measure free chlorine and total chlorine separately and quantitatively. Non-kit testing should only be used for a quick, preliminary check, signaling that a more reliable, quantitative test may be necessary.