Chlorine is widely utilized in water management systems, including public drinking water supplies and recreational swimming pools, primarily for its ability to destroy disease-causing microorganisms. When introduced into water, this disinfectant quickly forms several chemical species. The concept of “total chlorine” accounts for all these different forms to accurately assess a water system’s disinfection status. Understanding its composition is necessary for maintaining water quality and ensuring the ongoing safety of the water.
Defining Total Chlorine and Its Components
Total chlorine is a measurement representing the entire quantity of chlorine present in a water sample. It is the sum of two distinct forms: free chlorine and combined chlorine. This aggregate measurement provides a complete picture of the chlorine residual, which is the amount of chlorine remaining after the initial demand of the water has been met.
Free chlorine represents the portion of the disinfectant still available to actively sanitize the water and neutralize contaminants. Combined chlorine is the portion that has already reacted with organic or inorganic compounds, such as ammonia or nitrogen-containing materials. The total chlorine reading is a foundational metric because it encompasses both the active sanitizing agent and the byproducts of the disinfection process.
The Functional Difference Between Free and Combined Chlorine
Free chlorine is the highly active disinfecting agent in water, existing primarily as hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Hypochlorous acid is particularly effective because it easily penetrates the cell walls of harmful microbes, disrupting their functions and rendering them harmless. Maintaining a sufficient concentration of free chlorine is the primary goal of water treatment, ensuring continuous protection against contamination.
Combined chlorine, often referred to as chloramines, forms when free chlorine reacts with nitrogenous compounds introduced by human or environmental waste. Once chlorine is bound, its ability to disinfect is significantly reduced. Its presence indicates that the disinfectant has been consumed by contaminants in the water. High levels of combined chlorine are also responsible for the unpleasant chemical odor and the eye or skin irritation commonly associated with pools.
For example, in recreational water, the ideal level of free chlorine typically ranges between 2 and 4 parts per million (ppm) to provide effective sanitation. If the combined chlorine concentration exceeds 0.5 ppm, it suggests a heavy contaminant load and signals a need for corrective action to restore the water’s balance. The functional difference is distinct: free chlorine actively cleans the water, while combined chlorine signals the consumption of that active cleaner.
Measuring Total Chlorine and Interpreting the Results
Measuring total chlorine is a routine practice that helps operators understand the chemical balance of a water system. Common field testing methods, such as the DPD (N,N-Diethyl-p-phenylenediamine) method, use specialized reagents to produce a color change proportional to the chlorine concentration. A sample is first tested for free chlorine, and a second test, including a reagent like potassium iodide, is then performed to measure the total chlorine.
The concentration of combined chlorine is not measured directly but is determined mathematically by subtracting the free chlorine reading from the total chlorine reading. This calculation is essential for monitoring the water’s overall health. In a well-managed swimming pool, the total chlorine reading should be very close to the free chlorine reading, indicating a minimal amount of combined chlorine.
Acceptable total chlorine levels vary significantly between water types. In municipal drinking water systems, the maximum allowable residual is typically up to 4.0 milligrams per liter (mg/L), which ensures disinfection is maintained throughout the distribution network. For recreational water, where the free chlorine level is intentionally higher to handle bather load, the total chlorine should ideally not exceed the free chlorine by more than 0.2 ppm.