Which of the following is a common test for the presence of chlorine in water?

The orthotolidine test is a well-established method used to detect the presence of chlorine in water. Chlorine, often used as a disinfectant in water treatment processes, can be measured using this test due to the distinct color change that occurs when orthotolidine reacts with chlorine compounds. This reaction leads to the formation of a yellow-colored compound, enabling quantification of chlorine concentration.

The other options present different testing methodologies that do not specifically target chlorine levels. For instance, the fluorometric test is generally used for detecting fluorescence and is not tailored for chlorine measurement. The redox test measures the overall electron transfer reactions in a solution rather than focusing solely on chlorine. The biochemical oxygen demand test assesses the amount of organic matter in water through microbial respiration but does not provide information about chlorine presence. Thus, the orthotolidine test stands out as the specific choice for identifying chlorine in water.