What should be done to find a small chlorine leak?

To identify a small chlorine leak, holding a rag dipped in a strong ammonia solution near the suspected leak is effective because of the chemical reaction that occurs between chlorine and ammonia. When these two substances come into contact, they produce a distinctive white cloud or mist, which indicates the presence of chlorine gas. This visual confirmation is a practical method for detecting leaks in locations where the chlorine might be escaping.

The other methods suggested are not suitable or could even be dangerous. For instance, spraying hydrochloric acid near a chlorine leak could create toxic gases as hydrochloric acid and chlorine can react vigorously. Releasing carbon dioxide does not provide a reliable method for tracing the leak, as it won't interact with the chlorine in a way that marks its presence. Applying citric acid would also fail to provide clear results and may not react with chlorine in a useful manner. Therefore, using ammonia is the best and safest method for detecting small chlorine leaks.