Which residual test for chlorine would be least desirable?

The least desirable residual test for chlorine in this context is the OTA (Ortho-Toluidine Method). This method, while historically used for measuring chlorine levels, presents several challenges, particularly in terms of safety and reliability. The OTA method involves potentially hazardous chemicals, which may raise safety concerns for laboratory personnel and limit its application in certain environments.

Additionally, the sensitivity of the OTA method may vary based on water quality factors such as temperature and pH, making it less reliable compared to more modern testing methods. Other methods, such as DPD (N,N-diethyl-p-phenylenediamine), are more commonly used in practice due to their accuracy, ease of use, and safety profile. DPD testing provides straightforward and reliable measurements of free and total chlorine, making it preferable for environmental monitoring and regulatory compliance.

While other methods like SNORT (which is a less common test for chlorine levels) and Methyl Orange offer their own benefits, they do not carry the same level of concern associated with the OTA method. Therefore, the reliance on OTA as a testing method in contexts where safety and precision are paramount makes it the least desirable option.