Which residual test for chlorine is considered least desirable?

The least desirable residual test for chlorine among the options provided is the OTA (Orthotolidine Test). This test involves a chemical reaction wherein orthotolidine forms a color change in the presence of chlorine. While effective in determining free chlorine levels, the Orthotolidine Test has significant drawbacks that limit its desirability.

One of the primary concerns with the OTA is its potential health risks. Orthotolidine is a carcinogen, which poses safety and health issues for personnel conducting the testing. Moreover, the test is not as accurate as some of the other methods available, particularly when measuring low concentrations of chlorine. This inaccuracy can lead to misleading results, impacting water quality assessments.

In contrast, other tests, such as the DPD (N,N-diethyl-p-phenylenediamine) method, are widely used due to their reliability and safety profile. DPD tests are known for their accuracy and ease of use, making them a preferred choice for measuring chlorine residuals in water.