How Wait Time Effects Water Test Results

Mom always said, “patience is a virtue.” But is it? In the world of water testing for instance, patience is not always an advantage. In fact, patience can end up causing some odd and potentially incorrect results.

Iron, copper, and a few other tests require the operator to wait a few minutes before reading the results in order to give the reagents time to properly “digest” (react with the other chemicals). On the other hand, there are tests where waiting around could really skew the results. This is especially true when it comes to testing chlorine or bromine using DPD reagents.

Both chlorine and bromine are what we call “halogens.” All halogens react with DPD reagents in the same manner; they produce a pink-to-red color. When performing a standard color-matching test or a drop test using FAS-DPD, the reaction is the same. However, letting the treated sample “sit” before matching colors allows air, heat, and/or light to react with the treated sample, causing an ever-increasing red color to develop. The result — a false-high reading.

With the FAS-DPD method, an operator adds an oxidizer to a treated sample that has already turned pink/red after the addition of a powdered indicator. As the titrating reagent is added, the sample goes colorless at the endpoint of the reaction. The sample must remain colorless for at least 30 seconds to be considered the correct endpoint. This is where waiting can make all the difference. If the sample turns pink/red again within 30 seconds, then the true endpoint was not reached. At this point, the operator would continue to add the titrating reagent until the sample remained colorless for 30 seconds.

The takeaway…if you want accurate results, read the instructions first to be certain you allow the right amount of time to elapse before taking a reading. Remember — inaccurate results lead to improper treatment!