Free Cyanide Forms During Determination of Free Cyanide in Drinking Water

Abstract

Easily detectable amounts of free cyanide (FCN) were formed when deionized water was treated like drinking water and preserved and tested for FCN. This occurred when either ascorbic acid or thiosulfate was used to dechlorinate, though higher FCN concentrations were observed with ascorbic acid. The amount of FCN observed was up to 50–60 µg/L but strongly depended on the concentration of ascorbic acid used. The amount of FCN observed was less dependent on the amount of thiosulfate used. The FCN was observed immediately after the samples were preserved, tended to increase—primarily during the first 24 h—and persisted for at least five days. This demonstrates the potential to get false positive FCN results on drinking water samples that a US public water system would be required to report in its annual Consumer Confidence Report. Since drinking water sampling, preservation, and testing is prescriptive, there are few available ways to avoid these false positives.