Refinement of Weak Acid Dissociable (WAD) Method for Measuring Weak Metal Cyanide Complexes in Aqueous Samples
Abstract
The primary objective of this study was to correct deficiencies that had been identified in the current weakacid dissociable (WAD) cyanide analytical method. These deficiencies became apparent when the method was applied to groundwater and surface water samples dominated by iron–cyanide complexes (>50% of the total cyanide content); the analytical method, as is, is fine for determining WAD cyanide in any aqueous sample where weak metal cyanide complexes and free cyanide ions dominate the cyanide speciation (i.e., iron–cyanide complexes are <50% of the total cyanide content). It was also determined that the means required to correct these deficiencies depended upon which iron–cyanide complex was present (i.e., ferricyanide/ferrocyanide or an iron–pentacyano methylamino species, Fe(CN) 5 NHCH 3 4− ). For the samples dominated by the ferri- and ferrocyanide complexes, addition of the prescribed amount of zinc acetate under mildly acidic conditions was sufficient to entirely remove these complexes from solution via formation of an iron cyanide precipitate. However, the high-temperature distillation step of the analytical method was found to destabilize the iron–cyanide precipitate thus formed, resulting in the recovery of some of this strong acid dissociable cyanide complex as WAD cyanide. To rectify the method deficiency, an intermediate filtration step using a 0.45-micron filter was introduced to remove the precipitate before distillation is performed. A slightly different modified method was required when the iron–cyanide complexes were dominated by the iron–pentacyano methylamino complex, that is, Fe(CN) 5 NHCH 3 4− . In this instance, it was necessary to add three times the prescribed amount of zinc acetate to remove the complex from the solution. Also, the precipitate was comprised of smaller size particles, and hence, a smaller filter size (0.2 micron) was required to remove the precipitate before distillation. These modifications to the current WAD method provided a more accurate determination of the WAD cyanide content of the sample that is dominated by iron–cyanide complexes, as verified by the EPA-approved available cyanide method OIA-1677. However, for samples dominated by weak metal–cyanide complexes, the current WAD cyanide analytical method is deemed appropriate.