Introduction
In 1935 the procedure for the determination of cyanide in water involved the conversion of cyanide salts by acid solution and distillation into an absorber solution containing sodium or potassium hydroxide. Removal of sulfide was to add an excess of lead salt to the solution prior to distillation.[1] In 1939 a method was developed to isolate and concentrate low levels of cyanide from aqueous solutions by aeration of a heated acidic sample solution and collection of the generated hydrogen cyanide in a basic absorber solution.[2] Today’s EPA accepted methods are essentially the methods of 1939.
Discussion
Current accepted EPA methodology for total cyanide involves the aeration of a boiling, strongly acidic sample solution that transports cyanide as hydrogen cyanide from the sample solution to a basic absorber solution in the inert gas carrier stream. Common interferences, such as oxidizers, nitrate nitrogen and sulfide are assumed to be treated prior to, during, and/or after the distillation but before the final measurement step. Final measurement is made by conversion of cyanide to cyanogens chloride with strong chlorine solution and a subsequent color reaction of the cyanogens chloride with a pyridine – barbituric acid reagent. The resulting chromophore is obeys Beer’s Law being proportional to concentration and allowing the concentration of unknowns to be calculated.
The problem with this method is that the interferences thought to be adequately mitigated are not and can cause negatively biased or positively biased results to be reported. In many instances, negative bias is a result of components of the sample for which there is no easy spot test and that may actually not still be present by the time the laboratory distills the sample. In some instances these interfering components actually destroy cyanide during the distillation process. An example is sodium sulfite. Sodium sulfite use as a dechlorination chemical is increasing as regulations are beginning to require zero discharges for residual chlorine. Since chlorine is used as a disinfectant in wastewater treatment, sodium sulfite is added to remove the chlorine just prior to release of the effluent into the receiving stream. The problem is that during distillation sulfite reacts with cyanide to form cyanate which does not distill. Also, if sulfite reacts in the distillation and becomes sulfur dioxide it is carried along with the hydrogen cyanide into the receiving solution where it converts back to sulfite and converts cyanide to cyanate. Since the reaction of sulfite with cyanide is more rapid in basic than in neutral solutions it is likely that during the storage of basic cyanide sample the sulfite is gradually decomposing cyanide.
Another example is thiocyanate. There is no spot test for thiocyanate, it is rarely analyzed, and it is very likely to be present in wastewater samples. Thiocyanate when distilled by itself process no apparent interference with the approved distillation colorimetric method, however, if known amounts of cyanide is added to samples that contain thiocyanate recoveries are low. Samples that contain both thiocyanate and nitrate produce cyanide during the distillation process causing positively biased results. In fact, nitrate can react with almost all organic compounds in water during the distillation and create cyanide. This is especially disturbing considering both nitrate and organic compounds will be present in almost every wastewater.
Newly discovered interferences that result from attempts to mitigate sulfide interference have been addressed to some degree in the latest methods Update Rule posted in the March 12, 2007 Federal Register. ASTM Committee D19.06 has also just completed a Cyanide Sampling and Preservation Guide that discusses sulfide and sulfur interferences. Basically, what was discovered by EPA and the ASTM is that the procedures that have been in place for so many years that were thought to be effective are not. In many cases falsely negative results were, and still are, being reported simply because laboratories are following the prescribed procedures.
Conclusion
Cyanide procedures that depend on distillation to separate cyanide from the sample have been found to be inadequate at accurately measuring cyanide in the presence of components that are in most samples. Fortunately, new technology offers some solutions. Methods and instrumentation developed recently accurately determine cyanide when these interferences are present. Progress is being made to convince the EPA and the regulated community that the results being reported by the currently approved are not accurate and that the newer technology should be allowed for compliance reporting.
[1] A.E. Childs and W.C. Ball, Analyst 60, 294-9 (1935)
[2] W.O. Winkler, J. Assoc. Official Agr. Chem. 22, 349-55 (1939)
William Lipps
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