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