Determination of Total Cyanide Without Distillation

Introduction

The EPA has defined Total Cyanide as the amount of cyanide ion liberated by distillation with a sulfuric acid magnesium chloride solution followed by either colorimetric, titrimetric, or ion selective electrode measurement. The high heat and low pH of the distillation process have been demonstrated as causing low reproducibility and questionable accuracy, as well as being known to produce both false positive and false negative results depending upon the sample matrix.

Distillation

In the absence of interference, simple cyanides such as HCN, KCN, and NaCN are determined readily by each of the determinative steps, however, to determine total cyanide metal cyanide bonds must be broken and cyanide separated to produce simple cyanide. In all the approved EPA methodology this is accomplished by distillation from acid solution. Although distillation is assumed to eliminate, or at least minimize, most interferences the high temperature and strong acid solutions can potentially introduce significant positive or negative bias.

Thiocyanate and Sulfide are commonly occurring compounds in industrial waters and wastewaters, and are the most common and significant interferences in distillation based cyanide methods. While there are ways to remove sulfide, there are no adequate solutions for the removal of thiocyanate prior to analysis. The assumption that thiocyanate interference can be compensated for by the analysis of Total Cyanide including Thiocyanate and subtracting thiocyanate determined by a separate analysis depends on the accuracy of the cyanide measurement made in the presence of thiocyanate. Because thiocyanate can produce both positive and negative interferences, without an accurate analysis of each matrix one can never be sure whether to add or subtract.

Development of a Distillation Free Method for the Determination of Total Cyanide Recognizing that most interferences in accepted total cyanide determination methods stemmed from the distillation step that was supposed to eliminate interferences researchers developed a distillation free total cyanide method based on segmented flow on-line UV digestion gas diffusion with amperometric detection.[1] This novel method that quantitatively liberates most strong metal cyanide complexes, and accurately measures total cyanide from 0.002 mg/l to 5.00 mg/l at a rate of two minutes per sample is available from OI Analytical as Total Cyanide by UV-Digestion Amperometric Detection.

There are no known spot tests to detect thiocyanate, sulfite or thiosulfate accurately so no treatment can be applied to these samples. Recovery data indicates that cyanide can be accurately measured when analyzed by this non-distillation method, and that cyanide is not recovered when these same samples are distilled.

Conclusions

Approved total cyanide methods requiring a preliminary distillation step often produce unreliable data in complex real world samples. A method available from OI Analytical Total Cyanide by On-Line UV Digestion Amperometric Detection solves most interference problems by producing reliable data in the presence of known interferences.

Although the method has not been EPA approved (EPA defines total cyanide as cyanide determined after distillation) it is still advantageous to analyze cyanide by a method known to produce correct results. Analyzing strictly for compliance purposes can result in serious errors including potential under-reporting when cyanide is actually present, or in over-reporting when cyanide is not even there.

[1] Ljiljana Solujic, Emil B. Milosavljevic, and Michael R. Straka, Analyst,1999, 124, 1255-1260

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