Lachat Method: Cyanide 10-204-00-1-X2
By means of a passive miniature distillation device, MICRO DIST, the cyanide in the samples is released by digesting and acidifying cyanide complexes, converting them to hydrocyanic acid (HCN). The cyanide ion is trapped in a 1.0 M sodium hydroxide absorbing solution which is diluted to 0.25 M solution during the distillation. By means of flow injection analysis, the 0.25 M NaOH distillate is converted to cyanogen chloride by reaction with chloramine-T, pyridine and barbituric acid to give a red-colored complex. The absorbance of this complex is measured at 570 nm by measuring the peak area resulting from the sample. The peak area is proportional to the concentration of the cyanide in the sample.
The determinative step of this method is described here only so that a complete method is provided. Any approved continuous-flow method may be used for the determination of cyanide in the 0.25 M NaOH MICRO DIST distillates.
Note: Other approved distillation procedures may use different concentrations of NaOH in the absorber or trapping solution. Because the determinative method’s sensitivity is pH dependent, it is important to ensure that the pH of the absorber solution from any distillation procedure is adjusted to match the pH of calibration standards specified in the approved determinative method being used.
USEPA Reference Method “Methods for the Chemical Analysis of Water and Wastes”, EPA-600/4-79-020, Revised March 1983 and 1979, Method 335.2 specifies that absorbance be measured at 578 nm using a batch spectrophotometer. USEPA Reference Method “Methods for the Determination of Inorganic Substances in Environmental Samples”, EPA-600/R-93-100, August 1993, Method 335.4 specifies that absorbance be measured at 570 nm. The latter wavelength is specified in this Modified Method because Method 335.4 uses an automated continuous flow-through detector similar to that used in the following flow injection method.
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