Hach DR/700 Procedures Manual page 577

CYANIDE,
continued
Acid Distillation
For USEPA reporting purposes, samples must be distilled.
All samples to be analyzed for cyanide should be treated by acid
distillation except when experience has shown that there is no difference
in results obtained with or without distillation. With most compounds, a
one-hour reflux is adequate.
If thiocyanate is present in the original sample, a distillation step is
absolutely necessary as thiocyanate causes a positive interference. High
concentrations of thiocyanate can yield a substantial quantity of sulfide
in the distillate. The "rotten egg" smell of hydrogen sulfide will
accompany the distillate when sulfide is present. The sulfide must be
removed from the distillate prior to testing.
If cyanide is not present, the amount of thiocyanate can be determined.
The sample is not distilled and the final reading is multiplied by 2.2.
The result is mg/L thiocyanate.
The distillate can be tested and treated for sulfide after the last step of
the distillation procedure by using the following lead acetate treatment
procedure.
a) Place a drop of the distillate (already diluted to 250 mL) on a disc of
hydrogen sulfide test paper that has been wetted with pH 4.0 Buffer
Solution.
b) If the test paper darkens, add 2.5 N Hydrochloric Acid Standard
Solution drop-wise to the distillate until a neutral pH is obtained.
c) Add a 1-g measuring spoon of lead acetate to the distillate and mix.
Repeat Step a.
d) If the test paper continues to turn dark, continue adding lead acetate
until the distillate tests negative for sulfide.
e) Filter the black lead sulfide precipitate through filter paper and funnel.
Neutralize the sample to pH 7 and analyze for cyanide without delay.
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