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CYANIDE,
continued
Acid Distillation
For USEPA reporting purposes, samples must be distilled.
All samples 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, keep adding lead
acetate until the distillate tests negative for sulfide.
e) Filter the black lead sulfide precipitate through filter paper
and funnel. This sample should now be neutralized to pH
7 and analyzed for cyanide without delay.
162
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