Hach DR/700 Procedures Manual page 574

CYANIDE,
continued
a) Place a drop of sample on a disc of hydrogen sulfide test paper that
has been wetted with pH 4 Buffer Solution.
b) If the test paper darkens, add a 1-g measuring spoon of lead acetate to
the sample. Repeat Step a.
c) If the test paper continues to turn dark, keep adding lead acetate until
the sample tests negative for sulfide.
d) Filter the lead sulfide precipitate through filter paper and a funnel.
Preserve the sample for storage with 5 N Sodium Hydroxide Standard
Solution or neutralize to a pH of 7 for analysis.
Fatty Acids
Caution: Perform this operation in a hood as quickly as possible.
When distilled, fatty acids will pass over with cyanide and form soaps
under the alkaline conditions of the absorber. If the presence of fatty
acid is suspected, do not preserve samples with sodium hydroxide until
the following pretreatment is performed. The effect of fatty acids can be
minimized as follows:
a) Acidify 500 mL of sample to pH 6 or 7 with Acetic Acid Solution.
b) Pour the sample into a 1000-mL separatory funnel and add 50 mL of
hexane.
c) Stopper the funnel and shake for one minute. Allow the layers to
separate.
d) Drain off the sample (lower) layer into a 600-mL beaker. If the
sample is to be stored, add 5 N Sodium Hydroxide Standard Solution to
raise the pH to above 12.
ACCURACY CHECK
Standard Solution Method
Caution: Cyanides and their solutions, and the hydrogen cyanide
liberated by acids, are very poisonous. Both the solutions and the gas
can be absorbed through the skin.
To assure the accuracy of the test, prepare the following standard solutions.
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