Hach DR/850, Procedures Manual, Page: 159 / 460

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b) If the test paper darkens, add a 1-g measuring spoon of
lead acetate to the sample. Repeat Step a. (Purchase lead
acetate from a local supplier.)
c) If the test paper continues to turn dark, keep adding lead
acetate until the sample tests negative for sulfide.
d) Filter the black lead sulfide precipitate using the apparatus
listed under Optional Apparatus. 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. (Prepare a 1:10 dilution of Acetate Acid
concentration in water.)
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.
Prepare a 100 mg/L cyanide stock solution weekly by dissolving
0.2503 grams of potassium cyanide in deionized water and
diluting to 1000 mL.
CYANIDE,
continued