Hach DR2400 - Page 269

Cyanide
Cyanide
Cyanide_PP_Other_PPZ_Eng_Ody.fm Page 5 of 8
d. If step 3 suggests the presence of oxidizing agents, add two level,
1-g measuring spoonfuls of Ascorbic Acid (Cat. No. 6138-26) per liter
of sample.
e. Withdraw a 25-mL portion of sample treated with ascorbic acid and
repeat steps 1 to 3. If the sample turns blue, repeat steps 4 and 5.
f. If the 25-mL sample remains colorless, preserve the remaining sample to
pH 12 for storage with 5 N Sodium Hydroxide Standard Solution
(usually 4-mg/L) (Cat. No. 2450-53).
g. Perform the procedure given under Interferences, Reducing Agents, to
eliminate the effect of excess ascorbic acid, before following the cyanide
procedure.
Sulfides
Sulfides will quickly convert cyanide to thiocyanate (SCN
–
). To test for the
presence of sulfide and eliminate its effect, pretreat the sample as follows:
a. Place a drop of sample on a disc of Hydrogen Sulfide Test Paper
(Cat. No. 25377-33) that has been wetted with pH 4 Buffer Solution
(Cat. No. 12223-49).
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
(Cat. No. 7071-34) until the sample tests negative for sulfide.
d. Filter the lead sulfide precipitate through Filter Paper (Cat. No. 1894-57)
and a Funnel (Cat. No. 1083-67). Preserve the sample for storage with 5 N
Sodium Hydroxide Standard Solution (Cat. No. 2450-53) or neutralize to
a pH of 7 for analysis.
Fatty Acids
Caution: Perform this operation in a hood and as quickly as possible.
When distilled, fatty acids will pass over with cyanide and, under the alkaline
conditions of the absorber, will form soaps. If the presence of fatty acid is
suspected, use the following pretreatment before preserving samples with
sodium hydroxide.
a. Acidify 500 mL of sample to pH 6 or 7 with a 4:1 dilution of glacial Acetic
Acid (Cat. No. 100-49).
b. Pour the sample into a 1000-mL separatory funnel and add 50 mL of
Hexane (Cat. No.14478-49).
c. Stopper the funnel and shake for one minute. Allow the layers to
separate.
d. Drain off the lower, sample layer into a 600-mL beaker. If the sample is to
be stored, add enough 5 N Sodium Hydroxide Standard Solution
(Cat. No. 2450-53) to raise the pH above 12.
Acid Distillation
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.