Hach DR/700 Procedures Manual page 573

CYANIDE,
continued
or sample distillation procedures should be adjusted to approximately
pH 7 with 2.5 N Hydrochloric Acid Standard Solution. When
significant amounts of preservative are used, correct for volume
additions; see Sampling and Storage, Volume Additions, (Section I) for
more information.
Oxidizing Agents
Oxidizing agents such as chlorine decompose cyanides during storage.
To test for their presence and to eliminate their effect, pretreat the
sample as follows:
a) Take a 25-mL portion of the sample and add one drop of
m-Nitrophenol Indicator Solution, 10 g/L. Swirl to mix.
b) Add 2.5 N Hydrochloric Acid Standard Solution drop-wise until the
color changes from yellow to colorless. Swirl the sample thoroughly
after adding each drop.
c) Add 2 drops of Potassium Iodide Solution, 30 g/L, and 2 drops of
Starch Indicator Solution to the sample. Swirl to mix. The solution will
turn blue if oxidizing agents are present.
d) If Step c suggests the presence of oxidizing agents, add 2 level 1-g
measuring spoonfuls of ascorbic acid per liter of sample.
e) Withdraw a 25-mL portion of sample treated with ascorbic acid and
repeat Steps a to c. If the sample turns blue, repeat Steps d and e.
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 mL/L).
g) Perform the procedure given under Interferences, Reducing Agents,
to eliminate the effect of excess ascorbic acid, before performing 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:
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