Hach DR/850 Procedures Manual page 158

CYANIDE,
continued
Solution. If significant amounts of preservative are used, correct
for the volume added; see Correction for Volume Additions in
Section 1 for more information.
Oxidizing Agents
Oxidizing agents such as chlorine decompose cyanides during
storage. To test for their presence and 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 the addition of each
drop.
c) Add two drops of Potassium Iodide Solution, 30 g/L, and
two 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
two 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, adjust 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 following the cyanide procedure.
Sulfides
Sulfides 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 that has been wetted with pH 4 Buffer Solution.
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