Hach DR/850 Procedures Manual page 161

CYANIDE,
continued
c) Add Sodium Arsenite Solution drop-wise until the sample
turns colorless. Swirl the sample thoroughly after each
drop. Count the number of drops.
d) Take another 25-mL sample and add the total number of
drops of Hydrochloric Acid Standard Solution counted in
Step a.
e) Subtract one drop from the amount of Sodium Arsenite
Solution added in Step c. Add this amount to the sample and
mix thoroughly.
f) Using 10 mL of this sample, continue with Step 3 of the
cyanide procedure.
Reducing Agents
a) Adjust a 25-mL portion of the alkaline sample to between
pH 7 and 9 with 2.5 N Hydrochloric Acid Standard
Solution. Count the number of drops added.
b) Add four drops of Potassium Iodide Solution and four
drops of Starch Indicator Solution to the sample. Swirl to
mix. The sample should be colorless.
c) Add Bromine Water drop-wise until a blue color appears.
Count the number of drops, and swirl the sample after the
addition of each drop.
d) Take another 25 mL sample and add the total number of
drops of Hydrochloric Acid Standard Solution counted in
Step a.
e) Add the total number of drops of Bromine Water counted
in Step c to the sample and mix thoroughly.
f) Using 10 mL of this sample, continue with Step 3 of the
cyanide procedure.
Metals
Nickel or cobalt in concentrations up to 1 mg/L do not interfere.
Eliminate the interference from up to 20 mg/L copper and 5 mg/L
iron by adding the contents of one HexaVer Chelating Reagent
Powder Pillow to the sample and then mixing before adding the
CyaniVer 3 Cyanide Reagent Powder Pillow in Step 4. Prepare a
reagent blank of deionized water and reagents to zero the
instrument in Step 13.
161