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Cyanide
Cyanide
Page 6 of 8
If thiocyanate is present in the original sample, a distillation step is absolutely
necessary as thiocyanate causes a positive interference. High concentrations of
thiocyanate can yield a substantial quantity of sulfide in the distillate. The
"rotten egg" smell of hydrogen sulfide will accompany the distillate when
sulfide is present. The sulfide must be removed from the distillate prior
to testing.
If cyanide is not present, the amount of thiocyanate can be determined. The
sample is not distilled and the final reading is multiplied by 2.2. The result is
–
mg/L SCN
.
The distillate can be tested and treated for sulfide after the last step of the
distillation procedure by using the following lead acetate treatment procedure.
a. Place a drop of the distillate (already diluted to 250 mL) on a disc of
Hydrogen Sulfide Test Paper (Cat. No. 25377-33) that has been wetted
with pH 4.0 Buffer Solution (Cat. No. 12223-49).
b. If the test paper darkens, add 2.5 N Hydrochloric Acid Standard Solution
(Cat. No. 1418-32) drop-wise to the distillate until a neutral pH is
obtained.
c. Add a 1-g measuring spoon of lead acetate (Cat. No. 7071-34) to the
distillate and mix. Repeat step 1.
d. If the test paper continues to turn dark, keep adding lead acetate until the
distillate tests negative for sulfide.
e. Filter the black lead sulfide precipitate through filter paper
(Cat. No. 1894-57) and a funnel (Cat. No. 1083-67). Neutralize the liquid
filtrate to pH 7 and immediately analyze for cyanide.
Distillation Procedure
The following steps describe the distillation process using distillation apparatus
(Cat. No. 27744-00 or -02 for 115 VAC or 230 VAC, respectively) and cyanide
glassware (Cat. No. 22658-00) offered by Hach:
a. Set up the distillation apparatus for cyanide recovery, leaving off the
thistle tube. Refer to the Hach Distillation Apparatus Manual. Turn on the
water and make certain it is flowing steadily through the condenser.
b. Fill the distillation apparatus cylinder to the 50-mL mark with 0.25 N
Sodium Hydroxide Standard Solution (Cat. No. 14763-53).
c. Fill a clean 250-mL graduated cylinder to the 250-mL mark with sample
and pour it into the distillation flask. Place a stirring bar into the flask
and attach the thistle tube.
d. Arrange the vacuum system as shown in the Hach Distillation Apparatus
Manual, but do not connect the vacuum tubing to the gas bubbler.
Turn on the water to the aspirator to full flow and adjust the flow meter
to 0.5 SCFH.
e. Connect the vacuum tubing to the gas bubbler, making certain that air
flow is maintained (check the flow meter) and that air is bubbling from
the thistle tube and the gas bubbler.
f. Turn the power switch on and set the stir control to 5. Using a 50-mL
graduated cylinder, pour 50 mL of 19.2 N Sulfuric Acid Standard
Solution (Cat. No. 2038-49) through the thistle tube and into the
distillation flask.
Cyanide_PP_Other_PPZ_Eng_Ody.fm
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