Annex II
4. Procedure
a. Add 500 mL sample, containing no more
than 100 mg CN/L (diluted, if necessary,
with distilled water) to the boiling flask. Add
50 mL NaOH solution to the gas washer and
dilute, if necessary, with distilled water to
obtain an adequate depth of liquid in the
absorber. Connect the train, consisting of
boiling flask air inlet flask, condenser, gas
washer, suction flask trap and aspirator.
Adjust suction so that approximately 1 air
bubble/s enters the boiling flask. This air rate
will carry HCN gas from flask to absorber
and usually will prevent a reverse flow of
HCN through the air inlet. If this air rate does
not prevent sample back-up in the delivery
tube, increase air-flow rate to 2 air bubbles/s.
Observe air purge rate in the absorber, where
the liquid level should be raised not more
than 6.5 to 10 mm. Maintain air flow throughout the reaction.
b. Add 50 mL 1 + 1 H2S04 through the air
inlet tube. Rinse tube with distilled water and
let air mix flask contents for 3 min. Add
20 mL MgCI2 reagent through air inlet and
wash down with stream of water. A precipitate that may form redissolves on heating.
C. Heat with rapid boiling, but do not
flood condenser inlet or permit vapours to
rise more than halfway into condenser.
Adequate refluxing is indicated by a reflux
rate of 40 to 50 drops/min from the condenser lip. Reflux for at least 1 h. Discontinue
heating but continue air flow. Cool for 15 min
and drain gas washer contents into a separate
container. Rinse connecting tube between
condenser and gas washer with distilled
water, add rinse water to drained liquid and
dilute to 250 mL in a volumetric flask.
d. Determine cyanide content by titration
method (C) if cyanide concentration exceeds
1 mg/L or by the . colorimetric method (D) if
the cyanide concentration' is less. Use titration, the electrodeoprohe method on the spot
test to approximate eN. ' anntent. Alternatively, use the cyanide-sell!ctive:a!ectrodein the
concentration range 0.05 to, lOmg' , «N/L
(Method E).
e. Distillation gives quantitative recovery
of even refractory cyanides such as iron
complexes. To obtain complete recovery of
cobalticyanide use ultraviolet radiation pretreatment. If incomplete recovery is suspected, distill again by refilling the gas
washer with a fresh charge of NaOH solution
183
and refluxing 1 h more. The cyanide from the
second reflux, if any, will indicate completeness of recovery.
f. As a quality control measure, periodically test apparatus, reagents and other
potential variables in the concentration
range of interest. As an example, a minimum
98% recovery from 1 mg CN/L standard
should be obtained.
412 C. Titrimetric Method
1. General Discussion
a. Principle: CW in the alkaline distillate
from the preliminary treatment procedure is
titrated with standard silver nitrate (AgN03)
to form the soluble cyanide complex,
Ag(CN)\O;. As soon as all CW has been
complexed and a small excess of Ag+ has
been added, the excess Ag+ is detected by the
silver-sensitive indicator, p-dimethylaminobenzalrhodanine, which immediately turns
from a yellow to a salmon colour. The distillation has provided a 2: 1 concentration. The
indicator is sensitive to about 0.1 mg Ag/L. If
titration shows that CW is below 1 mg/L,
examine another portion colorimetrically.
2. Apparatus
Koch microburette. 5-mL capacity.
3. Reagents
a, Sodium hydroxide solution, NaOH, 1 N.
b. Sulphuric acid solution, H2S04 , 1 N.
c. Indicator solution: Dissolve 20 mg pdimethylaminobenzalrhodanine in 100 mL
acetone.
d. Standard silver nitrate titrant, 0.0192
N: Dissolve 3.27 gAgN03 in 1 L distilled water.
Standardize against standard NaCI solution,
using the argentometric method with K2Cr04
indicator, as directed in Chloride, Section
407A.
Dilute 500 mL AgN03 solution according
to the titre found so that 1.00 mL is equivalent to 1.00 mg CN.
4. Procedure
a. From the absorption solution take a
measured volume of sample so that the titration will require approximately 1 to 10 mL
AgN03 titrant. Dilute to 250 mL or some
other convenient volume to be used for all
titrations. For samples with low cyanide
4. Procedure
a. Add 500 mL sample, containing no more
than 100 mg CN/L (diluted, if necessary,
with distilled water) to the boiling flask. Add
50 mL NaOH solution to the gas washer and
dilute, if necessary, with distilled water to
obtain an adequate depth of liquid in the
absorber. Connect the train, consisting of
boiling flask air inlet flask, condenser, gas
washer, suction flask trap and aspirator.
Adjust suction so that approximately 1 air
bubble/s enters the boiling flask. This air rate
will carry HCN gas from flask to absorber
and usually will prevent a reverse flow of
HCN through the air inlet. If this air rate does
not prevent sample back-up in the delivery
tube, increase air-flow rate to 2 air bubbles/s.
Observe air purge rate in the absorber, where
the liquid level should be raised not more
than 6.5 to 10 mm. Maintain air flow throughout the reaction.
b. Add 50 mL 1 + 1 H2S04 through the air
inlet tube. Rinse tube with distilled water and
let air mix flask contents for 3 min. Add
20 mL MgCI2 reagent through air inlet and
wash down with stream of water. A precipitate that may form redissolves on heating.
C. Heat with rapid boiling, but do not
flood condenser inlet or permit vapours to
rise more than halfway into condenser.
Adequate refluxing is indicated by a reflux
rate of 40 to 50 drops/min from the condenser lip. Reflux for at least 1 h. Discontinue
heating but continue air flow. Cool for 15 min
and drain gas washer contents into a separate
container. Rinse connecting tube between
condenser and gas washer with distilled
water, add rinse water to drained liquid and
dilute to 250 mL in a volumetric flask.
d. Determine cyanide content by titration
method (C) if cyanide concentration exceeds
1 mg/L or by the . colorimetric method (D) if
the cyanide concentration' is less. Use titration, the electrodeoprohe method on the spot
test to approximate eN. ' anntent. Alternatively, use the cyanide-sell!ctive:a!ectrodein the
concentration range 0.05 to, lOmg' , «N/L
(Method E).
e. Distillation gives quantitative recovery
of even refractory cyanides such as iron
complexes. To obtain complete recovery of
cobalticyanide use ultraviolet radiation pretreatment. If incomplete recovery is suspected, distill again by refilling the gas
washer with a fresh charge of NaOH solution
183
and refluxing 1 h more. The cyanide from the
second reflux, if any, will indicate completeness of recovery.
f. As a quality control measure, periodically test apparatus, reagents and other
potential variables in the concentration
range of interest. As an example, a minimum
98% recovery from 1 mg CN/L standard
should be obtained.
412 C. Titrimetric Method
1. General Discussion
a. Principle: CW in the alkaline distillate
from the preliminary treatment procedure is
titrated with standard silver nitrate (AgN03)
to form the soluble cyanide complex,
Ag(CN)\O;. As soon as all CW has been
complexed and a small excess of Ag+ has
been added, the excess Ag+ is detected by the
silver-sensitive indicator, p-dimethylaminobenzalrhodanine, which immediately turns
from a yellow to a salmon colour. The distillation has provided a 2: 1 concentration. The
indicator is sensitive to about 0.1 mg Ag/L. If
titration shows that CW is below 1 mg/L,
examine another portion colorimetrically.
2. Apparatus
Koch microburette. 5-mL capacity.
3. Reagents
a, Sodium hydroxide solution, NaOH, 1 N.
b. Sulphuric acid solution, H2S04 , 1 N.
c. Indicator solution: Dissolve 20 mg pdimethylaminobenzalrhodanine in 100 mL
acetone.
d. Standard silver nitrate titrant, 0.0192
N: Dissolve 3.27 gAgN03 in 1 L distilled water.
Standardize against standard NaCI solution,
using the argentometric method with K2Cr04
indicator, as directed in Chloride, Section
407A.
Dilute 500 mL AgN03 solution according
to the titre found so that 1.00 mL is equivalent to 1.00 mg CN.
4. Procedure
a. From the absorption solution take a
measured volume of sample so that the titration will require approximately 1 to 10 mL
AgN03 titrant. Dilute to 250 mL or some
other convenient volume to be used for all
titrations. For samples with low cyanide