184
concentration « 5 mglL) do not dilute. Add
0.5 mL indicator solution.
b. Titrate with standard AgN03 titrant to
the first change in colour from a canary yellow to a salmon hue. Titrate a blank containing the same amount of alkali and water.
As the analyst becomes accustomed to the
end point, blank titrations decrease from the
high values usually experienced in the first
few trials to 1 drop or less. with a corresponding improvement in precision.
5. Calculation
(A - B) x 1000
mg CN/L = - - - - -
mL portion used
where:
250
x---------:mL portion used
A = mL standard AgN03 for sample and
B = mL standard AgN03 for blank.
6. Precision and Accuracy
For samples containing more than 1 mg CN/L
that have been distilled or for relatively clear
samples without significant interference, the
coefficient of variation is 2 %. Extraction and
removal of S2- or oxidizing agents tend to
increase the coefficient of variation to a
degree determined by the amount of manipulation and the type of sample. The limit of
sensitivity is approximately 0.1 mg CN /L, but
at this concentration the end point is indistinct. At 0.4 mglL, the coefficient of variation is four times that at CN concentration
levels> 1.0 mg/L.
412 D. Colorimetric Method
I. General Discussion
a. Principle: CW in the alkaline distillate
from preliminary treatment is converted to
CNCI by reaction with chloramine-Tat pH < 8
without hydrolysing to CNO-. (CAUTION:
CNCI is a toxic gas: avoid inhalation). After
the reaction is complete, CNCI forms a redblue dye on addition of a pyridine-barbituric
acid reagent. If the dye is kept in an aqueous
solution, the absorbance is read at 578 nm. To
obtain colours of comparable intensity, have
the same salt content in sample and standards.
4 The Measurement Process in Chemistry
b. Interference: All known interferences
are eliminated or reduced to a minimum by
distillation.
2. Apparatus
Colorimetric equipment: One of the following
is required:
a. Spectrophotometer, for use at 578 nm,
providing a light path of 10 mm or longer.
b. Filter photometer providing a light path
of at least 10 mm and equipped with a red
filter having maximum transmittance at 570
to 580 nm.
3. Reagents
a. Chloramine- T solution: Dissolve 1.0 g
white, water-soluble powder in 100 mL water.
Prepare weekly and store in refrigerator.
b. Stock cyanide solution: Dissolve approximately 2 g KOH and 2.51 g KCN in 1 L
distilled water. (CAUTION: KCN is highly
toxic: avoid contact or inhalation). Standardize against standard silver nitrate (AgN03)
titrant as described in Section 412C.4, using
25 mL KCN solution. Check titre weekly
because the solution gradually loses strength:
1 mL= 1 mgCN.
c. Standard cyanide solution: Based on the
concentration determined for the KCN stock
solution, calculate volume required (approximately 10 mL) to prepare 1 L of a 10 llg
eN/mL solution. Dilute with the 2 g NaOH/L
solution. Dilute 10 mL of the 10 llg CN/mL
solution to 100 mL with the 2 g NaOH/L solution: 1.0 mL = 1.0 llg CN. Prepare fresh daily
and keep in a glass-stoppered bottle. (CAUTION: Toxic -take care to avoid ingestion).
d. Pyridine-barbituric acid reagent: Place
15 g barbituric acid in a 250-mL volumetric
flask and add just enough water to wash sides
of flask and wet barbituric acid. Add 75 mL
pyridine and mix. Add 15 mL conc. hydrochloric acid (HCI), mix and cool to room
temperature. Dilute to mark with water and
mix. This reagent is stable for up to 1 month:
discard if a precipitate develops.
e. Sodium dihydrogen phosphate, 1 M:
Dissolve 138 g NaH2P04 • H20 in 1 L distilled
water. Refrigerate.
f. Sodium hydroxide solution: Dissolve 2 g
NaOH in 1 L distilled water.
4. Procedure
a. Preparation of calibration curve: Prepare
a blank ofNaOH solution. From the standard
concentration « 5 mglL) do not dilute. Add
0.5 mL indicator solution.
b. Titrate with standard AgN03 titrant to
the first change in colour from a canary yellow to a salmon hue. Titrate a blank containing the same amount of alkali and water.
As the analyst becomes accustomed to the
end point, blank titrations decrease from the
high values usually experienced in the first
few trials to 1 drop or less. with a corresponding improvement in precision.
5. Calculation
(A - B) x 1000
mg CN/L = - - - - -
mL portion used
where:
250
x---------:mL portion used
A = mL standard AgN03 for sample and
B = mL standard AgN03 for blank.
6. Precision and Accuracy
For samples containing more than 1 mg CN/L
that have been distilled or for relatively clear
samples without significant interference, the
coefficient of variation is 2 %. Extraction and
removal of S2- or oxidizing agents tend to
increase the coefficient of variation to a
degree determined by the amount of manipulation and the type of sample. The limit of
sensitivity is approximately 0.1 mg CN /L, but
at this concentration the end point is indistinct. At 0.4 mglL, the coefficient of variation is four times that at CN concentration
levels> 1.0 mg/L.
412 D. Colorimetric Method
I. General Discussion
a. Principle: CW in the alkaline distillate
from preliminary treatment is converted to
CNCI by reaction with chloramine-Tat pH < 8
without hydrolysing to CNO-. (CAUTION:
CNCI is a toxic gas: avoid inhalation). After
the reaction is complete, CNCI forms a redblue dye on addition of a pyridine-barbituric
acid reagent. If the dye is kept in an aqueous
solution, the absorbance is read at 578 nm. To
obtain colours of comparable intensity, have
the same salt content in sample and standards.
4 The Measurement Process in Chemistry
b. Interference: All known interferences
are eliminated or reduced to a minimum by
distillation.
2. Apparatus
Colorimetric equipment: One of the following
is required:
a. Spectrophotometer, for use at 578 nm,
providing a light path of 10 mm or longer.
b. Filter photometer providing a light path
of at least 10 mm and equipped with a red
filter having maximum transmittance at 570
to 580 nm.
3. Reagents
a. Chloramine- T solution: Dissolve 1.0 g
white, water-soluble powder in 100 mL water.
Prepare weekly and store in refrigerator.
b. Stock cyanide solution: Dissolve approximately 2 g KOH and 2.51 g KCN in 1 L
distilled water. (CAUTION: KCN is highly
toxic: avoid contact or inhalation). Standardize against standard silver nitrate (AgN03)
titrant as described in Section 412C.4, using
25 mL KCN solution. Check titre weekly
because the solution gradually loses strength:
1 mL= 1 mgCN.
c. Standard cyanide solution: Based on the
concentration determined for the KCN stock
solution, calculate volume required (approximately 10 mL) to prepare 1 L of a 10 llg
eN/mL solution. Dilute with the 2 g NaOH/L
solution. Dilute 10 mL of the 10 llg CN/mL
solution to 100 mL with the 2 g NaOH/L solution: 1.0 mL = 1.0 llg CN. Prepare fresh daily
and keep in a glass-stoppered bottle. (CAUTION: Toxic -take care to avoid ingestion).
d. Pyridine-barbituric acid reagent: Place
15 g barbituric acid in a 250-mL volumetric
flask and add just enough water to wash sides
of flask and wet barbituric acid. Add 75 mL
pyridine and mix. Add 15 mL conc. hydrochloric acid (HCI), mix and cool to room
temperature. Dilute to mark with water and
mix. This reagent is stable for up to 1 month:
discard if a precipitate develops.
e. Sodium dihydrogen phosphate, 1 M:
Dissolve 138 g NaH2P04 • H20 in 1 L distilled
water. Refrigerate.
f. Sodium hydroxide solution: Dissolve 2 g
NaOH in 1 L distilled water.
4. Procedure
a. Preparation of calibration curve: Prepare
a blank ofNaOH solution. From the standard