SOP for ESTIMATION OF CYANIDE

PRINCIPLE:

Hydrogen cyanide is liberated from an acidification sample by distillation and purging with

 air. The HCN gas is collected by passing it through an NaOH scrubbing solution. Cyanide 

concentration in scrubbing solution is the determined by colorimetric procedure

REAGENTS:

1. Sodium hydroxide solution: Dissolve 40 gm of NaOH in water and dilute to 1 liter
2. Magnesium chloride reagent : Dissolve 510 gm of magnesium chloride in water             and dilute to 1 liter.
3. Sulphuric acid : 1 + 1
4. Lead acetate
5. Sulfamic acid

PROCEDURE FOR DISTILLATION:

1.    Add 500 ml sample containing not more than 10 mg CN / L (dilute if necessary with distilled water) 

to the boiling flask. If a higher CN content is anticipated, use the spot test to approximate the required dilution. Add 10 ml NaOH solution to the gas scrubber and dilute, if necessary, with distilled water

 to obtain an adequate liquid depth in the absorber. Do not use more than 225 ml total volume of 

absorber solution.

When S^2- generate from the distilling flask is anticipated add 50 or more mg powdered PbCO₃ to the absorber solution to precipitates.

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 enter 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 backup 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.

2.    Add 2 g sulfamic acid through the air inlet tube and wash down with distilled water.

3.    Add 50 ml 1 + 1 H2SO4 through the air inlet tube. Rinse tube with distilled water and let air mix

 flask content for 3 min. Add 20 ml MgCl2 reagent through air inlet and wash down with stream of 

water. A precipitate that may form re-dissolves on heating.

4.    Heat with rapid boiling, but do not flood condenser inlet or permit vapors to rise more than half way

 into condenser. Adequate refluxing is indicated by a reflux rate of 40 to 50 drop/min from the 

condenser lip. Reflux for at least 1 hr. Discontinue heating but continue air flow for 15 min. Cool and quantitatively transfer absorption solution to a 250 ml volumetric flask. Rinse absorber and its

 connecting tubing sparingly with distilled water and add to flask. Dilute to volume with distilled water 

and mix thoroughly.

 

5.    Determine cyanide concentration in the absorption solution by colorimetric procedure.

COLORIMETRIC METHOD

PRINCIPLE: CN in alkaline distillate is converted to cyanogenchloride by reaction with

 chloramines – T solution at ph < 8 without hydrolyzing to CNO^-. CHCl forms red – blue 

colour on addition of pyridine – barbituric acid reagent.

MINIMUM DECTION LIMIT: 0.01 mg/l

REAGENTS:

1. Chloramine – T solution: Dissolve 1 gm of chloramines -T powder in 100 ml distilled water.

2. Stock Cyanide solution: Dissolve 1.6 gm of NaOH and 2.51 gm of KCN in distilled water. 1 ml = 1 mg

3. Standard Cyanide solution: Dilute 10 ml of stock solution to 1 lit with NaOH dilution 

solution. 1 ml = 10μ CN

4. Working cyanide solution: Dilute 10 ml of standard cyanide solution to 100 ml with 

NaOH dilution solution. 1.0 ml = 1.0μ CN

6.    Pyridine – barbituric acid solution: Place 15 gm of barbituric acid in 250 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 (HCl), mix, and cool to room

 temperature, Dilute to volume and mix until barbituric acid is dissolved.

      The solution is stable for approximately 6 months if stored in an amber bottle 

under refrigeration; discard if precipitate develops.

7.    Acetate buffer: Dissolved 410 g sodium acetate trihydrate, NaC₂H₃O₂.3H₂O in 500 

ml of water. Add glacial acetic acid to adjust to pH 4.5, approximately 500 ml.

8.    Sodium hydroxide dilution solution: Dissolve 1.6 g NaOH in 1 L distilled water.

PROCEDURE:

a)    Preparation of standard curve: Pipette a series of standards containing 1 to 10 µg CN

 into 50 ml volumetric flask (0.02 to 0.2 µg CN/ ml). Dilute to 40 ml with NaOH dilution solution. Use 40 ml of NaOH dilution solution as blank. Develop and measure absorbance in 10 mm
cells as described in below for both standard and blank. For concentrations lower than 

0.02 µg CN/ml use 100 mm cells.

Recheck Calibration curve periodically and each time a new reagent is prepared.

b)   Colour development: Pipette a portion of absorption solution into a 50 ml volumetric 

flask and dilute to 40 ml with NaOH dilution solution. Add 1 ml acetate buffer and 2 ml 

chloramines-T solution, stopper, and mix by inversion twice. Let stand exactly 2 min.

Add 5 ml pyridine-barbituric acid reagent, dilute to volume with distilled water, mix 

thoroughly, and let stand exactly 8 min. Measure absorbance against distilled water at 578 nm.

Measure absorbance of blank (0.0 mg µg CN/L) using 40 ml NaOH dilution solution and

 procedure for colour development.

CALCULATION:

CYANIDE (CN^-) mg/L  =  ABS / SLOPE

 

REFERENCE:  APHA – 4500-CN ^– E