Thiis method may be applied to absorption solution which contain 0.002 to 0.025 mg of cyanide. Absorption solutions with higher cyanide content may be dilute with sodium hydroxide solutions, c(NaOH) = 0.4 mo;/L. This method is not applicable if oxides or sulfur dioxide reach the absorption vessel during separation of the cyanides. Other interferences include substances that influence the action of the chloramine-T so;ution.
Principle
Reaction of the cyanide ions with the active chlorine of chloramine-T, leading to the formation of cyanogen chloride which reacts with pyridine to form a glutacondialdehyde, which in turn condenses with two molesof barbituric acid to form a red-violet dye.
Reagents
Sodium hydroxide, solution, c(NaOH) = 0.4 mol/L
Potassium cyanide, c(KCN) = 10 mg/L of CN - , dissolve 25 mg of potassium cyanide in the sodium hydroxide solution and dilute with the same sodium hydroxide solution to 1000 mL in volumetric flask.
Chloramine-T (C7H7ClNNaO2S3H2O), solution, dissolve 0.5 g of chloramine-T trihydrate in water in a 50 mL volumetric flask and dilute to the mark.
Pyridine/barbituric acid, solution, place 3 g of barbituric acid in a 50 mL volumetric flask, wash down the walls of the flask with just enough water to moisten the barbituric acid add 15 mL 0f pyridine and swirl to mix. Add 3 mL of hydrochloric acid solution 1.12 g/mL and dilute to the mark with water.
Equipment
Photometer with cells of optical path length 10 mm.
Usual laboratory equipment.
Procedure
Transfer the contents of the absorption vessel to a 25 mL volumetric flask. Rinse the absorption vessel three times with approximately 3 mL portions of water, transfer the rinsings to the flask, dilute to the mark with water and mix.
Transfer by means of a pipette, 10 mL of this solution into second 25 mL volumetric flaskand add whilst mixing , 2 mL of the buffer solution of pH 5.4, 4 ml of the hydrochloric solution 1.12 g/mL and 1 mL of the chloramine-T solution 0.5 g/50 mL Stopper the flask and leave for 5 mnt.
Add 3 mL of the pyridine/barbituric acid solution, dilute with water to the mark and mix.
Measure the absorbance at 578 nm in a cell of optical path length 10 mm against a reference liquid. Carry out the measurement 20 mnt after addition of the pyridine/barbituric acid solution.
Measure the absorbance of the blank test solution similarly.
Preparation of standard solutions
Transfer, by means a pipette 2, 5, 20 and 25 mL respectively of the standard potassium cyanide solution into series of four 250 mL volumetric flask. Dilute to the mark with sodium hydroxide solution 0.4 mol/L and mix.
Photometric measurements
Measurement the absorbance of the standard and the blank tes solution with photometer.
Plotting the graph
Plot a graph of absorbance against the cyanide contents, in mg, of the solutions. The relationship between absorbance and concentration is linier. Check the graph from time to time especially if new packages of chemicals are used.
Check the absolute values of the standard solutions by titration with silver nitrate solution.
Expression of result
Total cyanide = (ma-mb) x 1000 / f1 x f2 x Vs = mg/L
ma = is the cyanide content, in mg, of the test solution read from the calibration graph.
mb = is the cyanide content, in mg, of the blank test solution
Vs = Is the volume, in mL, , of the sample
f1 = 0.4, as only 40% of the contents of the absorption vessel are used for the determination
f2 = 0.97as the volume of the sample is increased by addition of preservatives immediately after sampling.
Literature
ISO 6703/1-1984
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