Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 157

cement to a solid glass support rod (Fig. 2.20). The isolation and concentration of
hydrogen cyanide from an aqueous solution is accomplished by adding 1.5L of sample to
the flask and sealing with a stopper. After a short equilibration period either 8.5 or 20.5ml
of 0.02N sodium hydroxide is pipetted through a small hole in the large stopper into the
petri dish suspended in the air space above the sample. Seal the system and absorb the
hydrogen cyanide diffusing from the surface of the test solution into the alkali solution.
The air space between the sample and alkali serves to separate the nonvolatile and ionic
species in solution from the collected hydrogen cyanide. A 2h collection period is
generally sufficient to allow for an analytically measurable amount of hydrogen cyanide
to be transferred from the sample to the sodium hydroxide. The small stopper in the lid is
removed and an 8 or 20ml aliquot of alkali absorbent containing the dissolved hydrogen
cyanide is transferred to a 10 or 25ml volumetric flask, respectively.
In the spectrometric finish, 25ml portions of solutions referred to above are acidified to
about pH 5.6 with 3 or 6 drops of 2.25N acetic acid, respectively. Four drops of
chloramine-T solution (1.0g 100mL −1 ) are added and the solution left to stand for about
2min. A single solution containing a phosphate buffer and chloramine-T [247] can be
used to lower the pH of the 0.02N sodium hydroxide and to halogenate the cyanide [247].
Then 1.0 or 2.5ml respectively of the pyridine-barbituric acid reagent [24] are added and
the solution made up to the mark with deionised water. After about 10min the intensity of
colour produced at 582nm is measured against a blank. The cyanide content expressed as
hydrogen cyanide is determined from a suitable calibration curve.
The detection limit achieved in the diffusion method was approximately 1.3µg cyanide
L −1 . The amount of hydrogen cyanide collected by the diffusion procedure is temperature
dependent. This effect is minimal, however, in the temperature range of 15°C to about
22°C. At higher temperatures the rate of hydrogen cyanide displacement increases
rapidly.
In the diffusion method, ammonia (50mg L −1 ), sulphide (1mg L −1 ) and phenol (50mg
L −1 ) resulted in only about an 8%, 13% and 0% reduction in the coloured development
by the pyridine-barbituric acid method for solutions containing 40µg cyanide L −1 . The
displacement of hydrogen cyanide by diffusion from 1.5L test solutions containing 25µg
cyanide L −1 chemicals: 130 mg L −1 chloride, 7.4mg L −1 SO 4 2− , 3.0mg L −1 NO 3 − ,
100mg was virtually unaffected by the individual presence of the following L −1 SCN − ,
50mg L −1 NH 3 − , 0.1mg L −1 Na 2 S-S, 50mg L −1 phenol, 50mg L −1 o-cresol, 5mg L −1
Zn 2+ , 5mg L −1 Cd 2+ , 5mg L −1 Pd 2+ , 1.0mg L −1 Fe(CN) 6 4− or Fe(CN) 6 3− .

Table 2.16 Determination of HCN by the diffusion procedure for sodium cyanide spiked
water samples at 20°C

HCN conc (µgL −1 )

Amount of total cyanide added (as
HCN)

(µgL −1 )
Expecteda Sample pH value Natural waters Determined

Well 7.92 27.2 26.2 18.8