Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 548

then 2.5mole L −1 sulphuric acid is added drop-wise until the solution becomes just red
coloured to methyl red indicator. The solution is then rendered just alkaline (ie yellow
colour) and buffered to pH 4.0. The solution is then heated and purged with air to liberate
hydrogen cyanide which is collected in cadmium nitrate solution. Cyanide is then
determined in this solution using a cyanide electrode (silver iodide silver sulphide, Orion
94-06 or equivalent) and a reference electrode (Orion 94-02 double junction or
equivalent).
In many of the previously described procedures for total cyanide, high acidity is used
as a means of decomposing complex cyanides prior to the determination of free cyanide
ion. Csikai and Barnard [54] used a low acidity decomposition procedure (EDTA
displacement at pH 4) and, in so doing, avoid interference by thiocyanate as illustrated in
Table 8.17.

Table 8.18 Recovery of cyanide from complex metal cyanides by EDTA displacement
procedure without sulphamate added

Sample
nature a

Thiocyanate nitrate
(present+added)

Cyanide (total)

mg L −1 mg L −1 mg L −1

present

mg
L −1
added

mg
L −1
found

%
Recovered

Cyanide added
as complex c of

DIW 0 0 0 1.00 1.00 100 Fe

DIW 0 0 0 1.00 1.00 100 Fe

DIW 0 0 0 1.00 0.99 99 Fe

DIW 0 0 0 1.00 0.96 96 Fe

DIW 0 0 0 5.00 4.95 99 Fe

DIW 0 0 0 5.00 4.85 97 Fe

DIW 0 0 0 1.00 1.00 100 Fe

DIW 0 0 0 1.04 1.07 103 Ni

DIW 10 100 0 1.04 0.81 78 Ni

DIW 10 100 0 5.00 4.95 99 Fe

DIW 10 100 0 1.00 1.01 101 Fe

10 b

DIW 100 0 1.00 0.91 91 Fe

DIW 10 100 0 1.00 1.01 101 Fe

DIW 10 100 0 5.00 5.04 101 Fe

DIW 10 100 0 5.00 5.17 103 Fe

DIW 50 200 0 1.00 1.00 100 Fe

DIW 50 1000 0 1.00 1.00 100 Fe