Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 587

Fig. 10.2 Manifold arrangement and cylindrical flow through electrode used for
the determination of cyanide. All tubes were 0.5mm bore
Source: Reproduced with permission from Elsevier Science [15]
The current was continuously monitored by measuring the voltage drop across a 10kΩ
precision resistor connected in series with the counter electrode, with an electronic
voltameter (Radiometer GVM 30) coupled to a Varian A-25 strip chart recorder.
Current voltage curves were recorded with a PAR 174 polarographic analyser under
hydrodynamic conditions and at a scan rate of 1mV sec −1 . The relationship between
limiting current and velocity of the fluid was measured with the aid of a waters M-5000
A pump at volume flow rates of between 0.1 and 10ml min −1 .
Free cyanide was separated by micro diffusion in Conway cells. The volume of 0.1mol
L −1 sodium hydroxide used as absorbing solution was between 0.1 and 1.0ml according
to the amount of free cyanide present in the sample. The free cyanide released upon
acidifying the samples (up to 5ml) with 0.5ml of 3mol L −1 sulphuric acid was
quantitatively recovered after 3h of distillation at room temperature (20-25°C). The
absorption solution was subsequently injected directly into the analyser. A 0.1mol L −1
solution of sodium hydroxide was used as the carrier stream.
Of the numerous substances tested, only sulphide, iodide and thiocyanate interfere
seriously in this method and they can be eliminated by distillation or precipitation.
Oxidants such as hypochlorite or hydrogen peroxide, which are common agents for
detoxification of cyanide effluents, must be fixed by addition of arsenite or sulphite since
they are reduced at the silver electrode and this results in a cathodic current. This is a
disadvantage of the method compared with potentiometric techniques in which oxidants
and reductants usually do not interfere. However, such compounds can be eliminated by
simple chemical treatment ordinarily performed at the sampling stage.