Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 586

reagent is added, and an optical absorbance measurement of the resultant blue dye is
made at 620nm wavelength. Volatile aldehydes and ketones interfere in the final colour
development. The detection limit is about 10µg L −1 .
The micro diffusion chloramine-T spectrophotometric method [13] for the
determination of free cyanide in non saline waters has been applied to industrial effluents.
10.4.3 Spectrofluorometry
McKinney et al. [14] have described a rapid fluorometric method for the determination of
cyanide in mine waters. The method is based on the liberation of naphtho (2, 3-c) (1, 2,
5) selenadiazole from the reagent Pd 2 1 2 Cl 4 by cyanide at 25°C and pH 8. The naphthol
(2, 3-c) (1, 2, 5) selenadiazole is then extracted into hexane and its fluorescence is
measured at 520nm with excitation at 377nm. The calibration graph is approximately
rectilinear for up to 100µg of cyanide per 3ml of hexane. Mercuric ions and sulphide
interfered.
10.4.4 Continuous flow analyses
Pihlar et al. [15] developed the amperometric method of determining cyanides in
industrial effluents to permit reliable and reproducible continuous flow measurements
and flow injection analysis of discrete samples of galvanising plant effluents at the µg L −1
level. The method is based on the measurement of the diffusion current arising from the
oxidation of silver to dicyanoargenate(I). By use of the flow injection principle and a
manifold arrangement, absolute amounts of less than one nanogram of cyanide can be
determined precisely in volumes as small as 10µL and at a rate exceeding 100 samples
per h.
Fig. 10.2 outlines a straight manifold arrangement and cylindrical flow through
electrode. For operation in the injection mode, the sample is introduced into the carrier
stream by means of a microsyringe through a septum in the injector and passes through
20cm of 0.5mm bore tubing before entering the detector. The amperometric flow through
electrode consists of a 0.9mm diameter silver wire (99.9% Ag) mounted centrally by
means of two Teflon stoppers in a stainless steel (or nickel) tube of 1.9mm bore which
serves as a counter electrode. The cell hold-up volume is 130µL. The mercurous sulphate
reference electrode is bridged into the cell through capillary tubing as shown in Fig. 10.2.
The potential of the silver electrode is controlled at −0.5V vs the MSE by means of a
potentiostat.