Anions in surface, ground, mineral, borehole and pure waters - Determination of Anions in Natural and Treated Waters - page 457

oven at 40-45°C The precipitate is drawn off from the membrane and collected in the
reaction vessel.
To convert bromide to bromate the isolated precipitate is poured into 5ml distilled
water and treated with 4ml 0.35mol L −1 hypochlorous acid/ sodium hypochloride at pH 7.
(This reagent is obtained by partial neutralisation with a normal solution of sulphuric acid
AnalaR of a sodium hypochlorite solution of appropriate grade, eg BDH AnalaR, low in
bromide).
After mixing, the 9ml of solution is evaporated to dryness on an IR stand equipped
with a 350W lamp. At this stage, bromide is quantitatively oxidised to bromate. The
residue remaining after evaporation is taken up in 20ml distilled water and transferred
into a 25ml volumetric flask. The pH is adjusted to 7.5 with N sulphuric acid, then the
volume is adjusted to 25ml.
The polarograph is set in the pulse mode under the following conditions: nitrogen
stripping 10min, mercury dropping frequency 0.5s, scanning rate 5mV s −1 . The half-wave
potential for bromate in these experimental conditions is 0.64V vs SCE.
The reaction medium is a self-supporting electrolyte. No further additions are
necessary and may even hinder by their side effects such as formation of deposits and
coprecipitation of bromate (with calcium chloride), superposition of the waves (with
alkali-chlorides) and finally hindering convection currents (with lanthanum trichloride).
A stable diffusion current is obtained at pH 1-2.
Using this method Shirashi et al. [14] found levels of bromide in the range 20-280mg
L −1 in Belgian ground water samples.

6.2.3.3 Ion chromatography

Ion chromatography using an amperometric detector has been used to determine down to
10µg L −1 bromide in rain water and groundwaters [15]. The equipment features an
automated ion chromatograph, including a programme controller, an automatic sampler,
an integrator and an amperometric detector. A fixed potential is applied to the cell. Any
electroactive species having an oxidation-reduction potential near the applied potential
will generate a current that is directly proportional to the concentration of the
electroactive species.
The detection limit for bromide is 0.01mg L −1 and the relative standard deviation is
less than 5% for bromide concentrations between 0.05 and 0.5mg L −1 . Chloride interferes
if the chloride to bromide ratio is greater than 1000:1 for a range of 0.01-0.1mg L −1
bromide; similarly, chloride interferes in the 0.1-1.0mg L −1

range if the ratio is greater

than 5000:1. In the latter case, a maximum of 2000mg L −1

of chloride can be tolerated.

Recoveries of bromide ranged from 97 to 110%.

6.2.4 Iodide

6.2.4.1 Spectrophotometric method

Iodide has been determined in groundwaters [16] by utilising the reaction between cerium
(IV) and arsenic(III) which is catalysed by iodide ion. This reaction is stopped by the