Anions in seawater - Determination of Anions in Natural and Treated Waters - page 377

monochloride, is indicated by the disappearance of the violet iodine colour from a
chloroform layer present in the titration flask.
3.18.2 Spectrophotometric method
Sugawara [105] has described a method for the determination of iodide in seawater.
Various workers [106,107] have modified this procedure.
Matthews and Riley [106] preconcentrated iodide by co-precipitation with chloride
ions. This is achieved by adding 0.23g silver nitrate per 500ml seawater sample.
Treatment of the precipitate with aqueous bromine and ultrasonic agitation promote
recovery of iodide as iodate which is caused to react with excess of iodide under acid
conditions, yielding I 3 − . This is determined either spectrophotometrically or by
photometric titration with sodium thiosulphate. Photometric titration gave a recovery of
99.0 ±0.4% and a coefficient of variation of ±0.4% compared with 98.5 ±0.6% and
±0.8% respectively for the spectrophotometric procedure.
Shizuo [107] allowed the silver halide precipitate obtained in the co-precipitation
process to stand in contact with the solution for more than 20h to ensure quantitative
collection of iodide on the precipitate. They then evaporated the oxidised iodate solution
to 5-10ml and again allowed the solution to stand for more than 12h before the
colorimetric determination. No interference occurred by bromine compounds. The errors
were then within ±3%.
3.18.3 Cathodic stripping voltammetry
Luther et al. [93] have described a procedure for the direct determination of iodide in
seawater. By use of cathodic stripping square wave voltammetry, it is possible to
determine low and sub-nanomolar levels of iodide in seawater, freshwater and brackish
water. Precision is typically ±5% (1σ). The minimum detection limit is 0.1-0.2nM (12
parts per trillion) at a 180s deposition time. Data obtained on Atlantic Ocean samples
show similar trends to previously reported iodine speciation data. This method is more
sensitive than previous methods by 1-2 orders of magnitude. Triton X-100 added to the
sample enhances the mercury electrode's sensitivity to iodine.
3.18.4 Ion chromatography
Ito et al. [99] have described an ion chromatographic method for determining trace iodide
in concentrated salt solutions. The method had a detection limit of 5µg of iodide L −1 , and
a relative standard deviation ( n =5) of 3% at 0.1mg of iodide L −1 .
In a more recent work Ito [103] has described a simple and highly sensitive ion
chromatographic method with ultraviolet detection for determining iodide in seawater. A
high-capacity anion-exchange resin with polystyrene-divinylbenzene matrix was used for
both preconcentration and separation of iodide. Iodide in artificial seawater (salinity, 35
‰) was trapped quantitatively (98.8±0.6%) without peak broadening on a
preconcentrator column and was separated with 0.35M sodium perchlorate+0.01M
phosphate buffer (pH 6.1). On the other hand, the major anions in seawater, chloride and