Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 493

Table 7.7 Amounts of free iodide and total inorganic iodine in a range of United
Kingdom potable waters

Sample source

Total iodine /µg L −1 Free iodide /µg L −1

London (borehole) 28.0 28.0

Harrogate 2.2 0.9

File 7.8 7.8

Bristol 4.1 2.1

Nottingham 14 3.5

Amesbury 4.3 0.9

Oxford 4.3 1.3

Gloucester 3.2 3.2

Dunoon 1.1 0.9

Nuneaton 2.8 1.6

Source: Reproduced with permission from the Royal Society of Chemistry [38]

(b) reducing the concentration of the sodium chloride reagent; and
(c) reducing the concentration of the potassium thiocyanate reagent.

These changes caused a corresponding decrease in sensitivity. The manifold system
shown in Fig. 7.8 has two different sets of reagents. Set A was used for the determination
of total inorganic iodine in water over the range 0.2-5.0µg L −1 of iodide and set B was
used for the determination of free iodide in water over the range 0.4-5.0µg L −1 of iodide.
A comparison of the concentration of free iodide and total inorganic iodine in a range of
potable waters determined by the methods described below is shown in Table 7.7.
The results in Table 7.8 show that it is necessary to adopt a standardised sampling
procedure. Samples were made up in 0.3% potassium carbonate solution in
polypropylene calibrated flasks, centrifuged and then stored in polystyrene bottles (30ml
universal containers, Sterilin Ltd., Teddington, Middlesex). Fresh potable water was
treated in the same way, but allowed to stand for 3 days before analysis.

Reagents for the determination of total inorganic iodine (System A)
All chemicals used should be of analytical-reagent grade and glass distilled water should
be used in preference to deionised water.
Standard iodide solution—4g L −1 . Dissolve 0.5232g of potassium iodide, previously
dried in an oven at 105°C for 2h, in distilled water and dilute to 100ml in a calibrated
flask.
Standard iodide solution—40mg L −1 . Dilute 10ml of the standard iodide solution (4g
L −1 of iodide) to 1000ml with distilled water in a calibrated flask (stable for 1 month).