Anions in seawater - Determination of Anions in Natural and Treated Waters - page 372

and excess iodide to give iodonium ions, I 3 − , which are detected spectrophotometrically
at 350nm. In the total-iodine procedure a pre-oxidation step is therefore required; here
bromine-water was chosen. Interference from nitrite ions, which in acid also oxidise
iodide to iodonium ions, is suppressed by sulphamic acid which destroys the nitrite ions.
Originally the objective of this work was only the automatic iodate-iodine procedure.
This was needed to give the extra precision called for in an earlier study [79] which
suggested that most of the observed variation in iodate-iodine results for the deeper
waters (>200m) of the oceans is due to analytical imprecision. However, in addition to
the original objective the procedure for total-iodine has developed. This development
stemmed from the need to test the likelihood of iodonium ions, produced in the iodate
procedure, being reduced by substances occurring naturally in seawater. This problem
was defined when Truesdale [80] showed that molecular iodine is reduced rapidly in
some seawaters; he found that the oxidising capacity of 260µg L −1 of iodine-iodine added
to a filtered coastal seawater disappeared within 30min. The effect of such a process on
the iodate method would be to lower the observed iodate concentrations. Further, the
effect, if it occurred, would be expected to have its maximum effect in coastal and
oceanic surface waters where primary productivity is highest; these waters also appear to
contain the lowest recorded iodate concentrations. To test for the iodonium-ion reduction,
a pre-oxidation step including iodine-iodine was incorporated in the analytical method for
iodate-iodine. Having accomplished the iodine-water pre-oxidation step successfully, it
was logical to attempt the bromine-water pre-oxidation and thereby produce the total-
iodine method.
Methods for the following three determinations were described by Truesdale [78].
(1) Determination of iodate without pre-oxidation
Iodate in the buffered sample is reacted with sulphamic acid (to destroy nitrite) and
potassium iodide to produce the iodonium ion I 3 − which is determined
spectrophotometrically at 350nm.
(2) Determination of iodate with pre-oxidation
Iodine water is added to an acetic acid sodium acetate buffered sample to reoxidise
to iodate any iodine-containing substances produced by reduction of iodate by
naturally occurring reducing substances present in the sample. Total iodate (ie iodate
present in the original sample as iodate plus additional iodate produced by iodine
water treatment) is then reacted with phenol solution at pH 5.4 to destroy excess free
iodine and then with sulphamic acid and potassium iodide to produce the iodonium
ion which is estimated spectrophotometrically at 350nm.
This determination will test for the presence of naturally occurring reducing agents
in seawater which by their action on iodonium ions could lead to an underestimate in
iodate concentration. (The use of the method on anoxic waters containing sulphide is
a prime example of when this precaution should be taken.)
(3) Determination of total iodine with pre-oxidation
This procedure is the same as that described under (2) except that the iodine water is
replaced by bromine water, and the buffer contains added sodium bromide (ie
sodium bromide-acetic acid-sodium acetate instead of the acetic acid-sodium acetate
buffer used in method (1)).