Anions in seawater - Determination of Anions in Natural and Treated Waters - page 410

sulphate with a precision of 0.8% RID at the 200mg L −1

sulphate level. Instrument

operating conditions are seen in Table 3.15.

Table 3.16 Comparison of results for sulphate by inductively coupled plasma emission
spectrometry with certified values for standard waters

Sulphate content (mg L −1 )

Sample

ICPES Certified value

EPA Quality Control Standard 2 7.3 7.2

IOS Standard Seawater 2800 2781

Source: Reproduced with permission from Elsevier Science [218]

Of the elements characteristically present as major components of non saline waters, only
calcium produced a slight interference. A weak calcium line at approximately 190.734nm
partially overlaps the sulphur line such that 1000mg L −1 Ca produces an apparent
sulphate signal of 25mg L −1 sulphate. Correction for this interference is easily achieved
by establishing the relationship between calcium concentration and apparent sulphate
signal and inserting this information in the controlling software. The effect of calcium
was then automatically subtracted during the measurement of sample.
Non saline water samples are normally acidified to stabilise them during storage. There
is an effect due to hydrochloric acid concentration on the sulphur emission signal. This
effect is conveniently overcome by making the acid content of samples and standards
identical at, for example 1 vol.%.
The accuracy of the inductively-coupled plasma procedure was assessed by analysing
waters of known sulphate composition, and by comparing measured sulphate values for a
wide range of samples with those obtained for the same waters by an automated
spectrophotometric procedure. Table 3.16 shows good agreement between the sulphate
measurements obtained and the nominal values for International Standard Sea Water and
an EPA Quality Control Standard.

3.29.4 Polarography
Polarography has also been used to determine sulphate [219]. In this method sulphate is
precipitated as lead sulphate in a 20% alcoholic medium, followed by polarographic
determination of the excess lead. It is claimed that there is little interference in this
method; 25-30 samples can be analysed in a 5h period with a precision of 0.2% or less.
In this procedure up to 70ml seawater, 10ml 0.01M lead nitrate, 20ml 95% ethanol,
0.2ml of 0.1% methyl red maximum current suppressor and two drops 3M nitric acid are
added to a 100ml volumetric flask which is then diluted to 100ml with distilled water.
The dropping mercury electrode had a drop time of 3-4s under an open head of 50cm
Hg. A saturated calomel electrode was the reference electrode. Before recording, the
solutions were well shaken. After recording, the electrodes were well rinsed with distilled