Anions in seawater - Determination of Anions in Natural and Treated Waters - page 350

100ml seawater or brine sample is titrated in 30-50 burette increments with 3-5ml
standard 0.1M hydrochloric acid using a pH electrode.
The accuracy of the method was tested experimentally by running duplicate titrations
on distilled water, artificial seawater with and without sulphate and artificial Dead Sea
waters. For each run, alkalinity was calculated by two methods:
(1) by the conventional Gran plot which presumes that the glass electrode is properly
calibrated and
(2) by the method that applies the titration data for in situ calibration of the glass
electrode by the slope correction algorithm.
The precision of the latter method when applied in the distilled water runs was found to
be significantly better than the conventional method. This should be attributed to the non-
stability of the glass electrode which is corrected for by the proposed algorithm.
3.3.2 Spectrophotometric methods
Various workers have discussed the determination of total alkalinity and carbonate [9-
11] and the carbonate-bicarbonate ratio [11] in seawater. A typical method utilises an
autoanalyser. Total alkalinity (T m-equiv. per litre) is found by adding a known (excess)
amount of hydrochloric acid and titrating back with sodium hydroxide solution; a pH
meter that records directly and after differentiation is used to indicate the end-point. Total
carbon dioxide (C m-equiv. of HCO 3 − per litre) is determined by mixing the sample with
dilute sulphuric acid and segmenting it with carbon dioxide free air, so that the carbon
dioxide in the sample is expelled into the air segments; the air is then separated from the
sample and passed into buffered phenolphthalein solution, thereby lowering the pH and
diminishing the colour of the phenolphthalein. The reduction in colour is measured
colorimetrically (540nm). The concentration of carbonate is given by 2 (T—C) m-equiv.
L −1 , and the concentration of bicarbonate is 2C—T m-equiv. L −1 .
A computer program has been used to calculate the magnitude of systematic errors
incurred in the evaluation of equivalence points in hydrochloric acid titrations of total
alkalinity and carbonate in seawater by means of Gran plots. Hansson [12] devised a
modification of the Gran procedure that gives improved accuracy and precision. The
procedure requires approximate knowledge of all stability constants in the titration.
3.4 Arsenate
3.4.1 Spectrophotometric method
Haywood and Riley [13] have described a spectrophotometric method for the
determination of arsenic in seawater. Adsorption colloid flotation has been employed to
separate phosphate and arsenate from seawater [14]. These two anions, in 500ml filtered
seawater, are brought to the surface in less than 5min, by use of ferric hydroxide (added
as 0.1MFeCl2, (2ml) as collector, at pH 4, in the presence of sodium dodecyl sulphate
(added as 0.05% ethanolic solution (4ml)) and a stream of nitrogen (15ml min −1 ). The