Anions in seawater - Determination of Anions in Natural and Treated Waters - page 398

Laurylbenzenesulphonate Na salt

5×10 − 6 †

0.547

ClO 4 -

NaClO4

1×10 −6 †

0.557

TiCl 4

×10 − 6

Ti4+ 0.551

*Sample solution 1 0ml; organic phase 5ml; phosphorous 0.366µg; measured against reagent
blank
†Tolerance limit
Source: Reproduced with permission from Elsevier Science [172]

A commonly used procedure for the determination of phosphate in seawater and estuarine
waters uses the formation of the molybdenum blue complex at 35-40°C in an
autoanalyser and spectrophotometric evaluation of the colour produced. Unfortunately,
when applied to seawater samples depending on the chloride content of the sample, peak
distortion or even negative peaks occur which make it impossible to obtain reliable
phosphate values (Fig. 3.15). This effect can be overcome by the replacement of the
distilled wash-wash solution used in such methods by a solution of sodium chloride of an
appropriate concentration

Table 3.13 Determination of phosphorus in river and seawater and recovery test

Sample Recovery test

Phosphorus
found µg L-1

Sample
taken (ml)

P in
sample
(ng)

P added
(ng)

P found
(ng)

Recovery
(%)

Seashore of Seto Inland Sea
Kojima

46.0 5 230 248 477 100

Shibukawa 17.4 8 139 148 391 101

Tamano 15.3 8 122 248 381 103

Ushimado 12.8 8 102 248 351 100

Nishiwaki 20.9 8 167 248 419 101

The samples were adjusted to about pH 3 with dilute sulphuric acid after sampling, and filtered
through a membrane filter (0.45µm). An appropriate amount of sample was transferred to a 25ml
test tube, 1ml of 4.5M sulphuric acid was added, and the mixture was heated at 95°C for
30min.The seawater samples were diluted to 25ml with distilled water. A 10ml sample was used,
except for Yoshii River sample D and the Kojima seawater for which 5ml samples were used. In
the recovery tests, the river water was diluted to 10ml with distilled water after the addition of
248ng of phosphorus (as phosphate), and the seawater was diluted to 25ml with distilled water
after the addition of phosphorus
*The values are the means of three determinations
Source: Reproduced with permission from Elsevier Science [172]