Anions in seawater - Determination of Anions in Natural and Treated Waters - page 404

Spencer and Brewer [115] have reviewed methods for the determination of silicate in
seawater. Various workers [193-196] have studied the application of molybdosilicate
spectrophotometric methods to the determination of silicate in seawater. In general, these
methods give anomalous results due, it is believed, to erratic blanks and uncertainty
regarding the structure of the silicomolybdate formed.
A more promising approach to the analysis of silicate is that of Thomsen and Johnson
[197], who used flow injection analysis. These workers achieved an analysis rate of 80
samples per h with a detection limit of 0.5M Si and a precision better than 1% at
concentrations above 10µmol L−1 Si.
Brzezinski and Nelson [198] have described a solvent extraction procedure for the
spectrophotometric determination of nanomolar concentrations of silicic acid in seawater.
Beta silicomolybdic acid was formed by the reaction of silicic and molybdic acids at
low pH. The combined acid was extracted into n-butanol and reduced with a mixture of
p-methylaminophenol sulphate and sulphite. After phase separation, the samples were
cleared by chilling in ice followed by centrifuging, and the colour was determined
spectrophotometrically at 810nm. The molar absorbance of the mixed acid was 229,000
in seawater and the precision was ±2.5nmol L −1 s i licon for concentrations less than or
equal to standard aqueous analyses. Sensitivity in seawater was 70% of that in distilled
water because of a significant salt effect. Natural concentrations of arsenate, arsenite and
germanic acid caused negligible interference. Phosphate interference was equivalent to
10 −1 2 nmol L −1 silicon over a broad range of phosphate concentrations.
3.28.2 Flow injection analysis
Flow injection analysis is a rapid method of automated chemical analysis that allows a
quasi-continuous recording of nutrient concentrations in a flowing stream of seawater
[134,199]. The apparatus used for flow injection analysis is generally less expensive and
more rugged than that used in segmented continuous flow analysis. A modified flow
injection analysis procedure, called reverse flow injected analysis [179,200], was adopted
by Thompson et al. [197] and is adapted here for the analysis of dissolved silicate in
seawater. The reagent is injected into the sample stream in reverse flow injection
analysis, rather than vice versa as in flow injection analysis. This results in an increase in
sensitivity [179].
This analytical procedure is based on the work of Truesdale and Smith [201,202], who
studied the optimum analysis conditions for segmented continuous flow analysis. The
sample is combined with a molybdate solution at a pH between 1.4 and 1.8 to form the β-
molybdosilic acid. After an appropriate time for reaction, a solution of oxalic acid is
added, which transforms the excess molybdate to a non-reducible form. The oxalic acid
also suppresses the interference from phosphate by decomposing phosphomolybdic acid.
Finally, a reductant is added to form molybdenum blue. Both ascorbic acid and stannous
chloride were tested as reductants.
The kinetics of each step in the reaction have an important influence since the reaction
mixture would typically have a residence time of less than 1min in the flow injection
analysis manifold. Previous studies [203, 204] indicate that the reaction requires several
minutes to go to completion. Fig. 3.18 shows the formation rate of β-molybdosilic acid