Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 256

phosphomolydic acid. These workers examined the standard molybdenum blue method
using 1cm cuvettes. The 'standard methods' version has been examined with polystyrene
cuvettes over the concentration range 0.01-0.5mg L −1 . Erratic performance encountered
at concentrations greater than about 1mg L −1 is shown to be due to incomplete colour
development during the prescribed measurement interval. Deviation from Beer's law
behaviour occurring near 1mg L −1 P is shown to be due to a transition from
'molybdenum blue' formation to 'heteropoly blue' formation. Incorporation into the
standard method of a mild (75°C) short-term (20min) heating of the developing samples
provides effective quantitation of the higher concentration range by a linear calibrations
function and makes measurement time less critical. Example calibrations and colour
development plots are presented and discussed in terms of the molybdenum blue and
heterpoly blue formations. Support is given to the involvement of molybdate dimers in
the 'blue' formation under the conditions of the 'standard method'.
Traces of orthophosphate in non saline waters have been determined by the stopped-
flow time difference analysis technique in combination with the molybdenum blue
method and preconcentration solvent extraction (with isobutyl alcohol) of 12-
molybdophosphate [608]. The reductant was stannous chloride. The apparent molar (PO 4
3− ) absorptivity of the coloured product was ~2×105M −1 cm −1 at 730nm, and the detection
limit was 0.3µg of phosphate L −1 . The calibration curve was linear from 0 to 5µg of
phosphate L −1 , with a relative standard deviation <3% and a recovery of 100 ± 6% at a
concentration of 20µg of phosphate L −1 .
Hassan and Salem [609] have described a sensitive spectrophotometric method for the
rapid determination of phosphate in non saline water based on the reaction between
phosphomolybdate and resazurine (at 495nm) or p -resolic acid (at 470nm). Phosphate can
be determined in concentrations of 0.02-0.08mg L −1 and 0.02-0.16mg L −1 with
resazurine and p -rosolic acid reagents, respectively.
Quin and Fu [610] have described a spectrophotometric method using potassium
persulphate as the oxidising agent for determining total nitrogen (at 200-210nm) and
total phosphorus at 700nm) in river water. The method is reported to have recoveries of
98.0-108.0% for total nitrogen and of 92.3-107.0% for phosphorus; the corresponding
detection limits were 0.010 and 0.019mg of nitrogen and phosphorus.
Taga and Kan [611] describe a flotation-spectrophotometric determin-ation of
phosphate in river water. The method has a relative standard deviation of 1.45% (n=6) for
0.4µg of phosphorus.
To and Randall [612] have reported on the use of ascorbic acid as a reductant and
compared its reliability to that of a method using stannous chloride for reducing
phosphate to the phosphomolybdenum blue complex. They report that, using ascorbic
acid, the colours are more stable than occurs when stannous chloride is used.
Consequently, the ascorbic acid method is more amenable to automation, although there
are some limitations to the method. In the preparation of the combined reagent, care
should be taken that all reagents reach room temperature and that they are mixed in order
as prescribed. Otherwise turbidity and sometimes even a precipitate may form.
The ascorbic acid solution and the combined reagent are very unstable. They have to
be prepared weekly and stored in the refrigerator when not in use. Otherwise the reagents
will turn yellow and become useless.