Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 262

Source: Reproduced with permission from Elsevier Science [632]
heating the acidified solution for 30min in a water bath at about 90°C. In this work the
water samples were made 0.35mol L −1 in sulphuric acid in stoppered test-tubes, heated
for about 45min in a water bath at about 95°C cooled to room temperature and injected
into the carrier system.
Workers at Tecator Ltd [633-635] have described flow injection methods for the
determination of between 0.025 and 5mg L −1 of phosphate. These methods are fairly
conventional employing to stannous chloride reducing system.
Fogg and Bselbu [636] developed a sequential flow injection voltammetric
determination at a glassy electrode of phosphate (and nitrite) by injection of reagents into
a sample stream. Good precision was obtained in the range 5-500µM phosphate.
A flow-injection method for determining phosphate concentrations in the field is
described by Worsfold et al. [617]. The colour-forming reagents are injected into the
sample stream and the absorbance is measured with a compact double-beam photometric
detector having light-emitting diodes and photodiodes. The response is linear over the
range 0-2000µg of phosphate L −1 and the detection limit is 2µg of phosphate L −1 ; there is
no interference from 10mg of silicate L −1 .
Motomizu and Oshima [637] determined orthophosphate in non saline waters by a
spectrophotometric method involving flow injection coupled with solvent extraction. The
ion pair formed between molybdophosphate and malachite green was extracted into
benzene-4-methyl-2-pentanone and the absorbance was measured at 630nm. The carrier
stream was 0.005M in sulphuric acid and the reagent stream contained ammonium
molybdate, malachite green and sulphuric acid. The calibration graph was linear for
concentrations <30ng of phosphate mL −1 on injecting 10µL samples. The detection limit
was 0.1ng of phosphate mL −1 , and the sampling rate was 40 samples/h.
Schulz and Thiele [638] described a modified flow injection analysis procedure based
on the formation of molybdenum blue using stannous chloride as reducing agent for the
determination of orthophosphate in water, waste water and sludge samples. The results
were compared statistically to those obtained by the standard method DJN 38402 part 71.
Pauer et al. [639] described a flow injection analysis method for the determination of
phosphate using the molybdenum blue colorimetric method with tin(II) chloride
stabilised with hydrazinium sulphate as reducing agent. The method was suitable for 10-
500µg L −1 phosphate. Precision of 0.50% at 500µg L −1 phosphate was obtained. Silicon
concentrations of less than 30mg L −1 did not interfere with the technique. Interference by
the preservative mercury(II) was negated by adding the same amount to both samples and
standard solutions. A carry over of less than 1% was achieved at 90 samples per hour.
Yuan and Qu [640] and Freeman et al. [132] have studied the application of flow
injection analysis to the determination of phosphates in non saline waters. The
application of this technique is also discussed under multianion analysis in sections
14.4.1.3 to 14.14.1.6.