Multianion analysis, miscellaneous methods - Determination of Anions in Natural and Treated Waters - page 722

transformation into hydride and subsequent measurement by atomic absorption
spectrometry).
The simultaneous presence of arsenic, generally as arsenate and phosphate in non
saline water (the arsenate/phosphate ratio being usually high) has given rise to the
development of manual conventional methods for determination of these species with or
without a prior separation process. The reaction on which these determinations are based
is in most cases the formation of a heteropolyacid with molybdate, with photometric
detection of Mo(V), molybdenum blue, generated with the aid of a suitable reductant, or
voltammetric detection, taking advantage in this case of the modification of the redox
potential of Mo(VI) caused by the formation of the heteropoly acid.
Flow injection analysis has proved to be a suitable technique for speciation as well as
for simultaneous determinations; however, it has found very limited use for the
determination of phosphate and arsenate (voltammetric detection), but not for the
resolution of ternary mixtures of this type or for arsenic speciation.
Linares et al. [31] have suggested several photometric flow injection methods for
arsenic speciation (as AsO 2 − and arsenate) and resolution of binary (arsenate-phosphate)
and ternary (AsO 2 -arsenate-phosphate) mixtures based on heteropoly acid formation and
on the use of a simple flow injection configuration which, by means of a selective valve,
allows fixing the most suitable conditions to this end.
In this method [31] the presence of a selecting valve in the flow injection analysis
system allows obtaining a suitable medium for the indicator reaction to develop with one,
two, or the three species of the mixture. The methods are simple and reliable and cover
wide ratio ranges for mixture resolution, and the errors encountered in their application to
real samples are relatively small.
14.4.1.6 Sulphate, orthophosphate and triphosphate
Coetzee and Gardner [32] have described a method for the determination of these anions
by flow injection analysis with a lead(II) ion selective electrode as a detector.
The system was optimised for the determination of sulphate ion over the concentration
range of 1-1000mg L −1 . This entire range is accessible with a single reagent solution of
10−5 M lead perchlorate in ethanol, which is mixed with an aqueous carrier stream,
making the method particularly useful for samples containing a wide range of sulphate
concentrations. The maximum sampling rate is a function of sulphate concentration; for
100ppm sulphate, the rate is 20 samples/h. Main interferences are such multicharged
anions as phosphates. The same flow system, but with an aqueous rather than ethanolic
reagent, was used for the determination of orthophosphate and triphosphate ions. While
calibration curves tended to be non-linear for these ions, reproducibility was adequate for
many analytical purposes.
One objective of this work was to study the applicability of ion selective electrodes as
detectors in flow injection analysis, particularly when potentially complicating factors,
such as precipitate formation, are present. The lead(II) ion selective electrode was chosen
for this purpose because many anions form sufficiently insoluble lead(II) salts to allow
their determination with lead ion. Examples of solubility products (as pK SP ) are as
follows: SO 4 2 −, 7.79; CrO 4 2− , 12.55; PO 4 3− , 43.5 (38°C); Fe(CN 6 4− , 18.02; and MoO 4