Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 777

18.5.1 Phosphate content in the range up to 0.4mg L − 1

Glassware for sampling should be cleaned and allowed to stand overnight filled with
sulphuric acid (density 1.84g mL − 1) , then rinsed several times with water and stored
filled with water. On no account should the glassware be allowed to come into contact
with detergents or alkaline liquids. This acid hardening treatment need only be repeated
occasionally.
Phosphate is readily adsorbed on to a plastic surface (see section 18.4.2) and for this
reason, good quality borosilicate glass containers are preferred. They should be
conditioned as described above and stored in a dark cool place. The use of plastic
containers for samples in the higher phosphate concentration range may be acceptable.
Some losses may occur, but they may be small in relation to the phosphate present.
Ideally, waters should be analysed as soon as possible after sampling as there is no
generally satisfactory method of preservation. Refrigeration (but not freezing) is
reasonably effective after short periods of storage, eg a few days.
Some samples may require filtering on site to ensure that changes are not occurring
during transit and storage.
The determination of orthophosphate, pyrophosphates and poly-phosphates (ie
condensed phosphates) and organophosphates has been discussed [6].

Interference effects in phosphomolybdenum blue methods for phosphate
There is little detailed information concerning the effect of interfering substances on the
method. Generally, no important interference problems are likely with unpolluted saline
and fresh waters; but the effect of interferences should be considered particularly in
polluted samples and at very low phosphate levels. The most likely sources of
interferences and implications are discussed below with references, and by Burton [7]
who also highlights other problems associated with the analysis of phosphorus
compounds.

Arsenic
Arsenic present as arsenate causes serious interference in the method. The extent of
interference is time dependent because the colour of the arsenomolybdate develops
slowly. However, as a guide 0.025mg L −1 As may give a response equivalent to about

0.015mg L −1 P (40ml sample volume).

Silicon
The method will tolerate 10mg L −1 of silicon present as silicate [8] but high
silicon/phosphorus ratios in samples where the phosphate level is very low may cause
significant errors.

Chromium
Cr(VI) is reported to interfere at 1mg L −1 level [9].

Oxidising agents
The action of oxidising agents in this method is complex. They may destroy the reducing