Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 773

18.2 Chloride
No preservation is required. The sample can be held for up to 28 days after sampling.
Sample bottles, made of glass or plastic must be halide free. In particular, high density
polyethylene containers must not be employed as these are likely to contain traces of
water extractable chloride ions. Low density polyethylene is suitable.
No preservative is needed for the chloride itself. If prevention of decomposition of
other sample components is necessary, hydrochloric acid, chlorides and disinfectants
containing halogens, mercury or silver must not be used for preservation of the sample.
18.3 Sulphate
Samples may be collected in glass or polyethylene bottles and should be analysed within
6h of collection according to one authority [4] and 28 days of collection according to
another authority [3]. Samples under anaerobic conditions should not be stored at 4°C for
more than 2 days [4]. Samples low in organic matter may be kept for longer periods, but
tests should be carried out to ensure that samples are sufficiently stable. To eliminate the
risk of air oxidation to sulphate of samples containing sulphide or sulphite, sample bottles
should be filled to exclude air. A settled unfiltered sample may be taken for sulphate
determination if desired. Alternatively, samples containing suspended matter should be
filtered through a well washed glass fibre filter capable of retaining particles of over
about 1µm size before analysis. If the sample is to be analysed by direct barium
titrimetry, its pH should, before analysis, be adjusted to 6-8 by addition of dilute
hydrochloric acid or sodium hydroxide.
18.4 Nitrite and nitrate
18.4.1 Estuarine waters
Biological instability of samples can be due to bacterial oxidation of ammonia to nitrite
which can be further oxidised to nitrate and, also, denitrification whereby nitrate is
reduced to nitrite and then to nitrogen gas. Both of these mechanisms will produce
changes in the content of oxidised nitrogen or nitrite.
To overcome instability the samples are filtered on collection and stored frozen in
polyethylene bottles. Before analysis, they are completely thawed out and mixed and, if
the total analysis time is to exceed 1 day 1ml chloroform added per litre of sample. The
samples should be stored at 4°C between analyses.
18.4.2 Non saline waters
In the case of anaerobic bacteria-free samples no pretreatment of the sample is needed
other than an initial suitable dilution if the concentration is greater than 50mg L −1 total