Chemistry Reference
In-Depth Information
increase accuracy, the taking of several separate samples may be consid-ered as an
alternative to aliquotting. The fixing process is as follows [17].
Place the sample carefully, with a minimum of aeration, in a bottle until it is almost
full. Add 2.0 ± 0.ml of 0.75M sodium carbonate, mix, and then add 2.0 ± 0.2ml 0.5M
zinc acetate per 100ml sample and mix again.
The whole sample is normally taken for analysis, but if volatile organic compounds are
present, the sample can be centrifuged or filtered and the solid portion retained. If only
part of the sample is taken for analysis special attention must be paid to thorough mixing
before removing the portion to be analysed.
It has been found that the presence of a large amount of suspended matter (10mm on
bottom of bottle) in the sample reduces the effectiveness of the preservatives. This was
due to anaerobic bacterial action within the settled solid.
The stabilised sample is stable for up to 5 days.
18.7
Fluoride
No sample preservation is needed. Samples can be stored for up to 28d without risk of
losses.
18.8
Cyanide
In a sample preservation procedure described by Csiki and Barnard [18] 600ml of a
wastewater sample is transferred to a well-rinsed screwcap polyolefin bottle and
stabilised by adding 5ml of 50% sodium hydroxide solution (this sample can be stored for
several weeks without significant change in the total cyanide content). To remove
sulphide, 600ml of the well-mixed sample is transferred to a 1L beaker containing a
magnetic stirring bar. 0.2g of cadmium carbonate is added and the solution stirred for a
few minutes. If a yellow precipitate forms, the cadmium carbonate addition collected is
repeated until no yellow precipitate is observed. The solution is filtered promptly through
paper and 500mL collected in a graduated cylinder.
18.9
Chromate and dichromate
Stollenwerk and Grove [19] investigated the effect on the concentration of hexavalent
chromium of sample preservation techniques. Acidification of samples could reduce
hexavalent chromium to the trivalent form. The rate of such reduction was increased by
impurities such as nitric oxide present in nitric acid. Organic matter could also accelerate
reduction of hexavalent chromium in acidified samples, and could also reduce the
chromium even in unacidified samples. The best method for preserving hexavalent
chromium involved filtration and cooling of the sample. Storing the sample at 4°C might
preserve the concentration of hexavalent chromium for some time, depending on the
reactivity of other constituents in the sample, but should not be relied on. Measurement of
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