Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 778

agent, or subsequently reoxidise the phospho-molybdenum blue.

Nitrite
A concentration of 1mg L −1 N (as nitrite) may be tolerated in the presence of 0.1mg L −1
phosphorus but the interference of nitrite is both complex and variable and appears to be
related to exposure to the air.

Nitrate
Nitrate nitrogen may be tolerated up to 20g L −1 (as N) provided that the absorbance is
measured within 2h of colour development.

Sulphide
Interference from sulphide is complex, variable, and, dependent on conditions, reacting
with both antimonate and molybdate. Complete removal is advocated (by oxidation to
sulphate or by aspiration with nitrogen) [10].

Removal of interferences
There is no single pretreatment that will systematically deal with all inter-ference
problems. There are, however, certain processes that will have a selective effect and the
analyst must use his discretion in their application.
The effect of oxidising agents and arsenate may be overcome by treatment with an
excess of metabisulphite/thiosulphate in acid solution [11]. The presence of sulphur
dioxide has no influence on the final production of molybdenum blue, but the treatment
can only be applied to determinations of inorganic phosphorus and total phosphorus, ie
where hydrolysis is acceptable, as opposed to the determination of dissolved reactive
phosphorus.
A slight excess of sulphamic acid [12,13] is effective in breaking down nitrite; 100mg
of the acid will deal with a nitrite concentration of 10mg L −1

N in a 40ml aliquot of test

solution.
A solvent extraction procedure [14] that is also effective in reducing certain
interferences by virtue of the dilution effect is described in section 18.4.2.

18.5.2 Phosphate contents in oligotrophic waters in the range up to 25µg L −1

In this method the phospho-molybdenum blue complex is formed and the colour
extracted into a small volume of n-hexanol [6]. Because of the extreme sensitivity of the
method and of phosphate losses by adsorption on glass sampling bottles, the samples in
this method are collected in iodised polyethylene bottles.
Low density polyethylene or nylon (babies feeding type) bottles are suitable but high
density polyethylene or polypropylene bottles are not. The plastic bottles must be treated
with iodine to prevent the adsorption of phosphorus which can lead to significant errors.

Iodisation procedure [ 15 ]
Remove any rubber washers from the bottle caps and place a small quantity of iodine
(about 0.1g) in the bottle. Replace the cap and store at 60°C for long enough to ensure
uniform absorption of iodine (usually 24h) during which time the bottle should be rotated