Rolling Revision of the WHO Guidelines for Drinking-Water Quality ...

electrochemical detection mode, it is suitable for the determination of trace levels of
both nitrate and nitrite in surface and groundwaters (Crompton, 1992). Applicable
ranges for other methods are (Clesceri, et al., 1989):
• Nitrate electrode method, 0.14-1400 mg NO - 3 N l -1
• Cadmium reduction method, 0.01-1 mg NO3 - N l -1
• Titanous chloride method, 0.01-10 mg NO - 3 N l -1
• Hydrazine reduction method, 0.01-10 mg NO - 3 N l -1
• Automated cadmium reduction method, 0.5-10 mg NO - 3 N l -1
• Devardas alloy method, >1 mg NO - 3 N l -1
Two methods of nitrate determination are outlined here: Devardas alloy method and
the cadmium reduction method.
Devardas alloy method (reduction to ammonia)
This method is applicable to nitrate concentrations exceeding 1mg l -1 (Ballance,
1996a). The procedure can be performed on the original sample or on the residue
remaining in the flask after the distillation process required in the determination of
ammonia nitrogen. This distillation step also serves to eliminate interference and thus
is applicable to analysis of wastewater and polluted surface water.
If a delay between sampling and analysis cannot be avoided, changes in the nitrogen
balance due to biological activity can be delayed through storage at a temperature just
above freezing point, with or without preservatives. Examples of preservatives are 0.8
ml concentrated sulphuric acid or 40 mg of mercury (as mercuric chloride) per litre of
sample. If sulphuric acid is used for preservation, the sample should be neutralised to
pH7 before analysis is begun.
Devardas alloy is composed of 59% aluminium, 39% copper and 2% zinc. Use of this
alloy reduces nitrate to ammonia by nascent hydrogen. The ammonia is subsequently
distilled and its concentration determined by titration. Nitrites are also reduced by
Devardas alloy and their concentration can be determined rapidly and readily.
Subtraction of the nitrite fraction from the total oxidised nitrogen will give the nitrate
concentration.
Nitrate nitrogen (as N) = (a–b) * 100 – n mg l -1
V
where: a = volume of 0.00714mol l -1 acid used for titration of the distillate of the
sample
b = volume of 0.00714mol l -1 acid used for titration of distillate of the blank
(ml)
V = volume of the undiluted sample (ml)
n = concentration of nitrite nitrogen in mg l -1 N, determined separately.
Cadmium reduction method
Nitrate is reduced to nitrite in the presence of cadmium. This method involves the use
of commercially available cadmium granules treated with copper sulphate and packed
in a glass column. The cadmium reduction column is illustrated in Figure 7.3.