Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 199

The effect of various potential interferences was also investigated. The results of these
experiments are shown in Table 2.23. Most of the common ions present in water samples
do not interfere, even with high concentrations. Those ions that showed serious
interference are rarely found in water samples at the highest concentrations examined.
Lake waters frequently contain a relatively high concentration of organic matter. In
determinations of nitrate by the copperised cadmium reduction method this organic
matter can interfere by chelating with copper in the reduction system. Hilton and Rigg
[400] overcame this difficulty by using an excess of zinc ions to protect the copper
catalyst from chelation. They devised a discrete analysis procedure based on these
principles for the determination of nitrate.
The Hilton and Rigg [400] method is summarised below:

Reagents
Copper sulphate solution, 2.28g L −1 CuSO 4 5H 2 O
Z inc(II) sulphate solution 51.0g L −1 ZnSO 4 7H 2 O
S odium hydroxide solution 0.2mol L −1 (8g L −1 ) sodium hydroxide

Table 2.23 Effect of diverse ions on the determination of nitrate by the multipoint
reaction-rate procedure with the cadmium alloy wire column.

Interferent a Ion: nitrate b Error (%) Interferent a Error c (%) Ion: nitrate b

Cyanide 100 −99.5 Copper 100 −50.5

10 −7.0 10 None

1 None Iron(III) 10 None

Dichromate d

10 −21.5 Magnesium 100 +6.5

1 −10.4 10 none

Iodate 100 −7.5 Mercury 100 −2.5

Iodide 100 −14.1 Tin(II) 10 −88.5

10 None 1 −15.2

Sulphide d

−12.0 Nickel d 10 100 −11.0

1 −2.6 10 None

Cobalt 100 −3.5 Phenol 100 −6.1

a The following ions in 100 concentration ratio showed negligible interference: acetate, bromide,
carbonate, fluoride, oxalate, perchlorate, sulphate, sulphite, cadmium, calcium, manganese, zinc,
silicate, urea, and hypochlorite
b Nitrate concentration 2mg L −1
c None means < 2%
d Permanent decrease in the reduction efficiency of the column; regeneration is needed
Source: Reproduced with permission from Elsevier Science [399]