Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 218

gas-sensing method, however, was unaffected by the presence of chloride in high
concentrations and also enabled the operator to compensate for the presence of trace
quantities of free ammonia in the sample.
Csiky et al. [457] used disposable clean-up columns of chemically bonded
alkylaminosilica for the selective removal of humic substances prior to measurements
with a nitrate ion selective electrode. Using the clean-up procedure, recoveries averaged
95% and reproducibility was independent of humic acid concentration. Without clean-up,
recovery was always greater than 100%, increased with humic acid concentration and
changed with time. The nitrate concentrations found in non saline waters after clean-up
were in good agreement with determinations by ion chromatography.
O'Hara and Okazaki [458] studied the effect of surfactants on the determination of
nitrate in river and stream waters using a nitrate ion selective electrode. The interferences
due to anionic surfactants and their elimination by the addition of cationic surfactants was
examined. The interference of dodecylbenzenesulphonate below 1mg L −1 was
unimportant.
Fig. 2.29 Effect of Manoxol OT on the response of nitrate-selective electrodes
with solidsilver contact (a) and internal reference solution (B) in the
presence of 10−1M potassium nitrate
Source: Reproduced with permission from the Royal Society of
Chemistry [459]
The interference of 1-10mg L −1 dodecylbenzenesulphonate could be mostly eliminated
by the addition of a cationic surfactant such as cetyltrimethylammonium bromide. An
anomalous response behaviour occurs in the presence of some cationic surfactants.
Hulanicki et al. [459] also studied the effect of surfactants on the response of nitrate
selective electrodes with polyvinyl chloride membranes. The electrodes investigated had
either an internal reference solution or a solid-silver contact. The latter type was found to
be less sensitive to surfactant interferences as shown in Fig. 2.29.