Anions in sewage effluent - Determination of Anions in Natural and Treated Waters - page 575

Whenever practicable the temperature should be as close as possible to 20°C Place the
beaker on a heat shield on a magnetic stirrer and adjust the stirring speed to achieve rapid
mixing but without vortexing. Immerse the fluoride selective ion electrode and reference
electrode in the solution and record the millivolt potential P 0 to ±0.2mV. The time for the
reading to reach equilibrium may extend to 5min for low fluoride levels. The Gran's Plot
procedure is designed to overcome interferences in the determination of fluoride and
should be used in all these cases.
To the above solution is added 1.0 ± 0.01ml of standard fluoride solution (100µg F
mL −1 or 10µg F mL −1 depending on fluoride content of sample) and the equilibrium
millivolt potential P 1 is recorded.
The above operation is repeated a further four times and the equilibrium millivolt
potential recorded each time: P 2 , P 3 , P 4 and P 5 .
T he potentials P 0 to P 5 are plotted versus the fluoride concentration added on 10%
volume corrected Gran's Plot paper. The fluoride concentration is thus determined: C F µg
mL −1 .
Fluoride concentration, mg kg −1 dry solids:
9.5 Nitrate
9.5.1 Spectrophotometric method
Dodin et al. [8] observed that when a solution of nitrate at pH 2.0 is irradiated with
ultraviolet light, the nitrate will oxidise methyl orange. They used this as the basis of a
spectrophotometric method for determining nitrate in industrial effluents.
Slicko and Tawfik [9] have described an automated technicon on-line method for
overcoming phosphate interference in the cadmium reduction method for the
determination of nitrate. It involved removal of phosphate by precipitation with calcium
chloride prior to the sample entering the reduction column.
9.5.2 Ion selective electrodes
Nitrate levels sewage in amounts down to 1mg L −1 have been determined by specific ion
electrodes [10,11]. Petts [11] used a non-porous plastic membrane nitrate selective
electrode and compared results obtained with this electrode and those obtained by a
standard spectrophotometric method in a sewage works effluent. The results indicate that
the selective ion electrode is suitable both for laboratory and plant monitoring