Chemistry Reference
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interfering chloride, it must be initially removed by precipitation with silver sulphate if
more accurate determinations are to be made. The elimination of chlorides was also
performed by treatment with cationic ion exchange resin, prepared in the silver form. In
this study better results were obtained by silver sulphate precipitation.
In Table 2.34 nitrate determinations on well water and spring water samples by ion
selective electrodes and phenoldisulphonic acid spectrophotometry are compared. Good
agreement was generally obtained between results obtained by the two methods.
Potentiometric titration using an Orion 93-07 nitrate-selective electrode and an Orion
90-02 reference electrode has been used [453] to determine nitrate in lake waters. Of the
variants of this procedure, the Gran method [454] was found to be most suitable. Six
known additions (0.0, 0.10, 0.15, 0.20, 0.25 and 0.30ml of 10-2mol L
−1
sodium nitrate)
were added to 10 ml of the sample and after each addition the electrode potential was
checked and recorded. Analysis was performed with constant stirring and in the presence
of 0.1ml of silver fluoride to each 10ml of sample in order to maintain a constant ionic
strength and to eliminate interferences from such as chlorides or sulphates.
Table 2.34
Comparision of specific ion electrode and PDSa method for nitrate-nitrogen
determination in well water
Nitratenitrogen
No of
samples
Nitrate-nitrogen
mg
L
−1
Conductivity
mmho
Chlorides
mg
L
−1
Electrode
b
PDS
b
level range
mg
L
−1
Min
Max
Min
Max
<5.0
4
2.92
2.60
0.45
1.10
76
140
5.0-10.0
45
8.78
8.02
0.50
3.15
40
750
10.1-20.0
71
15.24
14.45
0.70
3.15
111
740
20.1-30.0
12
26.15
25.00
0.80
2.70
95
600
>30.0
16
35.94
34.86
1.80
2.90
500
580
Total
148
a
phenoldisulphonic acid
b
mean value
Source: Reproduced with permission from the American Waterworks Association [452]
For calculating the nitrate concentration by Gran's method, the known transformation of
the Nernst equation was employed:
where
E
is the emf of the electrochemical cell, S the electrode function of the nitrate
electrode (found to be 55mV per decade). The graph obtained represents the dependence
between antilog
E*S
and the added quantity of standard solution. The intersection of the
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