Chemistry Reference
In-Depth Information
In the 3, 4 xylenol method [20] a calibration graph is prepared by transferring with a
semi-micro burette 1.0ml each of 100µg ml L
−1
nitrate solutions, containing between 10
and 100µg mg L
−1
of nitrate, into two-necked 100ml round bottom flasks. Evaporate each
solution nearly to dryness, heating each flask gently on a hot plate. Cover the flask and
cool in ice. Add 5ml of 80% sulphuric acid followed by 1ml of 2% 3, 4-xylenol solution.
A reagent blank is taken through the same procedure. The flask is covered and shaken for
60s. The contents of the flask are transferred into a separating funnel with 80ml of ice-
cooled distilled water dispensed from a wash bottle.
10ml of toluene is added to each solution, and the funnel shaken for 60s and the phases
allowed to separate. The lower aqueous layer is discarded and the toluene extract washed
with 10ml of distilled water by shaking the flask for a further 1min. 5ml of 1% sodium
hydroxide solution is
Table 5.2
Effects of various ions on the relative absorbances of nitrate ion values given
are percentage changes in absorbance
NO
3
−
to
ion ratio
Ion
1:1
1:10
1:100
1:1000
F
−
0.77
4.26
4.26
6.60
Cl
−
1.16
6.59
2.32
80.62
Br
−
4.65
9.30
29.45
75.96
I
-
4.65
2.71
3.88
39.92
CO
3
2-
4.65
4.65
9.30
2.30
PO
4
3−
0.77
1.16
3.87
3.10
NO
2
-
0.38
10.79
11.24
11.62
SO
3
2−
0.77
5.42
6.98
9.30
NH
4
+
0.77
2.71
9.30
9.30
Source: Reproduced with permission from the Royal Society of Chemistry [20]
added from a pipette to convert the phenol into a phenoxide. The funnel is shaken for 1
min and the phases allowed to separate. The stem of the funnel is rinsed with few drops
of the yellowish lower layer. The absorbance of the remaining portion is measured at
432nm using matched 1cm silica cells with distilled water as reference.
With rainwater samples, transfer 10-50ml of rainwater (filtered if necessary) into a
two-necked 100ml round bottomed flask and evaporate nearly to dryness on a hot plate.
Cover the flask and cool in ice. Then proceed as described above for the preparation of
the calibration graph.
The effects of various interferents results are shown in Table 5.2. There are no serious
interferences from most of the anions studied, including nitrite and chloride, to which
most methods for nitrate determination are normally intolerant. However, bromide
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