Chemistry Reference
In-Depth Information
Table 2.52
Comparison of procedures for the determination of sulphate in plant digests
and non saline waters
mg L
−1
SO
4
2−
Plant
% S (dry matter)
Water
sample
Sample
Flow
injection
analysis
Manual
gravimetric
method
Flow
injection
analysis
Manual
turbidimetric
method
01
0.30
0.25
109
11
10
12
0.25
0.24
114
18
18
16
0.32
0.29
122
25
23
22
0.30
0.27
125
21
23
36
0.26
0.26
134
16
15
79
0.46
0.46
192
32
29
Source: Reproduced with permission from Elsevier Science [771]
When 500mg L
−1
of calcium (as calcium chloride), hydrogen carbonate (as sodium
hydrogen carbonate) or magnesium (as magnesium chloride) was added to the sulphate
standards, no interference was detected for the water analysis systems. Also, recoveries
ranging from 97 to 102% were assessed for the water system; interference from organic
matter thus seems unlikely. It can be seen from Table 2.52 that the results obtained by the
proposed methods compare well with those obtained by the standard turbidimetric
procedure for water samples [772]. Colour interference can be corrected by running
blanks, if necessary; this is easily done in the flow injection system by withdrawing the
barium chloride from the R, reagent.
Non-turbidmetric flow injection analysis has also been used to determine sulphate in
water. One such system employs dimethylsulph-onazo(III) as the reagent [773].
A diagram of the analytical system is shown in Fig. 2.52. It was not necessary to
saturate the reagent solution with barium sulphate. However, it was necessary to saturate
the carrier solution with barium sulphate to obtain high sensitivity and good
reproducibility. Moreover, when not in use, it was necessary to fill the reaction coil with
ethanol and water (1:1) to obtain high sensitivity.
Calcium interference was a serious problem when this method was applied to waters.
This was overcome by passing the samples through a cation exchange resin, located just
after the sample injection valve. The suppressive effect of calcium ion was almost
eliminated by inserting the cation exchange column. Columns either 8 or 15cm in length
showed almost similar effects. For the determination of 10mg L
−1
sulphate, other ions did
not interfere within 5% negative errors up to the following concentrations: 30mg L
−1
of
magnesium and ammonium, 50mg L
−1
of
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