Chemistry Reference
In-Depth Information
Table 9.1
Interference effects in the determination of fluoride by ion selective electrode
Fluoride level
(mg/kg
−1
)
Interference
mg/kg
−1
50
200
5000
Aluminium sulphate (AI)
500
−2
−7
+200
2000
−3
+233
−8
5000
−3
−8
−400
Ferric nitrate (Fe)
2000
−1.3
−3
+210
20000
−1.3
+11
+150
Potassium dihydrogen phosphate (P)
80000
+0.7
+9
+250
Calcium chloride (Ca)
100000
+1.7
+8
-
Magnesium sulphate (Mg)
20000
+1.7
+9
+250
Source: Reproduced with permission from the Bureau of National affairs Inc. US [7]
In this method approximately 10±0.02g of homogenised wet sludge is weighed into a
polypropylene beaker, weight W
2
. 10.0±0.1ml of 2M perchloric acid is pipetted into a
50ml polypropylene beaker and both beakers treated on a water bath to between 85 ± 5°C
2M perchloric acid is added carefully to the sludge which is then removed from the water
bath and allowed to cool to room temperature. The resulting mixture is transferred to a
polypropylene volumetric flask (100ml) and diluted to volume with water.
Measurement of fluoride concentration
An appropriate volume V ml of the prepared extract is pipetted into a 100ml
polypropylene graduated flask (see below) followed by 25ml of 0.2M perchloric acid.
Expected F− level
(mg/kg
−1
dry
sludge
)
Volume V of extract
(mL)
Standard fluoride solution
(mg L
−1
)
20-800
25.0
10.0
200-2000
10.0
10.0
400-4000
5.0
10.0
800-8000
25.0
100
4000-40000
5.0
100
Add 50.0 ± 0.1ml of pH 8.3 citrate buffer and make to the mark with deionised water and
mix well. Ensure that the solution is blue to the bromocresol green end-point indicator at
this stage ie pH 5.4 ± 0.2. 50ml of this solution is pipetted into a 100ml polypropylene
beaker containing a plastic covered stirring bar. Ensure the temperature of the solution is
within ±1°C of other solutions within the batch and solutions used for calibration.
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