Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 486

1µg L −1 fluoride in a 500ml sample.

7.13.2 Flow injection analysis
Flow-injection potentiometry using a combination fluoride-selective electrode was used
to determine fluoride in potable water [29]. Sample throughput was 30-40 samples/h,
based on triplicate injections at a rate of 120 samples/h. Recoveries after spiking potable
water with fluoride concentrations ranging from 0.01 to 1mg L −1 were in the range of 91-
106%.

7.13.3 Atomic absorption spectrometry
A method for determining fluoride in potable water and seawater, based on the formation
of aluminium fluoride in an electrothermal graphite furnace, followed by molecular
absorption at 227.56nm was developed by Gomez Gomez et al. [30]. The detection limit
was 8ng mL −1 and the peak-height precision at 0.2µg of fluoride mL −1 was 5%.

7.13.4 Ion selective electrodes
Various workers have discussed the applications of ion selective electrodes to the
determination of fluoride in potable water [16,31,32]. Reported detection limits are in the
range 0.1 to 1mg L −1 . Nicolson and Duff [16] have carried out a detailed study of an
optimum buffer system for use with a fluoride selective electrode. They examined 11
buffer systems to determine maximum complexing ability, with optimal reproducibility
and sensitivity, in the determination of fluoride in water by the direct potentiometric
electrode method. A buffer system containing triammonium citrate was found to have the
best masking ability as well as satisfying the other criteria. Only aluminium and
magnesium posed interference problems. It is recommended that a minimum
decomplexing time of 20min (and preferably 24h) be allowed and fresh buffer solutions
be prepared after a maximum storage period of 2 weeks.
The composition of TISAB 111 M buffer is as follows: 58g sodium chloride, 57ml
glacial acetic acid, 4g EDTA, 2.43g tri-ammonium citrate.

Table 7.5 Interference limits (mg L −1 ) of several species

0.1mg L −1 F − Recovery limit 1.0mg L −1 F − Recovery limit

Buffer Ion

100% 95% 100% 95%

TISAB III M Al >500 >500 35 100

BO 2 tri-ammonium >100 >100 >100 >100

BO 3

citrate >100 >100 >100 >100

Ca >1000 >1000 >1000 >1000

H 2 BO 3

>500 >500 >500 >500