Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 490

c If F − concentration is above this limit, dilute analyte with diluted buffer (20ml L −1 )
Source: Reproduced with permission from the American Chemical Society [34]
temperature and the electrode potentials in a buffered solution containing 20µg L −1
fluoride were 0.16°C and 0.18mV and 0.05°C and 0.08m V respectively. If the
temperature is kept within ±0.2°C, the electrode can be operated for weeks without
calibration drift.
It has been observed by previous workers [35] that the electrode response drifted while
the fluoride solution was stirred, but, when stirring was stopped, the electrode assumed a
stable potential within 5-30min, depending on the fluoride concentration. Kissa [34]
examined this procedure and concluded that the apparently stable potential resulted from
a reduced transport of fluoride ions to the electrode and did not always represent an
equilibrium potential in the bulk of the solution. Under the conditions outlined in this
modified method, a stable electrode potential can be obtained in a short time in stirred
solutions as dilute as 20µg L −1 fluoride.
The modified test conditions proposed by Kissa [34] are outlined in Table 7.6.
7.13.5 Ion chromatography
The application of this technique is discussed under multianion analysis in sections 7.28.2
and 12.6.1.
7.13.6 Miscellaneous
Crosby et al. [36] examined five spectrophotometric methods, titration with standard
thorium nitrate, and an ion selective electrode method, for the determination of fluoride
in potable water. The methods were examined for reproducibility, sensitivity, range,
specificity, and, in the case of the spectrophotometric methods, for colour stability and
temperature effects. Of the spectrophotometric methods, the alizarin complexan
procedure is particularly suitable for samples containing only small amounts of fluoride.
The electrode method surpasses all the colorimetric methods for speed, accuracy and
convenience.
7.14 Hypochlorite
7.14.1 Titration method
The application of this technique is discussed under multianion analysis in section
14.3.1.1.