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Fluoride in drinking water is well recognized for its both beneficial and detrimental effects on
human health. Excessive amounts of fluoride in drinking water have affected the well-being of
thousands of people in the dry climate zone of Sri Lanka. Fluoride content in aqueous samples
is frequently analyzed by fluoride ion selective electrode (ISE) method; however, the TISAB
(Total Ionic Strength Adjusting Buffer) reagent used in the ISE method is expensive and costs
around Rs. 25,000.00 per liter and therefore, ISE is not a cost effective method. The main
objective of this project is to develop a cost effective TISAB solution that could produce
comparable results to commercial off-the-shelf TISAB solutions.
At the initial stages, three TISAB solutions were developed and their performance was
evaluated with respect to a WTW branded TISAB solution (TISAB-W). TISAB-1 is a 1.0 M
NaCl solution, TISAB-2 is a 1.0 M NaCl solution buffered to pH=5.5 and TISAB-3 is 1.0 M
NaCl solution containing of EDTA buffered to pH=5.5. Fifty six (56) natural water samples
were analyzed for fluoride using a WTW fluoride electrode connected to a pH/ION 340 i meter
and the above mentioned TISAB solutions. The data produced by TISAB-W was considered
as the accurate fluoride concentration of the samples for statistical evaluation. All the
experiments were duplicated. In addition, other physico-chemical parameters, including pH,
conductivity, hardness and metal ion concentrations were also analyzed.
The fluoride concentration of the samples varied from 0.292 to 5.521 mgL
-1
. Approximately
90% and 50% fluoride data produced by TISAB-1 and -2 solutions were significantly different
from TISAB-W data and the results were unacceptable. Often, TISAB-1 and -2 solutions
produced significantly lower fluoride concentrations than TISAB-W. On the other hand,
approximately 75% of samples tested using TISAB-3 produced acceptable results. This
observation suggests that that most of the fluorides ions present in the analyzed samples are
complexed to metal ions and a strong metal ion chelating agent is required to decomplex them.
In the second stage of the research, TISAB solutions containing strong metal chelating agents
including CDTA, triethanolamine, and tartrate were produced. The initial results are
encouraging and more data is required to evaluable their performance
