The effect of the chelating ligand in TISAB solutions on the potentiometric determination of fluoride in aquatic samples with ion selective electrode

Abstract

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Potentiometric determination of fluoride with ion-selective electrode is one of the frequently used methods to quantify fluoride in aqueous samples. The method requires using TISAB (Total Ionic Strength Adjusting Buffer) solution to improve the accuracy because it measures the activity of an analyte rather than its concentration. One of the main disadvantages of this method is high cost of commercial TISAB solutions. In this research project, fluoride concentration in well water samples was determined using a fluoride ion-selective electrode and several TISAB solutions developed in the laboratory. The main objective of this study was to evaluate the accuracy of the measured fluoride concentration. The data generated in this study will be used to develop a cost effective TISAB solution that can produce statistically comparable results to commercial TISAB solutions. During the study, eight (08) TISAB solutions were developed and seventy five (75) well water samples were analyzed by using these buffers. All the buffer solutions contained 1.0 M NaCI, 0.1 M sodium acetate pH buffer, adjusted to pH 5.5 and a metal complexing ligand to break metal-fluoride complexes to free fluoride ions to the solution to be determined by the fluoride selective electrode. The ligands used in this study were CDTA, EDTA, HE-EDTA, triethanol-amine, citrate, tartrate and phosphate. One buffer was developed without any ligand where acetate was the pH buffer and the metal chelating ligand. In addition to the above buffers. WTW brand TISAB (140100) solution was used as the reference. WTW brand 340i pH/ion meter equipped with a WTW brand fluoride electrode were used for fluoride analysis. All the analyses were carried-out in duplicate. Finally the samples were also analyzed by YS19500 photometer for fluoride using the Zirconyl-Cyanine R colorimetric method. In addition to fluoride concentration, pH and conductivity of the samples were also measured. The measured conductivities of the samples were in the range of 53.3 to 481.0 uS cm-1 and the fluoride concentrations were in the range of 389 to 1.961 mg dm-1. Sample pH valued were in the range of 6.4 to 8.5. The standard deviations of the duplicate fluoride measurements were less than 5% for all data. According to the results, the highest fluoride concentrations were produced by the WTW (commercial TISAB) buffer and the lowest fluoride concentrations were produced by the buffer without a ligand (acetate only buffer) at all times. Furthermore, WTW and CDTA TISAB solutions produced comparable results for most of the samples. Interestingly, Zirconyl-Cyanine R colorimetric method which is generally accepted as a standard method for total fluoride analysis produced low fluoride concentrations compared to WTW, CDTA, EDTA and HE-EDTA buffer for most of the samples.