Analytical Chemistry Laboratory 2

Direct Potentiometric Titration of Fluoride IonMr. *****Department of Chemical Engineering and Chemistry, Mapua Institute of Technology

In this experiment, the concentration of fluoride in drinking water will be determined by a potentiometric method.. The activity of fluoride in solution can be determined with a solid-state fluoride selective electrode. Four standard solutions were measured for its potentials, and a straight line was obtained. An unknown sample, in this case, sample # 3 (6 mL) was analyzed from the potential reading of 82.1 mV. The concentration of the fluoride was therefore computed using linear regression and the result was 2.07 ppm.

Objective The main objective of the experiment is to determine the concentration of Fluoride ion in a sample drinking water. Discussion Methodology Overview To measure a potential, we create a voltaic cell with the indicator and reference electrodes. We measure the voltage of the cell, giving a reading of the indicator electrode potential relative to the reference electrode. We can relate this to the analyte activity or concentration using the Nernst equation. The standard fluoride solution and the total ionic strength adjustment buffer (TISAB) can be purchased readily from commercial sources. Fluoride is a "general protoplasmic poison." When present as a soluble salt, it is readily absorbed from the alimentary tract. A 2% solution of sodium fluoride kills mucosal cells, and its ingestion leads to a severe hemorrhagic gastroenteritis; in part this corrosiveness is due to a toxic action on mucosal capillaries and is seen even after 50 mL portions of water were transferred to 100 mL volumetric flasks, and were diluted to the mark with TISAB solution. A 5 ppm F- solution was prepared by diluting 5 mL of the 100 ppm standard to 100 mL in a volumetric flask. Aliquots of 2.50-, 5.0-, 10.0-, 25.0-mL of the 5 ppm solution were transferred to 100 mL volumetric flasks as well as 6 mL of the sample, 25 mL of TISAB solution were added, and diluted to the mark. (These solutions corresponded to 0.5, 1.0, 2.5, and 5.0 ppm F-, respectively, in the sample.) After thorough rinsing and drying with paper tissue, the electrodes were immersed in the 0.5 ppm standard. It was stirred mechanically for 3 min; the potential was then recorded. It was repeated similarly with the remaining standards and the sample. parenteral administration. The systemic actions of fluoride are presumably related to the inhibition of one or more enzymes controlling cellular glycolysis and to the binding or precipitation of calcium as CaF2.

A linear graph is derived when concentration is in logarithmic form. Instrumentation The apparatus for this experiment consists of a solid state fluoride electrode, a saturated calomel electrode, and a pH meter.potential 120 100 80 60 40 20 0 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 log [concentration]

pH measurements with an electrode involve the measurement of potentials. The pH meter acts like a voltage measuring device designed for use with high-resistance electrodes.

The commonly used reference electrode is the saturated calomel electrode. It is designed to make measurements with high-resistance electrodes. Interpretation The potential data obtained from the pH meter readings is shown below with its corresponding concentration.Potential (mV) 113.4 100.9 84 82.1 60.5 Concentration (ppm) 0.5 1 2 x 5

In the presence of the total ionic strength adjustment buffer, a plot of potential vs. log [conc.] should be linear. To determine the amount of fluoride in a solution of unknown concentration, we must first calibrate the electrode by measuring the potential for a series of fluoride standard solutions. The unknown concentration, x, was found to be 2.07 ppm from the graph. The effect of all the common interferences and activity effects can be overcome by addition of a total ionic strength adjustment buffer (TISAB) to the standard and sample solutions. Sources of error include fluoride ion activity changes due to the ionic strength of solution, temperature, which can affect the measured potential through the Nernst equation which governs the electrode potential response and through the several equilibria that fluoride may have with species present in solution, and substances that complex fluoride in solution. Conclusion Potentiometric methods relate the potential generated at an indicator electrode to the concentration of an analyte in solution. The unknown concentration of the drinking water was obtained from the graphical visual from the experimental data. It was found out to be 2.07 ppm by using the method of linear regression for accuracy. References 1. Christian, G.D., Analytical Chemistry, 6th edition, New Jersey, John Wiley, 2004.

2. Skoog, D. A., and Leary, J. J., Principles of Instrumental Analysis, 4th edition, Saunders, New York 1992. 3. Harris, D.C., Quantitative Chemical Analysis, 3rd edition, Freeman, New York, 1991.