Experiment No.
Potentiometric Measurements
Experiment No. 1 POTENTIOMETRIC MEASUREMENTS
Objectives
1. 2. 3. To have an understanding of potentiometric measurements using various kind of electrode To determine and compare the pH of different samples using standard pH meter To determine and compare the amount of fluoride ion in different samples of toothpaste and mouth wash using Fluoride-ion selective electrode.
Reagents
Buffer solutions (pH = 4, 7 and 9) shampoo samples TISAB (total ionic strength adjustment buffer) solution 0.1 M F stock standard solution
Apparatus
beakers Pipets Iron stand Iron clamp Fluoride-ion selective electrode burette pH meter volumetric flask
Procedure
I.
pH determination of hair shampoos A. Calibration of the electrode (pH meter)
1. 2. Calibrate the pH meter using different buffer solutions (pH 4, 7, and 9) The pH meter should be adjusted to read the pH of the buffer solution at room temperature.
B. pH measurements of the samples
1. 2. Measure the pH of 5.00 mL concentrated sample by placing it in a 25-mL beaker. Rinse the electrode well with distilled water and blot it with soft tissue paper. Avoid wiping it as this may impart static charge to the glass membrane. 3. In a 100-mL beaker prepare a dilute solution of each sample by pipetting 5.00 mL of sample and diluting it with 45.0 mL distilled water (use a burrette to obtain more accurate volume). This will make a 1:10 dilution. Make sure to mix the solution well. 4. Measure and record the pH of the dilute solution. Note: Calibrate the pH meter after several use by measuring the pH of standard buffer solutions.
Experiment No. 1 II.
Potentiometric Measurements
Determination of Fluoride in toothpaste and mouth wash
A. Sample preparation 1. Toothpaste Weigh 0.20 g of the sample toothpaste in a 300-mL beaker and add 50 mL TISAB solution. Stir the solution thoroughly and boil for 2 min. Cool the suspension and transfer it in a 100-mL volumetric flask. Dilute to the mark with distilled water and mix well. Set aside for potential measurement. 2. Mouth wash and tap water Transfer 25.0 mL of the sample mouthwash in a 100-mL volumetric flask and add 50 mL of TISAB solution. Dilute to the mark with distilled water and mix well. Set aside for potential measurement.
B. Determination of fluoride from sample using Calibration Curve Method 1. Prepare 5 ppm fluoride solution by diluting 25.0 mL of the 100 ppm standard to 500 mL in a volumetric flask. 2. Transfer 5.00, 10.0, 25.0 and 50.0 mL aliquots of the 5 ppm solution to 100 mL volumetric flask, add 50 mL of TISAB solution and dilute to the mark. These solutions correspond to 0.5, 1.0, 2.5, and 5.00 ppm fluoride ion in the sample). 3. Determine the potential of each solution and prepare the calibration curve (E vs. log C) 4. Determine the potential of the samples 5. From the graph determine the concentration of fluoride in the sample.
C. Determination of fluoride from tap water sample using Standard Addition Method 1. Prepare the following solutions in a 100 mL volumetric flask by pipetting known volume of previously prepared standard solution (5 ppm): Solution Composition 1 50 mL TISAB + 5 mL tap water + 0 mL standard 2 50 mL TISAB + 5 mL tap water + 10 mL standard 3 50 mL TISAB + 5 mL tap water + 20 mL standard 4 50 mL TISAB + 5 mL tap water + 30 mL standard 2. 3. 4. Measure the potential of each solution. Plot the E vs (CV)std Determine the concentration of fluoride in the sample.
Experiment No. 1
Potentiometric Measurements
Group No.
_________
Date:
___________________
Name:
__________________________________________
_______________________ (Affix your signature)
Experiment No. 1 POTENTIOMETRIC MEASUREMENTS
D A T A
I. pH Determination of various shampoos
S H E E T
pH Sample Shampoo Description Concentrated Dilute
Experiment No. 1
Potentiometric Measurements
II. Determination of Fluoride A. Direct Potentiometry 1. Calibration Curve Method
Solution Number
Concentration E (mV) ppm [M]
Solution Number
Concentration E (mV) ppm [M]
slope
Linearity (R)
Sample Stock solution without buffer Toothpaste
E (mV)
[F-]
Sample
E (mV)
[F-]
Mouthwash
Tap water
Experiment No. 1
Potentiometric Measurements
B. Standard Addition Method
Initial volume, Vo Concentration of standard, Cstd S
Solution Number
Vo
Vstd
CstdVstd
E, mV
Response (Vo + Vstd)10!((E2-E1)/S)
4 Concentration of F- in unknown, mg/L
Professors Signature
