Experiment 3: ISE (Ion-Selective Electrode) – Chloride Determination

Abstract

In this experiment, we calculated potential (mV) of three know concentrations of Cl- (2.500 x 10-2, 2.500 x 10-3 and 6.250 x 10-4) and of one unknown sample solution. The 2.500 x 10-2M NaCl solution was created by dissolving 0.3630 grams of NaCl (s) into 250ml of dH2O in a volumetric flask. To prepare the 2.500 x 10-3M NaCl solution, 10ml of the 2.500 x 10-2M solution was pipetted into a 100ml volumetric flask, filled to the mark and mixed. For the 6.250 x 10-4M solution, 25ml of 2.500 x 10-3M solution was pipetted into a separate 100ml volumetric flask, filled to the mark and mixed. These three solutions were used to plot the calibration curve. The unknown sample solution was created by dissolving 0.5ml of mustard into 100ml of dH2O in a separate 100ml volumetric flask. A voltmeter was used to calculate the potential (mV). The silver electrode was coated with AgCl-Ag2S powder, soaked it in 0.1M KCl solution for 5 minutes and then in dH2O for another 5 minutes. The shorting cables were used to standardize the voltmeter at 0mV. After each calculation of our separate solutions, the electrodes were rinsed and placed back into dH2O. It was found that the electrode was 91.6% efficient and determined the unknown solution to have a chloride ion concentration of 5.623 x 10-4.

Data and Results:

Table 1: Preparation of the first standard solution

Weight of NaCl used (grams) Volume of volumetric flask (L) Concentration (M) of NaCl

0.3630 0.2500 2.482 x 10-2

Table 2: Data and Results obtained using the Chloride Electrode

[NaCl] (M) log10[NaCl] aCl- ln aCl- Potential (mV)

6.211 x 10-4 -3.207 6.039 x 10-4 -7.412 218

2.485 x 10-3 -2.605 2.354 x 10-3 -6.052 195

2.485 x 10-2 -1.605 2.127 x 10-2 -3.850 136

-14287562865000Graph 1: The effect of the concentration of NaCl (M) on the measured potential of the solutions (mV) against the natural logarithm (ln) of the calculated activities (ln aCl-)Table 3: Electrode Efficiency

Absolute Value of Slope 23.382

Temperature 23o

Ideal Slope 25.52

Electrode efficiency 0.9162

-123825638175Graph 2: The effect of the change of concentration of NaCl (M) on the log10[NaCl] against the natural logarithm of the calculated activities (ln aCl-)

Table 4: Determination of [Cl-] in sample

Sample Potential (mV) From grapy ln aCl- From graph log10[Cl-] [Cl-] mg Cl-/g of sample (mg/g)

Mustard (2) 222 -7.45 -3.25 5.623 x 10-4 730

Calculations

Molarity of solutions

Moles = mass/RFM

Moles = 0.3630/(22.99 + 35.45)

Moles = 0.00621149897 moles of NaCl

Molarity of 1st…