E7
USE OF ION-SELECTIVE ELECTRODES
This exercise does not require a report.
PURPOSE
To calibrate common ion-selective electrodes
To investigate their accuracy over a range of concentrations
To analyse simple solutions, using standard quantitative methods.
PROCEDURE
I. Fluoride Electrode
Ia. Calibration
1.
In plastic beakers, pipette 25 mL of the following standards and 25 mL of TISAB.
• 0.1 mg/L
• 1 mg/L
• 10 mg/L
• 100 mg/L
• 1000 mg/L
2.
3.
Soak the fluoride ISE in 1000 mg/L fluoride solution for 2 minutes. Rinse well with distilled
water.
Measure the standards in increasing order of concentration.
Ib. Analysis of tap water by calibration graph and standard addition
4.
Prepare a tap water sample as in step 1.
5.
Measure the sample.
6.
Pipette 1 mL of the 100 mg/L standard to the sample, stir and re-measure. This determines the
sample concentration by standard addition.
Ic. Investigation of matrix interference
7.
Measure the following solutions, which all contain 5 mg/L fluoride:
• distilled water – no TISAB
• 0.01 M HCl – no TISAB
• 0.01 M NaOH– no TISAB
• 0.01 M Fe (III) – no TISAB
• distilled water – with TISAB
• 0.01 M HCl – with TISAB
• 0.01 M NaOH– with TISAB
• 0.01 M Fe (III) – with TISAB
Calibrating the pH electrode 1
I.
Rinse the electrode with distilled water and dry gently with tissue. Place it in the pH 7
buffer.
II.
Turn the meter on, and press the temperature button.
III.
Measure the temperature of the solutions, and adjust the temperature setting.
IV.
Press the pH button.
V.
Using the knob above the Offset label, adjust the reading to 7.
VI.
Clean the electrode as before and place in the 4 or 9 buffer, depending on whether the
solution you will be measuring is acidic or alkaline. With the Slope control, set the
meter to 4 or 9, as appropriate.
VII. Measure the pH of the pH 7 buffer. If it is outside the range of 6.9-7.1, repeat steps 16.
VIII. If the repeat calibration fails, report this to your teacher, and record it in the instrument
log.
II. Using the pH electrode
8.
Calibrate the electrode using the 7 and 4 buffers.
9.
Measure the pH the following HCl solutions:
• 1M
• 0.1 M
• 0.01 M
• 0.001 M
10.
11.
12.
13.
Measure the pH of the pH 9 buffer. If it is in error by more than 0.2 pH units, use the Slope
control to correct it.
Measure the pH of the following NaOH solutions, progressing in order of increasing
concentration:
• 0.001 M
• 0.01 M
• 0.1 M
After measuring the NaOH solutions, re-measure the pH of the 0.01 M HCl solution, without
recalibration.
Rinse the electrode in distilled water and return to the 3 M KCl storage solution.
CALCULATIONS
I. Fluoride Electrode
•
for the fluoride standards, plot mV versus log10 F
•
determine the concentration range where the mV response is linear
•
draw a line of best fit through the linear region only
•
calculate the slope of the line-of-best-fit (this is equal to the working sensitivity, S)
•
using the line of best fit, determine the F concentration for the tap water
•
calculate the F concentration, using the standard addition data and the equation in your theory
notes
•
for the 5 mg/L solutions in Part Ic, determine their concentrations from the calibration
•
calculate the relative error for each of these solutions
1
This method applies to the Hanna meter. If you are using a different meter, you will need to write a different
set of instructions for yourself.
II. Using the pH electrode
•
calculate the true pH of each HCl and NaOH solution
•
calculate the error for each solution (measured pH – true pH)
•
plot a graph of error vs true pH for all solutions, except those with the added NaCl
QUESTIONS
1.
Where do your results indicate that the pH electrode is most accurate? How does this compare
with normal performance?
2.
Where is the linear region for the fluoride ISE? How does this compare with the manufacturer’s
specifications?
3.
How does your value of the working sensitivity compare with the manufacturer’s value?
4.
How do your fluoride concentrations for tap water compare with Hunter Water’s standard of 1
mg/L? Which value – normal calibration or standard addition - do you believe is more accurate?
5.
Explain the results observed for Parts Ic.
E7 - USE OF ION-SELECTIVE ELECTRODES
I. Fluoride Electrode
Solution
mV
0.1 mg/L F
1 mg/L F
10 mg/L F
100 mg/L F
1000 mg/L F
Tap water
Tap water + 1 mL 100 mg/L
Solution
mV - no TISAB
mV with TISAB
distilled water
HCl
NaOH
Fe (III)
II. Using the pH electrode
Solution
1 M HCl
0.1 M HCl
0.01 M HCl
0.001 M HCl
0.001 M NaOH
0.01 M NaOH
0.1 M NaOH
0.01 M HCl
Exact Molarity
pH