pH Electrode Measurements:
Influence of Ionic Strength and Temperature

Purpose: In this laboratory you will measure some of the errors that can occur in electrochemical pH measurement. The most common instrumental measurement is that of the pH of solution. However, using glass pH electrodes one often observes large errors due to the existence of cations similar to the proton in solution. In particular, Li, Na, and K can give rise to proton measurement errors in concentrated solutions. These cations can exchange with the protons in the glass membrane of the pH electrode resulting in erroneous potentials across the glass membrane and thus erroneous pH. Another source of error is the change in proton activity proton due to the ionic strength of solution. Also, temperature can effect several equilibria giving rise to complicated measurement errors. Temperature will affect the equilibria of solution phase species, as well as chemical species used in the reference electrode.

References: M. F. Ryan Science 165 851 1969; J. H. Fossun, et. al, Analytical Chemistry 23 491 1951

Apparatus: Digital pH meter with 4 significant figures, combination glass pH electrode, pH paper, thermometer

Reagents: Calibration buffer solutions, Tris buffer, NaCl

Solutions

• 100 mL each of 2.0, 1.5, 1.0, 0.5, 0.1, 0.01, and 0.001 M aqueous NaCl solutions. Start with the 1 L of the 2.0 M solution and make more dilute samples using dilution. Be sure to use distilled water to prepare all solutions.
• Buffer solution from Sigma 7­9 Tris(hydroxymethyl) aminomethane ('TRIS') by adding 0.124 g to 100 mL of distilled water.

Procedures

pH in NaCl solutions:

1. Set up the pH meter by connecting the pH electrode in the plug at the rear of the instrument.
   
   
2. Adjust the pH meter using appropriate calibration buffer solutions.
   
   
3. Place 100 mL of distilled water in a beaker. Measure the temperature of this sample, adjust the pH meter and measure the pH.
   
   
4. Repeat this same procedure for 'tap' water.
   
   
5. Use pH indicator paper to measure the pH of both the distilled and tap water solutions.
   
   
6. Measure the pH of each of the NaCl solutions. Be sure to compensate for any minor temperature changes in these solutions.
   
   
7. Plot the measured pH versus the ionic strength of solution. Comment on the effect of ionic strength.

Temperature effects:

1. Measure the pH of the tris buffer solution between 0 and 40 ºC in 5 ºC increments at two pH meter settings: one at 25 ºC and the other at the appropriate temperature.
   
   
2. Perform the same measurements on the 0.1 M NaCl solution.
   
   
3. Plot pH versus temperature of the tris and NaCl solutions both with and without temperature compensation. Comment on the observed effects.

Report Questions

1. How does a glass pH electrode work?
   
   
2. Why is the glass electrode potential affected by alkali and alkali earth ions? Do other species interfere?
   
   
3. Was the measured pH of distilled water different than that of the tap water? Should they have been different? Comment on the effects of dissolved in aqueous solutions.
   
   
4. Do the pH meter measurements agree with those of the pH paper? Which do you think is more accurate? Which do you think is less prone to chemical interference problems?