Experiment 5: Method/Instrumentation

The titration system is comprised of a central unit controlling an automatic burette and a potentiometer. This system is able to measure difference of potential as precisely as 0.01 mV, and the burette can deliver precise volumes as small as 5 uL, when using a 5 mL burette. In this experiment we use a semi-micro combination pH electrode, Orion Ross^TM. The titration unit is an ME-10 analyzer, manufactured by McIntosh Analytical Systems. A Windows interface program is used to control the unit during any titration. At the end of the titration, a data file containing the volume added and potential, together with burette increments and derivatives is stored on disk. The data are then analyzed using conventional software packages, i.e., spreadsheets or graphing programs to determine equivalence volumes and equilibrium constants.

The Glass Electrode/Reference Electrode Assembly

This potentiometric system (probe) allows one to follow the changes in proton activity ( i.e., concentration at constant ionic strength ). To obtain accurate results, this electrochemical system must behave according to the Nernst relationship. If during a titration the temperature of the titration cell and the ionic strength of the solution is maintained constant then the potential given by the electrode assembly is

E_mv=E_G + kT ln[H+]

where

E_G: is the potential normal of the glass electrode (G) given for [H+]=1M.
k: a constant function of R and F
T: Temperature in Kelvin.

E_G encompasses the reference potential, the correction for departure from ideal behavior (activity coefficients) and various function potential arising from the fabrication of the assembly, it also includes terms coming from activity connection since it is actually the activity of the proton that is probed.

Experimental determination of E_G and kT

One needs

a) to maintain the temperature constant, and
b) to maintain the same ionic strenght

If the first condition is achieved by using a thermostat circulation bath and a titration cell with an external water jacket, the second requirement necessitates to have a prior knowledge of the solution (i.e., the total concentration of all the different ions in solution). This latter condition may be critical if one titrates dilute solutions, it can be less critical if the solution contains enough electrolytes to screen out important changes in ionic strength. In any case, a good practice consists in adjusting titrant and sample to the same ionic strength. Let's assume, for example, that the ionic strength of the sample is

I=0.01

In order to check the validity of the electrode assembly one can realize a titration of a solution containing only a salt: i.e., perform an experiment consisting of adding some volumes of 0.01 M HCl to a 0.01 M KCl solution.