-
A. K. Covinaton and Jennifer M. Thain
I
University of Newcastle upon Tyne Newcastle upon Tyne, England
The Dissociation Constant of HF An undergraduate experiment with the lanthanum fluoride ion-selective electrode
A
recent contribution (1) to THIS JOURNAL suggested that the number of different types of equilibrium experiments available for undergraduate practical physical chemistry courses was limited and drew attention to the possibility of using ion-selective electrodes for this purpose. Unfortunately the price of these is high, but since they are finding increasing use for analysis and for process and control measurcments, it is important that undergraduates receive some instruction in their usage. We present here an experiment with the lanthanum fluoride electrode (8) based on a paper by Vanderborgh (8), but his experimental technique and method of analysis of results have been refined to demonstrate some important principles. From pF and pH measurements, Vanderborgh (8) calculated the pK. of hydrofluoric acid at various ionic strengths fixed by the addition of the sodium perchlorate. However, for all accurate measurements of equilibrium constants from cells it is best to avoid liquid junctions and the corresponding uncertainties associated with them. I n this experiment measurements are made on the two pairs of cells
and 0.1 M HCl for cell IIb) are prepend from the stock solutions by dilution, and, for the first, addition of dried solid sodium chloride in carefully weighed amount (carelessness here will lead to a systematic error in the result for pK.). For the glsss electrode measunments, solution measurement in order of increasing acidity is advisable because of the etching action on the glass electrode. Surprisingly the potentials of glass electrodes in these solutions are remarkably steady, however, different electrodes yield different results suggesting the presence of acid errors (see below). I t is advisable to repeat measurements in the standard solutions s t the end of the experiment.
LaFs INaF (c,), HCl (ex) I AgCI I Ag LaF8I NaF (0.01) NaCl (0.001) 1 AgCI I Ag
in which the lanthanum single crystal membrane is replaced by two lanthanum fluoride electrodes of the second kind. For the right hand cell, assuming the polarity of silver-silver chloride is positive, the electrode reactions would be
Ia. Ib
and Ag 1 AgCI / NaF (c,) KC1 (cn) I glass Ag 1 AgCll HCl (0.1) lglass
IIa IIb
where cl/ez = 0.1 and cl = 0.01 to 0.001 mol 1-1 (6 cells in all). The second cell of each pair (b) is used for standardization of the electrode prior to transfer to the first cell (a). The choice of cT/c9ratio in cell IIb is to allow the 'a&umption that
