Question

A lithium ion-selective electrode gave the potentials given in the table below for the following standard solutions of LiCl and two samples of unknown concentration

Solution ([Li+]/M)	Potential vs. SCE, mV
0.100 M	+1.0
0.050 M	-30.0
0.010 M	-60.0
0.001 M	-138.0
Unknown 1	-48.5
Unknown 2	-75.3

1. Draw a calibration curve of the electrode potential versus log[Li⁺] and determine if the electrode is displaying nernstain behaviour.
2. Use a linear least squares procedure to determine the concentration of the unknowns

Answer 1

Solution ([Li+]/M)	Potential vs. SCE, mV	log[Li+]	[Li+]/M for Unknown
0.100	+1.0	-1	-
0.050	-30.0	-1.30	-
0.010	-60.0	-2	-
0.001	-138.0	-3	-
Unknown 1	-48.5	-1.7 (Calculated from plot)	0.02
Unknown 2	-75.3	-2.1 (Calculated from plot)	0.008

According to Nernst equation,

0 -20 -40 (-1.7, -48.5) -60 -80 (-2.1, -753) -100 -120 -140 -2.2 -1.4 -2.6 -2.4 -2.0 -1.8 -1.6 -1.0 -3.0 -2.8 -1.2 log[Li] Potential vs. SCE, mV

RT In(Li nF E E

So,

E - constant x log(Li E X

If we plot electrode potential(E) versus log[Li⁺], the data points are fitted with straight line. So, given electrode is displaying nernstain behaviour.

From the plot the concentration of Unknown 1 and Unknown 2 are 0.02 (M) and 0.008 (M) respectively.