This graph shows that over the concentration range 10-^1 to 10-^5 M the calibra-
tion is linear with a slope of 0.029 V/log (a(Cu^2 +)). Below about 10-^6 M the line
curves, since the solubility of the crystal becomes significant. Interferences from
Cu(I), Ag(I) and Hg(II) are troublesome.
If a calibration curve is constructed, direct measurement of solution concen-
trations within that range may be made.Example
Calibration performed with a ISE selective to Mg^2 +ions gave a value of S =
0.0296, and E=0.411 V for a solution of a(Mg^2 +) =1.77¥ 10 -^3 M.What is the activity of Mg^2 +ions when E=0.439 V?Substitution gives a(Mg^2 +) =1.998¥ 10 -^4 M.The method of standard additions (Topic B4) has the advantage that comparison
is made in the same matrix solution, so that interferences and other effects are
the same for the unknown and for the standard.
Potentiometric titrations are discussed in Topic C5.C3 – Potentiometry 73
150100500–50–100
–7 –6 –5 –4 –3 –2 –1 0
log (a)Electrode potential (mV)Fig. 4. Electrode potential response for a copper ISE. (a) Against activity (solid line); (b)
against concentration (dashed line).