Teachers' guide 2 - National STEM Centre

Apparatus, materials, and reagents
by means of the equilibrium constant, K 1 , for equation (1) and the ionic product
of water,
where E~b is a constant involving E~, K1 and K w •
Construction and operation of the electrode
2 or 3 cm of antimony stick is cleaned with emery paper and a copper wire
soldered on to one end. It can be mounted in a glass tube and sealed in with
sealing wax. The electrode is then cleaned again and kept under water.
To calibrate the electrode, the cell:
Sb(s), Sb 20 3(s) I [H+(aq)] 1 Cu 2 (aq) ICu(s)
is set up with a standard copper electrode and the e.m.f. determined with the
antimony electrode immersed in buffer solutions of known pH. (A new salt
bridge should be used with each new solution.) The calibration curve thus
obtained will then enable the electrode to be used to determine the pH of
solutions, the pH of which is not known. The cell is relatively insensitive to
small temperature changes. A typical calibration curve is shown in figure A.22.
pH
13
2
40 60
Figure A.22
A calibration curve for use with an antimony electrode.
80 Potentiometer reading I em
Tablets for making solutions of pH 4, 7, and 9 can be purchased from British
Drug Houses Limited or their' Universal buffer mixture' can be used to make
the whole range of solutions required. An alternative is to make up all the
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