Instruction Manual
Potassium Electrode
12
pH Effects
Hydrogen ion interferes with measurements of low levels of potassium ion although the electrode
can be used over a wide pH range. Table 3 should be used to determine the minimum pH at which
low level potassium measurements can be made without more than a 10% error due to hydrogen ion
interference.
Electrode Life
A potassium electrode will last six months in normal laboratory use. On-line measurement might
shorten operational lifetime to several months. In time, the response time will increase and the
calibration slope will decrease to the point calibration is difficult and electrode replacement is
required.
Electrode Storage
The electrode may be stored in 1.0X10
-2
M potassium standard for short periods of time. For storage
over 3 weeks, rinse and dry the potassium membrane electrode and cover the tip with any protective
cap shipped with the electrode(s). The reference portion of the combination electrode (or the outer
chamber of the reference electrode) should be drained of filling solution, if refillable, and the rubber
insert placed over the filling hole.
ELECTRODE THEORY
Electrode Operation
A potassium ion electrode consists of an electrode body containing an ion exchanger in a sensing
module. This sensing module contains a liquid internal filling solution in contact with a gelled
organophilic membrane containing a potassium selective ion exchanger.
An electrode potential develops across the membrane when the membrane is in contact with an
potassium solution. Measurement of this potential against a constant reference potential with a
digital pH/mV meter or with a specific ion meter depends on the level of free potassium ion in
solution. The level of potassium ions, corresponding to the measured potential, is described by the
Nernst equation:
E = Eo + S log X
where:
E = measured electrode potential
Eo= reference potential (a constant)
S = electrode slope (~ 56 mV/decade)
X = level of potassium ions in solution
The activity, X, represents the effective concentration of the ions in solution. Total potassium
concentration, Ct, includes free potassium ions, Cf, plus bound or complexed potassium ions, Cb.
Since the potassium electrodes only respond to free ion, the free ion concentration is:
Cf = Ct - Cb