Thermo Fisher Scientific Ion Selective Electrodes Potassium - Manual (Page 13)

Instruction Manual Potassium Electrode
13
The activity is related to the free ion concentration, Cf, by the activity coefficient,γ ,by:
X =
γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = 1/2
Σ CxZx 2
where:
Cx = concentration of ion X
Zx = charge of ion X
Σ = sum of all of the types of ions in the solution.
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient,
γ , is constant and the activity, X, is directly proportional to the concentration.
To adjust the background ionic strength to a high and constant value, ionic strength adjuster (ISA)
is added to samples and standards. The recommended ISA for potassium is sodium chloride, NaCl.
Solutions other than this may be used as long as ions that they contain do not interfere with the
electrode's response to potassium ions.
The reference electrode must also be considered. When two solutions of different composition are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur
from the interdiffusion of ions into the two solutions. Electrode charge will be carried unequally
across the solution boundary resulting in a potential difference between the two solutions, since
ions diffuse at different rates. When making measurements, it is important to remember that this
potential be the same when the reference is in the standardizing solution as well as in the sample
solution or the change in liquid junction potential will appear as an error in the measured electrode
potential.
The composition of the liquid junction filling solution in the reference electrode is most important.
The speed with which the positive and negative ions in the filling solution diffuse into the sample
should be as nearly equal as possible, that is, the filling solution should be equitransferant. No
junction potential can result if the rate at which positive and negative charge carried into the sample
is equal.
Strongly acidic (pH = 0-2) and strongly basic (pH = 12-14) solutions are particularly troublesome to
measure. The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask
their effect on the junction potential with any concentration of an equitransferant salt. One must
either calibrate the electrode(s) in the same pH range as the samples or use a known increment
method for ion measurement.
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through checking each of the system
components in turn: the meter, the glassware, the electrode(s), the reagents, the sample, and the
technique.