CALIBRATION
Electrode characteristics change with time and usage and solutions with known values are used to establish
the standardisation point and electrode slope values.
CALOMEL REFERENCE
A reference electrode having an internal reference element of mercury/mercurous chloride. The filling solu-
tion is usually saturated potassium chloride with mercurous chloride. Widely used for pH measurement but
not recommended for use at temperatures >50°C.
CATION
A positively charged ion (Na
+
, Ca
++
, NH
4
+
, H
+
, etc).
COMBINATION ELECTRODE
An electrode which combines both sensing and reference elements in one housing.
CONCENTRATION (units: M, ppm, g/l
-1
)
The mass or total concentration of an ion in a solution includes both free ions bound to complexing species.
The ISE’s measured potential of a solution gives the activity of a given ion in solution, a decomplexing agent
is required to liberate bound ions to measure total concentration.
DEBYE-HUCKEL ACTIVITY EQUATION
Equation which approximates the mean activity coefficient of each ion to the ionic strength of a solution.
DISSOCIATION CONSTANT (K
a
; units: mol/m
3
)
Indicates the extent to which a molecular species dissociates in solution, thus the smaller the value of K, the
less the species has dissociated. Expressed as K=[A
+
] + [B
-
] / [AB].
DIVALENT ION
A doubly charged ion. S
--
is a divalent anion. Ca
++
, Mg
++
are divalent cations.
DOUBLE JUNCTION REFERENCE ELECTRODE
Used in place of a single junction reference electrode when the fill solution contains an ion which interferes
or reacts with the ion being analysed. Two separate filling solutions are used, an outer non-interfering solution
contacts the sample and an inner fill solution completes the electrical circuit of the reference cell.
DRIFT
Slow changes seen in electrode potential. Can often be minimised by careful attention to factors such as
temperature constancy.
ELECTRODE POTENTIAL
The potential difference developed between an electrode and a solution when the electrode is immersed in
that solution.
ELECTROLYTE
A compound that in solution dissociates into ions. Strong electrolytes are good conductors of electricity.
FILLING SOLUTION
A solution used in electrodes to make contact between an internal element and a membrane or sam
ple. The
filling solution of the internal reference element contains a fixed level of chloride. The concentration of this
ion determines the zero potential of the electrode.
The filling solution is saturated with silver chloride to prevent erosion of the internal reference ele
ment.
Reference filling solution is a concentrated salt solution contacting the sample solution and the internal reference
element.
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