GLOSSARY OF TERMS
ABSOLUTE MILLIVOLTS (mV)
Most pH/Ion meters have a mV mode which allows the potential developed by the electrode to be directly
displayed. This mode can be useful when assessing electrode performance and is normally employed when
working with ion selective electrodes.
ACID
A solution which dissolves in water with the formation of hydrogen ions and reacts with a base to form a
salt.
ACTIVITY
Effective concentration of free ions and ion pairs in a solution. In dilute solutions (<10
-4
M) ionic concentra-
tion and activity are practically identical. The ionic activity rather than concentration determines the extent
and rate of a chemical reaction.
ACTIVITY COEFFICIENT
Ions of similar size and charge have similar activity coefficients. For solutions of low ionic strength (<10
-4
M)
the activity coefficient is nearly 1 and lower for solutions of a higher ionic strength. The relationship be-
tween activity and concentration is described by the equation a
i
= C
i
fi
,
where:
a
i
= activity of ion i
C
i
= concentration of ion i
f
i
= activity coefficient
ALKALI
See Base
ANION
A negatively charged ion (Cl
-
, NO
3
-
, CO
3
-2
, etc).
ASYMMETRY POTENTIAL
The potential developed across a glass pH membrane when both sides of the membrane are in contact with
an identical solution. This term is also used to define an electrode potential developed in a pH7 buffer.
AUTOMATIC TEMPERATURE COMPENSATION (ATC)
A temperature sensor either incorporated into the pH electrode or in a separate probe which will correct the
electrode response for changes in temperature (the T term in the Nernst Equation). ATC does not correct
for changes that occur in the pH of samples with temperature. Meters that calibrate by automatically
recognising buffers (ABR) will have the buffer temperature characteristics programmed into their software
and calibrations will be automatically compensated. Meters that rely on manual calibration do not automati-
cally compensate for temperature and reference needs to be made to the tables supplied with the buffers
which relate to buffer value and temperature.
BUFFER
A solution which maintains a pH value (hydrogen ion concentration) when acids or alkalis are added.
BUFFER CAPACITY
Quantifies the ability of a solution to resist pH change when subject to the addition of strong acid or base.
BASE
A substance which dissolves in water with the formation of hydroxyl ions and reacts with acids to form
salts. Has a pH >7.
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