Chemistry Reference
In-Depth Information
incorrect calculation of the pH. Over the past 10 years, this problem has
been drastically reduced and circumvented by using glass membranes of
special composition. An example is the membrane with high Li
+
content. In
this case, Li
+
is exchanged with H
+
but not with K
+
and Na
+
, because the
latter ions are too big to fit in the small holes left by Li
+
in the membrane.
In addition, other sources of errors can occur owing to imperfections in
the glass-membrane construction or due to limitations related to the refer-
ence electrodes and the diaphragm. Temperature also plays an important
role in pH measurement. However, for these sources of errors reference is
made to the literature for more detailed information
15
.
The influence of temperature on the glass electrodes used in this work is
explained below. On the basis of the inclination R
T
/
z
i
F, and the knowledge
of k, the pH of unknown solutions can be determined starting from the mea-
sured potential. In practice, these parameters are obtained via calibration
of the glass electrode on the basis of two or more buffer solutions. However,
since both the inclination and factor k are functions of temperature, and
the activity of the protons in the buffer also changes with a varying tem-
perature (chemical equilibrium), the measurement of the pH of unknown
solutions is, in principle, only correct with the calibration temperature.
Hence, it would be necessary for measuring the pH with different tempera-
tures to recalibrate the glass electrode with every change in temperature.
An alternative is to derive an empirical relation which takes into account
the occurring changes. Manufacturers of glass electrodes often offer an
equation with the glass electrode which can be used in this relation. An
example of such a relation is given in Equation 3.5, which applies for the
Schott glass electrode used in this research and which is based on calibra-
tion of the electrode at 298.0 K:
(
)
-
298
pH
+
7
T
T
-
273
175
m
m
pH
=
[3.5]
T
m
in which
pH
T
represents the corrected pH; in fact this is the pH at 298.0 K
and
pH
m
is the pH measured with a temperature
T
m
, which is different from
the calibration temperature of 298.0 K. So this
pH
m
is not correct because,
on the one hand, it was calculated on the basis of an incorrect inclination;
(namely the one valid with 298.0 K) and, on the other hand, because of the
temperature dependency of the pH on the calibration buffers. Equation 3.5
corrects both effects. In order to verify the correctness of this equation, the
pH
m
of unknown solutions was measured, and from this the
pH
T
was cal-
culated, and, simultaneously, the
pH
m
of the same solution was measured
after the glass electrode was calibrated with the temperature
T
m
.This means
that for the latter no temperature correction is needed or that
pH
m
=
pH
T
.
From this, it appeared that Equation 3.5 is valid in the temperature area
T
m
± 10 K.
Deviations between the pH values obtained via the above-
