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Interference
An ion-selective electrode does not respond exclusively to the
ion for which it is designed, i.e. other ions may interfere. The
electrode's degree of selectivity as regards the primary ion, x,
compared to the interfering ion, i, is expressed by the selectivity
constant, K
. By introducing K
xi
general formulation can be made:
E = E
+ S • RT/zF • ln[a
0
Very few ions interfere in Pb
tions"). However, if compensation by calculation is necessary, it
should be noted that the K value is not an absolute constant. It
varies to some degree with sample composition, time, tempera-
ture, and also from electrode to electrode. When utmost accu-
racy is needed, the K value should be determined under condi-
tions as close to the actual measurements as possible.
K values can be determined fairly accurately by measuring
the electrode potential in solutions of identical activity, a, with
respect to lead ions, E
calculating the selectivity constant as follows:
(E2-E1) • 2 • F/R • T • ln 10
K
= 10
xi
If the degree of interference, K
activity is less than 10
-3
-1
10
and 10
, correction can be made in the manner described
above. If the degree of interference is greater than 10
measurement must be replaced by an indirect method, e.g. the
standard addition method. Concentration levels which result in
interference are stated in "Specifications".
Due to the formation of Pb(OH)
also dependent on the pH of the sample. In general, measure-
ments should not be performed outside the pH range 3 to 7. If
necessary, the pH of the sample should be adjusted.
into the Nernst equation, a more
xi
+ ∑(K
2+
Pb
2+
measurements (see "Specifica-
, and the interfering ion, E
1
l2/zi-1l
/a
IZ/ZiI
• a
i
xi
-3
, no correction is needed. If it is between
2+
, Pb
, lead determinations are
2
13
IZ/ZiI
• a
)]
xi
i
, and then by
2
, relative to the lead
-1
, direct
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