150
Ion Selective Electrode
12.2
An ion selective electrode (ISE) is designed to measure the concentration of an ion in a dilute solution. A membrane
situated on the face of the ISE determines the selectivity of the ISE. The membrane is composed of a matrix of PVC, a
layer of ionophore (a chemical which binds or surrounds the target chemical), and another matrix of PVC. In the 2950
system, this membrane is permanently connected to a disposable electrode which resides in the module module. Sample
is injected through the sipper into the module and is diluted at a ratio of approximately 1 part sample to 27 parts buffer.
Upon exposure of the intact ISE to the sample and buffer, ions pass through the membrane and are trapped by the
ionophore. When equilibrium between the membrane and the external solution is reached, the potential across the
membrane is compared to the potential measured by a reference electrode submerged in the same solution. The
potential, measured in millivolts (mV), is then converted to a concentration through a series of algorithms including the
modified Nicolski equation (a derivative of the Nernst equation). The relationship between the potential detected and the
concentration of a solution is not linear, but rather semi-logarithmic. The slope of the semi-log plot of the modified Nicolski
equation is approximately 57 mV per decade concentration.
An ammonium or potassium ISE is designed to measure the ammonium ion concentration [NH4
+
] or the potassium ion
concentration [K
+
] in solution. However, as is the case with most ISEs, other ions similar in shape, size, or charge may
also pass through the membrane and interfere with the measurement. In the case of ammonium and potassium, an
ammonium sensor will respond to ammonium, but will also respond to a lesser extent to potassium. Likewise, a potassium
sensor will also respond to ammonium to a certain extent. To correct for these interferences, during a calibration, the
instrument exposes the ammonium sensor to potassium and vice versa to collect data on the level of interference of each
ion with each sensor. The selectivity coefficient (SC) represents the extent to which an ion interferes with a sensor. Taking
into account the selectivity coefficients, the instrument software then applies the Nicolski equation to determine the
response of each sensor to the ion of interest.
Measurement Methodology
12.3
The 2900 Series employs a steady state measurement methodology. A typical enzyme sensor response is shown in
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.
Typical Enzyme Sensor Response
Figure 12.2
When sample or calibration standard is dispensed into the sample module, it is diluted into 600 microliters of buffer. The
enzyme sensor response increases and plateaus. After several seconds, the sample module is flushed with buffer and the
sensor response decreases.
The net response is the difference between the plateau current (i
plat
) and the initial baseline current (i
ib
). Typical net
responses for the 2900 Series are between 10 and 25 nA (nanoamps) for YSI calibration solutions.
