Siemens RAPIDLAB 248 Operator's Manual page 126

Page G-2
Potentiometry
Using an established convention, the activity of ions as measured by the
sensors can be expressed as concentration. Ionic strength is the primary
variable affecting the activity coefficient of ions in solution. The normal
ionic strength of blood plasma water is 160 mmol/kg
strength of calibrating solutions to 160 mmol/kg sets the activity
coefficients of ionic species in the calibrations equal to those of blood
plasma water at ionic strengths close to normal. Both calibrations and
measurements may then be expressed in units of concentration rather than
activity.
Amperometry
Amperometry is an electrochemical technique used to determine the
amount of analyte in solution by applying a fixed voltage between two
electrodes in an electrochemical cell, then measuring the current flowing.
The measuring electrode is negatively charged and serves as a cathode in
the electrical system. The reference electrode is positively charged, and
serves as the anode. Both electrodes are attached to an external voltage
source.
As the sample comes into contact with the two electrodes, a known voltage
is applied to the cathode. This voltage attracts molecules from the analyte
in solution to the cathode causing a chemical reaction (reduction) that uses
electrons. The electrons are replaced immediately in the sample solution by
a separate reaction (oxidation) that takes place at the anode. The two
reactions result in a current flow that can be measured. The current
measured is directly proportional to the concentration of analyte (reacting
at the cathode) present in the sample.
Sensors
Reference Sensor
The reference sensor contains a silver (Ag) wire, coated with layer of silver
chloride (AgCl) surrounded by a saturated potassium chloride (KCl)
solution. By making sure that the concentration of chloride ions (Cl
remains unchanged in the solution, the reference sensor maintains a
constant electrical potential. KCl is added to the reference sensor solution
chamber to maintain a saturated solution of KCl at 37°C.
A permeable cellulose membrane separates the KCl solution from the
sample. During analysis a diffusion potential, created between the sample
and KCl solution, provides the fixed half-cell potential required for
measurement.
The Ag wire conducts the potential to the measurement device where it is
compared to the potential of the measuring sensor. The potential difference
measured reflects the concentration of analyte in the sample.
248 Operator's Manual
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. Controlling the ionic
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