Bayer HealthCare Rapidlab 800 - Manual (Page 625)

Operating Principles IĆ5
I = 1/2
∑
m * z 2
where
I = ionic strength of the solution
z = the charge number of the ions in solution
m = concentration of the ion (mol/L)
The activity coefficient generally decreases with increasing ionic strength. 10
Using an established convention, the activity of ions that are measured by sensors
can be expressed in terms of concentration. This convention contains the
assumption that the normal ionic strength of blood plasma water is 160 mmol/kg.
11
Because ionic strength is the primary variable affecting the activity coefficient of
ionic species in solution, controlling the ionic strength of calibrating solutions to
160 mmol/kg sets the activity coefficients of ionic species in the calibrating
solutions equal to those of blood plasma water at sample ionic strengths close to
normal. Both calibrations and the expression of measured quantities may then be
made in units of concentration instead of activity.
12
Understanding the Reference Sensor
The reference sensor for the 800 systems works with certain measuring sensors in
the measurement module to create an electrochemical cell. It provides a fixed
potential, which is independent of analyte activity. The system compares the fixed
potential of the reference sensor to the measured potential from the following
sensors:
Sensor System
pH 840, 850, 860
Na + 850, 860
K + 850, 860
Cl – 850, 860
Ca ++ 850, 860