ST-200 aQua Operator’s Manual
Comparative method of measurement is utilized. First, the analyzer
measures the potential developed when the sample is aspirated through
the electrodes. Next, cal A is aspirated through the electrodes.
The difference in the two potentials is related logarithmically to the
concentration of ions in the sample divided by their respective
concentrations in the calibrant solution. Since the difference in potentials
and the concentration of the ions in the calibrant solution are known, the
analyzer can calculate the concentration of the ions in the sample solution,
in accordance with the Nernst equation, rewritten as:
E-Eº = S ln (Ci (x) / Ci (s)) or Ci(x) = Ci(s) x 10ˆ (E-Eº)/ S
Where: E = ISE potential developed in sample solution
Eº = ISE potential developed in the calibrant solution
S = Electrode Slope calculated during calibration
Ci(x) = Concentration of ion “I” in the sample
Ci(s) = Concentration of ion “I” in the calibrant solution
“S”, the slope, is determined during calibration using Cal A and B, which
are known levels of ions.
The ST-200 aQua electrolyte analyzer aspirates and measures Cal A,
followed by Cal B. The ST-200aQua finally aspirates additional Cal “A”
solution and measures it. The two Cal “A” readings are compared for drift
and each measurement is checked for instability (noise). The slope is
calculated from the difference between the second Cal A reading and the
Cal B reading. A slope outside the built-in software limits will be flagged
and displayed as low or high. The ST-200 aQua electrolyte analyzer will
detect excessive drifts or noise and display the appropriate error message.
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