Siemens Rapidlab 1200, Руководство оператора, Страница: 53 / 448

Rapidlab 1200 Operator’s Guide: System Overview and Intended Use 1-33
02087462 Rev. V
Amperometry
Amperometry is an electrochemical technique used to determine the amount of a
specific substance in solution by applying a fixed voltage between 2 electrodes in
an electrochemical cell, and then measuring the current generated as a result of a
reaction which produces or consumes electrons (oxidation or reduction,
respectively).
The electrochemical cell contains 2 electrodes: the anode, which is positively
charged and the cathode, which is negatively charged. The measuring electrode,
which is frequently composed of platinum (Pt) or another noble metal, can be
either the anode or the cathode. Each electrode is attached to an external voltage
source.
As the sample comes in contact with the 2 electrodes, a known voltage is applied
between the anode and the cathode. The analyte to be measured is either an
oxidizable or reducible species. If the analyte is an oxidizable species, it will
diffuse to the anode where it is oxidized. If the analyte is a reducible species, it
will diffuse to the cathode, where it is reduced. In either case, the electrochemical
reaction produces a current flow between the anode and cathode that can be
measured by a device, such as a milli/micro ammeter. The current measured is
directly proportional to the concentration of substance (oxidizable or reducible)
present in the sample solution.
pH and Blood Gases
The Rapidlab 1200 systems analyze blood samples for pH, p O
2
, and p CO
2
.
Hydrogen Ion Activity or pH
The notation of pH expresses the hydrogen ion activity in a solution as the
negative logarithm of the hydrogen ion concentration. The hydrogen ion is
actually the determinant of the acidity of blood or plasma. Normal cellular
metabolism requires an exacting environment where hydrogen ion concentration
must be maintained within narrow limits. Hydrogen ion activity reflects the
acid-base balance within blood. Acids are substances that donate hydrogen ions;
bases are substances that remove hydrogen ions from solution. The lungs, kidneys
and blood bases all work to maintain the acid-base status within the strict limits
for normal cell functioning.
Expressed in concentration units, hydrogen ion concentrations are very small
numbers that are cumbersome to use. (For example, the common neutral pH of
7.00 is 0.0000001 mol/L.) In 1909 Sorenson converted the numbers
mathematically to simplify their use and described the notation pH:
+ − =
cH pH
10
log
where (H
+
) is the molar concentration of hydrogen ions.
5