RADIOMETER ABL800 FLEX User manual

Contents
1. Potentiometric measuring principles
2. Amperometric measuring principles
3. Optical measuring principles
4. User-defined corrections
5. Performance specifications
6. Parameters
7. Solutions and gas mixtures
Index
Date of Issue
Reference
manual
ABL800 FLEX

SYSTEM PERFORMANCE AND WARRANTY DISCLAIM
Radiometer cannot provide or verify instrument performance characteristics and accept
warranty claims or product liability claims if the recommended procedures are not carried out,
if accessories other than those recommended by Radiometer are used, or if instrument
repairs are not carried out by authorized service representatives.
The instructions given in the Operator’s Manual for the ABL800 FLEX must be observed in
order to ensure proper instrument performance, and to avoid electrical hazards.
TRADEMARKS
ABL™, Deep Picture™, QUALICHECK™and Radiometer™are trademarks of Radiometer
Medical ApS, Denmark.
ABL is registered in the USA.
QUALICHECK is registered in the USA and in some other countries.
COPYRIGHT
The contents of this document may not be reproduced in any form or communicated to any
third party without the prior written consent of Radiometer Medical ApS.
While every effort is made to ensure the correctness of the information provided in this
document Radiometer Medical ApS assumes no responsibility for errors or omissions which
nevertheless may occur.
This document is subjected to change without notice.
©Radiometer Medical ApS, DK-2700 Brønshøj, Denmark, 2004. All Rights Reserved.

Contents
1. Potentiometric measuring principles........................................................... 1-1
Overview......................................................................................................... 1-1
General information ........................................................................................ 1-2
Reference electrode......................................................................................... 1-8
pH electrode .................................................................................................... 1-9
pCO2electrode .............................................................................................. 1-14
Electrolyte electrodes.................................................................................... 1-22
References..................................................................................................... 1-34
2. Amperometric measuring principles ........................................................... 2-1
Overview......................................................................................................... 2-1
General information ........................................................................................ 2-2
pO2electrode................................................................................................... 2-4
Metabolite electrodes .................................................................................... 2-12
3. Optical measuring principles........................................................................ 3-1
Overview......................................................................................................... 3-1
Optical system................................................................................................. 3-2
Correcting for interferences............................................................................. 3-7
Measurement and corrections.......................................................................... 3-9
References..................................................................................................... 3-14
4. User-defined corrections............................................................................... 4-1
Overview......................................................................................................... 4-1
General information ........................................................................................ 4-2
Correction factors for oximetry parameters and bilirubin............................... 4-4
Electrolyte and metabolite parameters ............................................................ 4-7
5. Performance characteristics......................................................................... 5-1
Overview......................................................................................................... 5-1
Definition of terms and test conditions ........................................................... 5-2
Performance test results – chart description.................................................... 5-5
Performance test results - pH .......................................................................... 5-8
Performance test results – pCO2.................................................................... 5-10
Performance test results – pO2...................................................................... 5-13
Performance test results – cK+....................................................................... 5-16
Performance test results – cNa+..................................................................... 5-18
Performance test results – cCl–...................................................................... 5-20
Performance test results – cCa2+.................................................................... 5-22
Performance test results – cGlu..................................................................... 5-24
Performance test results – cLac..................................................................... 5-26
Performance test results – ctHb..................................................................... 5-28
Performance test results - oximetry............................................................... 5-30
Performance test results - bilirubin................................................................ 5-40
Additional information about FLEXMODE ................................................. 5-46
Interference tests............................................................................................ 5-47
References..................................................................................................... 5-55
6. Parameters..................................................................................................... 6-1

Contents ABL800 FLEX Operator's Manual
Overview......................................................................................................... 6-1
General information ........................................................................................ 6-2
Measured parameters....................................................................................... 6-5
Input parameters............................................................................................ 6-14
Derived parameters........................................................................................ 6-17
Units and numerical format of derived parameters ....................................... 6-22
List of equations............................................................................................ 6-27
Oxyhemoglobin dissociation curve (ODC)................................................... 6-43
Conversion of units ....................................................................................... 6-48
Default values................................................................................................ 6-50
Altitude correction......................................................................................... 6-51
References..................................................................................................... 6-52
Index
Date of Issue

ABL800 FLEX Operator's Manual Contents
Warnings/Cautions
Throughout the manual, the descriptions may contain operational precautions and
warnings.
Definitions
Notice Definition
WARNING Warning alerts users to potential serious outcomes to
themselves or the patient (such as death, injury, or serious
adverse events).
PRECAUTION Precaution alerts users to exercise special care necessary for
the safe and effective use of the device. Precaution may
include actions to be taken to avoid effects on patients or
users that may not be potentially life threatening or result in
serious injury, but about which the user should be aware.
Precaution may also alert users to adverse effects on the
device by use or misuse, and the care necessary to avoid such
effects.
NOTE Notes give practical information.
WARNING/
CAUTION In this manual a distinction between a warning and a caution is not made. Any
notice that alerts the user to possible dangers of any kind is given the title
WARNING/CAUTION.
All WARNING/CAUTION notices that appear in this manual are listed here in
alphabetical order.
List of
WARNING/
CAUTION
Notices (NOTES are not presented in list form.)
• S5370 Cleaning Additive:
Very toxic by inhalation, in contact with skin and if swallowed. Danger of
cumulative effects. May cause sensitisation by inhalation and skin contact.
Toxic to aquatic organisms, may cause long term adverse effects in the
aquatic environment. After contact with skin, wash immediately with plenty
of water. Wear suitable protective clothing. In case of accident or if you feel
unwell seek medical advice immediately (show the label if possible). The
material and its container must be disposed of as hazardous waste.
• Electrolyte for E1001 Reference Electrode:
Irritating to eyes, respiratory system and skin. In case of contact with eyes,
rinse immediately with plenty of water and seek medical advice
• Gas cylinders:
Pressurized container. Non-flammable compressed gas. Do not breathe gas.
Gas mixtures containing less than 19.5 % oxygen may cause suffocation.
Protect from sunlight and do not expose to temperatures exceeding 50 C
(122 F). Store and use with adequate ventilation. Keep away from oil and
grease. Do not refill.
o
o

1. Potentiometric measuring principles
Overview
This chapter describes the potentiometric measuring principles and the pH, pCO2
and electrolyte electrodes that are based on this principle.
Introduction
Contents This chapter contains the following topics.
General information......................................................................................... 1-2
Reference electrode.......................................................................................... 1-8
pH electrode..................................................................................................... 1-9
pCO2electrode................................................................................................. 1-14
Electrolyte electrodes....................................................................................... 1-22
References........................................................................................................ 1-34

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
General information
Potentiometric
method The potential of an electrode chain is recorded using a voltmeter, and related to the
concentration of the sample (the Nernst equation).
An electrode chain describes an electrical circuit consisting of a sample, electrode,
reference electrode, voltmeter, membranes, and electrolyte solutions.
Sample
Electrolyte
solution Electrolyte solution
Reference
electrode Electrode
V
Membrane Membrane
Voltmeter
Every element in the electrode chain contributes a voltage to the total potential
drop through the chain. Thus:
• When immersed in the appropriate electrolyte solution, both electrodes have
separate potentials.
• The membrane junctions between the sample and electrolyte solutions also have
separate potentials.
The potentiometric measuring principle is applied to pH, pCO2, and electrolyte
electrodes.
Nernst equation The complete electrode chain potential therefore, is the sum of these separate
potentials and is the quantity measured by the voltmeter.
E= E + E
total 0sample
where the final unknown potential (Esample) can be calculated knowing the total
electrode chain potential (Etotal) and the standard potential (E0).
Having measured the unknown potential (Esample), the Nernst equation is then
applied to determine the activity (ax) of the species under study:
EE T
na
sample =+ 023R
Flog
.x
where:
E0= standard electrode potential
R =
gas constant (8.3143 Joule ×K−1 ×mol−1)
T= absolute temperature (310 K (37 oC ))
Continued on next page
1-2

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
General information, Continued
Nernst equation
(continued)
n = charge on the ion
F =
Faraday constant (96487 coulomb ×mol−1)
x
a= activity of
x
The Nernst equation is rearranged to express the activity as a function of the
potential Esample. Having measured Esample the activity can be calculated since all
other quantities are already known. Finally the analyzer converts activity to
concentration.
Strictly speaking, the potential of an electrode chain or the magnitude of current
flowing through an electrical chain is related to the activity of a substance, and not
its concentration.
Activity expresses the ‘effective concentration’ of a species, taking non-ideality of
the medium into account.
Activity and concentration are related by the following equation:
ax= γcx
where:
ax= the activity of the species x
γ= the activity coefficient of species x under the measurement conditions
(for ideal systems γ= 1)
cx= the concentration of species (mmol/L)
NOTE: To be exact, activity is related to the molality of species x, i.e., the number
of mmoles per kg of solvent. However molality is converted to concentration
(molarity).
The analyzer automatically converts activities into concentrations [1]. The term
concentration is therefore used in explanations of the measuring principles for each
of the electrodes further on in this chapter.
The potentiometric measuring principle is applied in the pH, pCO2, and electrolyte
electrodes. It is slightly different for the pCO2electrode, however, since the Nernst
equation is not directly applied.
Calibration is an analytical process defining the functional relationship between the
obtained readings or analytical responses and the concentration or other quantities
present in the calibration material (liquid or gas). Thus, a calibrating solution or a
gas mixture (for pCO2calibrations) is drawn into the measuring chamber and the
analyzer adjusts itself to measure the known value of the liquid or gas.
Calibration
Continued on next page
1-3

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
General information, Continued
The electrodes are active elements and must be calibrated regularly. Signals from
the electrodes change because of, e.g., protein build-up, worn-out membranes,
aging electrodes, etc.
Calibration
(continued)
The responses from the electrodes when measuring on the calibrating solutions are
checked to ensure that the amplified signals from the electrodes are converted to
accurate values for an unknown sample. The relationship between the electrode
amplifiers’ output and the pH/pCO2/electrolyte electrodes are simple mathematical
functions. Calibration data can therefore be determined by relating the electrode
signals during the calibration process to the values of the calibrating solutions.
Calibration line The calibration line expresses the relationship between the potential measured at
an electrode, and the concentration of the species specific to the electrode. The
calibration line forms the basis of the scale used by the analyzer to convert
electrode chain potentials to concentrations. Each electrode has a different
calibration line.
The pH electrode is used as an example to illustrate how the calibration line is
derived from two calibration solutions with known pH.
The calibration solutions give the following two points: −64 mV at pH 6.802 (Cal
2) and −100 mV at pH 7.398 (Cal 1)
Within the coverage range 6.300 to 8.000 the pH electrode is linear, and the
relationship between potential and pH is linear, so a line can be drawn between the
two points, as shown below:
Calibration line
pH
Measured
potential
(mV)
−
100
−
64
6.802
pH of Cal 2 sol. 7.398
pH of Cal 1 sol.
−
97
7.346
pH of sample
This is a two-point
calibration. In one-point
calibration, only the position
of the calibration line is
determined. The slope of the
calibration line is maintained
from the last 2-point
calibration.
The calibration line is stored in the computer and is used during measurement to
convert the potential measured at the pH electrode during sample analysis to an
actual pH value.
Continued on next page
1-4

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
General information, Continued
Calibration line
(continued) To describe the actual condition of the electrode, its calibration line is compared to
the calibration line of the theoretical electrode.
Theoretical calibration line
pH
Measured
potential
(mV)
−112.4
−75.5
6.8 7.4
The theoretical electrode is
defined to measure the
following:
−112.4 mV at pH 7.400,
−75.5 mV at pH 6.800.
The position and slope of the calibration line compared to the theoretical
calibration line are described by the status and sensitivity.
Sensitivity The electrode sensitivity illustrates the slope of the calibration line compared to the
slope of the theoretical electrode.
The sensitivity of the theoretical electrode is 100 % or 1.00.
Theoretical calibration line
Slope= −61.5 mV/pH
Sensitivity = 100 %
pH
Measured
potential
(mV)
−112.4
−75.5
6.8 7.4
2-point calibration line
Slope = −58.4 mV/pH
Sensitivity = 95 %
If an electrode
has a sensitivity
of 95 % or 0.95,
its sensitivity is
5 % lower than
the theoretical
electrode.
The sensitivity of an electrode is calculated as:
(%)
)802.6398.7(5.61 398.7802.6 −×
−
=atPotentialatPotential ySensitivit
where 61.5 = sensitivity of theoretical electrode.
Each electrode has its own sensitivity limits.
Continued on next page
1-5

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
General information, Continued
Status Status reflects the deviation from the theoretical electrode at pH 7.400 and,
therefore indicates the position of the calibration line.
Theoretical calibration
line drawn through the 1-
point calibration point.
pH
Measured
potential
(mV)
ECal 1
pHCal 1
Theoretical calibration line for the
theoretical pH electrode with known
potential of −112.4 mV at a pH =
7.400.
ECal 1 nom
ECal 1 nom(theo)
E=−112.4 mV
pHCal 1 nom
(7.400)
pHStatus
∆E
∆pH
A calibration
line with the
same slope as
the theoretical
calibration line
(−61.5 mV/
pH) is drawn
through this
point.
The calibration line for the actual electrode deviates from that of the theoretical
electrode. The status value describes this deviation.
Status of the actual pH electrode at pH 7.400 is calculated as:
1005.61 400.7.400.7.
400.7 ×
−
−= atelectrltheoreticaofPotentialatpotentialMeas Status
Each electrode has its own status limits.
Drift Drift of an electrode is a measure of stability obtained by comparing the last
accepted calibration with the previous calibration.
The following drift values are used:
• Drift 1 - obtained on Cal 1 and/or Gas 1;
• Drift 2 – obtained after a 2-point calibration.
The obtained drift values should not exceed the calibration drift tolerances. The
drift tolerances can be changed in the Setup program, but Radiometer recommends
using the default drift tolerances. Too narrow drift tolerances will cause electrode
drift errors even for normal electrode fluctuations. If the drift tolerances are made
wider, no warning will be given if the electrodes should become unstable.
Significant measurement errors could result.
Continued on next page
1-6

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
General information, Continued
Calibration
materials The following calibration materials are used:
Calibration Material Used for...
Calibration Solutions 1 and 2: the exact composition of
the calibration solutions is given in the barcode on the
bottle label, which can be read into the analyzer using the
barcode reader, or entered manually via the keyboard.
Calibration of the pH,
and electrolyte
electrodes
Gas 1 and Gas 2: each gas has a precise composition
essential for determining the accuracy of the analyzer in
each pCO2measurement; the exact composition of the
calibration solutions is given in the barcode on the bottle
label, which can be read into the analyzer using the
barcode reader, or entered manually via the keyboard.
Calibration of the
pCO2electrode
The Chemical Reference Laboratory at Radiometer is responsible for the accuracy
of the calibrating solutions. Traceability certificates for the individual solutions are
enclosed in Chapter 7: Solutions and Gas Mixtures.
Measuring time The measuring time of the electrode is independent of the electrode type. Electrode
signals are registered at 0.982 second intervals during both calibrations and
measurements. The registration of each electrode signal begins after the samples,
calibration solutions, and calibration gases are in position in the measuring
modules. The duration of each calibration is predetermined, as is the number of
updatings of the electrodes’ signals.
In general, the updatings from an electrode response are numbered from 1 to upd.
last, where updating number 1 is the first updating and upd.last is the last. The
diagram below schematically illustrates the electrode response that is calculated on
uncorrected electrode updating values.
Updatings
Signal
Updatings
Upd. 1 Upd. last
1-7

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
Reference electrode
Description The reference electrode is used in the measurement of pH and electrolyte
parameters and is located in the pH/Blood Gas module.
The reference electrode maintains a stable, fixed potential against which other
potential differences can be measured. The potential is not altered by sample
composition.
A fixed potential is maintained at the reference electrode by the following
equilibrium reactions:
AgCl ⇔Ag + Cl
+−
Ag + e ⇔Ag
+−
These reactions are possible because the electrode is made from a Ag rod coated
with AgCl to provide the Ag/Ag+equilibrium and determine the reference
potential.
The electrolyte solution acts as a salt-bridge
solution that maintains an electrical contact
between the coated Ag wire and the sample.
The solution is 4 M sodium formate
(HCOONa), adjusted to pH 5.5 with
hydrochloric acid.
The chloride concentration in the electrolyte
solution is adjusted in accordance with the
chloride concentration in the rinse solution,
to reduce Cl−exchange across the
membrane, thereby obtaining a more stable
potential.
The electrode is encased in the electrode
jacket: The rubber ring seals the electrode in
the jacket to prevent evaporation or leakage
of the electrolyte solution.
Electrode contact
Electrolyte
A
g rod coated
with AgCl
3-layer membrane
Electrode jacket
The membrane consists of three separate membranes:
Membrane Function
Inner To limit diffusion through the membrane and stabilizes the whole
membrane system.
Middle To prevent protein interference.
Outer To reduce the interchange of sample or rinse solution and
HCOONa solution.
Packaging The E1001 reference electrode comes in a box with an insert explaining the
preparation of the electrode and its use.
1-8

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
1-9
pH electrode
Description The pH electrode (E777) is a pH-sensitive glass electrode. The pH-sensitive glass
membrane is located at the tip and seals the inner buffer solution with a constant
and known pH.
The air bubble allows for expansion
of the inner buffer solution when the
electrode is thermostatted to 37 oC.
The potential difference across the
glass membrane is due to a change
in the charge balance at the
membrane.
The glass membrane is sensitive to
H+ions. The metal ions in the glass
are exchanged with protons on either
side of the membrane, from the inner
buffer solution on one side and the
sample on the other.
A difference in the ion exchange on either side of the membrane occurs if the H+
concentration (and therefore pH) is unequal on both sides. The number of positive
and negative ions is no longer equal, so the potential difference across the
membrane changes. If the H+concentrations on either side of the membrane are
equal, the potential difference will theoretically be 0 mV.
Electrode contact
Glass membrane
Inner buffer
solution
Electrode
The theoretical sensitivity of the pH electrode at 37 oC being equal to −61.5 mV
per pH unit, using pH = −log [H+], and converting concentration to activity, the
Nernst equation can be expressed as:
Nernst equation
EE615pH m
sample 0
=− ×.
V
Designation The following symbols are used:
−61.5 mV/pH = Theoretical sensitivity of the pH electrode at 37 oC
E(pH,Cal2) = Potential of the pH electrode chain from a calibration
measurement on Cal 2 solution
E(pH,Cal1) = Potential of the pH electrode chain from a calibration
measurement on Cal 1 solution
E0(pH,Cal1) = Standard potential of the pH electrode chain with a nominal
pH = 7.4 (the approximate pH of Cal 1 solution)
Continued on next page

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
pH electrode, Continued
Designation
(continued) pH(Cal1,nom) = Nominal pH of Cal 1 solution (pH = 7.4)
pH(Cal1) = pH of Cal 1 solution
E(pH,Cal1prev) = Potential of the pH electrode chain from the previous
calibration measurement on Cal 1 solution
Sens(pH,prev)
fraction = Sensitivity of the pH electrode from the previous 2-point
calibration
pH(Cal1,prev) = pH of Cal 1 solution in the previous calibration
measurement
pH(Cal2) = pH of Cal 2 solution
Sens(pH) = Relative sensitivity of the pH electrode chain.
Sensitivity The sensitivity of the pH electrode (SenspH) is obtained from the calibration line
obtained from a 2-point calibration on Calibration Solutions 1 and 2 (Cal 1 and Cal
2), and is calculated from the following equation:
[] pH(Cal1)pH(Cal2)61.5 Cal1)E(pH,Cal2)E(pH,
Sens(pH) −×−
−
=(fraction)
The sensitivity of the pH electrode should fall between 0.92 - 1.03 or 92 -103 %.
Status The status of the pH electrode is calculated from the following equation:
pH(Cal1)nom)pH(Cal1,2
61.5- Cal1)(pH,ECal1)E(pH,
Status(pH) 0−+
−
=
The status of the pH electrode should fall between a pH of 6.7 and 8.1.
Drift Drift 1 is calculated from the following equation:
[]
prev)pH(Cal1,pH(Cal1)
prev)Sens(pH,61.5- Cal1prev)E(pH,Cal1)E(pH,
1(pH)Drift −−
×
−
=
NOTE: Under normal circumstances, pH(Cal1)−pH(Cal1,prev) = 0. However in
instances where the Cal 1 solution container has been replaced between two
consecutive calibrations, pH(Cal1)−pH(Cal1,prev) ≠0.
The default drift tolerances set by Radiometer for Drift 1 are ±0.020.
Drift 2 is calculated from the following equation:
[]
Drift 2(pH) E(pH,Cal2) E(pH,Cal1prev)
-61.5 Sens(pH,prev) pH(Cal2) pH(Cal1,prev)=−
×−−
The default drift tolerances set by Radiometer for Drift 2 are ±0.020.
Continued on next page
1-10

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
pH electrode, Continued
Measurement The sample pH is calculated as follows:
pH(sample) = E(pH,sample) E(pH,Cal1)
61.5 Sens(pH) pH(Cal1)
−
−× +
Corrections The measured pH value is then corrected for systematic deviations from the
reference method using the following equation:
Equation A:
pH(sample,corr.) = A0× pH(sample) + A1
where:
pH(sample) = uncorrected pH value of the sample
pH(sample,corr.) = corrected pH value of the sample.
A0= instrument-dependent correction factor
A1= instrument-dependent cut-off
Equation A+:
When an additional correction is needed, equation A is first used together with the
constants for the FLEXMODE (195 and 165 µL, no message) mode. Then the
obtained results are put back into equation A as pH(sample) and then treated again,
using the constants for the specific sample handling to obtain the corrected value.
Corrections are as follows:
ABL8XX
FLEX Mode A0A1Equation
35/25/15 S195 0.9964 0.0150 A
S95 0.9964 0.0150 A
S85 0.9964 0.0150 A
C95 1.007
−0.053 A+
C55 1.025
−0.1880 A+
FLEXMODE (no message) 0,9964 0.0150 A
FLEXMODE (message 874) 1.007 −0.0530 A+
FLEXMODE (message 873) 1.007 −0.0530 A+
FLEXMODE (message 872) 1.0216 −0.1639 A+
FLEXMODE (message 871) 1.025 −0.1880 A+
Continued on next page
1-11

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
pH electrode, Continued
Corrections
(continued) ABL8XX
FLEX Mode A0A1Equation
35/25/15 FLEXMODE (message 870) 1.030 −0.216 A+
(cont.) FLEXMODE (message 869) 1.030 −0.216 A+
30/20/10 S85 0,9964 0.0150 A
C55 1.025
−0.1880 A+
FLEXMODE (no message) 1.0006
−0.0035 A+
FLEXMODE (message 872) 1.0209 −0.1575 A+
FLEXMODE (message 871) 1.025 −0.1880 A+
FLEXMODE (message 870) 1.030 −0.216 A+
FLEXMODE (message 869) 1.030 −0.216 A+
05 S165 0,9964 0.0150 A
S95 0,9964 0.0150 A
S85 0,9964 0.0150 A
C95 1.007
−0.053 A+
C55 1.025
−0.1880 A+
FLEXMODE (no message) 0,9964 0.0150 A+
FLEXMODE (message 874) 1.007 −0.053 A+
FLEXMODE (message 873) 1.007 −0.053 A+
FLEXMODE (message 872) 1.0216 −0.1639 A+
FLEXMODE (message 871) 1.025 −0.1880 A+
FLEXMODE (message 870) 1.030 −0.216 A+
FLEXMODE (message 869) 1.030 −0.216 A+
00
Continued on next page
1-12

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
pH electrode, Continued
Stability criteria The following stability criterion must be met to obtain a stable electrode response
during 1- and 2-point calibration:
pH(limit)i).updpH(sample,last).updpH(sample, ≤−
The following stability criterion must be met to obtain a stable electrode response
during measurement:
pH(limit)i).updpH(sample,last).updpH(sample, ≤−
where:
pH(sample,upd.last) = pH value from the last updating with a measurement
on calibration solution or sample. (The last updating
is number 30).
pH(sample,upd.i) = pH value for a given updating with a measurement
on calibration solution or sample. (The relationship
must be fulfilled for at least one of the updating
numbers 20 or 21).
pH(limit) = pH limiting value for the stability criterion (0.005).
1-13

1. Potentiometric measuring principles ABL800 FLEX Reference Manual
pCO2electrode
The pCO2electrode (E788) is a combined pH and Ag/AgCl reference electrode
mounted in a plastic jacket, which is filled with a bicarbonate electrolyte.
Description
The jacket is covered by a 20
µm silicone membrane
moulded on a 50 µm nylon net.
The net both reinforces the
silicone membrane and serves
as a spacer in order to trap a
layer of the electrolyte between
the membrane and the glass tip
of the electrode. The electrolyte
also contains glycerol to
prevent collection of air
bubbles in the electrode jacket
thus improving electrode
stability.
Electrolyte
A
g/AgCl reference band
Membrane
Electrode contact
Electrode jacket
The membrane allows any uncharged molecules of CO2, O2, N2to pass through it.
Charged ions such as H+will not pass. Consequently, dissolved CO2from the
sample will diffuse into the thin layer of bicarbonate electrolyte until the
equilibrium is reached.
This produces carbonic acid:
H2O + CO2⇔H2CO3
Carbonic acid dissociates according to the following equilibrium reaction:
HCO H HCO
23 23
⇔+
+−
The release of H+ions changes the H+concentration, and therefore the pH of the
solution on one side of the pH-sensitive glass membrane.
The concentration gradient of H+ions on the other side of the membrane affects
the potential difference across the glass membrane. This change in potential across
the glass membrane is measured by the voltmeter.
Nernst equation The Nernst equation is used to convert the potential reading into a pH value:
mV)(pH5.61
0glass
×
−
=
EE
where:
Eglass = potential difference across the glass membrane
E0= standard electrode potential
61.5 mV/pH = theoretical sensitivity of the pH electrode at 37 oC
Continued on next page
1-14

ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
pCO2electrode, Continued
Nernst equation
(continued) The pH value is related to the partial pressure of CO2in the sample by the
following equation:
2
CO2
-
3
aCO
HCO
log+pK=pH
α ×pc
where:
pKa= −log Ka, the equilibrium constant for the dissociation of carbonic acid in
water
α
CO2= solubility coefficient for CO2in water
The bicarbonate concentration
[
]
HCO3
-is so large compared to
[
that it can be
considered constant. At constant temperatures
]
H+
α
CO2is also constant. So the
equation can be simplified to:
pH = K' -log CO2
p
where:
K' is a constant incorporating the equilibrium constant for carbonic acid (Ka), the
bicarbonate concentration, and the solubility coefficient
α
CO2.
2
3
CO
HCOH −+ ×
=cc Kais the equilibrium constant for carbonic acid.
pCO2of the sample is then calculated from the equation above.
Designation The following symbols are used:
pCO2(Gas1),
pCO2(Gas2) = Pressure of CO2in Gas 1 or Gas 2, respectively
FCO2(Gas1),
FCO2(Gas2) = Fraction of CO2in Gas 1 or Gas 2, respectively
BGas 1 or 2 = Pressure inside the measuring chamber during a
measurement on Gas 1 or Gas 2 respectively
pHO
2= Water vapor pressure (6.2751 kPa at 37 oC)
E(CO2,Gas1),
E(CO2,Gas2) = Potential of the
pCO2electrode from a measurement on
Gas 1 or Gas 2, respectively
Sens(pCO2,theo) = Theoretical (absolute) sensitivity of the pCO2electrode
at 37 oC
Sens(pCO2,prev) = Relative sensitivity of the pCO2electrode from the
previous 2-point calibration
Continued on next page
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