Ion-selective electrode device for polarographic measurement of oxygen

An ion-selective electrode device for measuring oxygen concentrations comprises a noble metal reduction electrode connected to a voltage source; a reference electrode connected to said reduction electrode; an ion-selective carrier or ligand membrane disposed in front of the reduction electrode and containing a cation-selective ligand and being permeable to hydrogen ion; a closure membrane to seal the device against the outside atmosphere, the said closure membrane being permeable to oxygen and impermeable to water, and an electroyte containing the ligand cation of said ligand membrane, the said electrolyte being disposed between the said ligand membrane and said closure membrane.

BACKGROUND OF THE INVENTION 
The invention relates to a device for the polarographic measurement of 
oxygen. 
The prior art devices of this type have two important shortcomings. 
On the one hand, even highly purified noble metals used in the electrodes 
do not prevent the transmission of foreign matters to the electrodes which 
thus exhibit drift phenomena which interfere with a long service time. 
On the other hand, there occurs an oxygen consumption by the measurement 
itself which is so high that it interferes with the diffusion field in the 
most frequently measured organic tissues. As a consequence, the true 
oxygen concentration can be obtained either not at all or only after 
overlong times for the measurement. 
It is therefore an object of the invention to avoid these shortcomings. 
SUMMARY OF THE INVENTION 
This is accomplished by a device comprising a source of potential voltage; 
a reduction electrode for the oxygen composed of a noble metal and 
connected to said voltage source; a reference electrode connected to said 
reduction electrode; a ligand membrane provided in front of the reduction 
electrode, the ligand membrane containing a cation-selective carrier and 
being permeable to hydrogen ions; a closure membrane to seal the 
electrodes against the outside space, the closure membrane being permeable 
to oxygen and impermeable to water, and an aqueous electrolyte containing 
the ligand cation of the ligand membrane, the said electrolyte being 
disposed between said ligand membrane and said closure membrane. 
By virtue of this device the selective action of the ligand membrane causes 
only the reaction material to come in contact with the reduction electrode 
while no such contact takes place with the materials of which the 
electrolyte or the opposite electrode are composed. Chemical changes in 
the reduction electrode are thus avoided and the time of constant 
operation of the device is substantially extended. Furthermore, the 
particle flow of the oxygen is drastically reduced. An interference of the 
concentration field of the oxygen through the measurement activity is 
therefore extremely small. 
The signal produced is measured by means of an amplifier which in order to 
preserve the above advantages should have an input value of about 
10.sup.12 .OMEGA.. 
The device operates with particular success if the ligand is Na-selective. 
It has been found that in that case an easily indicated signal which is 
highly proportional to the actual oxygen concentration is produced. 
The ligands or ion-sensitive molecule may be 
3,6-dioxaoctanediacid-bis-diphenylamine, or preferably may consist of 
N,N,N,N-tetrabenzyl-3,6-dioxaoctane diamide or 
N,N'-dibenzyl-N,N'-diphenyl-1,2-phenylenedioxydiacetamide. 
In a special embodiment of the invention the ion selective reference 
electrode is covered by a corresponding reference ligand membrane which 
contains an ion-sensitive molecule or ligand different from the cation 
ligand of the ligand membrane. This will protect the reference electrode 
from physical or chemical damage. 
The electrolyte preferably consists of 0.1 M HCl and 0.01 M NaCl. NaCl and 
HCl are required for the function of the oxygen electrode 2 if the 
electrode consists of platinum covered with an Na.sup.+ membrane. On the 
other hand 0.01 M KCl should be used for the electrode if the reference 
electrode consists of platinum covered with a K.sup.+ membrane. 
Preferably, the voltage provided is 200 mV. In this manner at the boundary 
layer of the platinum a Po.sub.2 dependent oxygen potential is generated. 
The resistance of the electrode is: 
##EQU1## 
R.sub.Po.sbsb.2 =resistance of polarization layer (Helmholtz layer) 
R.sub.membrane =resistance of Na.sup.+ membrane 
U.sub.Po.sbsb.2 =oxygen potential 
U.sub.volt =voltage of basic source 
The total resistance is in the range of 10.sup.10 Ohms and decreases by 
less than one order of magnitude with an increasing oxygen potential. 
As already pointed out the membranes are preferably made of PVC impregnated 
with a solvent containing the ion-sensitive molecule. The membrane is for 
instance made by setting up a solution of PVC in dibenzyl ether. The 
solution is then mixed with about 0.6% by weight of the Na-ligand or 2.7% 
weight of the K-ligand (such as valinomycin) and subjected to drying. 
Thus, membranes of a thickness down to a few microns are obtained. 
These membranes are then placed onto a body of PVC comprising the platinum 
electrodes and are welded together with the main body chemically by using 
again a small amount of solvent. The thus formed structure is then covered 
with a membrane of PVC or Teflon which does not contain a ligand. The 
space between the two membranes in the device is thereafter filled with an 
electrolyte. 
It should be noted that the electrolyte does not contain the ligand itself, 
but only the ions. 
With the device of the invention oxygen concentrations can be measured in a 
range of pressure from 1/100 mm mercury up to several psi. The measurement 
can be carried out amperometrically, that is with a measuring resistance 
of about 10.sup.6 .OMEGA. which will cause a stationary particle flow in 
the measuring system. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawing.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A source of polarization voltage 1 of 200 mV is connected with the 
reduction electrode 2 which preferably consists of platinum. Another 
electrode, the reference electrode 3, is arranged in annular manner around 
the reduction electrode. 
In front of the reduction electrode a PVC membrane (20) is arranged which 
contains an ion selective cation molecule or ligand and is permeable to 
hydrogen ions and oxygen. 
In front of the other electrode, the reference electrode 3, a similar 
membrane, the reference ligand membrane 30, is provided which, however, 
contains an ion-sensitive molecule or ligand which is different from the 
ligand of the ligand membrane 20. 
As has been noted above the device can be operated also without the 
reference ligand membrane. 
The ion-sensitive molecule provided in the ligand membrane may consist of 
any of the products listed above and in the example was formed of 
3,6-dioxaoctanediacid-bis-diphenylamine. 
An electrolyte 5 is disposed so as to cover the entire device. The device 
is furthermore sealed against the exterior space by a closure membrane 6 
which may be of polymerized tetrafluoroethylene synthetic plastic, 
commercially available under the trade name "Teflon". If no reference 
ligand membrane 30 is provided the electrolyte may consist only of 0.1 M 
HCl and 0.01 M NaCl. If a reference ligand membrane such as the membrane 
30 is provided the electrolyte must additionally also contain the ion of 
the ligand used in the latter membrane, e.g. KCl. 
The operation of the device is as follows: In the exterior space there is a 
specific concentration of the oxygen 7. The oxygen then will diffuse 
through the closure membrane 6 which may be made of Teflon and furthermore 
will penetrate together with the hydrogen ions from the electrolyte 5 into 
the ligand membrane 20. After reduction of the oxygen water is formed with 
the hydrogen ions according to the equation 
EQU O.sub.2 +4e.fwdarw.20.sup.-- +4H.sup.+ =2H.sub.2 O 
This reaction results in a change of potential of the electrode 2-20 in the 
chain of potential formed by the electrodes 2-20, 3-30, the electrolyte 5 
and the voltage source 1. This change of potential is then indicated by 
means of the amplifier 4. As already noted the amplifier 4 has an input 
resistance of about 10.sup.12 .OMEGA.. 
It will be understood that the electrolyte in case of the use of a 
reduction membrane and a reference membrane must contain the ions of both 
ligands. For instance, if the ions are Na.sup.+ and K.sup.+ in the case 
of an Na-ligand membrane used on the electrode 2 and in case of a K.sup.+ 
ligand membrane on the reference electrode 3 the electrolyte must contain 
both Na.sup.+ and K.sup.+ ions. 
The polarization voltage is generated by a battery-driven, stabilized, and 
highly insulated power supply which is connected in series to the oxygen 
electrode. The slope of the calibration curve increases with higher 
polarization voltage. At a polarization voltage of -200 mV there was 
obtained a linear calibration curve with a slope of approximately 40 mV 
per decade of oxygen tension. 
The sealing of the cathode with an ion-selective PVC membrane provides the 
following advantages: 
1. The cathode is protected against deposition of metals and other 
reducible species in an almost ideal way. 
2. The platinum interface is not in contact with an aqueous phase. 
3. The ions which are supposed to be involved in the electrochemical 
reaction can be selected. 
The oxygen sensor described is very suitable for tissue measurements 
because of its small convection sensitivity, its high sensitivity in the 
low PO.sub.2 range and its relatively small drift. 
The device can be used for PO.sub.2 measurements in: 
1. gases, 
2. blood, 
3. tissues (brain, heart, liver, kidney, skeletal muscle), 
4. skin (noninvasive measurements of arterial Po.sub.2). 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.