Electrochemical system for measuring the partial oxygen pressure in a gaseous or liquid atmosphere

Electrochemical system for measuring the partial pressure of oxygen in a gaseous or liquid atmosphere, in which the electrolyte is a halogenide mixture doped with peroxide ions. This system constitutes an oxygen gauge permitting measurements at temperatures below 200.degree. C.

The present invention relates to an electrochemical system for measuring 
the partial pressure of oxygen in a gaseous or liquid atmosphere, which 
can also be called an electrochemical oxygen gauge. 
It is known to obtain such oxygen gauges by associating a reference 
electrode and a working electrode with a solid electrolyte layer. 
In the prior art systems, the solid electrolyte may be an oxide. In such an 
electrolyte, the 0.sup.= ions provide the ionic conduction and are brought 
into operation by the reactions of the electrode. Such a gauge is 
described, for example, in French patent application No. 2,243,625, which 
utilizer, as the electrolyte, a solid solution of zirconium and yttrium 
oxide, a solid solution based on thorium. However, the use of these gauges 
is confined to relatively high temperatures, e.g., above 40.degree. C., by 
the fact that these oxides are very poor conductors at low temperature. 
It has also been proposed to overcome these drawbacks by providing 
electrolytes in which the ionic conductivity is furnished by ions other 
than oxygen, e.g., by halogenides. It is in this way that Pelloux et al, 
in "Solid State Ionics" Vol. 1, 1980, p. 343, have proposed an electrolyte 
consisting of SrCl.sub.2 doped with KC1 and containing a small quantity of 
SrO. In French patent application No. 2,486,244, entitled (in English 
translation) "Potentiometric System Usable as Sensor for Determining the 
Pressure of a Gas" it is proposed to use a fluoride mixture of the type 
PbSnF.sub.4, in which the fluoride ions provide the conductivity. 
While these electrolytes enable the construction of usable oxygen gauges, 
they nevertheless have the drawback of exhibiting a relatively long 
response time, the response time of the gauge being defined as the time 
necessary for the e.m.f. of the gauge to become substantially equal to the 
theoretical e.m.f., after a change in the oxygen pressure. Such a drawback 
is detrimental to those applications in which it is desired to rapidly 
detect a change in the content of oxygen. 
Accordingly, it is an object of the present invention to provide a system 
which makes it possible to overcome the drawbacks of the prior art and 
which, in particular, exhibits a relatively short response time while 
being usable at low temperatures, particularly below 200.degree. C. 
This and other objects which will appear are achieved in accordance with 
this invention by an electrochemical system for measuring the partial 
pressure of oxygen in a gaseous or liquid atmosphere which comprises a 
working electrode responsive to the oxygen, a solid electrolyte consisting 
at least in part of a halogenide mixture capable of providing the ionic 
conduction by means of the halogen ions, a reference electrode consisting 
of a gas, or a solid, or a mixture of solids, said reference electrode 
being capable, at least in its immediate vicinity, of maintaining constant 
the chemical potential of the halogen in the electrolyte, this system 
being characterized in that the electrolyte is doped with a peroxide which 
provides the reactive component of the working electrode. 
The advantages of the doping resides in the fact that, for such an oxygen 
gauge, it is the peroxide ions which are used in the electrode reaction. 
This permits improving its reversibility and reducing the response time of 
the gauge. 
In addition, such an electrolyte exhibits good conductivity at ordinary 
temperatures and no electronic conductivity. 
In accordance with a further characteristic of the invention, the 
electrolyte is selected from mixed halogenides which are good conductors 
and which provide sites capable of accepting the peroxide ions without 
decomposition. 
Preferably, the halogenide is a fluoride mixed with lead and tin having the 
formula PbSnF.sub.4. 
According to a further characteristic, the electrolyte consists of 
PbSnF.sub.4 in which the peroxide is present in the proportion of 0.2 to 
3% (atomic percentage relative to the fluoride), and preferably 0.5 to 2%. 
The reference electrode may be constituted, at least partly, by the mixture 
of a metal and its halogenide corresponding to the halogen of the 
electrolyte, the metal being selected from tin, lead, bismuth and silver. 
As the measuring electrode, one can utilize a porous electrode consisting 
of an inert metal such as, for example, platinum and silver. This 
electrode may then be obtained by depositing a metallic coating layer on 
the electrolyte. 
In another embodiment, the measuring electrode consists of an 
electronically conductive oxide. It is preferably obtained by oxidation, 
in a corresponding oxygen atmosphere. One can also produce a tablet, by 
means of a powder of said oxide, pressed into contact with the 
electrolyte. Preferably the electronically conductive oxide is the dioxide 
of ruthenium. 
The invention will be better understood from the description of a preferred 
embodiment, which follows, given with reference to the accompanying 
drawing. This description provides an illustration of the invention, but 
is not under any circumstances to be considered as a limitation.

In FIG. 1, there is illustrated an oxygen gauge 1 comprising a tablet 2 of 
solid electrolyte. This solid electrolyte consists of a fluoride mixed 
with lead and tin doped with barium peroxide, in proportion of 0.5% of 
peroxide relative to the fluorine (atomic percentage). This doping was 
obtained by dissolving the barium peroxide in a mixture of powders of tin 
fluoride and lead fluoride. 
To carry out this dissolution the mixture of powders is melted in a sealed 
tube in a vacuum of 10.sup.-4 millibar and this is maintained at a 
temperature of 250.degree. C. for two hours. 
The percentage of 0.5% of peroxide was so chosen that the peroxide ions are 
stable, given that the barium ions become attached to lead sites during 
doping. 
The electrolyte tablet 2 is in contact at its lower surface 9 with the 
reference electrode 10 consisting of a compressed mixture of tin and tin 
fluoride powders. A current collector grid 11 of platinum is disposed 
below that face 12 of electrode 10 which is opposite to the contact face 
with the face 9 of the electrolyte. 
On its upper face 3 the electrolyte is in contact with the measuring 
electrode 4 which consists of a porous disc of ruthenium dioxide to which 
is connected a conductive lead 6 surrounded by a protective sleeve 7 and 
whose extremity 8 is connected to an electronic millivoltmeter having high 
input impedance. The measuring electrode was obtained by producing a 
tablet of RuO.sub.2 powder compressed simultaneously with the electrolyte 
and the reference electrode mixture so as to form a unitary tablet. Thus 
there is obtained an assembly which permits very good exchange reactions 
between, on the one hand, the electrolyte and the reference electrode and, 
on the other hand, the electrolyte and the measuring electrode. 
The assembly is placed in a cup 13 comprising a bottom 14 and a cylindrical 
wall 15. This cup is made of a fluid-tight resin and is open toward the 
top. 
The assembly constituted by cup 13 and electrolyte 2 is completely sealed 
so that the reference electrode has no contact with the ambient in which 
the working electrode is located. 
The current collector grid 11 is connected, by a conductive lead 17 of 
platinum surrounded by a sleeve 18, to the same millivoltmeter as the 
conductive lead 6. 
FIG. 2 shows a system particularly suitable for studying the operation of 
the oxygen gauge shown in FIG. 1. 
The gauge 1 is placed on a support 30 whose surface 31 is spherical. The 
support 30 is of alumina. The assembly is placed in the lower, spherical 
portion 32 of a housing 33 made of Pyrex glass. The housing 33 comprises 
an upper portion 34. The upper portion 34 and the lower portion 32 are 
connected together by a sealed cylindrical rim 16. The upper portion 34 
comprises an outlet tube 41 and a orifice 42 both of which provide sealed 
passages. The lower portion 32 similarly comprises an inlet tube 43 and 
tube pass-through 44 and 45 permitting the exit of conductor leads 8 and 
17. The tube 43 is extended into the interior of the housing by an alumina 
tube 60 whose lower extremity opens above the measuring electrode. 
A temperature gauge 46 is placed inside the housing and its upper extremity 
extends in sealed manner through the upper portion through orifice 42. The 
temperature gauge consists of a platinum sensor. A heater 50 surrounds the 
lower portion 32 of housing 33 and a metal shielding armature 51 is placed 
between the housing and the heater. 
The above described system has been used to test the oxygen gauge shown in 
FIG. 1. This gauge may be represented by the following electrochemical 
chain: Pt, Sn-SnF.sub.2 /PbSnF.sub.4 -BaO.sub.2 (0.5%)/RuO.sub.2 (powder), 
0.sub.2 (PO.sub.2). The variation of the e.m.f. of the gauge was measured 
as a function of the logarithm of the partial oxygen pressure inside the 
housing. It was observed that the linear variation of the e.m.f. as a 
function of P(0.sub.2) conformed to the theory of Nernst's law, and also 
that the slope of the straight line correspond to a degree of oxidation of 
the oxygen equal to (-1). This provides good confirmation that, by doping 
the electrolyte with the peroxide, the reactive component of the electrode 
has been provided. This measurement was performed at 160.degree. C. 
In addition it was determined, for that temperature, that the gauge 
embodying the invention provided improved response time relative to 
undoped gauges, because it was found that, after a partial pressure 
variation of 10.sup.-2 atmospheres at 0.2 atmosphere, the response time 
was 6 min., whereas it exceeded 10 hours when the electrolyte is not 
doped. 
The invention is not limited to the embodiments described but rather 
encompasses all variants thereof.