1 2
METAL-METAL SALT ELECTRODE AND METHOD FOR This wire is generally not silver for cost purposes. If elec
MAKING THE SAME trolyte should contact this new wire or connection, er
BACKGROUND OF THE INVENTION Latic fnd ,UnStable ^ ^^T^
This last limitation is the major cause of failure of this
1. Field of the Invention ^ type of electrode.
This invention relates to an improvement in an electrode for 4. Another disadvantage of the pressed pellet type of elecelectro chemical use for the measurement of electromotive trode is its resultant size and shape. It must be put into the force and the passage of electrical current therethrough, and forms of a compacted mass of rather large size to be cornto a method for making the same, Q mercially feasible.
2. Description of the Prior Art 5. Another disadvantage of the pellet type electrode is that Electrodes are used whenever it becomes necessary to con- it is produced in an essentially non-porous form.
vert from electronic conduction, conduction in a metal by A second property of electrodes which gives some indica
valence electrons, to conduction in an ionic solution or gel. In tion of performance is electrode impedance. This is commonly
.omc conducuon charge is transferred by means of ions of ,5 refem;d tQ ^ larizatjon. Genera„y jow electrode im.
much greater mass than electrons. An electrode is therefore a . . . . f. . „ . . . . ,
.° .... .. . . . .. pedance is desirable, especially in cases where substantial curtransducer much like a radio antenna. Electrodes are used in r „ „ . ,. . , ,. ...
batteries, chemistry and medicine. In batteries they are used rents must flow'If we v,sua,"e a certam number °f'°nS Wh,Ch
to convert chemical energy into electrical energy. In chemis- are caPable of exchanging charge with the metal phase, im
try, they may be used to study chemical reactions and to deter- 20 mediately adjacent to the metal but in the solution phase,
mine the activity (concentration) of various ions. In medicine when current is forced to flow through the electrode, these ad
they are used to monitor the living organisms ion movements, jacent layers of ions are used up quickly. It then takes addi
electrocardiogram, electroencephalogram, etc. tional energy to cause ions to move up to the metal phase. This
There are a number of properties of electrodes common to loss of energy at the electrode solution interface can be all these uses which are a measure of how well an electrode 25 characterized by impedance. Impedance is inversely related to functions. The first property is the potential measured electrode surface area and current density. Low impedance between an electrode and another standard electrode, or the type electrodes can be created artificially by providing a subpotential between two identical electrodes, the latter com- stantial atmosphere of mobile ions, capable of exchanging monly referred to as "offset potential." charges with the metal phase. This is done to some extent in
In the case of a silver, silver chloride electrode referred to as 30 metal> metal salt type pellet electrodes. Peilet type electrodes
the "standard hydrogen electrode," the potential £ at 25° C is are made to be non orous. This may be a disadvantage if one
determined by the chloride ion activrty according to the equa- ... ,QW eIectrode impedance. Porosity increases the
£ = 0 222 —0 059 log [Cl~] overall amount of material available for charge exchange with
The electrode can therefore be used to measure the activity of 35 the solution- P°rosity decreases sensitivity of the electrode to
chloride ions in a solution or be used as a reference electrode physical movement in the solution or motion of the electrode,
if placed in a solution of known chloride ion concentration. This sensitivity to physical movement is a common cause of
The limits of its applicability are determined by the stability signal artifact in biological measurements and a significant
and reproducibility of its potential. A good measure of its contribution to the state of the art would be an electrode
limitations is provided by immersing two like electrodes in the 40 which is insensitive to motion of the surrounding medium,
same solution and measuring the difference in potential Porosity on the other hand decreases the response time of the
between them. electrode to changes in ionic concentration in the solution
In theory one should read zero volts between any two elec- which is undesirable in the field of chemical measurements.
trades prepared by the same process but in practice there ex- As mentioned, pellet type electrodes must be fabricated from
ists some offset potential. Electrodes manufactured by con- 43 large partjcies, and this also tends to increase electrode im
ventional techniques deteriorate rapidly or show offsets rang- pedance. it would, therefore, be highly desirable to produce a
ing from hundreds of microvolts to tens of millivolts. The best ,ow im edanC6i stable electrode which not only overcomes
commercially available electrodes so far as offset potentials „„u,„„,„ „f 0„ Kllf ■ -„t,-_-i„ ■ ,„
', . ., ., ,, .. ,, f . the problems ot the prior art, but is extremely inexpensive to
are concerned are the silver, silver chloride pellet electrodes, Jq produce
These electrodes have offset potentials in the hundreds of
microvolts range. These electrodes have certain disadvantages SUMMARY OF THE INVENTION as follows:
1. Offset potentials are determined by stresses produced by It is therefore among one of the principal objectives of this the compression of the metal, metal chloride mixture dur- 55 invention to produce a low impedance, stable electrode ing fabrication. system which is of high quality and yet is extremely economi
2. Offset potentials are also a function of particle sizes. In cal to produce. In accord with the invention, there has now order to obtain a mixture which will flow easily in produc- been discovered an improved electrode comprising broadly a tion equipment, large free flowing particles must be used. metal-metal salt and a matrix material. This basic electrode These larger particles have less surface area per unit 60 material is coatable on an electrically conducting substrate to volume and tend to limit the homogeneity of the mixture provide an excellent quality electrode assembly.
on a microscopic scale. Because of this lack of The method of making the electrode comprises preparing in homogeneity and reduced surface area these electrodes admixture a metal and a metal salt and incorporating the mixhave higher offset potentials than those produced with ture into a matrjx mat„riai therefor, then applying that resultfiner particles. 65 ing mixture to an electrically conducting support.
3. In order to maintain low and constant offset potentials
any electrical connection made directly to the pellet elec- BRIEF DESCRIPTION OF THE DRAWING trode in contact with solution must be of the same metal
as in the mixture. In the case of the silver, silver chloride The invention will be hereinafter more fully described with
electrode this contact can be a silver wire around which 70 reference to the accompanying drawing in which:
the pellet is compressed or a silver disk pressing against FIG. 1 is a plan view showing an electrode made in the
the pellet. In the former, the electrode cost is substan- manner in accord with the invention.
tially increased. In the latter, an extra precision part must FIG. 2 is a sectional view taken along line 2—2 of FIG. 1.
be fabricated. In both cases a further solder connection or FIG. 3 is a sectional view taken along line 2—2 of FIG. 1
weld to a wire must be made in order to make connection. 75 disclosing the two-coat species of the present invention.
