Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1994-04-29
1995-10-24
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
20415313, 204402, 204412, 204415, 2041531, 204290R, 427 96, 427125, G01N 2726
Patent
active
054607108
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates in general to electro-analysis of liquid solutions, such as to detect and/or measure the concentration of a species in the solution by causing the said species to generate electrical signals at an exposed working surface of at least one electrode with the said working surface in contact with the solution, the electrode being one which has at least one dimension comparable to or smaller than the concentration boundary layer thickness of the solution.
FIELD OF THE INVENTION
The concentration boundary layer thickness is the distance from the electrode surface out into the solution over which the concentration of the analyte changes from its value at the surface of the electrode to its value in the bulk of the solution. Such a concentration variation exists because the analyte is being consumed by an electrochemical reaction occurring at the electrode surface. The boundary layer thickness is determined by the diffusion coefficient of the species being analysed. The diffusion coefficient in turn is different for different species and depends on the temperature and on the viscosity and dielectric constant of the solvent.
The liquid solution is typically a stream of liquid, and one non-limiting example of an application of the invention is in the monitoring of a fresh-water supply for its chlorine content. In practice, the dimension mentioned above (referred to herein as a "micro-dimension") is typically of the order of only about one micrometer.
Electrodes as defined above give a relative sensitivity (i.e. the ratio of electrical signal current generated to concentration of the species in the solution) that increases with the size of the electrode, while the magnitude of the measured current decreases. Current at these electrodes is independent of flow conditions.
It has been found that such elements, using gold as the electrode material, may be used to measure chlorine in water at concentrations down to around 0.1 ppm, and that the measured current is independent of the flow rate of solution past the electrode surface. These electrodes have to be cut and carefully polished before use, which would make them expensive for industrial use in applications such as water supply monitoring. They do however offer a number of advantages, among which may be mentioned the following: high sensitivity; relative immunity to movement in the analyte e.g. due to stirring; and the fact that they can be used in liquids of high electrical resistance.
SUMMARY OF THE INVENTION
The invention relates specifically to sensing elements incorporating electrodes for use in electro-analysis of liquid solutions; methods of making such elements; apparatus for use in electro-analysis of liquid solutions and including such elements; and methods of carrying out the analysis itself using such apparatus.
Among the objects of this invention are to provide versatile analytical sensing elements of high sensitivity and reliability, suitable for simple as well as complex analytical tasks, in which the construction of the elements is not only relatively inexpensive, but also appropriate to quantity production methods, by enabling the area and electromechanical characteristics of the working surfaces of the electrodes (i.e. the surface, having the above-mentioned micro-dimension, that is in actual contact with the test solution when in use) to be readily repeatable.
In practice, the working surface must be thoroughly clean. A further object of the invention is therefore to provide a sensing element in which this requirement is satisfied in a simple way.
It is also found, where the solution under test is a stream (e.g. a water supply), that because the operation of the electrodes depends on the rate of diffusion of the analyte species, the flow rate of the stream has to be carefully regulated. In current practice, the electrochemical method is supplemented with "spot" determinations made using colorimetric techniques. The invention aims to overcome these drawbacks.
According to the invention in a fi
REFERENCES:
patent: 2183531 (1939-12-01), Allison
patent: 3530046 (1970-09-01), Mochizuki et al.
patent: 4217194 (1980-08-01), Lubbers et al.
Planer George V.
Williams David E.
Bell Bruce F.
Capteur Sensors & Analysers Ltd.
Middleton James B.
Niebling John
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