Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Patent
1994-09-06
1996-07-16
Warden, Jill
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
422 98, 422 83, 436106, 436116, 436118, G01N 700
Patent
active
055364730
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present inventions relates to gas sensors and to methods of making them.
Applicants are particularly, though not exclusively, interested in gas sensors for use in monitoring gases, such as acid gases, e.g. H.sub.2 S, NO.sub.2 and SO.sub.2, in connection for example with industrial process control or environmental protection.
It is well known that organic polymers may form the gas sensing material in gas sensors. Such organic materials include conducting polymers which are normally p-type semiconductors whose conductivities are changed when exposed to oxidising gases such as NO.sub.x or reducing gases such as NH.sub.3. Sensors using such materials have been based on both electrical techniques and optical techniques (e.g. surface plasmon resonance).
Various organic polymers, such as polyaniline, which appear to be suitable as gas sensing materials are commonly deliberately `doped` to improve the specificivity and/or sensitivity of the materials towards particular gases. Such `doping` also increases the electrical conductivity and facilitates the detection of change in conductivity of the polymer on exposure to and interaction with the gases being sensed; the change in conductivity being used as a measure of the concentration of the sensed gases.
With some organic polymers, such as polyaniline, in order for doping to be achieved the cationic part of the dopant can only be a hydrogen ion. In such a case the doped form of the polymer can be regarded as the protonated form.
SUMMARY OF THE INVENTION
Applicants investigations have revealed that there is a disadvantage with gas sensors comprising such protonated polymers as the sensing material. Applicants have found that the protonated polymers are unstable as gas sensing materials in the sense that they have a tendency to produce inconsistent conductivity readings; thus unreliable results may be obtained. It is believed that this disadvantage is a consequence of the mechanism of the interaction between the gases being sensed and the polymer which involves a direct de-doping or de-protonating process resulting in undesired degradation of the material.
An object of the invention is to overcome or alleviate the disadvantages associated with protonated polymeric materials as mentioned above.
Accordingly, the present invention provides a gas sensor comprising a film or layer of non-protonated polyaniline as the gas sensing material.
In this specification the term "non-protonated" means that less than 1% of the protenatable imine nitrogen in the polyaniline is protonated.
The non-protonated polyaniline may be in the base (neutral) emeraldine form as depicted by the formula: ##STR1## or may be in the base (neutral) leuco-emeraldine form (that is the fully reduced form), as depicted by the formula: ##STR2##
Alternatively, the non-protonated polyaniline may be formed of a mixture of base emeraldine and base leuco-emeraldine forms.
The base emeraldine form appears to be stable up to about 200.degree. C. in air while the base leuco-emeraldine form appears to be stable up to about 200.degree. C. in an inert atmosphere such as argon or nitrogen.
According to another aspect of the invention a method of making non-protonated polyaniline comprises treating protonated polyaniline with an alkaline solution to de-protonate the material.
The alkaline solution may be an ammonium hydroxide solution.
It will be appreciated that the non-protonated material should not be contacted with a proton-donating species, such as an inorganic acid before being incorporated into a gas sensor.
Dry, solid non-protonated material may be obtained by separating the material from the alkaline solution, for example by filtering, washing the filter cake with water, and optionally in addition with an organic solvent such as isopropanol, after which the filter cake may be suitably dried, for example under vacuum at room temperature.
The dried non-protonated material may be stored in a dry inert atmosphere, for example in a vacuum desiccator.
Where the gas sensor is based o
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Agbor Napoleon E.
Monkman Andrew P.
Petty Michael C.
Scully Margaret T.
British Gas plc
Carrillo Sharidan
Warden Jill
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