Toxic sensor and method of manufacture

Details Toxic sensor and method of manufacture Toxic sensor and method of manufacture

1999-05-12

2001-06-19

Warden, Jill (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S414000, C204S432000, C205S780500

Reexamination Certificate

active

06248224

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to sensors such as for toxic compounds in gases or fluids and, in particular, to the manufacture and use of a detector for sensing ammonia.
2. Description of the Related Art
Toxic gas sensors are relied upon to an ever increasing extent to safeguard the health and safety of personnel entering a possibly hazardous area. With added attention being directed to worker's safety in confined spaces, there is a need for rapidly identifying gaseous or liquid agents, which (even if not asphyxiating or combustible) are nonetheless toxic, using compact, portable equipment. With an increasing integration of electronics, the ability to obtain an instrument read-out of toxic sensors is becoming simpler. However, there remains a need to provide sensors having improved accuracy and sensitivity to avoid inappropriate conclusions drawn from a sensor's output indication.
Electrochemical methods are frequently used for analyzing traces of gases. In methods of this kind, the measuring electrode of a galvanic cell is brought into contact with the gas to be examined, producing an electrical current which is proportional to the concentration of the compound to be measured. In the case where reducible gases (for example, oxygen, nitrogen dioxide, ozone or chlorine) is measured, there is a reaction on a measuring electrode which acts as a cathode, while oxidizable gases (for example, hydrogen sulfide, carbon monoxide, hydrogen) react on a measuring electrode acting as an anode.
Many electrochemical ammonia sensor cells in use today employ the Sevringhaus (potentiometric) Principle (DS-A-2009937). A pH glass electrode is employed as a measuring electrode,and the potentials between the pH electrode and a reference electrode are measured. The difference in potential serves as a measurement signal, related to the presence of ammonia through a pH-shift of the electrolyte according to the reaction NH
3
+H
2
O→NH
4
+
+OH
−
. This process suffers drawbacks in that the signal is logarithmically related to the ammonia concentration. Moreover, the time to fix the balance is extremely slow. Further, other gases (e.g., SO
2
, HCl, CO
2
) are able to change the pH-value of the electrolyte, falsifying the measurements.
Other commercially available ammonia sensors (such as those available from Sensoric GmBH & Co., operate on the amperometric principle. Sensors of this type respond to a direct transformation of ammonia passing through a gas-permeable membrane onto a catalytic working measurement electrode, according to the following equation:
NH
3
→N
2
+6H
+
+6e
Electrodes in these types of sensors include measure, reference and counter-electrodes in contact with a water-free organic gel electrolyte, cooperating with the electro catalyst to oxidize the ammonia to nitrogen. It is important for direct transformation that the electrolyte be a water-free medium. One example of a highly effective catalyst is platinum black. The reaction causes an electric current that is proportional to the concentration of ammonia in the measuring gas. Unfortunately, the oxidation rate of ammonia to nitrogen is not fast enough at higher concentrations, and intermediate by-products of the oxidation of ammonia causes the measuring electrodes to become partly blocked, resulting in a temporary poisoning of this electrode, with a continuous decline in the measuring signal if the ammonia gas is not withdrawn from the sensor. Furthermore, the selectivity of this sensor is not very high.
Another type of amperometric measurement procedure is known from GB 2,225,859. The ammonia passes a gas-permeable membrane into an electrolyte containing soluble non-oxidizable reagent. The reagent changes through a reaction with ammonia to a substance that is electrochemically oxidizable, preferably an organic ammonium salt such as hydrochloride of Tris(hydroxymethyl)-aminomethane (“Tris-HCl”). This transforms the ammonium salt into Tris(hydroxymethyl)-aminomethane, which is oxidized in a second stage, in place of the ammonia itself on the measuring electrode. A highly effective catalyst, either rhodium or gold, must be used with this procedure to produce oxidation of the newly formed amine. Unfortunately, other substances, such as by-products in the measuring gas (e.g., CO or alcohol) will also be measured. Another disadvantage is that a bias-potential is necessary to cause a reaction. This causes long warm-up periods, dependable on temperature, degrading the measuring behavior of the sensors, and impairing the ability to obtain a favorable zero-noise level. In this type of procedure, ammonia reacts only with a particular substance (Tris-HCl) added to the electrolyte, producing an electrochemical active species which later transforms itself at a measuring electrode, creating a measurement signal proportional to ammonia concentration.
A similar measuring procedure is known (U.S. Pat. No. 5,234,567) which uses a chemical species which reacts with ammonia to form a product which is more electrochemically active than ammonia. The chemical species is one of iodine or Nesslers reagent or a solution of manganese and silver nitrate. The preferred chemical reagent is iodine. The ammonia diffuses into the sensor and dissolves readily in water to product OH
−
(reaction 1), which is necessary for a secondary reaction forming iodine-ions (reaction 2).
NH
3
+H
2
O→NH
4
+
+OH
−
  (1)
6OH
−
+3I
2
→5I+IO
3
+3H
2
O  (2)
2I
−
→I
2
+2e  (3)
The measuring signal results from the reformation of iodine from the iodine-ion (reaction 3). This must occur at an elevated potential of the measuring electrode. A +300 mV bias potential is required between the reference and measuring electrodes. This requires a long warm up time to stabilize the zero reading of the sensor. Another disadvantage is that a very small side reaction can also form iodine-ions from iodine (reaction 4).
I
2
+3H
2
O→5I
−
+IO
3
−
+6H
+
  (4)
This side reaction is temperature dependent with a small increase of temperature substantially increasing the zero current, which again is a big disadvantage.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrochemical sensor for the analysis of gaseous and liquid ammonia and volatile amines.
A further object of the present invention is to provide an ammonia sensor having improved accuracy over its designated operating range.
Yet another object of the present invention is to provide a sensor which is relatively free of maintenance requirements, and which remains stable over extended periods of time.
It is an object of the present invention to provide a sensor for ammonia and its derivatives which does not require a bias potential and which has a stable zero-noise level despite fluctuating ambient temperatures and humidity. It is also an object of the present invention to provide a sensor of this type which is able to measure elevated ammonia concentrations over extended periods of time.
It is important that the sensor provide relatively rapid determinations of toxic concentrations, and do so simply without requiring complex external equipment.
These and other objects of the present invention are provided in a sensor effective for measuring a target component in a gaseous or aqueous samples, the sensor comprising:
a housing containing an electrolyte, the electrolyte including a stable oxidizable metal ion;
a measuring electrode in contact with said electrolyte, the measuring electrode including a support layer and a catalyst effective for catalyzing the oxidation of said stable oxidizable metal ion in the presence of said target component; and
a counter electrode in contact with said electrolyte.


REFERENCES:
patent: 3743589 (1973-07-01), Nicholas
patent: 4105525 (1978-08-01), Synnott et al.
patent: 4141800 (1979-02-01), Breuer et al.
patent: 4297173 (1981-1

Affiliated with

Also associated with

Rating

Toxic sensor and method of manufacture does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Toxic sensor and method of manufacture, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Toxic sensor and method of manufacture will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2481760