Electrochemical Cell for the Detection of Chlorine

The present invention relates to an electrochemical cell suitable for use in detecting and measuring chlorine gas in which a substantially non-aqueous organic electrolyte is used.

The need for detecting and measuring gaseous components of the environment has been increased because of regulations which deal with clean air, clean water and a clean work place. Numerous devices have been produced to measure such gaseous components one of which operates on electrochemical principles. Many electrochemical processes for the determination of gaseous components are known in the art. See for example Advances in Electrochemistry and Electrochemical Engineering, Volume 10, (J. Wiley & Sons, 1976).

Of particular importance in any electrochemical detecting sensor is that it have high accuracy, good reproductability, high sensitivity to the gas sought to be measured and little interference from other gaseous components of the environment. And, in the case of threshold limiting value (TLV) measurements, a fast response time. Presently, most electrochemical sensors operate in an aqueous electrolyte, see for example U.S. Pat. Nos. 3,776,832 and 3,992,267. The most commonly used aqueous electrolytes incorporate solutions of sulfuric acid because of their insensitivity to carbon dioxide which is frequently present in gaseous mixtures. Unfortunately, aqueous electrolytes are restricted by the range of electrical potential at which water decomposes and by the high vapor pressure of water. Aqueous electrolytes also have a very high dielectric constant and, therefore, can generally dissolve more gas. However, such high dissolution rates also create various measurement distortions.

Various attempts have been made to construct electrochemical cells useful in measuring and detecting chlorine. Unfortunately, all of the electrodes selective to chlorine dissolve in the electrolyte medium in the presence of chlorine. It is presently believed that no one has successfully produced a primary continuous chlorine detecting electrochemical cell which operates utilizing an aqueous electrolyte which does not rely upon the destruction of the sensing electrode.

The present invention is directed to an electrochemical cell in which a substantially non-aqueous organic electrolyte is used to sense and measure chlorine gas. It is an object of the present invention to provide an electrochemical cell which is extremely accurate down to below about 1 part per million chlorine (the present TLV value), has a fast response time (less than 60 seconds) and has an extended useful life (over one year).

Generally, the present invention comprises an electrochemical cell having a housing preferably made from polyethylene or other plastic material which is resistant to the electrolyte, nonabsorbent and inert with respect to chlorine. The cell includes therethrough a coextensive cavity for receiving an organic electrolyte. Sealingly positioned across one end of the cavity is a working electrode consisting of a gas diffusion membrane having a first gold catalytic member positioned on the cavity side thereof and electrically connected to a detecting circuit. Preferably, the working electrode is prepared by mixing gold powder with a TFE dispersion and painting the mixture onto a Zitex.RTM. (a porous fluorocarbon) which acts as the membrane material. The membrane and electrode are dried and sintered to obtain and achieve a good bond.

A counter electrode is sealingly positioned across the other end of the cavity. The counter electrode comprises a gas diffusion membrane having a second catalytic member preferably consisting of gold or platinum black positioned on the cavity side to which an electrical lead is connected. Preferably, the counter electrode includes an air reference electrode to which an electrical lead is connected thereto for maintaining the working electrode at the desired potential. The counter and reference electrodes are typically Teflon.RTM. bonded platinum black electrodes which are dispersion painted onto a Zitex.RTM. membrane, dried and sintered in the same manner as the working electrode.

Generally, it is desirable to include first and second end plates which are sealingly positioned across the respective ends of the housing. The first end plate is spaced away from the working electrode to provide a chamber adjacent thereto and includes a gas inlet for directing gas to be detected and measured to the membrane of the working electrode. Each plate is mounted by means of bolts which most advantageously pass through the housing and both end plates and are made from the same material. In applications where end plates are not desired, such as in small portable units, the membranes are exposed to the atmosphere and define the ends of the cell.

An electrolyte consisting of one molar lithium perchlorate dissolved in an organic solvent which is selected from the group consisting of .gamma.-butyrolactone and propylene carbonate fills the cavity. Preferably, lithium perchlorate is dissolved in propylene carbonate which is slightly hydrophilic and maintains the solution with about 1 percent water. Also suitable for use are other inorganic salts which provide some ionic conductivity and are soluble in the organic solvent, but which do not react with the electrodes or the solvent at the operating potential.

In a preferred embodiment of the invention, the cell comprises a third (reference) electrode which is useful for maintaining the potential difference between such reference electrode and the working electrode from between +500 to -700 mv. For best results, it has been found that the potential should be maintained between about +400 to -500 mv and preferably about -100 to -150 mv with reference to the platinum air electrode. The potential may be maintained in the acceptable +400 to -500 mv range by two means. The first involves a potentiostatic circuit which electronically maintains a fixed potential difference between the working electrode and the reference electrode. A second approach can be used when the potential of the reference electrode itself is in the +400 to -500 mv range. Here the potential of the working electrode is set at the same potential as the reference electrode, and the current necessary to perform this function is measured. In the -100 to -150 mv range there is no need to potentiostat the working electrode. This difference is provided by the gold working electrode and the platinum air electrode (reference electrode); i.e., the circuit is electrochemically potentiostated.

The present invention provides a generally linear response from about 0 to 6 parts per million chlorine with response to chlorine gas in about 30 to 60 seconds. Typically, cells of the present invention provide a 5 to 8 .mu.A/ppm output response. Other advantages will become apparent from a perusal of the following detailed description of the best mode for making and practicing the present invention taken in connection with the accompanying drawings.

FIG. 1 is an elevation of the electrochemical cell of the present invention;

FIG. 2 is an elevation along line II--II of FIG. 1 showing the working electrode bonded to Zitex.RTM. membrane;

FIG. 3 is a front elevation of a counter and a reference electrode bonded to a Zitex.RTM. membrane; and

FIG. 4 is a circuit diagram useful in detecting and measuring chlorine with the cells of the present invention.

The following is a detailed description of the best mode presently contemplated for the manufacture and use of the electrochemical cell of the present invention. With respect to FIG. 1, electrochemical cell 10 comprises housing 11 having a cavity 12 coextensively positioned therethrough. Housing 11 includes openings 13 to receive mounting bolts 19 and is preferably formed from a solid block of polyethylene such that cavity 12 may be easily formed by drilling through the block. As should be clear, however, the particular form or configuration of the housing or arrangement and configuration of the electrodes can be altered to suit the final instrument package into which the electrochemical cell is to be adapted.

Cell 10 includes working electrode 14 and counter/reference electrodes 16, described more fully below. Electrodes 14 and 16 are sealingly mounted at opposite ends of housing 11 by respective end plates 17 and 18. End plates 17 and 18 are secured to housing 11 by means of bolts 19 which pass through openings 13 in housing 11 and correlative openings 21 in electrodes 16 and 14. End plate 17 includes recess 22 to provide for sufficient area for diffusion of the gas into the membrane of the working electrode. Alternatively, sufficient space can be obtained by means of annular sealing means (not shown) such as and "O" ring which is preferably positioned between the end plate and working electrode. With respect to the reference air electrode, it has not been found necessary to provide a similar recess since sufficient air enters through the membrane exposed at the periphery of housing 11 and end plate 18.

End plate 17 also includes gas inlet 24 which in typical detection instruments is connected to a means for pumping environmental or atmospheric air into the cell at a closely controlled rate and outlet 25. The atmospheric air enters into and through the chamber formed by recess 22 and working electrode 14 and diffuses into the electrolyte contained in cavity 12 through the membrane portion of electrode 14. As mentioned above, sufficient air diffuses through the Zitex.RTM. membrane to supply oxygen to counter/reference elctrodes 16 which functions as the air electrodes in the preferred embodiment.

Referring to FIGS. 2 and 3 working electrode 14 comprises a Zitex.RTM. or Gortex.RTM. (porous fluorocarbon) membrane 26. Any other membrane material may be used. Such other material would most desirably have characteristics similar to Zitex.RTM., such as not becoming wetted by the electrolyte, large number of pores (e.g. 50% porous) having a small pore size, thin enough to avoid restricting the concentration of gas and nonreactive with the gas and solvent. Most importantly, it must permit diffusion of the desired gas without allowing the electrolyte to escape. Preferably membrane 26 is of the same basic configuration as housing 11 and includes openings 21 which cooperate with openings 13 to receive bolts 19. Working electrode 14 also includes catalyst portion 27 having a configuration and shape substantially the same as cavity 12 and preferably consisting of a high gold powder surface area (0.61 sq in) (e.g. 45 mg) (0.4 m.sup.2 /gm) and TFE (Teflon.RTM.) dispersion (e.g. 30% Teflon.RTM./70% water solution) sintered to membrane 26. In the manufacture of the electrodes, it is desirable to add the dispersion to the gold powder which had already been placed on the membrane. Catalyst portion 27 also includes tab 28 to which is electrically connected lead 31 (See FIG. 1).

Counter/reference electrode 16 includes membrane 32 made of Zitex.RTM. or Gortex.RTM. and a catalytic portion. In the case of a two electrode cell, the catalytic portion of the electrode structure would be manufactured in a manner similar to that of the working electrode, except it is preferable to use platinum black instead of gold. In the preferred embodiment, however, the catalytic portion is divided into first region 33 defining the counter electrode having an area of about 1.2 sq. inches and second region 34 defining the reference electrode having an area of about 1.2 sq. inches. Each region includes an associated tab 36 and 37 for connection with electrical leads 38 and 39. First and second regions of the catalyst portion preferably consists of Teflon.RTM. bonded platinum black (e.g. 35 mg each) sintered to membrane 32.

The electrolyte solution preferably comprises either propylene carbonate or .gamma.-butyrolactone into which is dissolved lithium perchlorate or other suitable supporting electrolyte. The lithium perchlorate is present in an amount of from 10.sup.-5 molar to saturated and preferably from 0.1 to 1.0 molar.

The electrical circuit for use in converting the output of the cell need only include a voltmeter with parallel resistors in the case of a two electrode cell as is well known in the art. In the three electrode cell, however, it is desirable to potentiostat the cell for optimum efficiency. Because cells of the present invention for measuring chlorine have an error of typically less than 4% due to temperature variations, a temperature compensation circuit is not required for span current. It is, however, desirable to include temperature compensation for zero current as shown in schematic diagram shown in FIG. 4. With respect to a three electrode cell, FIG. 4 is a schematic diagram of a circuit useful in converting the signals from the cells of the present invention into concentration of chlorine (ppm).

In a presently preferred embodiment the values in ohms of the resistors are as follows:

Potentiostat: Op. Amp Fairchild 747A, R.sub.1 =10K, R.sub.2 =1.3K, R.sub.3 =750 and R.sub.4 =100.

OPERATIONAL AMPLIFIER II and CIRCUIT: RCA 3160, R.sub.5 =8.06K and C.sub.1 =35.mu.f at 6 volts.

CALIBRATION: R.sub.6 =10K and R.sub.7 =5K

ZERO CURRENT ADJUST. CIRCUIT: Fairchild 747B, R.sub.8 =13.3K, R.sub.9 =5K.

THERMISTOR Th.sub.2 =30K at 25.degree. C.

OP. AMP. IV CIRCUIT: RCA 3160, R.sub.10 =2.5K, R.sub.11 =10K.

SPAN ADJUST: R.sub.12 =499K, R.sub.13 =5K.

METER AMP., R.sub.14 =100K, R.sub.15 =200K.

Table I below shows the effect of other gases on the cells of the present invention. As can be seen, the only positive interferent was NO.sub.2 and negative interference was H.sub.2 S. Their effect on the accuracy of the cell in detecting and measuring chlorine was negligible, however.

In Tables II and III below, linearity and response for electrochemical cells of the present invention are shown. These tests were conducted on cells used for continuous monitoring of Cl.sub.2, and were run at a flow rate similar to diffusion conditions. Normal instrument flow rates are between 750 and 800 cc/min. which provides better response times as well as a slightly higher output.

In temperature studies of cells of the present invention, amplifier output was measured and Table IV indicates the percentage deviation from base was plotted:

While presently preferred embodiments of the invention have been shown and described in particularity, it may be otherwise embodied within the scope of the appended claims.