Abstract:
An apparatus for detecting the concentration of an analyte in a carrier including a housing having a working end, a membrane covering at least a portion of the working end, the membrane being substantially permeable to the analyte and substantially impermeable to the carrier, wherein the housing and the membrane define a chamber within the housing, an electrolyte solution disposed within the chamber, a tin anode disposed within the chamber and in contact with the electrolyte solution, and a cathode disposed within the chamber and in contact with the electrolyte solution.

Description:
BACKGROUND  
       [0001]     The present application relates to sensors and, more particularly, to electrochemical cell sensors for determining the concentration of a dissolved/dispersed analyte.  
         [0002]     The measurement of the amount of gaseous oxygen dissolved in a volume of water is important in many applications including fish farming, waste water treatment and preventing corrosion and scale build-up in industrial boilers. Some dissolved oxygen sensors measure the partial pressure of oxygen in water, which is proportional to the amount of oxygen in the water (measured in milligrams per liter or parts per million).  
         [0003]     A galvanic-type sensor for measuring dissolved oxygen typically includes a pair of electrodes (i.e., an anode and a cathode) immersed in an electrolyte solution within a sensor body. The electrode materials are selected such that the electromotive force or cell potential between the cathode and anode is greater than −0.5 volts, thereby eliminating the need for applying an external voltage (as is done with polarographic-type sensors). An oxygen permeable membrane typically is provided to separate the electrodes from the sample being measured.  
         [0004]     Accordingly, as oxygen diffuses through the membrane, the oxygen is reduced at the cathode and a measurable electric current is generated within the cell. Higher oxygen concentrations in the sample results in more oxygen diffusing across the membrane, thereby producing more current. The current may be conducted through a thermistor to correct for permeation rate variation due to water temperature change such that the actual output from the galvanic sensor is a voltage.  
         [0005]     Galvanic sensors may utilize lead anodes. However, because of the health risks associated with lead, such sensors typically incorporate zinc, rather than lead, anodes. Unfortunately, zinc anodes tend to exhibit significant unstable background current due to the higher voltage potential difference between the anode and the cathode.  
         [0006]     Accordingly, there is a need for a galvanic sensor that does not exhibit significant unstable background current and does not have an electrode formed from lead.  
       SUMMARY  
       [0007]     In one aspect, the electrochemical cell sensor provides an apparatus for detecting the concentration of an analyte in a carrier including a housing having a working end, a membrane covering at least a portion of the working end, the membrane being substantially permeable to the analyte and substantially impermeable to the carrier, wherein the housing and the membrane define a chamber within the housing, an electrolyte solution disposed within the chamber, a tin anode disposed within the chamber and in contact with the electrolyte solution, and a cathode disposed within the chamber and in contact with the electrolyte solution.  
         [0008]     In another aspect, the electrochemical cell sensor provides an apparatus for detecting dissolved oxygen in a liquid carrier including a housing having a working end, a membrane covering at least a portion of the working end, the membrane being substantially permeable to the oxygen and substantially impermeable to the liquid, wherein the housing and the membrane define a chamber within the housing, an electrolyte solution disposed within the chamber, a tin anode disposed within the chamber and in contact with the electrolyte solution, and a silver cathode disposed within the chamber and in contact with the electrolyte solution.  
         [0009]     In another aspect, the electrochemical cell sensor provides a method for detecting dissolved oxygen in an aqueous liquid solution with a galvanic-type sensor including the steps of providing the sensor with a circuit having an anode including tin and a cathode including silver, positioning the anode and the cathode in an electrolyte solution, exposing the electrolyte solution to the dissolved oxygen such that the dissolved oxygen generates an electric current in the circuit, and monitoring the generated electric current.  
         [0010]     Other aspects of the electrochemical cell sensor will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a front elevational view, partially in section, of one aspect of an electrochemical cell sensor according to the present invention; and  
         [0012]      FIG. 2  is a graphical illustration of a voltammagram comparing a prior art sensor with the electrochemical cell according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     As shown in  FIG. 1 , a first aspect of the electrochemical cell sensor, generally designated  10 , includes a sensor housing  12 , a cathode  14 , an anode  16 , a membrane  18  and an electrolyte solution  20 . The housing  12  and membrane  18  may define a chamber  22  near the working end  24  of the sensor  10 . The cathode  14 , the anode  16  and the electrolyte solution  20  may be positioned within the chamber  22 .  
         [0014]     The cathode  14  may be formed from and/or may include silver and may have a diameter of, for example, approximately  5  mm. A first lead  26  may be connected to the cathode  14 . The anode  16  may be formed from and/or may include tin and may surround, at least partially, the cathode  14 . A second lead  28  may be connected to the anode  16 . The first and/or second leads  26 ,  28  may be connected to a processor, a monitoring device, an ammeter, a voltmeter or the like (not shown) such that an electrical signal may be monitored as analytes (e.g., oxygen) are reduced/oxidized at the electrodes (e.g., at the cathode).  
         [0015]     The cathode  14  and the anode  16  may be at least partially separated and/or electrically insulated from each other by a spacer  30 . The spacer  30  may be an epoxy or other polymeric material or the like capable of electrically insulating the cathode  14  from the anode  16 . The spacer  30  may include a recess  32  having a shoulder  34  for positioning the cathode  14  near the working end  24  of the sensor  10 . Furthermore, the spacer  30  may include a passageway  36  extending proximally from the shoulder  34  to accommodate the first lead  26 .  
         [0016]     The anode  16  may be electrically isolated from the surrounding sample medium (not shown) by the housing  12 , which may be an epoxy or other polymeric or electrically insulating material.  
         [0017]     At this point, those skilled in the art will appreciate that the sensor  10  may be any galvanic-type sensor having an anode and a cathode and may have various dimensions and structural configurations.  
         [0018]     The membrane  18  may be a permeable or semi-permeable membrane and may be impervious to the electrolyte solution  20  and to the surrounding sample medium (e.g., the gas or liquid carrier), but may permit analytes (e.g., dissolved oxygen) to diffuse from the sample medium into the electrolyte solution  20 . The membrane  18  may be formed from any appropriate membrane material such as, for example, a polyethylene or a polytetrafluoroethylene material. In one aspect, the membrane  18  may cover the working end  24  of the sensor  10  and may be secured to the housing  12  by an elastic ring  38  positioned within a groove  40 . In another aspect (not shown), the sensor  10  may not include a membrane  18  or an electrolyte solution  20 , leaving the cathode  14  and anode  16  directly exposed to the sample medium.  
         [0019]     The electrolyte solution  20  may be disposed within the cavity  22  and may be in direct contact with the cathode  14  and the anode  16 . The electrolyte solution  20  may include an aqueous solution of various salts, such as chloride salts or the like. For example, the electrolyte solution  20  may include an aqueous solution of about 0.1 M to about 1.5 M potassium chloride.  
         [0020]     Accordingly, when the sensor  10  is exposed to a sample medium containing, for example, dissolved oxygen, the oxygen may diffuse through the membrane  18  and into the electrolyte solution  20  at a rate proportional to the oxygen concentration in the sample medium. Without being limited to any particular theory, it is believed that the diffused oxygen migrates to the cathode  14 , where the oxygen is reduced, forming hydroxide ions. The hydroxide ions may then oxidize the tin anode, forming free electrons. The free electrons may be transported from the cathode  14  to the anode  16 , thereby generating an electric current. The amount of electric current generated may be correlated to the oxygen concentration in the sample medium to provide the user with a usable measurement of dissolved oxygen concentration.  
       EXAMPLE  
       [0021]     Electric current was conducted across two different sensors as a function of voltage applied between the cathode and anode of each sensor. The two sensors were tested in water-saturated air (21% oxygen). The electrolyte solution in each sensor was a potassium chloride aqueous solution. As shown in  FIG. 2 , curve A represents a sensor having a silver cathode and a zinc anode (i.e., a prior art sensor) and curve B represents a sensor having a silver cathode and a tin anode (i.e., a sensor according to an aspect of the present invention). Each curve includes a portion in which the current flow is an approximately linearly increasing function of voltage followed by a section in which the current is approximately constant at a reduction plateau despite increasing voltage.  
         [0022]     The primary defining property of a galvanic-type sensor is that it operates with zero externally applied potential. For best sensor stability, this potential should be near the center of the current plateau where current is proportional to oxygen partial pressure.  
         [0023]     In  FIG. 2 , curve B (i.e., silver cathode/tin anode) produces a current plateau that has minimal slope around zero potential, while curve A (i.e., silver cathode/zinc anode) produces a current plateau that curves upward at zero potential.  
         [0024]     Accordingly, the sensors of the present invention provide a more stable background current during operation then similar sensors having a silver cathode and a zinc anode. In addition, the sensors of the present invention avoid the health hazards associated with electrodes formed from lead. Therefore, the sensors of the present invention may be well-suited for the continuous or semi-continuous measurement of dissolved oxygen and other analytes in various environments such as lakes, streams, industrial tanks or wastewater treatment plants.  
         [0025]     Although the electrochemical cell sensor is shown and described with respect to certain aspects, modifications may occur to those skilled in the art upon reading the specification. The electrochemical cell sensor includes all such modifications and is limited only by the scope of the claims.