Abstract:
A chloramine amperometric sensor includes a sensor body with an electrolyte disposed inside the sensor body. A membrane is coupled to the sensor body and adapted to pass chloramine therethrough. A reference electrode is disposed in the electrolyte and coupled to a first conductor. A second conductor is coupled to a working electrode that is disposed proximate the membrane. The working electrode is constructed from a noble metal in non-compact form. The non-compact form can be a Gas Diffusion Electrode, which can include metal mesh, carbon paper, carbon cloth, metal/carbon powder loaded on a porous membrane or any combination thereof.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to quantitative analytic sensors. More specifically the present invention relates to a sensor that uses an electrode response to measure the concentration of chloramine in a solution. 
   Chloramine is often used in the treatment of water. While chloramine is generally not as powerful a disinfectant as chlorine, it is often used instead of chlorine because it persists longer and provides a number of other benefits. Thus, sensing chloramine provides useful information for such treatment systems as well as any other system where chloramine is used. 
   Amperometric sensors are generally known. In such sensors, a species of interest reacts electrically to generate an electrical response that is measured in the form of current flow. One example of a chloramine amperometric sensor is the Model 499A DO-54-99 (SQ6684) available from Emerson Process Management, Rosemount Analytical Division, of Irvine Calif. 
   Development of embodiments of the present invention is due, at least in part, to a recognition of limitations of current state of the art chloramine amperometric sensors. For example, current sensors generally use a sensing electrode that consists of a solid metallic disc or other shape that is generally a noble metal. The chloramine diffuses across a gas-permeable membrane, such as polytetrafluoroethylene (PTFE) and enters an electrolytic solution. The chloramine then reduces a second species such as I −  into I 2 . The reduced second species, such as I 2 , then obtains electrons from the sensing electrode to generate a current that is related to the quantity of chloramine. However, sensor linearity begins to drop off for higher concentrations of chloramine, about 2 ppm. It is believed that conventional sensors limit the access of the second species, such as I− to sensing electrode (cathode) due to the geometry of the sensing electrode. Another problem with current amperometric sensors for chloramine sensing is due to the activity of dissolved oxygen. If oxygen is dissolved in the chloramine containing specimen, the dissolved oxygen will reduce at a level similar to the chloramine, thus “clouding” the measured chloramine response. 
   SUMMARY OF THE INVENTION 
   A chloramine amperometric sensor includes a sensor body with an electrolyte disposed inside the sensor body. A membrane is coupled to the sensor body and adapted to pass chloramine therethrough. A reference electrode is disposed in the electrolyte and coupled to a first conductor. A second conductor is coupled to a working electrode that is disposed proximate the membrane. The working electrode is constructed from a noble metal in non-compact form. The non-compact form can be a Gas Diffusion Electrode, which can include metal mesh, carbon paper, carbon cloth, metal/carbon powder loaded on a porous membrane or any combination thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic view of a chloramine monitoring system in which embodiments of the present invention are particularly useful. 
       FIG. 2A  is a diagrammatic view of a conventional chloramine amperometric sensor. 
       FIG. 2B  is a diagrammatic view of a chloramine amperometric sensor in accordance with an embodiment of the invention. 
       FIG. 2C  is a diagrammatic view of a chloramine amperometric sensor in accordance with another embodiment of the invention. 
       FIG. 3  is a graph of the cyclic voltammetry curve of a Platinum electrode in a 100 ppm chloramine solution at pH 7. 
       FIG. 4  is a graph of a potential scan of a Platinum black loaded Gas Diffusion Electrode (GDE) in a 100 ppm chloramine solution at pH 7 illustrating cyclic voltammetry both in the presence and absence of chloramine. 
       FIG. 5  is a graph of a response curve of a sensor in accordance with an embodiment of the invention to different free chlorine species. 
       FIG. 6  is a calibration curve illustrating the relationship between output current and chloramine concentration (in the 1.00–8.00 ppm range) for the sensor illustrated in  FIG. 2C . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention provide a chloramine amperometric sensor that employs a working electrode with significantly higher porosity than previous working electrodes. One example includes using a Gas Diffusion Electrode (GDE) loaded with a powdered noble metal catalyst to measure chloramine concentration. Another example includes using a working electrode constructed from noble metal mesh. Amperometric sensors that employ screens on the anode are known. See, for example, U.S. Pat. No. 4,776,942. However, since the screen is used at the counter electrode, it provides no benefit for increasing access to the sensing electrode, where sensing process occurs. 
     FIG. 1  is a diagrammatic view of a chloramine monitoring system in which embodiments of the present invention are particularly useful. System  10  includes analysis device  12  and sensor  14 . Analysis device  12  can be any suitable device capable of generating meaningful chloramine information from sensor  14 . For example, device  12  can be an analyzer such as the Model 1054 Microprocessor Analyzer available from Rosemount Analytical Inc. Uniloc Division of Emerson Process Management. Device  12  can also be a transmitter that is adapted to generate chloramine data and transmit the data over a process communication loop. One example of such a transmitter is the Model 1181RC Transmitter available from Rosemount Analytical Uniloc. Sensor  14  is coupled to sample specimen container  16 , which may be a pipe for example. Sensor  14  has an electrical characteristic that varies in response to chloramine concentration in the specimen. 
     FIG. 2A  is a diagrammatic view showing a conventional chloramine amperometric sensor. Sensor  20  generally includes a sensor body  22  that contains a quantity of electrolyte  24 . Working electrode  26  (also referred to herein as the cathode, or sensing electrode) is supported within body  22  on support  28  such that it contacts membrane  32 . Reference electrode  30  (also referred to as an anode, or counter-electrode) is also disposed within electrolyte  24 , but is spaced apart from working electrode  26 . Electrode  30  can be any standard reference electrode such as Silver/Silver Chloride. Membrane  32  is disposed at one end of body  22  and is generally placed in contact with the chloramine containing sample. Membrane  32  can be a commercially available porous membrane sold under the trade designation Zitex G106 from Saint-Gobain Ceramics &amp; Plastics, Inc., of Wayne, N.J., but can be any suitable porous material that does not allow the electrolyte to leak from the sensor. Conductors  34  and  36  are coupled to electrodes  26  and  30 , respectively, to allow device  12  to measure the electrical characteristic of sensor  20  that varies with chloramine concentration. Working electrode  26  is formed from a solid disc of platinum, but can be any suitable noble metal, such as gold. As such, the only path of the electrolyte to the working electrode is the thin layer between the working electrode and the membrane. This limited contact results in reduced sensor output at high concentrations. 
     FIG. 2B  is a diagrammatic view showing chloramine amperometric sensor  40  in accordance with an embodiment of the invention. Sensor  40  bears some similarities to sensor  20  described with respect to  FIG. 2A  and like components are numbered similarly. Working electrode  42  is disposed proximate membrane  32 . Working electrode  42  provides substantially more accessibility to the electrolyte  24  than working electrode  26 . In one preferred embodiment, electrode  42  is a Gas Diffusion Electrode (GDE). In this example, electrode  42  is a GDE loaded with 80 percent platinum-black (powdered platinum) and carbon on a carbon cloth electrode (ECC). The platinum was distributed at a density of about 5.0 mg/cm 2 . The configuration used for working electrode  26  can be obtained from E-Tek, Inc. (www.etek-inc.com), of Somerset, N.J., USA, by specifying the loading and density listed above. In this embodiment, electrolyte  24  was a pH 10 buffer with potassium chloride (KCl) added as a supporting electrolyte. Preferably, electrolyte  24  has a pH between about 9.0 and 11.0  FIG. 2C  is a diagrammatic view of chloramine amperometric sensor  50  in accordance with an embodiment of the present invention. Sensor  50  includes many components that are similar to sensors  20  and  40 , and like components are numbered similarly. Sensor  50  includes working electrode  52  constructed from noble metal, in this case gold, in a mesh form. The mesh allows substantially more surface area for electrolyte interaction than a solid working electrode. Working electrode  52  is disposed proximate membrane  32  such that chloramine passing through membrane  32  will interact with both electrolyte and working electrode and generate an electrical response. Those skilled in the art will appreciate that while the platinum black loaded carbon cloth embodiment was described with respect to platinum, and the mesh embodiment was described with respect to gold, embodiments of the present invention can be practiced using any suitable noble metal in either form. Further, it is expressly contemplated that additional forms of presenting the noble metal catalyst to the chloramine are possible as long as they provide a surface area greater than that of a solid. For example, a number of noble metal spheres could be maintained proximate the membrane without departing from the spirit and scope of the invention. As used herein, “non-compact” is intended to mean any form that is not a unitary contiguous solid object. 
     FIG. 3  is a plot illustrating a Cyclic Voltammetry (CV) curve of the conventional sensor described with respect to  FIG. 2A . In  FIG. 3 , the reduction current peak at about −300 mV arises from the chloramine reduction. The reduction peak is thus in the oxygen reduction potential region. This overlap of chloramine reduction potential with oxygen reduction potential was a significant drawback for prior art amperometric chloramine sensors. 
     FIG. 4  illustrates a potential scan for sensor  40  described with respect to  FIG. 2B . The chart illustrates two runs, one run included testing solution containing chloramine at a concentration of 100 ppm at a pH of 7, while another run was performed in the absence of chloramine.  FIG. 4  illustrates sensor response to chloramine, and other free chlorine species.  FIG. 4  also illustrates the chloramine reduction potential at the GDE is in a more positive region, compared to  FIG. 3 , and thus has moved away from the oxygen reduction potential. Thus, it is believed that chloramine sensors in accordance with various embodiments of the invention will not suffer from interference from dissolved oxygen. 
     FIG. 5  is a graph of a response curve of sensor  40  to different free chlorine species. The x-axis represents a time period during which sensor  40  was introduced to various testing solutions. When introduced to the various testing solutions, sensor  40  eventually arrived at the following currents: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
                 1 ppm hypochlorite 
               0.0E+00; 
             
             
                 
                 1 ppm chloramine 
               1.5E−06; 
             
             
                 
               2.5 ppm chloramine 
               2.8E−06; 
             
             
                 
               2.5 ppm hypochlorite 
               6.0E−06; and 
             
             
                 
               1.7 ppm total chlorine (tap water) about 
               0.5–2.0E−06. 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 6  is a calibration curve illustrating the relationship between output current and chloramine concentration (in the 0–6 ppm range) for sensor  50 . As illustrated in  FIG. 6 , the output current of sensor  50  in response to chloramine concentrations in this range is highly linear. In fact, a linear equation can be fitted to the sensor response to virtually match the sensor response. Thus, a simple linear equation can be used in device  12  to relate sensor output to chloramine concentration. Prior art chloramine sensors are not believed to provide linearity to a concentration near 6 ppm. Thus, it is believed that sensors in accordance with the present invention will provide a predictable linear response in applications where prior art sensor responses would not be linear. Further, due to the enhanced response of sensors in accordance with embodiments of the invention, it is believed that interference from dissolved oxygen in the testing solution is significantly reduced if not eliminated altogether. 
   Although the present invention has been described with reference to present embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.