Abstract:
A molybdenum salt-palladium salt solution for CO detection is made reversible by addition of ferric ion. The system is made more CO specific by adding an interference control salt which forms white or colorless precipitates with interfering species. The operational and shelf life are extended by a mixture of counterions; the acetate counterion is particularly useful.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to chemical sensors and more particularly to carbon monoxide sensors. 
     The reaction of Carbon Monoxide (CO) with a variety of molybdenum compounds is reported as early as 1910 [C. Zenghelis, Z. Anal. Chem, 40, 429, (1910)] and the literature is reviewed in 1935 by J. Schmidt, &#34;Das Kohlenoxyd&#34;, Akad. Verlag, Leipzig, p 186, (1935). It is further presented in &#34;Spot Tests in Inorganic Analysis&#34; by F. Feigel in 1937. [F. Feigel, V. Anger, R. Oesper, &#34;Spot Tests in Inorganic Analysis&#34;, Elsevier Publishing Company, New York, p. 168 (1972)]. 
     In this relevant literature, three (3) basic equations are pointed out: 
     
         Mo.sup.+6 +CO→Mo.sup.+3 +CO.sub.2 (1) 
    
     
         Pd.sup.+2 +CO+H.sub.2 O→Pd.sup.0 +CO.sub.2 +2H.sup.+ (2) 
    
     
         Pd.sup.0 +Mo.sup.+6 →Pd.sup.+2 +Mo.sup.+3 (3) 
    
     The reaction in Equation (1) is very slow and, therefore, a catalyst (Pd +2 ) is used. The Pd +2 , even in very small quantities, adsorbs and simultaneously gets reduced by CO as shown in Equation (2). Pd 0 , in turn, enters into the reaction, Equation (3), producing molybdenum blue. Thus, a slightly yellow solution is changed to a blue color with the intensity of the blue color being directly related to CO exposure. As presented in Equations (1), (2) and (3), the reaction is not reversible and, therefore, has limited application in sensor technology. 
     To make the system reversible, therefore, there must be a secondary 
     The invention relates generally to chemical sensors and more reaction which converts Mo +3  back to Mo +6 , i.e., an oxidizer must be present. 
     The question of the reversible CO sensor is addressed by M. K. Goldstein in U.S. Pat. No. 5,063,164. That patent suggests several possible chemical recipes for this type of sensor, but does not address the criteria or requirements for a successful reversible sensor; nor does it address the chemistry or mechanisms to make the CO sensor completely specific. 
     Goldstein shows a solid state CO sensor having five components: (1) palladium salt, (2) molybdenum and/or tungsten salt or acid salt, (3) copper salt, (4) cyclodextrin molecular encapsulant which encapsulates at least one but not all of the other components, and (5) chloride salt, all impregnated into a substrate. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an improved CO sensor. 
     It is also an object of the invention to provide a reversible CO sensor. 
     It is another object of the invention to provide a specific CO sensor which eliminates interferences. 
     It is a further object of the invention to provide a CO sensor with extended lifetime. 
     The invention is a reversible CO sensor formed of an aqueous solution of (1) tungsten and molybdenum salts or acid salts, (2) palladium salt, (3) iron salt. The iron salt provides ferric ion as a reversing agent. The solution further can include a sodium salt which forms a white or colorless sulfide to eliminate interferences and increase specificity. The invention further includes a long-life CO sensor based on palladium with mixed counterions. The CO sensor is formed of an aqueous solution of (1) tungsten and molybdenum salts or acid salts, (2) palladium salt, (3) Pd(II) redox controlling counterion producing salt and (4) Fe +3  ion as the reversing agent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a diagram of a Pd(II) ion surrounded by water dipoles. 
     FIG. 2 is a diagram of a Pd(II) ion surrounded by sulfate and acetate counterions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The choice of a reversing agent depends on its ability to produce the desired reactions only, and nothing more. The first requisite is that the Mo +3  formed with the reaction of Pd +2 , CO and Mo +6  goes back to Mo +6  and no other valence states of the Mo. This requires a reversing compound whose redox potential closely matches that of Mo +3  going to Mo +6  so that the reaction proceeds spontaneously in the thermodynamic sense. The second requirement is that the selected oxidizing agent, when used, regenerates itself and not a series of compounds of various chemical formula and valence states. Based on these criteria, ferric ion is selected (irrespective of the counter ion) as the reversing agent in the formulation of a CO sensing chemistry. The reverse reaction proceeds as follows: 
     
         Mo.sup.+3 →3Fe.sup.+3 →Mo.sup.+6 +3Fe.sup.+2 (4) 
    
     Eventually, by air oxidation Fe +2  returns to Fe +3  to be available for reuse. 
     The amount of ferric ion (such as ferric chloride) used strictly depends on the dynamic range of concentrations of CO to be detected and the desired time delay for reversibility. In fact, if time is not a criterion, the reliance on oxygen in the air to cause the reversibility is perfectly acceptable. 
     The CO sensor chemistry is thus a solution of (1) tungsten or molybdenum salts or acid salts which provides the W +6  or Mo +6  ion, (2) palladium salt which provides the Pd +2  ion, and (3) iron (ferric) salt which provides the Fe +3  ion. The solution is typically aqueous, but other solvents might be used. The tungsten acid/salt may be selected from tungstosilicic acid and salts, thereof, tungsten trioxide, heteropolyacids of tungsten, ammonium tungstate and alkali metal, or alkaline earth metal salts of the tungstate ion. The molybdenum acid/salt may be selected from molybdosilicic acid and salts thereof, molybdenum trioxide, heteropolyacids of molybdenum, ammonium molybdate, and alkali metal or alkaline earth metal salts of the molybdate anion. The palladium salt may be selected from palladium sulfate, palladium sulfite, palladium pyrosulfite, palladium chloride, palladium bromide, palladium iodide, palladium perchlorate, CaPdCl 4 , Na 2  PdCl 4 , K 2  PdCl 4 . The iron (ferric) salt may be selected from ferric chloride, ferric sulfate, ferric bromide, ferric iodide, and ferric perchlorate. All the salts must be soluble. 
     In operation, the Mo +6  is reduced to Mo +3  by the CO in the presence of the Pd +2  catalyst. The Fe +3  then oxidizes the Mo +3  back to Mo +6 . 
     For the detection of CO between 0 and 500 ppm, a typical chemical system consists of: 
     Palladium Sulfate (0.04 wt. %) 
     Molybdosilicic Acid (0.2 wt. %) 
     Ferric Chloride (0.04 wt. %) 
     in aqueous solution. This composition is modified for other dynamic ranges. 
     The second part of the requirement for a CO sensor is that it be specific. All existent CO sensors including the one defined in U.S. Pat. No. 5,063,164 suffer from a variety of interferences of which hydrogen sulfide is most common. For example, palladium sulfide, molybdenum sulfide and copper sulfide (which would be formed in U.S. Pat. No. 5,063,164) are all black/brown which prohibits measurement of the yellow to blue color change when Mo +6  is reduced to Mo +3 . Iron sulfide, which is yellow/green can also cause some problems. The solution, therefore, is to add a fourth component to the system which not only preferentially forms a sulfide, but a white or colorless one which will not interfere with the CO measurement. To accomplish this, sodium chloride (2 wt. %) is incorporated into the system. 
     The four (4) component chemical system was extensively tested with a UV/VIS spectrophotometer. The following table shows the results of these tests. This formulation for a CO sensor indicates no response to the key Occupational Safety and Health Administration (OSHA) interferences. 
     
         ______________________________________INTERFERENCELEVEL          RESPONSE     OSHA LEVEL______________________________________Propane  (1000 ppm)              0            1000 ppmNO.sub.2 (30 ppm)  0            3 ppmButane   (800 ppm) 0            800 ppmSO.sub.2 (10 ppm)  0            2 ppmN.sub.2 O    (100 ppm) 0H.sub.2 S    (50 ppm)  0            10 ppmCO.sub.2 (100%)    0            10000 ppmMethane  (100%)    0            1000 ppm______________________________________ 
    
     Other salts can also be used to eliminate interferences and enhance specificity by producing white or colorless precipitates with the interfering species. These salts may be selected from chlorides and perchlorates of sodium, lithium, potassium, calcium, magnesium, aluminum, platinum, cobalt, etc. 
     Extending the shelf life and operational life of the CO sensor is very important. Doing so is difficult, and involves modification of the redox properties of the catalyst. This was achieved in the following way: 
     The catalyst is Palladium(II). When common salts of palladium (PdSo 4 , PdCl 2 , etc.) are dissolved in water, Pd(II) exists in the solvated form, as shown in FIG. 1. Water molecules form dipoles (δ -  -δ + ) which surround the Pd (II) ion. The sulfate counterion is also present. The counterions are also similarly solvated. 
     In the aquo form, Pd(II) is energetically very prone to reduction. When Pd +2  is surrounded by neutral molecules like water, ethanol, ethylene glycol, glycerol, and molecules having reducing functional groups, the operational and shelf life of the sensor chemistry are diminished drastically because of reduction without CO. Similar results occur with counterions like SO 4   -2 , Cl - , PO 4   -3 , HPO 4   -2 , etc. Because the species surrounding the Pd +2  determines its redox properties, the properties of Pd +2  can be controlled by carefully selecting its counterions. 
     When neat CO is passed through a solution of PdSO 4  in pure water, the solution turns black immediately. However, when PdSO 4  is dissolved in sodium acetate saturated water, it takes fifteen times longer to get dark. The acetate ions (CH 3  CO 2   - ) now surround the Pd(II) ion as shown in FIG. 2. The sulfate counterion is also present. Thus, the presence of acetate counterion greatly influences the redox properties of Pd +2 . This difference between sulfate and acetate counterions is used to extend lifetime. 
     The invention includes a CO sensing chemistry based on palladium with mixed counterions. By carefully adjusting the proportion of acetate and sulfate, or acetate and chloride counterions, the redox properties of Pd +2  are modulated; thereby prolonging the shelf life and operational life of the sensor. 
     Thus, the invention includes the addition of suitable counterions to the molybdenum/palladium combination. Ferric ion is added for reversibility and sodium salt for specificity as previously described. However, the bicounterion concept is applied to the palladium catalyst to achieve prolonged shelf life and operational life. 
     The counterion can be provided by the following: sodium acetate, potassium acetate, ammonium acetate, etc. The amount of counterion to be added is typically in the range of ten times the molar concentration of palladium. To accomplish this, sodium acetate (0.16 wt. %) is incorporated into the system. 
     Changes and modifications in the specifically described embodiments may be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.