Abstract:
A novel series of chalcogenide glass compositions that are relatively highly selective for a specific ion in solution. When utilized as a sensing element for electrodes or similar analytical devices, a modified surface layer is formed on the surface of the glass in contact with the test solution. The surface layer being characterized by a surface ionic conductivity ranging from 10 -7  to 10 -2  S/cm, a diffusion coefficient ranging from 10 -11  to 10 -6  cm 2  /s and having a ratio of electronic to ionic conductivity within the modified surface layer of not more than 1.0. Specific compositions having these unique morphology and transport properties are disclosed for the detection of cadmium, silver, thallium, mercury, copper and lead.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of electrochemical detection, and is more particularly directed to a series of novel chalcogenide glass compositions that may be used as sensor materials for the selective detection of metallic ions in solution. 
     2. Description of the Related Art 
     With the advent of ion selective electrodes (also known as ISEs), the use of electrochemical detection in industrial and medical applications has virtually exploded over the last few decades. ISEs provide a viable and convenient means for measuring the concentration of a specific ionic species in solution. In so doing, flowing samples can be continuously monitored such as to insure water quality, to identify contaminant levels in industrial effluent, and to attain optimum yields in chemical processes. Aside from the more conventional ISEs having an internal reference solution and electrode, solid state devices such as all solid state ISEs which are somewhat more durable are being developed. In addition, ion selective field effect transistors (ISFETs) are known which are so compact as to potentially enable implantation in patients, for example, to monitor critical ionic levels during surgery and the like. 
     The sensor materials utilized in ISEs and ISFETs to construct the sensing element or selective membrane of the device, generally need exhibit ionic conductivity and be selective or responsive to a specific ionic species. Known sensor materials include crystalline electrolytes such as pressed pellets of silver sulfide by itself or in combination with silver halides or other sulfide salts. See e.g. U.S. Pat. No. 3,591,464 to Frant. While these crystalline materials have utility, their overall performance in terms of selectivity can be altered in the presence of corrosive media such as acids and in the presence of strong oxidizing agents. In addition, the crystalline structure may be less durable in certain environments so as to detract from potential stability at the crystalline membrane surface over time. For these reasons, particularly in the case of solid state ISEs and ISFETs, there is a need to improve sensor materials known in the art to provide a stable device in varying environments and over long periods of usage. 
     Another type of sensor material known in the field is chalcogenide glass. Chalcogens, the elements of the sixth group of the Periodic Table including sulfur, selenium and tellurium as well as their binary, ternary and multi-component compounds with the elements of the third, fourth and firth groups such as germanium, phosphorous and antimony, can be readily obtained as amorphous vitreous solids. Chalcogenide glasses are promising membrane materials, particularly for solid-state ISEs and ISFETs, because they are generally more chemically stable in comparison with the corresponding crystalline materials. 
     Chalcogenide glass membranes comprising 60% selenium (Se), 28% germanium (Ge) and 12% antimony (Sb), and doped with elemental iron, cobalt and/or nickel were early on developed showing a selective response to iron (Fe +3 ) and copper (Cu +2 ) ions. C. T. Baker and I. Trachtenberg, J. Electrochem. Soc., 118. 329 (1971). Other vitreous chalcogenide compositions have since been disclosed which exhibit sensitivity to other metallic ions in solution including copper, lead, cadmium and silver for example. See, Yu. G. Vlasov and E. A. Bychkov, Ion-Selective Electrode Rev., 1987, Vol. 9, pp. 5-89 (made a part hereof and incorporated herein by reference). These chalcogenide membranes showed little change in selectivity even in the presence of strong acids and oxidation agents. Furthermore, the electric properties of these amorphous solids appear uneffected by occasional impurities which could otherwise negatively impact consistency of results. 
     In conjunction with their investigations respecting chalcogenide electrode materials, the present inventors observed that certain chalcogenide glass membranes develop a modified surface layer on the membrane surface upon contact with a test ionic solution. This modified layer is characterized by a considerable concentration of exchange sites for ion migration from the solution to the exchange sites and vice versa. It is believed that the direct ion exchange between solution and this modified surface layer is a potential-generating process that is extremely stable. Thus, the inventors next area of inquiry resided in developing specific chalcogenide formulations that would include this modified surface having a high concentration of ionic exchange sites, and further have the ability to affect a gradual transition from ionic conductivity at the membrane/solution interface to electronic conductivity for measurement. Excessive electronic conductivity would detract from the selectivity of the electrode, while too low a value of electronic conductivity would hinder its use in all solid state and ISFET devices. In combination with those attributes, the inventors sought specific chalcogenide compositions that exhibited high selectivity for specific ions in solution, even higher than conventional crystalline materials, so as to provide not only a more durable and stable electrochemical device but to provide a more sensitive device as well. 
     Thus, it is a primary object of the present invention to provide a chalcogenide glass sensor material for measuring the presence of ions in solution that is relatively highly selective for a specific ion in solution. 
     It is another object of the present invention to provide an improved chalcogenide glass sensor material that is durable and effective over extended periods of in-use service. 
     A further object of the present invention is to provide a chalcogenide glass sensor material that exhibits relatively high precision, stability and reproducibility of measurement. 
     It is also an object of the present invention to provide a chalcogenide glass sensor material that is relatively stable in the presence of oxidizing agents and corrosive media such as strongly acidic media. 
     Another object of the present invention is to provide a chalcogenide glass sensor material that exhibits both ionic conductivity and electronic conductivity in such a manner as to affect a gradual transition of ionic conductivity to electronic conductivity for measurement. 
     Yet another object of the present invention is to provide a chalcogenide glass sensor material that is particularly suitable for use in an all solid state ISEs and ISFETs constructed to achieve all the objects heretofore set forth. 
     A further object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of cadmium. 
     Another object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of copper. 
     Yet a another object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of silver. 
     Still a further object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of lead. 
     It is another object of the present invention to provide a novel chalcogenide glass sensor material for the selective detecting of thallium. 
     It is yet a further object of the present invention to provide a novel chalcogenide glass sensor material for the selective detection of mercury. 
     Still another object of the present invention is to provide an electrochemical device for the selective detection of metallic ions in solution comprising a novel chalcogenide glass sensor material as heretofore described. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by a novel series of chalcogenide glass compositions that are relatively highly selective for a specific ion in solution. The glass compositions made in accordance with this invention are characterized by a unique morphology, wherein a modified surface layer forms on the surface of the glass upon initial contact with an aqueous test solution. This modified surface layer has a thickness generally ranging from about 20 nanometer (nm) to 200 nm and exhibits a surface ionic conductivity ranging from 10 -7  to 10 -2  S/cm and a diffusion coefficient ranging from 10 -11  to 10 -6  cm 2  /S. It is believed that the specific morphology and transport properties of this modified surface layer of the chalcogenide glass provides a material having high ionic exchange current density at the glass membrane/solution interface resulting in enhanced sensor properties. 
     The ratio of electronic to ionic conductivity within the modified surface layer is preferably not more than 1.0. which affects a stable and reproducible transition from ionic conductivity at the modified membrane layer to electronic conductivity for measurement. Although the inventors do not wish to be limited by theory, it is hypothesized that glasses with higher ionic conductivity in the modified surface layer display less chemical stability and potential reproducibility, while glasses with lower surface ionic conductivity and comparable electronic conductivity reveal less sensitivity and selectivity to primary ions. 
     Specific compositions having these unique morphology and transport properties have been developed for the detection of cadmium, silver, thallium, mercury, copper and lead. These materials perform well and are highly selective and stable even in the presence of acidic solutions and strong oxidizing agents. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A series of chalcogenide glass sensor materials in accordance with this invention may be used to construct electrode membranes for the selective detection of a specific ion in solution. The glass materials are characterized by a unique surface morphology wherein a modified surface layer is formed on the membrane when contacted with an aqueous solution. The modified surface layer has a thickness ranging from about 20 to 200 nm, a surface ionic conductivity ranging from 10 -7  to 10 -2  S/cm, and a diffusion coefficient ranging from 10 -11  to 10 -6  cm.sup. 2 /s. The ratio of electronic to ionic conductivity within the surface layer is preferably not more than 1.0. 
     In one embodiment of the present invention, a chalcogenide glass sensor material is provided for the selective detection of cadmium, and comprises cadmium (Cd), silver (Ag), arsenic (As), iodide (I), antimony (Sb) and sulfur (S) in the amount ranges based on gram-atomic weight percents as follows: 
     
         ______________________________________Element   Gram-Atomic Weight % of Total______________________________________Cd        1-7Ag         8-30As        15-30Sb         2-10I          6-18S         remainder______________________________________ 
    
     Examples of suitable chalcogenide glass compositions in accordance with this invention for the selective detection of cadmium include Cd 1  Ag 30  As 24  Sb 2  I 6  S 37 , Cd 2  Ag 8  As 30  Sb 5  I 10  S 45 , Cd 2  Ag 17  As 23  Sb 3  I 12  S 43 , Cd 4  Ag 22  As 16  Sb 5  I 14  S 39 , and Cd 7  Ag 15  As 15  Sb 10  I 18  S 35 , with Cd 2  Ag 17  As 23  Sb 3  I 12  S 43  most preferred for purposes of the present invention. 
     A chalcogenide glass sensor for the detection of lead in accordance with the present invention comprises lead (Pb), silver (Ag), germanium (Ge), iodide (I) and sulfur (S) in the amounts as follows: 
     
         ______________________________________Elements  Gram-atomic Weight % of Total______________________________________Pb         3-15Ag        10-30Ge        10-25I          5-20S         remainder______________________________________ 
    
     Examples of suitable lead selective compositions include Pb 3  Ag 30  Ge 18  I 5  S 44 , Pb 4  Ag 10  Ge 25  I 12  S 49 , Pb 10  Ag 22  Ge 10  I 14  S 44 , Pb 13  Ag 19  Ge 13  I 15  S 40  and Pb 15  Ag 14  Ge 15  I 20  S 36 , with Pb 13  Ag 19  Ge 13  I 15  S 40  being most preferred. 
     A chalcogenide glass sensor for the detection of silver comprising silver (Ag), germanium (Ge) and selenium (Se) may be provided as follows: 
     
         ______________________________________Elements  Gram-atomic Weight % of Total______________________________________Ag         6-30Ge        25-40Se        remainder______________________________________ 
    
     Examples of silver selective compositions in accordance with this invention include Ag 6  Ge 40  Se 54 , Ag 15  Ge 35  Se 50 , Ag 19  Ge 30  Se 51  and Ag 30  Ge 25  Se 45 , with Ag 15  Ge 35  Se 50  being most preferred. 
     A chalcogenide glass sensor for the detection of thallium ions in accordance with the present invention comprises thallium (Tl), germanium (Ge) and selenium (Se) in the amount ranges as follows: 
     
         ______________________________________Elements  Gram-atomic Weight % of Total______________________________________Tl        20-35Ge        14-40Se        remainder______________________________________ 
    
     Examples of suitable thallium selective compositions include Tl 20  Ge 40  Se 40 , Tl 24  Ge 30  Se 46 , Tl 28  Ge 14  Se 58  and Tl 35  Ge 16  Se 49  with Tl 28  Ge 14  Se 58  being preferred. 
     A chalcogenide glass sensor for the selective detection of copper ions may be provided in accordance with the present invention comprising copper (Cu), silver (Ag), germanium (Ge) and selenium (Se) in the amount ranges as follows: 
     
         ______________________________________Elements  Gram-atomic Weight % of Total______________________________________Cu         5-20Ag         5-20Ge        18-32Se        remainder______________________________________ 
    
     Examples of suitable copper selective compositions include Cu 5  Ag 20  Ge 30  Se 45 , Cu 10  Ag 10  Ge 32  Se 48 , Cu 15  Ag 12  Ge 22  Se 51 , and Cu 20  Ag 5  Ge 18  Se 57 , with Cu 10  Ag 10  Ge 32  Se 48  being most preferred. 
     A chalcogenide glass sensor for the selective detection of mercury may be provided comprising mercury (Hg), silver (Ag), iodide (I) , arsenic (As), antimony (Sb) and sulfur in the amount ranges as follows: 
     
         ______________________________________Elements  Gram-atomic Weight % of Total______________________________________Hg         2-10Ag        25-40As         7-16I          2-30Sb        1-5S         remainder______________________________________ 
    
     Examples include Hg 2  Ag 25  As 16  Sb 4  I 23  S 30 ,Hg 4  Ag 35  As 10  Sb 1  I 25  S 25 , Hg 4  Ag 40  As 8  Sb 3  I 23  S 22 , Hg 7  Ag 34  As 7  Sb 5  I 30  S 17 , and Hg 10  Ag 26  As 12  Sb 4  I 20  S 28 , with Hg 4  Ag 35  As 10  Sb 1  I 25  S 25  being preferred. 
     Any conventional compounding procedure known in the art may be used for the preparation of the vitreous sensor materials. In a preferred embodiment, the elemental components of relatively high purity are measured and ground together in an agate mortar under an inert gas such as argon. The mixture is then sealed in a silica tube evacuated to about 0.1 to 0.01 Pascal (Pa). The tube is heated slowly in a rocking furnace or the like to an upper range temperature of about 1100°-1300° Kelvin (K). The melt should be allowed to mix and react for several hours ranging anywhere from 15-65 hours, then quenched to a solid amorphous glass or glassy/crystalline form and can be annealed to remove any strain. 
     The glass melts may then be cut into disks as is well known in the art and polished for use as the sensor element or membrane in an ion selective electrode, including convention ISE elements, as well as all solid state ISEs and ISFETs. 
    
    
     The following examples of the preparation of sensor materials and membranes made in accordance with the invention and the responses of selective electrodes using such membranes are for purposes of illustration only. 
     EXAMPLE I 
     Synthesis of Chalcogenide Glass Sensors 
     A. Preparation of Cadmium Selective Composition 
     The elemental components listed in Table IA below were measured, thoroughly mixed and ground in an agate mortar under dry argon flow. The components were then sealed in an evacuated silica tube and heated to about 340°-380° K. and maintained at that temperature for a period ranging from 20-25 hours. The temperature was then increased over time to about 450°-500° K. and maintained for 15-20 hours, followed by additional heating to 800°-850° K. and held for 15-20 hours and to a final temperature of 1100°-1150° K. for 15-20 hours. The melt was then quenched at a mean quenching rate of 2° to 4° Kelvin per second (K/s) in the temperature range of 400°-500° K. 
     
                       TABLE IA______________________________________Cadmium Chalcogenide Glass Sensor______________________________________   Cd   0.319 g   Ag   2.601 g   As   2.445 g   Sb   0.518 g   I    2.161 g   S    1.956 g______________________________________ 
    
     B. Preparation of Lead Selective Material 
     The elemental components listed below in Table IB were measured, thoroughly mixed and ground in an agate mortar under dry argon flow. The components were sealed in an evacuated silica tube and initially heated to a temperature ranging from 340°-380° K. and maintained at that temperature for about 30-35 hours. The temperature was then systematically increased to 450°-500° K. and held for 35-40 hours, to 800°-850° K. and maintained for 35-40 hours and finally to 1200°-1250° K. held for 35-40 hours. The melt was quenched at a mean quenching rate of 2° to 4° K./s in the temperature range of 400°-500° K. 
     
                       TABLE IB______________________________________Lead Chalcogenide Glass Sensor______________________________________   Pb   3.036 g   Ag   2.310 g   Ge   1.064 g   1    2.145 g   S    1.445 g______________________________________ 
    
     C. Preparation of Silver Selective Materials 
     The elemental components listed in Table IC below were mixed, ground in an agate mortar under dry argon flow and sealed in an evacuated silica tube. The tube was heated to 450°-500° K. and maintained at that temperature for about 10-15 hours. The temperature was then increased to 800°-850° K. and held for 15-20 hours, then to 1200°-1250° K. and maintained for 25-35 hours. The melt was quenched at a mean quenching rate of 2° to 4° K./s in the temperature range of 400°-500° K. 
     
                       TABLE IC______________________________________Silver Chalcogenide Glass Sensor______________________________________   Ag   1.996 g   Ge   3.134 g   Se   4.870 g______________________________________ 
    
     D. Preparation of Thallium Selective Materials 
     The elemental components listed below in Table ID were mixed, ground in an agate mortar under argon flow and sealed in an evacuated silica tube. The tube was heated slowly to about 450°-500° K. and maintained at that temperature for about 10-15 hours. After that the temperature was increased to 800°-850° K. and maintained for about 15-20 hours, followed by heating to 1200-1250° K. for 25-30 hours. The melt was quenched at a mean quenching rate of 2° to 4° K./s in the temperature range of 400°-500° K. 
     
                       TABLE ID______________________________________Thallium Chalcogenide Glass Sensor______________________________________   Tl   5.056 g   Ge   0.898 g   Se   4.046 g______________________________________ 
    
     E. Preparation of Mercury Selective Material 
     The elements listed below in Table IE were thoroughly mixed and ground in an agate mortar under dry argon flow. The components were then sealed in a silica tube and heated to an initial temperature ranging from 340°-380° K. for about 25-30 hours. The temperature was then increased to about 450°-500° K. and maintained for 20-25 hours, followed by heating to 800°-850° K. for 20-25 hours and to 1100°-1150° K. for 25-30 hours. The melt was quenched at a mean quenching rate of 2° to 4° K./s in the temperature range of 400°14 500° K. 
     
                       TABLE IE______________________________________Mercury Chalcogenide Glass Sensor______________________________________   Hg   0.851 g   Ag   4.007 g   As   0.795 g   Sb   0.129 g   I    3.367 g   S    0.851 g______________________________________ 
    
     All solid state ISEs were prepared from the above chalcogenide sensor materials. The material samples were cut from the melt as discs ranging in size from 5 to 10 millimeters (mm) in diameter and 1 to 3 mm thick. The discs were ground and polished for resistance measurement. Silver was evaporated on one side of the membrane and the edges were ground to remove any traces of silver which could short the sample. A metallic wire was attached to the silver coating using silver microadhesive. 
     The membrane was then sealed at the end of a plastic tube, with the silver coated surface being positioned on the interior of the tube and the metallic wire attached to a digital voltmeter. Electrochemical measurements were performed using chalcogenide glass sensor and silver/silver chloride reference electrode. Nitrate salts of corresponding metals were used for standard solution preparation. Nitric acid and potassium hydroxide were used to change pH of the standard solution. Selectivity coefficients were measured by mixed solution method. 
     EXAMPLE II 
     Comparison Of Sensor Materials Of The Present Invention With Conventional Crystalline Electrode Materials And Prior Art Chalcogenide Materials 
     A. Cadmium Chalcogenide Glass Sensor 
     The selectivity of the cadmium selective material prepared in Example IA above (Cd 2  Ag 17  As 23  Sb 3  I 12  S 43 ) was compared with that of conventional polycrystalline electrode material comprising CdS--Ag 2  S and with prior art chalcogenide glass sensor compositions comprising CdS--Ag 2  S--As 2  S 3  and CdI 2  --Ag 2  S--As 2  S 3 . 
     
                       TABLE II A______________________________________Sensor Membrane  Selectivity CoefficientsMaterial         log K.sub.Cd.spsb.2+.sub., Co.spsb.2+______________________________________New Material     4 (1) × 10.sup.-5Cd.sub.2 Ag.sub.17 As.sub.23 Sb.sub.3 I.sub.12 S.sub.43Prior Art Chalcogenides            4 (1) × 10.sup.-4CdS--Ag.sub.2 S--As.sub.2 S.sub.3CdI.sub.2 --Ag.sub.2 S--As.sub.2 S.sub.3.Conventional Crystalline            4 (1) × 10.sup.-4CdS--Ag.sub.2 S______________________________________ 
    
     It should be noted that the cadmium chalcogenide glass sensor of the present invention reveals better selectivity in the presence of alkali cations, cobalt and barium ions than conventional polycrystalline materials and prior art chalcogens. It is also 10 to 30 times less sensitive to the interference of nickel and thallium ions than conventional materials. Moreover, the acid stability of the inventive cadmium selective chalcogenide glass sensor is 30 to 100 times higher. 
     B. Lead Chalcogenide Glass Sensor 
     The selectivity of the lead selective material prepared in Example IB above (Pb 13  Ag 19  Ge 13  I 15  S 40 ) was compared with that of conventional polycrystalline electrode material comprising PbS--Ag 2  S and with prior art chalcogenide glass sensor compositions of PbS--Ag2S--As2S3 and PbI 2  --Ag 2  S--As 2  S 3 . The cadmium chalcogenide glass sensor of the present invention reveals better selectivity in the presence of cadmium and iron (III) ions than conventional polycrystalline materials. This material is also 10 to 100 times less sensitive to strong oxidizing agents such as hydrogen peroxide or potassium permanganate. 
     The inventive lead selective material exhibits greater selectivity in the presence of hydrogen ions, wherein the working pH ranges of the compositions in 0.01M lead nitrate solution are as follows: 
     
                       TABLE II B______________________________________Sensor MembraneMaterial          Working pH Range______________________________________New Material      1.3-6.5Pb.sub.13 Ag.sub.19 Ge.sub.13 I.sub.15 S.sub.40Prior Art Chalcogenides             3.5-6.5PbS--Ag.sub.2 S--As.sub.2 S.sub.3PbI.sub.2 --Ag.sub.2 S--As.sub.2 S.sub.3Conventional Crystalline             2.0-6.5PbS--Ag.sub.2 S______________________________________ 
    
     C. Silver Chalcogenide Glass Sensor 
     The selectivity of the silver selective material prepared in Example IC above (Ag 15  Ge 35  Se 50 ) in accordance with the present invention was compared with that of conventional polycrystalline electrode material comprising Ag 2  S and with prior art chalcogenide glass sensor compositions of Ag--As--S and Ag--As--Se. 
     
                       TABLE II C______________________________________Sensor Membrane  Selectivity CoefficientsMaterial         log K.sub.Ag.spsb.+.sub., Hg.spsb.2+______________________________________New Material     3.0 (9) × 10.sup.-4Ag.sub.15 Ge.sub.35 Se.sub.50Prior Art Chalcogenides            1.0 (3) × 10.sup.-3Ag--As--SAg--As--SeConventional Crystalline            2.0 (6) × 10.sup.-2Ag.sub.2 S______________________________________ 
    
     In comparison with conventional silver ion-selective electrodes based on Ag 2  S polycrystalline mixture, this silver chalcogenide glass sensor is 10 to 100 times more stable in strong acid and corrosive media such as concentrated nitric acid and reveals better selectivity in the presence of Hg 2+  ions. The inventive material is 2 to 6 times more selective in the presence of mercury ions in comparison with prior chalcogenide glass sensors and 40 to 130 times more selective in comparison with conventional crystalline ion-selective electrodes. 
     D. Copper Chalcogenide Glass Sensor 
     The selectivity of a copper selective material Cu 10  Ag 10  Ge 32  Se 48  prepared in accordance with the present invention was compared with that of conventional polycrystalline electrode materials such as CuS--Ag 2  S, Cu 2  S--Ag 2  S, CuS and Cu 2  S, and with prior art chalcogenide glass sensor compositions of Cu--Ag--As--Se. 
     
                       TABLE II D______________________________________Sensor Membrane  Selectivity CoefficientsMaterial         log K.sub.Cu.spsb.2+.sub., Ni.spsb.2+______________________________________New Material     4 (1) × 10.sup.-6Cu.sub.10 Ag.sub.10 Ge.sub.32 Se.sub.48Prior Art Chalcogenides            3 (1) × 10.sup.-5Cu--Ag--As--SeConventional Crystalline            3 (1) × 10.sup.-5CuS--Ag2S, Cu.sub.2 S--Ag.sub.2 S,CuS and Cu.sub.2 S______________________________________ 
    
     In comparison with conventional copper ion-selective electrodes based upon CuS--Ag 2  S or Cu 2  S--Ag 2  S polycrystalline mixtures, the inventive copper chalcogenide glass sensor is 10 to 30 times less sensitive to the presence of interfering hydrogen ions. It is also an order of magnitude more selective in the presence of nickel ions in comparison with prior chalcogenide glass sensors and conventional crystalline electrodes. 
     E. Thallium Sensor Materials 
     The selectivity of the thallium selective material Tl 28  Ge 14  Se 58  prepared in accordance with the present invention was compared with that of conventional polycrystalline electrode materials. The present inventors are unaware of any prior art chalcogenide systems for the detection of thallium. 
     In comparison with polycrystalline thallium ion-selective electrodes based on HgI 2  --TlI, AgI--TlI, or Tl 2  S--Tl 4  Hg 6  mixtures, thallium chalcogenide glass sensor is an order of magnitude more sensitive in diluted solutions of thallium ions, and its selectivity in the presence of alkali and hydrogen ions is 100 times higher. 
     F. Mercury Sensor Materials 
     The selectivity of the mercury selective material Hg 4  Ag 35  As 10  Sb 1  I 25  S 25  prepared in accordance with the present invention was compared with that of conventional polycrystalline electrode materials. The present inventors are unaware of any prior art chalcogenide systems for the detection of thallium. 
     In comparison with polycrystalline mercury ion-selective electrodes based on AgI, AgI--Ag 2  S, HgI 2  --HgS or HgI 2  --AgI--Ag 2  S mixtures, mercury chalcogenide glass sensor reveals much more stable response in diluted solutions of mercury (II) ions without mechanical grinding or polishing and is an order of magnitude more sensitive in the presence of hydrogen ions. 
     While several embodiments of the invention, together with modifications thereof, have been described in detail herein it should be understood and will be evident that various further modifications are possible without departing from the scope of the invention.