ELECTROCHEMICAL CELL

Abstract: Electrochemical cell including an ion-specific mem­ brane containing, in one aspect, Ag, S, and one of Sb or Te.

Description: The invention relates to an electrochemical analysis and more particularly to an ion-specific electrochemical cells according to the first part of claim 1 and a method of making an ion-specific membrane for use in an electrochemical cell. Such cells are known in which the ion-specific element is a silver-containing impervious membrane separating a reference and a sample solution containing an unknown amount of an ion to be measured, e. g. halide, arsenite, cyanide, or sulfide. US -A- 4 071 427 (CHENG et al.) describes an arsenite sensing cell employing a membrane containing a mixture of Ag₂S and Ag₃AsO₃. US -A- 3 563 874 (ROSS et al.) describes a halide sensing cell employing a membrane containing a mixture of silver sulfide and silver halide. US -A- 3 172 778 (VAN DE LEEST et al.) describes a halide sensing cell employing a membrane containing a mixture of Ag₃SBr and Ag₃SJ. US -A- 4 396 486 (MRUK et al.) describes an ion-selective electrode membrane made by pressing at 350 °F, a "coprecipitate of Ag₂S, Ag₂Se, CdS and CdSe." The above patent specifications are hereby incorporated by reference. The technical problem of the invention regarding this prior art is to provide improved membranes for ion-specific electrochemical cells. The solution to this problem is accomplished by the characterizing parts of claims 1 and 4 , respectively. The remaining claims go further in specifying the invention. I.e.: Some of the membranes contain Ag, S, and either Sb or Te; preferably, these membranes contain a compound containing Ag and one of Sb or Te. Some of the membranes contain Ag, S, and one of Sb, Te, As, or Se, and are made by combining an aqueous solution of a silver salt with an aqueous solution of a compound containing S and one of Se, Sb, Te, or As to form a precipitate containing Ag, S, and one of Se, Sb, Te, or As, and subjecting the precipitate to elevated pressure at a temperature lower than 100 °C to form the membrane. The membranes are believed to contain a new class of ternary compounds of Ag, S, and one of Se, Sb, Te, or Ar. In some preferred embodiments the ternary compound has the formula Ag₃QS₃, wherein Q is As or Sb. In other preferred embodiments the ternary compound has the formula Ag₄SZ, wherein Z is Se or Te. The membrane can also contain one of AgX, wherein X is a halogen; lead sulfide; cadmium sulfide; copper sulfide; mercury sulfide; silver cyanide; or silver thiocyanate. When such an additional component is present, the ternary silver compound makes up between 10 and 90 mole %, more preferably 45 to 60 mole %, and most preferably 50 mole %, of the membrane. The membranes of the invention are durable and provide the option of measuring any of the three elements of which they are composed. The membranes are also pressible under cold conditions (temperatures below 100 °C), and can be used as the pressible matrix for additional ion-specific materials which are desired to be employed but which do not press well themselves; examples of such materials are the silver halides, particularly silver iodide (for sensing iodide or cyanide); lead sulfide (for sensing lead); cadmium sulfide (for sensing cadmium); copper sulfide (for sensing copper); mercury sulfide (for sensing mercury); silver thiocyanate (for sensing thiocyanide); and silver cyanide (for sensing cyanide). The membranes of the invention also provide the advantage of resistance to strong solvents such as iodide and cyanide. It is believed that this solvent resistance may be due to the Te, Sb, As, or Se, which might bind the silver more tightly within the membrane and thus protect it. The membranes of the invention also have the advantage of providing close to Nernstian responses at low sample ion levels; this is perhaps a function of solvent resistance and the resultant small amount of silver which dissolves from the membrane and goes into solution. Incidentally, FR -A- 2 268 264 (AGENCE NATIONALE) tabulates a number of compounds for use in a membrane in an electrochemically cell, including compounds containing Ag, one of As or Sb, and S (AgAsS₂ and AgSbS₂ in molar ratios 1:1:2). The process described calls for heat treatments at temperatures as high as approximately 1000 °C (e.g. 700 °C (example 8), 500 °C (examples 1a, b, 4), 400 °C (examples 2, 5, 6), 100 °C for 5 h and then 400 °C for 10h (example 3)); such a process cannot be used to make, for example, the ternary compound Ag₄SSe, because Ag₄SSe is unstable (it expands and decomposes) at temperature higher than about 100 °C. US -A- 4 071 427 (CHENG et al.) specifically describes preparing a membrane "formed of silver sulfide intimately and uniformly mixed with silver arsenite" by dissolving As₂O₃ in NaOH, adding Na₂S'H₂O, and titrating with a slight excess of AgNO₃; thus it describes a membrane formed as a mechanical mixture of two binary compounds, and notes that silver sulfide is highly insoluble. US -A- 3 809 637 (HIGASHIYAMA et al.) describes a selective electrode including a disc in a "batch composition" comprising two binary compounds, one of which is a cuprous chalcogenide or a cuprous telluride or selenide, pressed and heated at high temperature. US -A- 3 591 464 (FRANT et al.) describes a selective electrode including a membrane formed as a compressed coprecipitate of Ag₂S and a compound selected according to the nature of the ion to which the electrode is to be responsive (the sulfides of Cu, Pb, and Cd for copper, lead, and cadmium-sensitive elements; AgSCN for thiocyanate-sensitive elements). A plurality of ways of carrying out the invention is described in detail below with reference to drawings , namely : Fig. 1 a diagrammatic side-elevation, in cross-section, of an electrochemical cell employing a membrane of the invention; Fig. 2 a diagrammatic representation of the cell of Fig. 1 used in conjunction with a reference cell. Certain of the components of the membranes of the invention occur naturally as minerals. Thioantimonite, believed to have the formula Ag₃SbS₃, occurs naturally as the mineral pyrargyrite, and the mineral prousite, believed to contain Ag₃AsS₃, and the mineral aguilarite, believed to contain Ag₄SSe, occur naturally. These minerals can be pressed to form membranes of the invention. Alternatively, membrane materials containing Ag, S, and either Se or Te, and believed to contain a compound having the formula Ag₄SZ, can be made by dissolving solid sodium selenide or telluride (commercially available) in a 0.2M sodium sulfate solution and then combining the resulting solution with a dilute solution of silver ion (e.g. AgNO₃). The resulting precipitate is separated out, washed, and dried. In more detail, the above procedure is as follows. In a 2 L beaker containing 1 L of deionized water, 25.05 g (0.148 moles) of reagent grade silver nitrate is dissolved. Separately, in a 500 ml flask containing 250 ml of deionized water, 8.86 g (0.0369 moles) of reagent grade sodium sulfide nonahydrate (Na₂S 9H₂O) is dissolved. To this sodium sulfide solution is added 4.61 g (0.0369 mole) of sodium selenide. The solution is stirred to dissolve all solids. The sulfide-selenide solution is transferred to a 250 ml separatory funnel suspended over the beaker containing the stirred silver nitrate solution. The sulfide-selenide solution is added dropwise to the stirred silver nitrate solution over a period of 30-60 minutes. After the addition is complete, the suspension is stirred at room temperature for an additional hour. The precipitate is collected by filtration, washed with deionized water and methanol, and finally air dried. The dry precipitate is stored in a brown glass bottle until use. A precipitate containing Te rather than Se is made according to the procedure described above, substituting sodium telluride (6.41 g, 0.0369 moles) for sodium selenide. Membrane materials containing Ag, S, and either As or Sb, believed to have the formula Ag₃QS₃, are made as follows. Reagent grade silver nitrate (25.48g, or 0.150 moles) is dissolved, with stirring, in 1 L of deionized water in a 2L beaker. Separately, in a 500 ml flask containing 250 ml of deionized water, 18.01 g (0.075 moles) of sodium sulfide nonahydrate is dissolved, with stirring. To the sulfide solution are added 8.49 g (0.025 moles) of antimony trisulfide. The solution is stirred until all solids have dissolved. The antimony-sulfide solution is added dropwise to the silver nitrate solution and the product is collected, washed, and dried, as described above for silver sulfoselenide. A precipitate containing AS rather than Sb is made according to the above procedure, substituting arsenic trisulfide (6.15 g, 0.025 moles) for antimony trisulfide. The above minerals or precipitates, alone or in combination with an additional membrane component, can then be made into membrane form, according to well-known methods, e.g. as described in Ross et al. U.S.P. 3,563,874. The membrane components are first each weighed out separately in the required proportions. In some instances there will be only one component, while in other instances at least one additional component, e.g. silver iodide, will also be present. The membrane components are combined and ground together using mortar and pestle to form a homogeneous mixture. The mixture is then cold-pressed into an electrode membrane, according to procedures well-known in the art. After cold-pressing, the membrane surface is highly polished, also according to well-known procedures. Polishing is important in obtaining maximum ion sensitivity. The membranes of the invention are used in an electrochemical cell such as that shown diagrammatically in Figure 1. Electrochemical cell 20 includes tube 22, which is open at both ends. The tube typically is formed of a liquid impervious, substantially rigid, electrically insulating material, such as epoxy (which is preferred), unplasticized polyvinylchloride, polytetrafluorethylene, glass or the like, substantially chemically inert to solutions being tested. One end of tube 22 is sealed with membrane 24 of the invention. Membrane 24 can be quite thick, for example, 1/4 inch, although thinner structures are preferred. Membrane 24 is preferably sealed across one end of tube 22 with an appropriate sealing compound such as an epoxy resin. Disposed internally of tube 22 and in electrical and physical contact with the inner surface of the membrane 24 is charge transfer means providing a fixed concentration of silver either in metallic or ionic form. This means is shown as a reference electrolyte 30, for example, an aqueous saturated solution of suitable salts. Immersed in electrolyte 30 is internal referrence electrode 32, for example the well-known Ag-AgCl element. This combination of electrolyte 30 and reference electrode 32 provides means for electrically contacting the internal face (e.g., the surface of the membrane contacting the electrolyte) at a substantially stable or fixed potential. The other open end of tube 22 is fitted with annular cap 34 having an aperture in which is sealed the usual coaxial cable 36, the central conductor of which is connected to internal reference electrode 32 and the peripheral conductor of which is intended to provide electrostatic shielding. As shown in FIG. 2, electrochemical cell in use is placed so that the outer surface of membrane 24 contacts solution 40 under test. A standard reference electrode 42 is also placed in contact with solution 40. Reference electrode 42 is preferably a double junction reference electrode such as that sold by HNU Systems, Inc., Newton, MA under model number ISE 40-20-00. This reference electrode employs two solutions: an internal solution of 4M KCl, saturated with AgCl; and an external, silver-free solution of 1 M NaNO₃. Both electrode 20 and reference electrode 42 are connected electrically to respective inputs of an electrometric device which is preferably the usual high-input impedance voltmeter. In operation of the assembly of FIG. 2, a potential, E ref of substantially fixed value (assuming constant temperature conditions) develops between reference electrode 42 and solution 40 independently of the ion concentration in the latter. Another potential, E m , will develop across membrane 24 between internal electrolyte 30 and solution 40. Because the potential, E int between reference electrode 32 and electrolyte 30 is also fixed, the total potential E t appearing between electrodes 42 and 20 will be the sum of E m ,E ref , and E int , and thus varies with E m only. E t can be readily measured on an electrometric device, thus indicating the presence and activity of the desired ions in solution 42. Other embodiment of this invention will occur to those skilled in the art which are within the scope of the following claims.

Claims: Electrochemical cell (20) comprising a reference electrode (32), an electrolyte (30), and an ion-specific membrane (24), arranged to detect an ion in solution, characterized in that said membrane (24) contains Ag , S and one of As , Sb , Se , or Te , as a ternary compound, having one of the formulas Ag₃AsS₃ Ag₃SbS₃ Ag₄SSe Ag₄STe . Cell of claim 1, wherein said membrane comprises between 10 and 90 mole % of said ternary compound. Cell of claim 1 or 2, wherein said membrane comprises one of AgX , X = a halogen; lead sulfide; cadmium sulfide; silver thiocyanate; silver cyanide; copper sulfide; or mercury sulfide. Method of making an ion-specific membrane for use in an electrochemical cell, combining an aqueous solution of silver salt with an aqueous solution of a compound containing S and one of As , Sb , Se or Te , to form a precipitate comprising a ternary compound Ag₃AsS₃ , Ag₃SbS₃ , Ag₄SSe or Ag₄STe , subjecting said precipitate to elevated pressure at a temperature lower than 100 °C to form said membrane. Method of claim 4, wherein prior to subjecting said precipitate to pressure said precipitate is combined with one of AgX , X = a halogen; lead sulfide; cadmium sulfide; silver thiocyanate; silver cyanide; copper sulfide; or mercury sulfide.

Inventors: ATWOOD, EDWARDS S., DRISCOLL, JOHN N.
Applicants: HNU SYSTEMS INC.
Application year: 1984
Main classification: G01N27
IPC codes: G01N27