EP0515413A1

EP0515413A1 - Ag 2?S MEMBRANE FOR THE DETERMINATION OF SILVER IONS OR HALIDE IONS IN SOLUTION

Info

Publication number: EP0515413A1
Application number: EP19910903309
Authority: EP
Inventors: Philippe Gérald ROBERT
Original assignee: Kodak Pathe SA; Eastman Kodak Co
Current assignee: Kodak Pathe SA; Eastman Kodak Co
Priority date: 1990-02-12
Filing date: 1991-02-04
Publication date: 1992-12-02
Also published as: CA2076020A1; FR2658296B1; JPH05504199A; FR2658296A1; WO1991012521A1

Abstract

L'invention concerne une membrane cristalline de sulfure d'argent pour le dosage d'ions argent ou d'ions halogénures en solution et son procédé de fabrication. La membrane selon l'invention comprend du sulfure d'argent dopé avec un ion capable de former soit avec Ag+ soit avec S2- un sel beaucoup plus insoluble que les espèces en solution. De préférence, la membrane contient aussi un composé hydrophobe inerte. La membrane est préparée par coprécipitation du sulfure d'argent et du dopant, suivie par une homogénéisation avec le composé hydrophobe et d'un pressage. La membrane peut être utilisée dans une électrode pour doser les ions argent ou halogénures en solution.The invention relates to a silver sulfide crystal membrane for the determination of silver ions or halide ions in solution and its manufacturing process. The membrane according to the invention comprises silver sulfide doped with an ion capable of forming either with Ag + or with S2- a salt much more insoluble than the species in solution. Preferably, the membrane also contains an inert hydrophobic compound. The membrane is prepared by coprecipitation of the silver sulfide and the dopant, followed by homogenization with the hydrophobic compound and pressing. The membrane can be used in an electrode to measure silver ions or halides in solution.

Description

AG S MEMBRANE FOR THE ASSAY OF SILVER IONS OR IONS

The invention relates to a membrane sensitive to silver ions and to halide ions, usable in an electrode. More specifically, the invention relates to a crystalline membrane based on silver sulfide which can be used in an electrode for measuring the activity of silver ions or halides in solution.

Selective membrane electrodes are well known in patents and the literature.

In general, these electrodes comprise a crystal membrane in the form of a compressed tablet of an ion-sensitive material whose activity is to be detected, placed at the lower end of the body of the electrode. One side of the membrane, called the inner side, is connected to the electrochemical measuring chain by a reference element constituted either by an internal electrolyte of known ionic activity in which a reference metal electrode is immersed, or by a suitable metal system . The other face of the membrane, called the external face, is brought into contact with the solution to be assayed. Between the two faces of the membrane there is a potential difference. This potential is measured by means of a high impedance • voltmeter connected to the reference element and to the solution to be assayed. The potential difference thus measured and the logarithm of the activity of the ions to be measured are linked by a linear relationship, the well-known Nernst equation.

Many membranes based on metal sulfides, in particular membranes based on Ag S are known in the art.

U.S. Patent 3,591,464 describes a crystal membrane electrode formed from an intimate mixture of Ag S and another compound chosen according to the nature of the ion whose l 'activity. Through example, to detect Cu ions, the mole percentage of cupric sulphide compared to silver sulphide is between 95% CuS / 5% Ag S and 1%

CuS / 99% Ag S. Thus, with Ag S, we combine CuS, PbS, CdS and AgSCN to measure Cu ++, Pb ++ respectively,

Cd ⁺⁺ , and SCN ^" .

U.S. Patent 4,400,243 describes a membrane comprising a mixture of at least four metal sulfides, each sulfide corresponding to an ion to be assayed. For example, the mixture may contain 15 to 65% by weight of silver sulfide, 15 to 65% by weight of cadmium sulfide, 30 to 65% by weight of lead sulfide, and 3 to 9% by weight of sulfide of copper. This mixture can be obtained by coprecipitation, for example, by adding to a solution of sodium sulfide a solution containing a mixture of 0.3 moles of cadmium nitrate, 0.3 moles of lead nitrate, 0.3 moles of nitrate silver and 0.1 mole of copper nitrate. This membrane makes it possible to dose the Ag ⁺ , Cd ⁺⁺ , Pb ⁺⁺ , Cu ⁺⁺ ions. U.S. Patent 3,563,874 describes a non-porous membrane sensitive to halide ions consisting of a mixture of silver sulfide and at least 5 mol% of silver halide, for example silver chloride for assaying Cl ^~ , the preferred mixtures vary between 10 moles and 90% AgCl. In the case of mixed membranes containing iodine sulfide and bromine sulfide, the minimum halide content must exceed 5 mol%.

US Patent 4 0.96 049 describes a membrane made of silver sulfide and comprising on its surface a sparingly soluble substance having an ion in common with the ion to be assayed, for example AgCl, AgBr or Agi for measure Cl-, Br ^~ and I- respectively.

US Patent 4,172,778 describes an Ag 3SBr or Ag3SI membrane for the assay of Ag ⁺ , I ^~ or Br ^~ .

For example, Ag 3SI is obtained by precipitation of equimolar amounts of sodium sulfide and sodium iodide with a slight excess of silver nitrate.

U.S. Patent 3,672,692 describes an Ag S membrane for detecting sulfide ions in solution. This patent teaches that the silver sulfide membrane must be of a relatively high degree of purity because the presence of ions or foreign molecules, such as AgO makes it porous and of low mechanical resistance. U.S. Patent 3,809,636 teaches to use silver sulfide and silver selenide and / or telluride to form an electrode for assaying silver ions. For example, the membrane contains from 10 to 30% by weight of silver sulphide and from 70 to 90% by weight of silver telluride. U.S. Patent 4,549,953 describes the combination of silver sulfide, silver selenide, silver telluride and mercury sulfide, for example Ag SSe or Ag STe obtained by co-precipitation to form a membrane to measure the activity of one of the three elements that make up the membrane.

In summary, the crystal membranes of the prior art based on Ag S comprise a mixture of silver sulfide and a sulfide of another metal in order to assay silver or this other metal, or a mixture of sulfide silver and a halogen salt to measure this halide.

One of the objects of the present invention is to provide a new crystalline membrane based on silver sulphide sensitive to silver ions and to halide ions in solution although it does not contain halide and which makes it possible to measure the ions silver or halides in diluted or saturated solutions in a rapid, reproducible and precise manner.

This object is achieved with a membrane according to the invention comprising silver sulfide doped with a ion capable of forming either with Ag or with S, a very insoluble salt, that is to say much more insoluble than the species in solution.

By "doped silver sulphide" is meant that the silver sulphide contains a doping agent in a proportion of less than 5 mol%, and preferably in a proportion of approximately 1 mol% and which does not occur in the electrochemical equilibria used.

Another object of the present invention is to obtain a membrane having improved mechanical properties and good resistance to chemical agents.

This object is achieved with an Ag S crystal membrane doped with an ion forming with S ^2- or Ag ⁺ a very insoluble salt, further containing a hydrophobic compound.

Another object of the present invention is to obtain an electrode using this membrane which is easy to use and inexpensive.

This object is achieved by incorporating this membrane interchangeably.

The electrode according to the invention measures the activity of silver ions or halides in solution. In dilute solution, that is to say for concentrations less than 10 ^~ mole / 1, the activity of the ion is practically equal to the concentration; beyond this, a correction factor is applied. Between the electrode and the reference element a potential difference is established which varies with the activity of the ion for which this membrane is selective. The relationship between the activity of. the ion and the potential thus measured is logarithmic:

E = + 2.3 RT / nF log A (Nernst equation) or

E is the measured potential, EE eesstt llaa ffrraaccttiioonn ddee ppoottee: ntiel due to the choice of the reference electrode and internal solutions. 2,3 RT / nF is a system constant, A is the activity of the ion in solution. The electrode according to the invention provides rapid and precise responses in accordance with the Nernst equation in most diluted or saturated solutions. It allows measurement of Ag ion activities up to 10 ^~ mole / 1 in silver halide solutions and Ag ⁺ ions up to 10 ^{~ 5} mole / 1 in AgNO or d solutions halide ions up to 10 ^~ mole / 1 in halide solutions.

In an embodiment of the present invention illustrated in Figure 1, the electrode is formed of a hollow resin rod (2) comprising: a) a reference electrode (8), composed of a 90:10 mixture by mass of silver and resin, immersed in an AgNO solution of known concentration (7), b) the Ag S membrane (3) according to the invention fixed in leaktight manner to one end of the body of the 'electrode. For example, the membrane can be fixed with an adhesive to the body of the electrode, in which case the body of the electrode constitutes with the membrane the interchangeable part, or be fixed with a seal, for example a silicone seal, in which case the membrane alone is interchangeable. In practice, the reference electrode is placed as close as possible to the membrane, for example by interposing a paper washer (4) of thickness <0.2 mm between the membrane and the reference electrode, in order to avoid potential drift due to the temperature difference between the membrane and the reference electrode. One can also place a temperature sensor (5), for example silicon, in the resin to automatically correct said potential drift. A sheath (6) of thermoplastic resin allows the electrical connections (1) coming from the temperature sensor (5) and the electrode to be isolated. reference (8) of the reference solution (7).

In another embodiment illustrated in Figure 2, the electrode does not contain a reference electrolyte. One of the faces of the membrane according to the invention (3) is brought into contact with the solution to be dosed, the other face being covered with metallic silver (10) and connected in an appropriate manner, for example by welding with the tin (9), to a metal wire (11) connected to the measurement system. This layer of metallic silver makes it possible to constitute the reference system.

In yet another embodiment (not shown), the electrolyte flows slowly, in a controlled manner, through the membrane.

In operation, the selective electrode is connected to a terminal of a high impedance millivoltmeter (> 10 Ω), the other terminal of which is connected to the reference electrode by means of a salt bridge.

The process for preparing the membranes comprises the following steps: 1) the silver sulfide is coprecipitated with less than 5 mol% of doping agent; the coprecipitate obtained is filtered, washed and dried,

2) the coprecipitate is homogenized,

3) a small amount of homogenized coprecipite is compacted under pressure and at room temperature to obtain a fine, practically non-porous pellet,

4) the pellet obtained is subjected to a heat treatment at a temperature between 90 ° C and 150 ° C. In a preferred embodiment, the coprecipite is homogeneously mixed with a hydrophobic substance.

The first step in the preparation of the membranes is therefore the coprecipitation of Ag S and the doping agent. The doping agent is intended to create imperfections in the structure of the Ag S crystal and on its surface, to obtain the necessary high conductivity.

In practice, the doping agent is chosen from cations or anions which form very insoluble salts with Ag or S ^~ , that is to say much more insoluble than the species in solution, which here are mainly halides d 'money. If we define the solubility constant ks of a salt as the product of the activities of the ions which constitute it, and the product of solubility pks as the inverse of the decimal logarithm of the solubility constant, we consider that a salt is "much more insoluble" than another when its solubility constant ks is at least a factor of 10 ^{~ 3} less than the solubility constant of the other salt, or else its solubility product pks is greater than at least 3 units to that of the other salt. Thus, as an doping agent, an anion or a cation forming with Ag ⁺ or S will be chosen a salt having a pks at least greater than 20 if, as is often the case, the most insoluble species encountered in the solution is Agi. (pks _A _ about 17 to 25 ° C), and preferably higher than that of Ag S which is about 49 to 18 ° C. For example, a chalcogenide anion such as Se ^2- , Te ^~ or a metal cation such as Pb ²⁺ , Cd ²⁺ , Cu ²⁺ , Hg ²⁺ , Pd ²⁺ , Tl ³⁺ , Au can be used as doping agent. ³⁺ , Rb ⁺ . The preferred cations are Hg ²⁺ , Pd ²⁺ , Tl ³⁺ , Au ³⁺ , Rb ⁺ . A particularly preferred cation is Hg ²⁺ .

The amount of doping agent can vary widely depending on the very nature of the doping agent, but remains less than 5 mol%. If the doping agent is Hg ²⁺ , a preferred range is 0.5-5 mole% relative to the number of moles of Ag S and more advantageously approximately 1% by moles is used relative to the number of moles of Ag S.

The doping agent is incorporated into Ag S by co-precipitation. For example, if the dopant is Hg ²⁺ , we introduce by double jet S ^2- and Ag ⁺ in the form salts, in the presence of an excess of S and in basic medium, preferably at a pH> 10, Hg ²⁺ being in the form of salt in the jet of Ag ⁺ .

The coprecipitate obtained is in the form of mixed crystals of Ag S and of dopant, for example if the dopant is

Hg, the coprecipite is formed of crystals of silver sulfide, mercuric sulfide and mixed sulfides of silver and mercury.

After washing and drying, the precipitate is homogenized for several minutes in order to break the crystal agglomerates, without however crushing the crystals. Preferably, the coprecipite is mechanically mixed with a hydrophobic compound in the form of fine particles, for example dry or in a solvent medium in a ball mixer. It is also possible to form a dispersion of particles of homogenized coprecipite and of hydrophobic substance in an organic solvent, such as chloroform, in which case, the solvent is allowed to evaporate while maintaining the dispersion. This hydrophobic compound reduces the porosity of the membrane, improves its mechanical properties and lubricates its surface. Any hydrophobic compound can be used in the form of particles having a size close to that of Ag S crystals, that is to say less than 25 μm, provided that this compound is inert, that is to say resistant chemical agents contained in the solutions to be analyzed, and is also resistant to the temperatures of the heat treatment, that is to say resistant to a temperature of the order of 150 ° C. - such as fluorocarbon polymers or halohydrocarbon polymers such as polytetrafluoroethylene , polyhexafluoropropylene or polyvinylidene fluoride. A preferred polymer is polytetrafluoroethylene Teflon (marketed by Dupont de Nemours). One can for example use from 0.5 to 20% by weight of Teflon 701N having a particle size of 20 μm, advantageously 1% by weight, or preferably Teflon DLX6000 having a particle size of 1 μm, at a rate of 0.5 to 20% by weight, advantageously 2.5% to 3% by weight.

The next step consists in compacting the homogenized coprecipite or the homogeneous mixture comprising the coprecipite and the hydrophobic compound in order to obtain a dense and practically non-porous pellet, having optimal conductivity and good mechanical properties and also to avoid inclusions. carbonated. During this step, distortion of the crystals must be avoided, which would lead to a loss of conductivity of the membrane. Compaction is carried out in a hydraulic press following a pressure cycle. The powder is compacted under vacuum between two surfaces of polished steel. The pressure P expressed in kiloPascals varies according to the time t expressed in minutes according to the equation:

P = 5 x 10 ² + b. (T) ^a x 10 ² where a is between 1 and 2 and b between 5 and 15.

The maximum pressure is from 55 x 10 ² kPa to 155 x 10 ² kPa. Then, a rest time of the order of 1 to 15 minutes is observed to stabilize the pellet.

Finally, a heat treatment makes it possible to homogenize the crystalline phase (by coalescence). The treatment is carried out in oil or preferably in air at a temperature between 90 ° C and about 150 ° C for 6 to 24 hours.

The membrane thus obtained has the form of a dark gray pellet, with a diameter of approximately 10 mm and a thickness of approximately 0.8 mm. It is practically non-porous, its density varies between 6.55 and 7.13 and its surface has no roughness.

Examples 1-3 which follow relate to the preparation of the membranes. Example 1

A first solution is prepared containing 0.98 mole of AgN0 ₃ , 0.01 mole of Hg (CH ₃ COO) ₂ , 0.035 mole of acetic acid in 100 g of water and a second solution containing 0.5 mole of Na 2S, 9H2O and 2.5 x 10 ^-3 mole of sodium hydroxide in 919 g of water. These two solutions are introduced with stirring by double jet into a basin containing 0.156 mole of Na 2S, 9H2O and 0.22 mole of sodium hydroxide in

1.5 liters of water at 50 ° C. The precipitation lasts 60 min. The pH is 11.3. It is filtered, the coprecipite is washed thoroughly with water and dried with acetone.

A sample of 5 g of precipitate is taken, 1% by weight of Teflon 701N is added. Dry mixing in a ball mixer, at room temperature for 15 min. A homogeneous powder is obtained.

423 mg of this powder are deposited in a densification matrix between the faces of two polished steel pellets. A load is applied to the entire hydraulic unit according to a pressure cycle, the pressure varying from 0 to 55 x 10 ² kPa. The membrane is left to stand under load for 15 min in the press. Its weight is 420 mg, its diameter 10 mm, its thickness 0.764 mm. Then, a heat treatment is carried out in air at 140 ° C for 14 h 30 min. Example 2

This membrane was prepared using the procedure of Example 1, except that one uses 2.5% Teflon ^® DLX6000 instead Teflon ^® 701N. The weight of the membrane is 0.423 mg and the thickness of 0.793 mm. Example 3

This membrane is prepared using the procedure of Example 1, except that 2.5% of Teflon® DLX6000 is used. The weight of the membrane is 0.423 mg and the thickness of 0.814 mm. Example 4

Two electrodes are produced as shown in FIG. 1 respectively comprising the membranes of Example 1 and of Example 2, by bonding the membranes to the lower part of the body of the electrode. The membrane / body of the electrode assembly is then interchangeable.

Between two measurements, the electrode is rinsed with water and gently wiped to dry it.

To calibrate these electrodes, they are placed in saturated or unsaturated solutions of AgX containing a known amount of KX whose ionic strength is adjusted to 0.1 by addition of KNO 3. Thus, the responses of the electrode respectively for known quantities of KC1 in a saturated or unsaturated solution of AgCl, of KBr in a saturated or unsaturated solution of AgBr, of Kl in a saturated or unsaturated solution of Agi.

Figures 3 to 6 show the responses of the electrodes expressed in VAg as a function of pAg (- log of the activity of Ag ions) in solutions at 40 ° C, for increasing concentrations of halides. Thus, from left to right in FIGS. 3 to 6, the VAg are those obtained for respective chloride concentrations of 10 ^{~ 5} , 10 ^{~ 4} , 10 ^{~ 3} , 10 ^{~ 2} , 10 ^-1 mole / 1 (pAg of 4 to 8 approximately), in bromide of l ^"5 , 10 ^{~ 4} , lθ ^{" 3} , lθ ^"2 , 10 ^_1 mole / 1 (pAg of 6.5 to 10.5 approximately), in iodide of 10 ~ ⁴ , 10 ^{~ 3} , 10 ^{~ 2} , 10 ^-1 mole / 1 (pAg from 11 to 14 approximately).

FIG. 3 reports the response of the electrode comprising the membrane 1, respectively in saturated solutions of AgCl, AgBr, Agi. FIG. 4 shows the response of the electrode comprising the membrane 1, respectively in unsaturated solutions of AgCl, AgBr, Agi.

FIG. 5 shows the response of the electrode comprising the membrane 2, respectively in saturated solutions of AgCl, AgBr, Agi. FIG. 6 shows the response of the electrode comprising the membrane 2, respectively in unsaturated solutions of AgCl, AgBr, Agi.

In all cases, there are three distinct areas: the area of chlorides whose concentration is between 10 ^{~ 5} and 10 ^{~ 3} mole / 1 (4 <pAg <6.5), the area of bromides whose concentration is included between 10 ^-3 and 10 ^~ mole / 1 (8.5 <pAg <11), the area of iodides whose concentration is between 10 ^~ and 10 ^~ mole / 1 (11 <ρAg <14). We also observe a common domain for chlorides whose concentration is between 10 ~ ³ and 10 ^-1 mole / 1 and for bromides whose concentration is between 10 and 10 mole / 1 (6.5 <pAg <8.5) . We see from Figures 3 and 5 that in saturated solutions, we get nernstian responses (i.e. a linear relationship between the read potential and the pAg according to the Nernst equation) for concentrations chloride between 10 ^{~ 3} and 10 ^" mole / 1 (6 <pAg <8), for bromide concentrations between 10 ^~ and 10 ^~ mole / 1 (6.5 <pAg <10.5), for iodide concentrations between 10 ^{~ 4} and 10 ^_1 mole / 1 (11 <pAg <14).

It can be seen from FIGS. 4 and 6 that the nernstian behavior in unsaturated solutions is essentially encountered in chloride solutions having concentrations between 10 ^-3 and 10 ^-1 mole / 1 (6 <pAg <8), bromide having concentrations between 10 ^{~ 4} and 10 ^-1 mole / 1 (8.5 <pAg <10.5)., iodide having concentrations between 10 ^~ and 10 ^~ mole / 1 (11 <pAg <13). Example 5

An electrode is produced, as illustrated in FIG. 2, by covering one face of the membrane, prepared in Example 3, with a layer of polymer containing metallic silver and by connecting the latter by a tin solder to a copper wire, itself connected to the measurement system. The wire / membrane assembly is placed in a resin cylinder and is easily interchangeable. "Figure 7 reports the response of this electrode in AgNO solutions of respective concentrations of 10 ^_1 , 10 ^{~ 2} , 10 ^{~ 3} , 10 ^{~ 4} , and 10 ^{~ 5} mole / 1 (pAg from 1 to 5). It can be seen that the responses of the electrode are practically in conformity with the Nernst equation for all the concentrations.

Claims

CLAIMS 1 - Membrane sensitive to silver ions or halides in solution, said membrane comprising silver sulfide doped with an ion capable of forming, either with Ag ⁺ or with S ^2- a salt much more insoluble than the species in solution, membrane characterized in that it contains a doping amount of doping agent less than 5 mole% relative to the number of moles of Ag S. 2 - Membrane according to claim 1, in which the doping agent is chosen from Rb ⁺ , Hg ² , Pd ²⁺ , Tl ³⁺ , Au ³⁺ , Se ^2- , Te ^2- . 3 - Membrane according to claim 1 or 2 wherein the doping agent is Hg ²⁺ . 4 - Membrane according to claim 3 wherein the molar ratio of Hg ² to Ag S is about 1%.

5 - Membrane according to claims 1 to 4 further comprising an inert hydrophobic substance.

6 - The membrane of claim 5, wherein said hydrophobic substance is a hydrophobic polymer resistant to a temperature of about 150 ° C.

7 - Membrane according to claim 6 wherein said hydrophobic polymer is polytetrafluoroethylene polyhexafluoropropylene and polyvinylidene fluoride.

8 - The membrane of claim 7, wherein the polymer is polytetrafluoroethylene and the weight ratio of polytetrafluoroethylene to silver sulfide is between 0.5 and 20.%. g - membrane according to claims 7 and 8, in which the polytetrafluoroethylene is in the form of particles having a size of less than 25 μm. 0 - Process for manufacturing membranes sensitive to silver ions or halides in solution, comprising the following steps: 1) the silver sulphide is coprecipitated with less than 5 mol% of doping agent; the coprecipitate obtained is filtered, washed and dried,

2) the coprecipite is homogenized, 3) a small amount of homogenized coprecipite is compacted under pressure and at room temperature to obtain a practically non-porous fine pellet, 4) the pellet obtained is subjected to a heat treatment between 90 ° C. and 150 ° C.

11 - Process according to claim 10, in which the doping agent is Hg ²⁺ and the co-precipitation takes place at a pH> 10.

12 - Process according to claims 10 or 11, wherein the coprecipite is mixed with an inert hydrophobic substance.

13 - Process according to claims 10 to 12 wherein the coprecipite is homogenized in a ball mixer with polytetrafluoroethylene, in step 2). 14 - Process according to claims 10 to 12 wherein the coprecipite homogenized in step 2) is mixed with polytetrafluoroethylene by forming a dispersion in an organic solvent and then drying this dispersion in air. 15 - Method according to any one of claims 10 to 14, wherein in step 3), the pressure is applied according to a pressure cycle and varies between 0 and 155 x 10 ² kPa.

16 - Process according to claims 10 to 15, wherein the heat treatment is carried out at a temperature between 90 ° C and 150 ° C in air or oil.

17 - An electrode for measuring the activity of silver ions or halides in solution, comprising the membrane defined in any one of claims 1 to 9.