Silver chloride equilibrium

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

Equilibrium curve for silver chloride. Silver chloride (s) is in contact with Ag+ and Cl- ions in aqueous solution. The product Q of the concentration of ions [Ag+] X [Cl-] is equal to Ksp (curved line)when equilibrium exists. If 0 > K,p, AgCI(s) tends to precipitate out until equilibrium is reached. If 0 < Ksp, additional solid dissolves. 

Despite this detailed familiarity with equilibrium, there is one facet we have not considered at all. What determines the equilibrium constant Why does one reaction favor reactants and another reaction favor products What factors cause sodium chloride to have a large solubility in water and silver chloride to have a low solubility Why does equilibrium favor the reaction of oxygen with iron to form FejAi (rust) but not the reaction of oxygen with gold As scientists, we cannot resist wondering what factors determine the conditions at equilibrium. 

Expression (2) applies to a solubility equilibrium, provided we write the chemical reaction to show the important molecular species present. In Section 10-1 we considered the solubility of iodine in alcohol. Since iodine dissolves to give a solution containing molecules of iodine, the concentration of iodine itself fixed the solubility. The situation is quite different for substances that dissolve to form ions. When silver chloride dissolves in water, no molecules of silver chloride, AgCl, seem to be present. Instead, silver ions, Ag+, and chloride ions, Cl-, are found in the solution. The concentrations of these species, Ag+ and Cl-, are the ones which fix the equilibrium solubility. The counterpart of equation (7) will be... 

Just as in expression (4), the concentration of the solid (silver chloride) does not appear in the equilibrium expression (20) it does not vary. 

The negative voltage shows that the state of equilibrium favors the reactants more than the products for the reaction as written. For standard conditions, the reaction will not tend to occur spontaneously. However, if we place Ag(s) in 1 M H+, the Ag+ concentration is not 1 M— it is zero. By Le Chatelier s Principle, this increases the tendency to form products, in opposition to our prediction of no reaction. Some silver will dissolve, though only a minute amount because silver metal releases electrons so reluctantly compared with H2. It is such a small amount, in fact, that no silver chloride precipitate forms, even though silver chloride has a very low solubility. 

It can be shown from a consideration of the overall stability constants of the ions [Ni( CN)4] 2 " (1027) and [ Ag( CN)2 ] (1021) that the equilibrium constant for the above ionic reaction is 1015, i.e. the reaction proceeds practically completely to the right. An interesting exercise is the analysis of a solid silver halide, e.g. silver chloride. 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

The equilibrium constant for this reaction is actually the solubility product, Ksp = [Ag+][C1 ], for silver chloride (Section 11.8). 

C18-0073. For the following salts, write a balanced equation showing the solubility equilibrium and write the solubility product expression for each (a) silver chloride (b) barium sulfate (c) iron(H) hydroxide and (d) calcium phosphate. 

Sometimes a metal electrode may be directly responsible to the concentration of an anion which either gives rise to a complex or a precipitate with the respective cations of the metal. Therefore, they are termed as second-order electrodes as they respond to an ion not directly involved in the electron transfer process. The silver-silver chloride electrode, as already described in Section 16.3.1.1.3, is a typical example of a second-order electrode. In this particular instance, the coated Ag wire when dipped in a solution, sufficient AgCl dissolves to saturate the layer of solution just in contact with the respective electrode surface. Thus, the Ag+ ion concentration in the said layer of solution may be determined by the status of the solubility product (Kvfa equilibrium ... 

For example, adding silver nitrate solution to test for Cl (aq) is effective due to the very low solubility of silver chloride. You can use the precipitation of an insoluble salt to remove almost all of a particular ion from a solution and, as a result, cause a shift in the position of equilibrium of the original solution. The common ion effect is important in the solubility of salts. The precipitation of insoluble salts is used to identify the presence of unknown ions. You will learn more about the common ion effect in Chapter 9. 

We consider a silver electrode covered with a silver chloride film in chloride solution. As shown in Fig. 4—21, the electron level of the silver-silver chloride electrode in ion transfer equilibrium is expressed by the real potential a.(A. A a w) of electrons in the silver part of the electrode as shown in Eqn. 4-27 ... 

Fig. 4-21. Electron energy levels of an ionic electrode of silver-silver chloride in ion transfer equilibrium cfia ) = Fermi level of electron in silver part of electrode snvAfCici-) = equivalent Fermi level to transfer equilibriiun of silver ions and chloride ions in silver-silver chloride electrode.

From Eqns. 4-27 and 4-28, the equilibrium electrode potential, , is obtained for the transfer of silver ions and chloride ions at the silver-silver chloride electrode as shown in Eqn. 4-29 ... 

If a hydrogen electrode be immersed in each solution since the Aystem is in equilibrium, the potential difference between these electrodes must be zero. Similarly for the chlorine ion, there will be a zero potential difference between two silver-silver chloride electrodes immersed in the two media. 

One gram of silver /3-alumina (see above) is placed into a fused quartz test tube about 2 cm in diameter and about 14 cm long. Five grams of lithium chloride is added. It is important that the lithium chloride used have a very low content of other alkali metal impurities, except Cs, since the ion exchange equilibria greatly favor the presence of the other alkali metals in the /3-alumina crystals over lithium. Essentially all of the impurity ends up in the crystals. The fused-quartz test tube is heated to 650° in a furnace. For crystals 1-cm in diameter the time to reach 99% equilibrium is approximately 16 hours. The molten salt is decanted and the crystals are allowed to cool to room temperature. Methyl alcohol containing about 10% propylamine or ethylenediamine is used to wash the product and thereby remove the silver chloride and residual lithium salts. The sample is dried at 400° and stored in a dessicator. The lithium /3-alumina crystals contain less than 0.05% Ag. If the lithium chloride used contains a trace of sodium or potassium, it can be prepurified by treatment with silver /3-alumina at 650°. Each gram of silver /3-alumina will remove about 30 mg of sodium from the melt. The molten lithium chloride, after decantation from the pretreatment silver /3-alumina, can be used to prepare the product, lithium 0-alumina. 

The preparation of this compound from silver (3-alumina is similar to the preparation of lithium /3-alumina. The melt consists of 10 g of potassium chloride. The exchange temperature is 800°. For crystals with diameters of 1 cm it takes about 16 hours to reach 99% of equilibrium. The potassium salts used should contain less than 0.1 wt % sodium. After decantation of the melt the crystals are washed with water containing 2% propylamine or ethylenediamine to remove residual potassium salts and silver chloride. The sample is dried at 200°. The potassium 0-alumina contains less than 0.05% silver. 

We stressed in Chapter 9 that Le Chatelier s principle is only a rule of thumb. We can work toward a deeper understanding and a quantitative understanding of the effect by considering the equilibrium constants involved. Suppose we have a saturated solution of silver chloride in water ... 

The equilibrium constant for this reaction is actually the solubility product, Ksp, for silver chloride (Section 11.10). It does not matter that overall the reaction is not a redox reaction so long as it can be expressed as the differ- ence of two reduction half-reactions. Because silver chloride is almost insol-i uble, we expect K to be very small (and E° to be negative). 

Silver chloride is a common source and intermediate product in many extractive metallurgical processes, for example it occurs in the anode slimes from copper refineries, the residues of leaching processes for base metals, as a product of the chlorination of impure gold—silver bullion, and in photographic waste. A novel process for the leaching and purification of silver chloride, which was devised by Parker et a/.,26 is based on the observation that silver chloride is very soluble in some dipolar aprotic solvents containing chloride ion but is much less soluble when water is present. The very different behaviour of the equilibrium... 

Consider the following equilibrium Ag + (aq) + Cl acj) AgCl(s). UseLe Chatelier s principle to predict how the amount of solid silver chloride will change when the equilibrium is disturbed by ... 

The solubility of an ionic compound increases dramatically if the solution contains a Lewis base that can form a coordinate covalent bond (Section 7.5) to the metal cation. Silver chloride, for example, is insoluble in water and in acid, but it dissolves in an excess of aqueous ammonia, forming the complex ion Ag(NH3)2 + (Figure 16.13). A complex ion is an ion that contains a metal cation bonded to one or more small molecules or ions, such as NH3, CN-, or OH-. In accord with Le Chatelier s principle, ammonia shifts the solubility equilibrium to the right by tying up the Ag+ ion in the form of the complex ion ... 

These electrodes are based on two equilibria the electrochemical equilibrium involving formation of the interfacial potential and the solubility equilibrium between the cation and its sparsely soluble salt. The most popular electrode of this type is the silver/silver chloride electrode. The electrochemical equilibrium is the same as for the Ag/Ag+ electrode described above (6.27) and the solubility equilibrium is... 

The difference between the electrical potentials in the two copper wires is determined by the difference [/l"(Cu) — e(Cu)] under equilibrium conditions with certain restrictions. (The single prime refers here to all parts of the cell to the left of the boundary between the two solutions, and the double prime to all parts to the right of the boundary.) The restrictions are that the boundaries between the various parts of the cell are permeable only to certain species. Without such restrictions the electrical potential difference of the electrons in the copper wires would be zero at equilibrium. The boundary between the copper and platinum or between the copper and silver is permeable only to electrons that between the platinum with adsorbed hydrogen and the first solution is permeable to hydrogen ions but not electrons that between the second solution and the silver chloride is permeable to chloride ions but not electrons and that between the silver chloride and silver is permeable only to silver ions. We ignore the presence of the boundary between the two solutions, for the present. The conditions of equilibrium in terms of the chemical potentials are then ... 

Throughout this discussion we have ignored the effect of the boundary between the two solutions. We have indicated that the solution to the left of the boundary is saturated with hydrogen and to the right of the boundary with silver chloride. The two solutions are not identical and not at equilibrium. There would be an electrical potential difference across the boundary. However, the solubilities of hydrogen and silver chloride in the solutions are very small. The chemical potential of the hydrogen ion is essentially independent of the concentration of hydrogen in the solution, and that of the chloride ion is essentially independent of the concentration of the silver chloride. Therefore, the two solutions are considered to be identical,... 

Consider the equilibrium between solid silver chloride and Ag+ and Cl- in a saturated aqueous solution AgCl(s) i Ag+(aq) + CI"Oq)... 

Commercial ISEs are widely available for various ions. Usually the ion-selective layer is made from an insoluble salt of the ion in question. For example, a chloride-detecting electrode can be made where the selective layer is a pellet of AgCl. Because of the very low solubility of silver chloride, the pellet never reaches equilibrium with the solution. Instead a small amount of chloride dissolves in the sample, leaving a relative surplus of silver atoms at the pellet... 

The following table lists the standard electrode potentials (in V) of some electrodes of the second kind.13 These consist of three phases. The metal is covered by a layer of its sparingly soluble salt and is immersed in a solution of a soluble salt of the anion. Equilibrium is established between the metal atoms and the solution anions through two partial equilibria one between the metal and its cation in the sparingly soluble salt and the other between the anion in the solid phase of the sparingly soluble salt and the anion in solution. The silver chloride electrode is preferred for precise measurements. 

Cubic and higher order polynomial expressions also arise naturally in a wide range of problems in chemistry, particularly in solubility and equilibrium problems. If we try to dissolve lead (II) chloride (PbCl2) instead of silver chloride, the solubility product expression becomes... 

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