Silver, metallic precipitation

The oxidation of an aldose or ketose by silver ions results in the reduction of the silver ion to silver metal. The silver is normally present as Ag (NH3)20H . A positive test has a coating of silver metal precipitating on the sides of the container as a mirror. 

Silver metal precipitates from solution and coats the flask, producing a smooth silver mirror, as seen in Figure 14.4. The test is therefore often called the Tollens silver mirror test. The commercial manufacture of silver mirrors uses a similar process. Ketones cannot be oxidized to carboxylic acids and do not react with the Tollens reagent. 

Chemical analysis methods maybe used for assay of silver alloys containing no interfering base metals. Nitric acid dissolution of the silver and precipitation as AgCl, or the Gay-Lussac-VoUiard titration methods are used iaterchangeably for the higher concentrations of silver. These procedures have been described (4). 

Silver nitrate is astringent and a protein precipitant, which is not medically desirable. Other forms of silver have been used to avoid this problem, including coUoidal silver, silver-protein preparations, and finely divided silver metal called Katadyn silver. 

We can easily demonstrate that reaction (3) can occur even when the half-cells are not separated. You did this in Experiment 7. A copper wire immersed in AgN03 solution caused copper to dissolve [the blue color of Cu+2(aq) appeared] and metallic silver was precipitated. The ratio of (Cu dissolved) to (Ag formed) was the same as that in our cell, hence the net reaction was the same. 

Our conclusions are again in agreement with experiment. The cell operates so as to dissolve zinc metal and precipitate silver metal. The voltage is indeed about 1.5 volts. Finally, experiment shows that one mole of zinc does react with two moles of silver ion, as required by the balance of electrons. 

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. 

That some silver does dissolve to form Ag+ can be verified experimentally by adding a little KI to the solution. Silver iodide has an even lower solubility than does silver chloride. The experiment shows that the amount of silver that dissolves is sufficient to cause a visible precipitate of Agl but not of AgCl. This places the Ag+ ion concentration below 10-10 M but above 10-17 M. Either of these concentrations is so small that we can consider our prediction for the standard state to be applicable here too—silver metal does not dissolve appreciably in 1 M HC1. In general, the question of whether a prediction based upon the standard state will apply to other conditions depends upon how large is the magnitude of °. If ° for the overall reaction is only one- or two-tenths volt (either positive or negative), then deviations from standard conditions may invalidate predictions that do not take into account these deviations. 

FIGURE 11.22 When ammonia is added to a silver chloride precipitate, the precipitate dissolves. However, when ammonia is added to a precipitate of mercury(l) chloride, mercury metal and mercury(ll) ions are formed in a redox reaction and the mass turns gray. Left to right silver chloride in water, silver chloride in aqueous ammonia, mercury(l) chloride in water, and mercury(l) chloride in aqueous ammonia. 

The reactivities of potassium and silver with water represent extremes in the spontaneity of electron-transfer reactions. The redox reaction between two other metals illustrates less drastic differences in reactivity. Figure 19-5 shows the reaction that occurs between zinc metal and an aqueous solution of copper(II) sulfate zinc slowly dissolves, and copper metal precipitates. This spontaneous reaction has a negative standard free energy change, as does the reaction of potassium with water ... 

A simple and economical method for recovering silver residues by dissolution in used photographic fixer (thiosulfate) solution, then precipitation by addition of zinc powder, is detailed [1]. After the acid digestion phase of silver recovery operations, addition of ammonia followed immediately by addition of ascorbic acid as reducant gives a near-quantitative recovery of silver metal, and avoids the possibility of formation of silver nitride [2],... 

Fig. 4.3.2 AgCI particles containing metal silver dots precipitated in 0.10 mol kg-1 mixtures of dioleyldimethylammonium chloride and polyoxyethylene(6) nonylphenyl ether-cyclohexane system. Wo = 3 [dioleyldimethylammonium chlorideJ/fAgNOj] = 33.3.

As regards the argentiferous black coppor, it might be dissolved directly In the sulphuric acid, and the silver afterwards precipitated by metallic copper, as is... 

Tin and lead are the most rapid precipitants of metallic silver from the nitrate cadmium, zinc, copper, bismuth, and antimony axe moro slow in their operation, and mercury still more tardy. Chloride of silver is rapidly reduced by most of the metals which form soluble chlorides, such as zinc, iron, cadmium, cobalt, and arsenic. Zinc, copper, and arsenic rapidly reduce the ammoniacal solution of oxide of silver. Of all the metallic precipitants, zinc and cadmium are the most effective but when zinc or antimony aro used, the separated silver contains these metals. 

The action of arsine on silver and mercury salts has attracted much attention owing to the important application to analytical methods for arsenic (p. 319). The action of arsine on a dilute aqueous solution of silver nitrate has long been known to yield metallic silver, arsenious acid and nitric acid.9 With more concentrated solutions the introduction of a few bubbles of arsine produces a deep lemon-yellow coloration, the liquid also acquiring an acid reaction. The coloration disappears after one or two days, silver is precipitated and the colourless solution contains arsenious and arsenic acids.10 If a rapid stream of arsine be passed into a concentrated solution of silver nitrate at 0° C. the whole liquid solidifies to a yellow crystalline mass which rapidly blackens with separation of silver. Lassaigne represented the reaction with the dilute solution by the equation... 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

An electrochemical immunoassay has been developed by Chu et al. [75], based on the precipitation of silver on colloidal gold labels which, after silver metal dissolution in an acidic solution, was indirectly determined by ASV at a glassy carbon electrode. The method was evaluated for a noncompetitive heterogeneous immunoassay of an IgG... 

To a stirred solution of 0.100 g (1.17 x 10 4 mmol) of U(OAr)3 in 25 mL of hexane was added 0.023 g (1.17 x 10 4 mmol) of AgBF4 dissolved in 5mL of THF. An immediate reaction took place, resulting in a color change from brown to a black suspension, with the formation of a precipitate of silver metal. The solution was stirred for ca. 5 min and filtered through Celite, yielding a yellow solution. After the filtrate was dried under reduced pressure, the residue was redissolved in a minimum volume of hexane and the resulting solution cooled to —40°C. After 24 hr, the product was isolated by filtration in 43% yield (0.044 g). 

Tollens test. Most aldehydes reduce Tollens reagent (ammonia and silver nitrate) to give a precipitate of silver metal. The free silver forms a silver mirror on the sides of the test tube. (This test is sometimes referred to as the silver mirror test.) The aldehyde is oxidized to a carboxylic acid. 

To begin, we need Horn Silver which occurs as the mineral Cerargyrite, consisting mainly of silver chloride. Pure silver metal dissolved into nitric acid then precipitated with a solution of sea salt will also work for this experiment. Wash the precipitate (which is silver chloride) with water then dry it. Next, mix it with three times its weight of Sal Ammoniac and grind it well together. Now gently sublime the powder and collect all that will come over. 

The fact that such a dissociation takes place can be proved easily by experiments. Silver ions (which are the product of this dissociation) can be precipitated by hydrogen sulphide gas as silver sulphide Ag2S, and also silver metal can be deposited on the cathode from such a solution by electrolysis (note that the dicyanoargentate ion, with its negative charge moves towards the anode when electrolysed). By applying the law of mass action to the dissociation mentioned above, we can express the dissociation constant or instability constant as... 

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