Spot Reaction Experiments Part X :
Topochemical Reactions FRITZ FEIGL
Laboratorio da Produ$& Mineral, Ministerio du Agricultura, Rio de Janeiro, Brazil
(Translated by Ralph E. Oesper, Uniuersity of Cincinnatz]
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F THE formation of materials involved nothing be- free by removing the crystallization retarders by yond certain cbemical reactions, then every chemical means of activated charcoal or other adsorbents. The production of nuclei, and likewise the growth of change that runs its course according to its stoichiometric formulation would always give rise to products crystals, proceed a t definite rates that are dependent of the same form-species, color, solubility, and the like. on the nature of the materials, the degree of snperEvery experienced chemist knows that this .does not saturation of the solution, pH, the nature of the cohappen in all cases, and he clearly recognizes the fact solutes, etc. It is obvious that the formation of nuclei that a chemical equation, even though it presents the has great influence in the production of a new solid most important part of the chemical event, nonetheless phase and the ease of detecting its presence. Furthercannot give a total picture of what happens when a more, it is clear that the size of the individual crystals material is formed. For example, variation of such is determined by the relation of the rate a t which conditions as concentration, temperature, pH, the nuclei are formed to the speed of growth. If nuclei order of adding the reactants, the presence of accom- are produced slowly, while the crystals grow rapidly, panying materials, etc., not only affect the initiation a few large crystals will result rather than numerous and completeness of a precipitation, but, a t times, small ones. Under reversed .conditions, the tendency cause the material to appear in different forms and grain will be to protluce many tiny crystals. If similar consizes, or even with different colors or solubilities. siderations hold for the production and growth of Furthermore, unexpected and unpredictable side reac- amorphous solids, or in the formation of solids by the tions may ensue, and to varying extents. It is clear decomposition of gaseous reactants, a determining then that besides a main chemical reaction, supple- influence of the conditions or the reaction environment mentary cbemical and physical factors play a signs- will be exhibited in these cases also. The production of cant part. Consequently, the production of a material nuclei and the rate of growth will be affected, and conby chemical means should be wiezued as the end result of sequently also the form of the solid reaction products. the combined actions of all the chemical and physical The efFect extends also to color and solnbility, since relationships that obtain during the entire process. A these properties are often linked with the particular special branch of chemistry deals with the dependence form of the material. An important environmental of specific forms on definite chemical-physical condi- factor in the chemical production of materials is that the tions. V. Kohlschnetter (1874-1939), the founder of production of nuclei, which is the primary process in this field of study, coined for it the apt terms "chemistry the formation of a solid phase from.molecular disperof aggregation forms" and "genetic formation of sions, is often affected by substrates on which the nuclei can settle. A well-known instance is the breakmaterials." ing down of supersaturated solutions by invisible dust The formation of solid products by reaction of dissolved materials is a condensation process, proceeding particles. If, therefore, a reaction theater is charged from molecular dispersions, since the molecule is the with an indifferentsolid on which nuclei are formed more smallest independent unit of a compound. The pro- rapidly, as soon as the solubility product of the desired duction of a new solid phase, whose forerunners are material is exceeded, a "local condensate" will be oboften colloidally dispersed, is governed by the pre- tained, either exclusively, or along with the normal liminary formation of nuclei. These consist af invisible "space condensate." Cases are known in which the molecular aggregates. The growth to v~sibleforms form-species and other properties are quite different in occurs only on such nuclei. Such nuclear growth is the two condensates. On the other hand, conditions called crystal development when crystalline products sometimes obtain in the formation of local condensates, are obtained. Crystal growth is now believed not to be which are lacking in the production of space condena single uniform process. The first step is the ad- sates. This occurs if the solid phase introduced into sorption of dissolved molecules on the existing crystal the reaction theater is per se not homogeneous. It faces, and the adsorbate then takes its proper place may have spots containing other materials (impurities) in the crystal lattice. Consequently, if the nuclei ad- or there may be local structural differences (surface sorb foreign solutes in place of their own molecular defects, crystal edges, etc.). Such factors have a species, the growth of the crystals may be retarded, or specific effect on the formation of nuclei. These ineven entirely prevented, or the product may not have fluences may be due to altered adsorption ability of the its usual crystal form. I t is common practice in pre- whole surface, to local differences in adsorbability, parative chemistry to keep the surfaces of the nuclei or to solnbility and reactivity of or a t particular areas 34.2
of the solid phase. Inhomogeneities and structural differences, that play a role here, are vanishingly few in comparison with the myriads of molecules uniformly distributed throughout the reaction space. Consequently, lacking direct contact, they have no effect on these molecules. On the other hand, their number is not negligible in comparison with the molecules in their immediate neighborhood. Furthermore, the extraneous solid phase introduced into a reaction space may be a precipitate produced there. Although this material may apparently be indifferent, the principle of inhomogeneity will also apply to its surface. Accordingly, in the limited area of a phase change, processes are sometimespossible which can be significant in determining the composition, form-species, and other properties of a local condensate. Reaction conditions in a limited space, which differ from those in a homogeneous extensive volume, are also encountered when a solid functions as a direct reactant. This situation may arise in two ways: (1) the reaction takes place directly on the surface involving molecules or atoms that are still part of the lattice; (2) dissolved constituents of the solid may react quite near their parent substance. In both cases, the resulting products may differ somewhat in the rate a t which they are produced, also in form and properties, from materials of the same composition but obtained by other procedures. When considering the conditions under which a chemical reaction takes place, it is therefore necessary to remember that things can happen a t interfaces or a t certain places in a reaction theater which can be of considerable importance to the chemistry of the aggregation form or to the genesis of the product. I t is a logical generalization of one of Kohlschuetter's terms to designate certain changes as topochemical reactions (GI. solror = place). These chemical changes, thermal decompositions, or electrolytic depositions are characterized by the fact that certain parts of a reaction theater exert a definite influence in bringing about the chemical change, and also affect the localization, form, and other properties of the resulting reaction products. In an extended sense, topochemical reactions include the fact that the crystal form deposited from a saturated solution may depend on the particular substratum used. Topochemical reactions can be classified into the following types: 1. Insoluble reaction products remain at the site of their formation. 2. Insoluble reaction products deposit at preferential areas of the reaction space. 3. Insoluble, soluble, or gaseous reaction products are formed at preferred localities and then leave their place of origin. 4. Dependence of the crystal form of a compound on the crystalline substratum.
Almost all branches of chemistry furnish instances of topochemical reactions. The formation of inorganic compounds is an especially fruitful field. Typical examples are the preparation of certain anhydrous metal chlorides (CrC13, FeC13, A1C13, etc.) and the formation of iron selenide. These chlorides cannot be
obtained by desiccating the corresponding hydrates; but are formed by passing chlorine over the heated metal. Similarly, ferrous selenide is best prepared by heating the metal in selenium vapor. Reactions involving solids, and likewise sintering processes, if not purely topochemical, doubtless involve such reactions as integral steps. Numerous examples can be drawn from analytical procedures. For example, the familiar Marsh test for arsenic, which includes the thermal decomposition of arsenic hydrides is a topochemical reaction. In this procedure black arsenic forms, rather than the gray variety obtained by reduction of arsenic solutions by stannous chloride, etc. Without exception, spot tests carried out on filter paper impregnated with difficultly soluble reagents are topochemical. The deposition of the reaction product is localized. As a consequence, the sensitivity of such tests is much greater than that obtainable with the same reactions carried out with dissolved reagents in test tubes, or as spot reactions on a nonporous surface. The protective layer effects, described in Part IV of this series,' which have certain applications in analysis, are topochemical. All electrolytic separations used for analytical purposes belong in this category. Many of the expedients employed to obtain adherent deposits are designed to secure the most advantageous conditions for obtaining the best form of deposit. Certain topochemical reactions, such as etching of sections and the taking of imprints, are used in metallography to determine structures and detect inhomogeneities. Topochemical processes that lead to corrosion or to making metals passive are still more significant. For instance, the use of lead and iron vessels in chemical industry is primarily possible only because these metals can be rendered passive. Topochemical reactions are the basis of photography. The production of silver nuclei when films or plates are exposed, likewise other processes such as intensification, sensitizing, and fixing are localized reactions. The importanre of topochernical reactions is particularlv great in chctnical technolog).. Both long-standing and quite recent processes belong in this category. Plating of metals, galvanizing, bronzing of iron and brass, hardening of cement,, mordant dyeing of textiles, sizing of paper, manufacture of white lead, production of good adsorbent alumina, activation of bleaching earths, are pertinent examples. When solid catalysts are used to bring about the reaction of gases, the actual catalytic effect often occurs on quite definite areas of the solid. Heterogeneous catalyses have found extensive industrial applications. The most striking proof of the topochemical nature of such effects is the enhancement of the catalytic action when mixed catalysts are used, or when the catalysts are pretreated. Further evidence is the ease with which surface catalysts are inactivated (poisoned). Petrology and mineralogy exhibit the results of topochemical processes that have proceeded on a gigantic FEIGL, F., THIS JOURNAL. 20,298 (1943)
scale and over tremendous periods of time. Instances are: formation of beds or deposits, new formations of minerals and pseudomorphoses (i.e., iron oxide to pyrites and marcasite). Finally, topochemical reactions also occur in living matter, where sometimes they are of great importance. The processes of assimilation and metabolism, the development of pathological deposits, and the deposition of the contents of plants, which often is highly localized, are cases in point. Many processes that occur in living cells are reactions within limited theaters, where topochemical factors can be of great moment. This paper was written to introduce this comparatively little known branch of chemistry to a larger circle of chemists, particularly students and their teachers. Several characteristic topochemical preparations that can be carried out as spot reaction experiments will he described and discussed. 53. Topochemical Detection of the Volatzlzzation of Mercury at Room Temperature. Mercury freezes a t -3g°C. and boils a t 357% At room t e m p e r a t u r e i. e., about 330 degrees below the boiling point-its vapor pressure is only 0.001 mm. It has long been known that mercury and its salts have a decided physiological action. Some years ago heated discussions were held as to whether prolonged exposure to minute amounts of mercury vapor is harmful. There is no doubt that metallic mercury continuously evolves its vapor, and consequently there is a t least potential danger in chemical or physical laboratories, where drops of mercnry are very likely to be spilled and lie hidden in cracks in the benches and flooring. Accordingly, a demonstration of this source of danger is of interest to those who work in such laboratories. The evaporation of mercury a t room temperatures can be revealed by a spot reaction experiment. This demonstration provides an excellent example of a topochemical reaction. It also illustrates other points quite nicely. This test is also applicable to amalgams, and, if modified, to organic rnercu~ials.~It depends on the action of free mercury on palladous chloride. The reaction : PdCL HgQ-+ Pdo HgC12, occurs almost instantaneously. The finely divided palladium is easily seen, and appears either gray or black, depending on the quantity set free. The reaction occurs if a drop of mercnry is merely allowed to roll over a strip of palladium chloride paper. The sensitivity is much higher if the mercnry reacts as vapor. Procedure: Filter paper is soaked with 1 per cent palladium chloride solution and then dried. A droplet of mercury is placed in a porcelain crucible, which is then covered with a sheet of the reagent paper, that is weighted down with a watch glass. At 30°C., a bright gray circle (outline of the upper edge of the crucible) appears within five minutes. The color gradually darkens, and becomes deep black in one twtwo hours. If the covered crucible is left overnight in a refrigerator, a dark gray ring will form by morning. At O°C. FEIGL,F., "Spot Tests," 3rd Edition. Elsevier Publishing
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the vapor pressure of metallic mercury is 0.0002 mm. Palladium chloride paper is yellow-brown, and consequently small quantities of reduced palladium do not show up as distinctly as they would against a white background. This greater contrast can be secured by holding the paper, after exposure to mercury vapor, over an open ammonia bottle. The color of the paper is discharged almost a t once, due to the formation of colorless Pd(NH&Cll, and the stain becomes more perceptible. If the original reaction was definitely positive, this intensification is superfluous. This test demonstrates a typical topochemical effect, since the finely divided palladium remains a t the site of its formation. The reaction is more rapid if the paper is dry rather than moist. This remarkable fact is a striking example of the reaction of a solid without intervention of a ~ o l v e n t . ~ In this test, the palladium does not deposit immediately and the color constantly deepens from light gray to black. This interval, in which there is no visible separation of palladium, signifies that the threshold of discernihility must be reached. During this period the mercury is constantly evaporating and eventually its concentration in the air space of the crucible reaches a value a t which it will affect the palladium chloride. As soon as this reaction begins, the evaporation equilibrium Hg F? Hg is disturbed and the vaporiza(Liquid)
(vapor)
tion is resumed. The continuous vaporization of the mercury can thus be followed by the aid of this topochemical test. The reagent paper serves as a trace catcher for mercury vapors. The test is a good example of an analytical application of the continuous disturbing of an equilibrium. Many tests for dissolved materials utilize this same principle. However, in ionic reactions the consumption of the reacting ion and its subsequent replenishment from a source in equilibrium are so rapid that it is not possible, a s in the present experiment, clearly to differentiate the effect of the test from that of the replenishment. The gradual deepening of the color of the palladium ring on the paper demonstrates also that after the initial coating of the chloride by the liberated metal the mercury vapor is able to diffuse through inhomogeneities and pores of the coating. This process is important in many topochemical processes and in the reactions of solids. 54. Charring of Carbohydrates by Adsorbed Aqueous Hydrochloric Acid. Many organic materials are charred by concentrated sulfuric acid. This familiar effect is due to the extraordinary dehydrating power of the acid. The elements of water are abstracted from the organic substance leaving free ~ a r b o n . Analo~ gously, high-boiling hygroscopic phosphoric acid also destroys paper and carbohydrates. The strange fact that paper moistened with hydrochloric acid is partially charred on beating has apparently not been included in a
FEIGL, F., L. I. MIRANDA, AND H. A. SUTER, THISJOURNAL,
21, 19 (1944).
' A test for free sulfuric acid is based on this action. See FEIGL. F.. 09. ~ i l .
the literature. At first sight, the action seems illogical. Aqueous solutions of hydrochloric acid when heated give off, depending on tpe concentration, either water or hydrogen chloride gas. This loss continues until an azeotropic mixture is attained. This contains 20.24 per cent HCl and boils constantly and completely a t lll°C. At this temperature, acid of this concentration does not char cellulose or similar materials. None the less, if a drop of either the dilute or concentrated acid is placed on filter paper, and the paper is then kept on a hot plate or in an oven (llO°C.) for a short time the acidified area chars definitely. This localized+. e., topochemical-chamng can be explained. Aqueous hydrochloric acid is adsorbed by paper; the binding probably is through the C1 atom. Adsorbed hydrochloric acid is not completely volatilized (as a constant boilingmixture) a t lll°C., but gives off water, which is replaced immediately by dehydrating the adsorbent. Accordingly hydrochloric acid not only effectuates chamng, but it remains on paper, even a t a temperature equal to or possibly above the boiling point of the constant boiling acid. Procedure: A piece of filter paper and a piece of cellophane are spotted with 5 N hydrochloric acid. The specimens are kept a t llO°C. for 10 minutes in an oven. The filter paper will be distinctly charred. In contrast, the cellophane, which does not adsorb hydrochloric acid, will be unaffected. A strip of moist Congo paper is then pressed against the specimens. The indicator is not changed by the cellophane, because the acid boiled away completely, whereas, the charred area on the paper turns the Congo paper deep blue. Consequently, there is still acid present, despite the fact that the paper was heated to the boiling temperature of the azeotropic HCl-water mixture. 55. Topochemical Effect with Amorphous Silica.= The red-brown, water-insoluble silver chromate is easily soluble in an excess of ammonia water, forming water-soluble silver ammine chromate [Ag(NH3)alaCrOa. The solution of this salt is yellow because it contains the ionsAg(NH3)s+and Cr04=.Solutionsof this ammine salt present the equilibrium: 2 Ag(NHa)z+ CrO4 2Ag+ 4NHs CrOy. The concentration of free Ag+ and Cr04= ions in this equilibrium is smaller than that corresponding to the solubility product of silver chromate. The formation of silver chromate is masked by the presence of ammonia. The equilibrium of themasked reaction is disturbed by the addition of all materials which consume ammonia. De-masking occurs and red-brown silver chromate precipitates. The silver ammine chromate solutions can thus be used to detect soluble or slightly soluble inorganic and organic compounds which exhibit acid characteristics. It is interesting to note that certain forms of silica show a characteristic topochemical behavior toward silver-ammine chromate. Pure silica in its crystalline form (quartz sand, rock crystal, etc.) does not react
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FEIGL, F., 'Zaboratory Manual of Spot Tests." translated by R. E. O~s~~~,Academic Press,New York, 1943, p. 155.
with this reagent, even though the specimen has been finely pulverized. In contrast, amorphous silica, either hydrous or ignited, reacts immediately. Consequently, the de-masking of this reagent furnishes a means of distinguishing between crystalline and amorphous silica. The action of silica on masked silver chromate solution is probably of a different nature than that due to slightly soluble acid compounds. The latter cause a precipitation of silver chromate as the result of a chemical reaction in which the equilibrated ammonia is consumed in the formation of an ammonium salt. The direct formation of an ammonium salt is not involved in the case of amorphous silica, particularly that contained in ignited products. It is much more likely that ammonia is rapidly adsorbed on the surface of the finely divided silica. This adsorption likewise disturbs the equilibrium and results in the precipitation of silver chromate on the surface of the amorphous silica. Consequently, silver chromate precipitates on those areas of the silica surface a t which ammonia is adsorbed. This is a typical topochemical effect, which can obviously be applied to analytical problems. Procedure: A pinch of amorphous silica is placed in a depression of a spot plate and one or two drops of [Ag(NH3)2]aCr04 solution added. If the specimen had been thoroughly washed and dried, red-brown silver chromate precipitates a t once. The same effect, but to a lesser degree, is obtained with amorphous silica that has been ignited. Amorphous silica is prepared by evaporating a mixture of sodium silicate and hydrochloric acid to dryness on a water bath. The residue is thoroughly washed with hot water. Part of this test material should be ignited. Silver-ammine chromate solution is prepared as follows: The precipitate obtained by mixing silver nitrate and potassium chromate solutions is washed well with hot water. It is then shaken with a deficiency of 6 N ammonia water. The suspension is allowed to stand for an hour, and then filtered. The filtrate should be stored in a tightly stoppered container. If the reagent becomes turbid after long standing, i t can be restored to usefulness by filtering. 56. Topochemical Reactions on the Surface of Metallic Silver and Copper. If base metals are exposed to the action of air or certain chemicals, oxides or other compounds are often produced on the surface of the metal. Common examples are the visible "rusting" of iron in the air, and the rapid formation of an invisible film of oxide on aluminum. Such typical topochemical processes also include the "tarnishing" of copper or silver when exposed to hydrogen sulfide. Frequently, these films of oxide or sulfide protect the underlying metal from further attack. Three topochemical reactions on metal surfaces will be discussed here. They are easily demonstrated and have interesting features. A brown deposit is quickly formed when silver foil is spotted with permanganate solution. Heat accelerates the reaction:
6Ag
+ ZKMnO, + HIO
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3 Ag,O
+ 2Mn02 + 2KOH
as the locale of the reaction, and the reaction product was h e d a t the site of its formation. The extent of the The foil, after washing with water, is stained with an free surface of the solid was not a significant factor. adherent mixture of the oxides. Part of the deposit may consist of silver manganite, Ag2Mn03. Silver wire However, this is not a universal rule, because many iuor powder coated in this way is used for pharmaceutical stances are known in which the available free surface purposes because it retains its bactericidal (oligo- of the solid reactant is the deciding factor for both the dynamic) powers much longer than bright silver. The initiation and the extent of topochemical processes. microorganisms cause the evolution of hydrogen sulfide, A striking example is furnished by contrasting the bewhich in turn produces a coating of nonbactericidal havior of magnesite in compact crystals and as a powder. silver sulfide on the metal. This decrease in the oligo- These forms differ decidedly in their behavior toward a dynamic action of silver is prevented by the manganese "nickel dimethylglyoxime equilibrium solution." This reagent consists of the filtrate obtained after dioxide, which consumes the hydrogen sulfide. Procedure: The grease is removed from a strip of the reaction of an aqueous solution of a nickel salt of silver foil by rinsing it with ether and alcohol. The a mineral acid with a 1 per cent alcoholic solution of metal is spotted with N / 5 permanganate solution and dimethylglyoxime DH2). Under these conditions, the warmed for a few minutes in an oven (llO°C.). After precipitation of the red Ni-dimethylglyoxime is incomrinsing with water, a brown stain (Ag20 MnOz) plete, since this salt is soluble in acid solutions. The will be found on the spotted area. The coating will per- filtrate from the partial precipitation thus presents the equilibrium : sist eveu if the foil is wiped with a wet or dry cloth. Another topochemical effect, whose chemistry is Niif+ + 2DH2 F? Ni(DHh . ,. +. 2H+ quite interesting, is observed if copper (or bronze) If H+ ions are removed from this saturated equifoil is spotted with a solution of selenious acid or, better, with acidified sodium selenite solution. A brown-black librium solution, an equivalent precipitate of Ni-dito black stain appears in a few minutes. This reaction methylglyoxime results. This reagent can thus he has heen known for a long time and has served as a test used to reveal all materials that are "basic," using the for selenite. The reaction can be explained as follows: term in its widest sense. I t acts toward them as an Metallic copper does not dissolve in nonoxidiziug acids irreversible indicator.= Magnesite is not soluble in water. It consumes H + because the equilibrium Cuo 2H+ + Cu++ 2H0 ions (MgCOa 2H+ + Mg++ COz H20) and lies almost entirely to the left. If Se08- ions are present they are reduced to free selenium by the accordingly gives a positive test with Ni-dimethylnascent hydrogen: SeOz 4H0 -+Seo 2H20. Part glyoxime equilibrium solution. The reaction occurs of the liberated Selenium immediately forms copper with practical speed, however, only if the specimen is selenide. Accordingly, if these successive steps are powdered. Small crystals, or the rock in compact form, added (any production of Cu+ is not taken into ac- react so slowly that no discernible red precipitate is count), the topochemical reaction on the surface of the obtained in 24 hours. In other words, the free surface determines the occurrence of this topochemical reaction. copper can he written: Procedure: A crystal of magnesite is placed in a depression of a ~ ~ o t - ~ l a A t epinch . df the ground ma3Cu Se0.6Ht CuSe 2Cut+ + 3Hg0 terial is put into an adjacent depression. Several drops An adherent dark stain of cupric selenide will be of equilibrium solution are placed on the test portions. formed by eveu small quantities of selenium. The powder turns pink within a few minutes, and soon Procedure: A piece of copper foil is scrupulously is definitely red. The compact specimen remains uncleaned. It is spotted with 1 per cent and 0.1 per cent affected even after several hours. If, however, the solutions of sodium selenite. On standing, a dark crystal is scratched with a needle, and replaced in the spot forms. This is particularly adherent if quite dilute reagent, a red streak quickly develops along the inselenite solutions are applied to the foil. jured surface. A tarnishing reaction+. e., a topochemical reaction Preparation of the equilibrium solution: 2.3 g. of between a solid and a gaseous reactant--can be nicely NiSOa.7Hz0,dissolved in 300 ml. of water, are treated illustrated. A concentrated solution of seleuious acid, with 2.8 g. dimethylglyoxime, dissolved in 300 ml. of or. solid Se02,is placed on silver foil. The specimen is alcohol. The suspension is filtered after 30 minutes. then placed in a drying oven (110" to 120°C.). A black, The clear filtrate is ready for use. It will keep for adherent spot of Ag8e develops only on the treated several weeks if stored in stoppered vessels. area. The reaction can he explained easily. Since 58. A Temperature-dependent Topochemical Effect SeOz sublimes at 315"C., it has an appreciable vapor &th Calcite. The Ni-dimethylglyoxime equilibrium pressure a t the temperature used in this demonstration. solution can he applied to demonstrate an interesting The reaction, 2Ag0 SeOa + AgzSe O2takes place, topochemical effect on the surface of calcite crystals. and the silver selenide adheres to the foil. Since this material is basic in the sense that it con57. A Topochemical Reaction of Magnesite that De- sumes H + ions it will give a red precipitate with the pends on its Free Surface. The preceding examples of ' FEIGL.F.,AND C. P. J. DA SILVA,Ind. Eng. Chem.. Anal. Ed., topochemical reactions all involved a solid which served 14. 376 (1942).
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347 59. Topochemical Detection of Zinc i n Zinc Oxide. Tech. nical zinc oxide, which is usually manufactured by burning the metal in air, often contains small amounts of unoxidized zinc. This contaminant can be detected with the aid of phosphomolybdic acid. The metal dissolves with production of hydrogen, which reduces the sexivalent molybdenum to lower oxides. These are deep blue and the product is called "molybdenum blue." Under the conditions prescribed here, the resulting colloidally dispersed molybdenum blue remains, a t first, a t the site of its formation. Later, it gradually diffuses throughout the whole solution that covers the unaltered zinc oxide surface. Consequently, this is an instance in which the product is formed topochemically and then leaves its birthplace. Traces of metal in metal oxides can be detected by applying this reaction. Procedure: A pinch of technical zinc oxide is placed in a depression of a spot plate. Two or three drops of 5 per cent water solution of phosphomolybdic acid are added, without stirring. Any invisible traces of metallic zinc develop deep blue points. Some time Calcium sulfate is formed in the second reaction and, as later, these inhomogeneities disappear, and the whole is well known, it is less soluble in warm water than in supernatant solution becomes light blue or green. For the cold. (The alcohol may accentuate this differ- comparison, the test should be repeated on a specimen ence.) The calcium carbonate surface seems to exert of zinc oxide prepared in the wet way, or by ignition of a considerable influence both on the rate a t which zinc nitrate. nuclei are formed, and also on the crystal growth of 60. Nuclear Effects i n the Reduction of Silver Salts. calcium sulfate. Probably, calcium sulfate is adsorbed The significance of nuclear processes in the development by calcium carbonate, and perhaps even is incorporated of a new, solid phase was pointed out in the introductory in its lattice. The consequence is that the calcite portion of this paper. A solid phase grows only on the surface is coated with a film of calcium sulfate, and this nuclei which are preformed. Consequently, the rate at protective layer prevents the penetration of H+ ions, which a solid phase deposits from a supersaturated sohand so prevents a continuation of the reaction that tion and the visibility of the precipitate are determined produces Ni-dimethylglyoxime. The nonoccurrence of by the rate a t which nuclei are produced and the speed this reaction a t higher temperatures shows that even with which they grow. Co-solutes may influence both traces of calcium sulfate are sufficient to provide a of these factors, and to an appreciable extent. Someperfectly coherent, impermeable coating. It is possible times they accelerate, but mostly they retard these to approximate the quantities of calcium sulfate neces- processes. In addition, the speed of precipitation may sary to form this protective layer if account is taken of be increased if solids are present, on which the nuclei the fact that the presentation of even minute quantities deposit. The activity of nuclei and a topochemical inof OH- ions is followed by precipitation of Ni-di- fluence of solid materials on the formation of nuclei can methylglyoxime. Consequently, the quantities of be demonstrated. The reduction of silver salts serves calcium sulfate must be less than the equivalent quan- admirably. tity of OH- ions that can be revealed by the equilibrium It has long been established that silver nuclei can speed up the action of slow reductants on both soluble solution. Procedure: A small crystal of calcite is treated on a and insoluble silver salts. The development of a spot plate with several drops of equilibrium solution "latent image" on an exposed silver halide Glm or plate prepared from nickel chloride. A similar specimen, in is based on this observation. The exposed areasan adjacent depression, is treated with the correspond- i. e., the silver nuclei contained in the latent imageing reagent prepared from nickel sulfate. Both crystals accelerate the reduction, by the developer, of the adshow a red tint in a few minutes. Several milliliters jacent unchanged silver halide. The acceleration is of the two reagents are placed in separate small test proportional to the number of nuclei present. These tubes, brought to boiling, and kept hot in boiling water. can be produced by insolating silver bromide that has A small, well-developed crystal of calcite is added to been formed on Glter paper by a spot reaction. If the each. The reagent containing C1- ions immediately de- spot, which contains invisible silver nuclei, is then posits red Ni-dimethylglyoxime, while the other, which placed in a solution that contains silver ions and a slow contains SO,- ions, remains unchanged, even after a reducing agent, the reduction occurs more quickly where nuclei are present than in other areas. long time.
equilibrium solution. In contrast with magneske, the reaction succeeds even with compact crystals or fragments of crystals, and a t room temperature. The surface of the crystals becomes red. However, if an equilibrium solution is prepared from nickel chloride and another from nickel sulfate, their reactions with calcite in the warm and cold are not identical. Both give a red deposit a t room temperature. With the nickel chloride solution the reaction is faster a t higher temperatures, an effect that is in line with expectation. In contrast, calcite shows no apparent reaction with hot nickel sulfate equilibrium solution, and the crystal appears to be totally unaffected. The basis of this diverse behavior is the presence of SO4- ions. The reactions can he written:
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(Continued on page 353)
SPOT REACTION EXPERIMENTS Velculescu7states that traces of silver (0.005 y Ag) can be detected by the following procedure. The precipitation of metallic silver can also be hastened from a mixture of a silver salt and a slow reductant, if silver ions are adsorbed locally. This effect is also illustrated in this demonstration experiment. Procedure: A drop of 1 per cent potassium bromide solution is placed on filter paper and treated with a drop of 0.02 N silver nitrate solution. The paper is b r i d y exposed and then thoroughly washed by being. placed in water. It is then bathed in a developing solution. A black stain of precipitated silver appears in a few minutes at the spot where silver bromide was formed, and where silver nuclei were produced by the action of the light.
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' VELCULESCU,A. J.. Z. anal. Chem., 90,111 (1930).
(Continued from page 347)
The developer contains 2 g. of pyrogallol and 2 g. of citric acid dissolved in 500 ml. of water. Just before using, 60 ml. are mixed with 2 ml. of N/10 silver nitrate. Two adjacent depressions of a spot plate are each charged with two drops of developer. The bottom of one depression is scratched with a glass rod. Within 30 seconds, silver will deposit, as a black stain, along the scratch. The comparison solution will remain clear for about six minutes, and then undergo a gradual reduction with resultant precipitation of colloidal silver. The reason for these totally different reduction patterns is that the glass or porcelain dust a t the scratched surface adsorbs Ag+ ions. These are thus concentrated locally, leading to a more rapid production of silver, which, in its turn, functions as the nucleus for the further reduction of the silver salt.
