SILVER

The iodides of mercury.If a solution of potassium iodide is added to solutions of a mercurous and a mercuric salt respectively, the corresponding iodides are precipitated. Mercuric iodide is the more important of the two, and as prepared above is a red powder which changes to yellow on heating to 150°. The yellow form on cooling changes back again to the red form, or may be made to do so by rubbing it with a knife blade or some other hard object.

Occurrence.Silver is found in small quantities in the uncombined state; usually, however, it occurs in combination with sulphur, either as the sulphide (Ag2S) or as a small constituent of other sulphides, especially those of lead and copper. It is also found alloyed with gold.

Metallurgy.Parkes's process.Silver is usually smelted in connection with lead. The ores are worked over together, as described under lead, and the lead and silver obtained as an alloy, the silver being present in small quantity. The alloy is melted and metallic zinc is stirred in. Zinc will alloy with silver but not with lead, and it is found that the silver leaves the lead and, in the form of an alloy with zinc, forms as a crust upon the lead and is skimmed off. This crust, which, of course, contains lead adhering to it, is partially melted and the most of the lead drained off. The zinc is removed by distillation, and the residue is melted on an open hearth in a current of air; by this means the zinc and lead remaining with the silver are changed into oxides and the silver remains behind unaltered.

Amalgamation process.In some localities the old amalgamation process is used. The silver ore is treated with common salt and ferrous compounds, which process converts the silver first into chloride and then into metallic silver. Mercury is then added and thoroughly mixed with the mass, forming an amalgam with the silver. Aftersome days the earthy materials are washed away and the heavier amalgam is recovered. The mercury is distilled off and the silver left in impure form.

Refining silver.The silver obtained by either of the above processes may still contain copper, gold, and iron, and is refined by "parting" with sulphuric acid. The metal is heated with strong sulphuric acid which dissolves the silver, copper, and iron present, but not the gold. In the solution of silver sulphate so obtained copper plates are suspended, upon which the pure silver precipitates, the copper going into solution as sulphate, as shown in the equation

Ag2SO4+ Cu = 2Ag + CuSO4.

The solution obtained as a by-product in this process furnishes most of the blue vitriol of commerce. Silver is also refined by electrolytic methods similar to those used in refining copper.

Properties of silver.Silver is a heavy, rather soft, white metal, very ductile and malleable and capable of taking a high polish. It surpasses all other metals as a conductor of heat and electricity, but is too costly to find extensive use for such purposes. It melts at a little lower temperature than copper (961°). It alloys readily with other heavy metals, and when it is to be used for coinage a small amount of copper—from 8 to 10%—is nearly always melted with it to give it hardness.

It is not acted upon by water or air, but is quickly tarnished when in contact with sulphur compounds, turning quite black in time. Hydrochloric acid and fused alkalis do not act upon it, but nitric acid and hot, concentrated sulphuric acid dissolve it with ease.

Fig. 88Fig. 88

Electroplating.Since silver is not acted upon by water or air, and has a pleasing appearance, it is used to coat various articles made of cheaper metals. Such articles are said to be silver plated. The process by which this is done is called electroplating. It is carried on as follows: The object to be plated (such as a spoon) is attached to a wire and dipped into a solution of a silver salt. Electrical connection is made in such a way that the article to be plated serves as the cathode, while the anode is made up of one or more plates of silver (Fig. 88,A). When a current is passed through the electrolyte silver dissolves from the anode plate and deposits on the cathode in the form of a closely adhering layer. By making the proper change in the electrolyte and anode plate objects may be plated with gold and other metals.

Compounds of silver.Silver forms two oxides but only one series of salts, namely, the one which corresponds to the mercurous and cuprous series.

Silver nitrate(lunar caustic) (AgNO3). This salt is easily prepared by dissolving silver in nitric acid and evaporating the resulting solution. It crystallizes in flat plates, and when heated carefully can be melted without decomposition. When cast into sticks it is called lunar caustic, for it has a very corrosive action on flesh, and is sometimes used in surgery to burn away abnormal growths.

The alchemists designated the metals by the names of the heavenly bodies. The moon (luna) was the symbol for silver; hence the name "lunar caustic."

Silver sulphide(Ag2S). This occurs in nature and constitutes one of the principal ores of silver. It can beobtained in the form of a black solid by passing hydrosulphuric acid through a solution of silver nitrate.

Compounds of silver with the halogens.The chloride, bromide, and iodide of silver are insoluble in water and acids, and are therefore precipitated by bringing together a soluble halogen salt with silver nitrate:

AgNO3+ KCl = AgCl + KNO3.

They are remarkable for the fact that they are very sensitive to the action of light, undergoing a change of color and chemical composition when exposed to sunlight, especially if in contact with organic matter such as gelatin.

Photography.The art of photography is based on the fact that the halogen compounds of silver are affected by the light, particularly in the presence of organic matter. From a chemical standpoint the processes involved may be described under two heads: (1) the preparation of the negative; (2) the preparation of the print.1.Preparation of the negative.The plate used in the preparation of the negative is made by spreading a thin layer of gelatin, in which silver bromide is suspended (silver iodide is sometimes added also), over a glass plate or celluloid film and allowing it to dry. When the plate so prepared is placed in a camera and the image of some object is focused upon it, the silver salt undergoes a change which is proportional at each point to the intensity of the light falling upon it. In this way an image of the object photographed is produced upon the plate, which is, however, invisible and is therefore called "latent." It can be made visible by the process of developing.To develop the image the exposed plate is immersed in a solution of some reducing agent called the developer. The developer reduces that portion of the silver salt which has been affected by the light, depositing it in the form of black metallic silver which closely adheres to the plate.The unaffected silver salt, upon which the developer has no action, must now be removed from the plate. This is done by immersing the plate in a solution of sodium thiosulphate (hypo). After the silver salt has been dissolved off, the plate is washed with water anddried. The plate so prepared is called the negative because it is a picture of the object photographed, with the lights exactly reversed. This is called fixing the negative.2.Preparation of the print.The print is made from paper which is prepared in the same way as the negative plate. The negative is placed upon this paper and exposed to the light in such a way that the light must pass through the negative before striking the paper. If the paper is coated with silver chloride, a visible image is produced, in which case a developer is not needed. The proofs are made in this way. In order to make them permanent the unchanged silver chloride must be dissolved off with sodium thiosulphate. The print is then toned by dipping it into a solution of gold or platinum salts. The silver on the print passes into solution, while the gold or platinum takes its place. These metals give a characteristic color or tone to the print, the gold making it reddish brown, while the platinum gives it a steel-gray tone. If a silver bromide paper is used in making the print, a latent image is produced which must be developed as in the case of the negative itself. The silver bromide is much more sensitive than the chloride, so that the printing can be done in artificial light. Since the darkest places on the negative cut off the most light, it is evident that the lights of the print will be the reverse of those of the negative, and will therefore correspond to those of the object photographed. The print is therefore called the positive.

1.Preparation of the negative.The plate used in the preparation of the negative is made by spreading a thin layer of gelatin, in which silver bromide is suspended (silver iodide is sometimes added also), over a glass plate or celluloid film and allowing it to dry. When the plate so prepared is placed in a camera and the image of some object is focused upon it, the silver salt undergoes a change which is proportional at each point to the intensity of the light falling upon it. In this way an image of the object photographed is produced upon the plate, which is, however, invisible and is therefore called "latent." It can be made visible by the process of developing.

To develop the image the exposed plate is immersed in a solution of some reducing agent called the developer. The developer reduces that portion of the silver salt which has been affected by the light, depositing it in the form of black metallic silver which closely adheres to the plate.

The unaffected silver salt, upon which the developer has no action, must now be removed from the plate. This is done by immersing the plate in a solution of sodium thiosulphate (hypo). After the silver salt has been dissolved off, the plate is washed with water anddried. The plate so prepared is called the negative because it is a picture of the object photographed, with the lights exactly reversed. This is called fixing the negative.

2.Preparation of the print.The print is made from paper which is prepared in the same way as the negative plate. The negative is placed upon this paper and exposed to the light in such a way that the light must pass through the negative before striking the paper. If the paper is coated with silver chloride, a visible image is produced, in which case a developer is not needed. The proofs are made in this way. In order to make them permanent the unchanged silver chloride must be dissolved off with sodium thiosulphate. The print is then toned by dipping it into a solution of gold or platinum salts. The silver on the print passes into solution, while the gold or platinum takes its place. These metals give a characteristic color or tone to the print, the gold making it reddish brown, while the platinum gives it a steel-gray tone. If a silver bromide paper is used in making the print, a latent image is produced which must be developed as in the case of the negative itself. The silver bromide is much more sensitive than the chloride, so that the printing can be done in artificial light. Since the darkest places on the negative cut off the most light, it is evident that the lights of the print will be the reverse of those of the negative, and will therefore correspond to those of the object photographed. The print is therefore called the positive.

1.Account for the fact that copper has been used for so long a time.

2.Write equations for the action of concentrated sulphuric and nitric acids upon the metals of this family.

3.How would you account for the fact that normal copper sulphate is slightly acid to litmus?

4.Contrast the action of heat on cupric nitrate and mercuric nitrate.

5.State reasons why mercury is adapted for use in thermometers and barometers.

6.How could you distinguish between mercurous chloride and mercuric chloride?

7.Write equations for the preparation of mercuric and mercurous iodides.

8.How would you account for the fact that solutions of the different salts of a metal usually have the same color?

9.Crude silver usually contains iron and lead. What would become of these metals in refining by parting with sulphuric acid?

10.In the amalgamation process for extracting silver, how does ferrous chloride convert silver chloride into silver? Write equation. Why is the silver sulphide first changed into silver chloride?

11.What impurities would you expect to find in the copper sulphate prepared from the refining of silver?

12.How could you prepare pure silver chloride from a silver coin?

13.Mercuric nitrate and silver nitrate are both white solids soluble in water. How could you distinguish between them?

14.Account for the fact that sulphur waters turn a silver coin black; also for the fact that a silver spoon is blackened by foods (eggs, for example) containing sulphur.

15.When a solution of silver nitrate is added to a solution of potassium chlorate no precipitate forms. How do you account for the fact that a precipitate of silver chloride is not formed?

SYMBOLATOMIC WEIGHTDENSITYMELTING POINTCOMMON OXIDESTinSn119.07.35235°SnO SnO2LeadPb206.911.38327°PbO Pb3O4PbO2

The family.Tin and lead, together with silicon and germanium, form a family in Group IV of the periodic table. Silicon has been discussed along with the non-metals, while germanium, on account of its rarity, needs only to be mentioned.

The other family of Group IV includes carbon, already described, and a number of rare elements.

Occurrence.Tin is found in nature chiefly as the oxide (SnO2), called cassiterite or tinstone. The most famous mines are those of Cornwall in England, and of the Malay Peninsula and East India Islands; in small amounts tinstone is found in many other localities.

Metallurgy.The metallurgy of tin is very simple. The ore, separated as far as possible from earthy materials, is mixed with carbon and heated in a furnace, the reduction taking place readily. The equation is

SnO2+ C = Sn + CO2.

The metal is often purified by carefully heating it until it is partly melted; the pure tin melts first and can be drained away from the impurities.

Properties.Pure tin, called block tin, is a soft white metal with a silver-like appearance and luster; it melts readily (235°) and is somewhat lighter than copper, having a density of 7.3. It is quite malleable and can be rolled out into very thin sheets, forming tin foil; most tin foil, however, contains a good deal of lead.

Under ordinary conditions it is quite unchanged by air or moisture, but at a high temperature it burns in air, forming the oxide SnO2. Dilute acids have no effect upon it, but concentrated acids attack it readily. Concentrated hydrochloric acid changes it into the chloride

Sn + 2HCl = SnCl2+ 2H.

With sulphuric acid tin sulphate and sulphur dioxide are formed:

Sn + 2H2SO4= SnSO4+ SO2+ 2H2O

Concentrated nitric acid oxidizes it, forming a white insoluble compound of the formula H2SnO3, called metastannic acid:

3Sn + 4HNO3+ H2O = 3H2SnO3+ 4NO.

Uses of tin.A great deal of tin is made into tin plate by dipping thin steel sheets into the melted metal. Owing to the way in which tin resists the action of air and dilute acids, tin plate is used in many ways, such as in roofing, and in the manufacture of tin cans, cooking vessels, and similar articles.

Many useful alloys contain tin, some of which have been mentioned in connection with copper. When tin is alloyed with other metals of low melting point, soft, easilymelted alloys are formed which are used for friction bearings in machinery; tin, antimony, lead, and bismuth are the chief constituents of these alloys. Pewter and soft solder are alloys of tin and lead.

Compounds of tin.Tin forms two series of compounds: the stannous, in which the tin is divalent, illustrated in the compounds SnO, SnS, SnCl2; the stannic, in which it is tetravalent as shown in the compounds SnO2, SnS2. There is also an acid, H2SnO3, called stannic acid, which forms a series of salts called stannates. While this acid has the same composition as metastannic acid, the two are quite different in their chemical properties. This difference is probably due to the different arrangement of the atoms in the molecules of the two substances. Only a few compounds of tin need be mentioned.

Stannic oxide(SnO2). Stannic oxide is of interest, since it is the chief compound of tin found in nature. It is sometimes found in good-sized crystals, but as prepared in the laboratory is a white powder. When fused with potassium hydroxide it forms potassium stannate, acting very much like silicon dioxide:

SnO2+ 2KOH = K2SnO3+ H2O.

Chlorides of tin.Stannous chloride is prepared by dissolving tin in concentrated hydrochloric acid and evaporating the solution to crystallization. The crystals which are obtained have the composition SnCl2·2H2O, and are known as tin crystals. By treating a solution of stannous chloride with aqua regia, stannic chloride is formed:

SnCl2+ 2Cl = SnCl4.

The salt which crystallizes from such a solution has the compositionSnCl4·5H2O, and is known commercially as oxymuriate of tin. If metallic tin is heated in a current of dry chlorine, the anhydrous chloride (SnCl4) is obtained as a heavy colorless liquid which fumes strongly on exposure to air.

The ease with which stannous chloride takes up chlorine to form stannic chloride makes it a good reducing agent in many reactions, changing the higher chlorides of metals to lower ones. Thus mercuric chloride is changed into mercurous chloride:

SnCl2+ 2HgCl2= SnCl4+ 2HgCl.

If the stannous chloride is in excess, the reaction may go further, producing metallic mercury:

SnCl2+ 2HgCl = SnCl4+ 2Hg.

Ferric chloride is in like manner reduced to ferrous chloride:

SnCl3+ 2FeCl3= SnCl4+ 2FeCl2.

The chlorides of tin, as well as the alkali stannates, are much used as mordants in dyeing processes. The hydroxides of tin and free stannic acid, which are easily liberated from these compounds, possess in very marked degree the power of fixing dyes upon fibers, as explained under aluminium.

Occurrence.Lead is found in nature chiefly as the sulphide (PbS), called galena; to a much smaller extent it occurs as carbonate, sulphate, chromate, and in a few other forms. Practically all the lead of commerce is made from galena, two general methods of metallurgy being in use.

Metallurgy.1. The sulphide is melted with scrap iron, when iron sulphide and metallic lead are formed; theliquid lead, being the heavier, sinks to the bottom of the vessel and can be drawn off:

PbS + Fe = Pb + FeS.

2. The sulphide is roasted in the air until a part of it has been changed into oxide and sulphate. The air is then shut off and the heating continued, the reactions indicated in the following equations taking place:

2PbO + PbS = 3Pb + SO2,PbSO4+ PbS = 2Pb + 2SO2.

2PbO + PbS = 3Pb + SO2,

PbSO4+ PbS = 2Pb + 2SO2.

The lead so prepared usually contains small amounts of silver, arsenic, antimony, copper, and other metals. The silver is removed by Parkes's method, as described under silver, and the other metals in various ways. The lead of commerce is one of the purest commercial metals, containing as a rule only a few tenths per cent of impurities.

Properties.Lead is a heavy metal (den. = 11.33) which has a brilliant silvery luster on a freshly cut surface, but which soon tarnishes to a dull blue-gray color. It is soft, easily fused (melting at 327°), and quite malleable, but has little toughness or strength.

It is not acted upon to any great extent by the oxygen of the air under ordinary conditions, but is changed into oxide at a high temperature. With the exception of hydrochloric and sulphuric acids, most acids, even very weak ones, act upon it, forming soluble lead salts. Hot, concentrated hydrochloric and sulphuric acids also attack it to a slight extent.

Uses.Lead is employed in the manufacture of lead pipes and in large storage batteries. In the form of sheet lead it is used in lining the chambers of sulphuric acidworks and in the preparation of paint pigments. Some alloys of lead, such as solder and pewter (lead and tin), shot (lead and arsenic), and soft bearing metals, are widely used. Type metal consists of lead, antimony, and sometimes tin. Compounds of lead form several important pigments.

Compounds of lead.In nearly all its compounds lead has a valence of 2, but a few corresponding to stannic compounds have a valence of 4.

Lead oxides.Lead forms a number of oxides, the most important of which are litharge, red lead or minium, and lead peroxide.

1.Litharge(PbO). This oxide forms when lead is oxidized at a rather low temperature, and is obtained as a by-product in silver refining. It is a pale yellow powder, and has a number of commercial uses. It is easily soluble in nitric acid:

PbO + 2HNO3= Pb(NO3)2+ H2O.

2.Red lead, or minium(Pb3O4). Minium is prepared by heating lead (or litharge) to a high temperature in the air. It is a heavy powder of a beautiful red color, and is much used as a pigment.

3.Lead peroxide(PbO2). This is left as a residue when minium is heated with nitric acid:

Pb3O4+ 4HNO3= 2Pb(NO3)2+ PbO2+ 2H2O.

It is a brown powder which easily gives up a part of its oxygen and, like manganese dioxide and barium dioxide, is a good oxidizing agent.

Soluble salts of lead.The soluble salts of lead can be made by dissolving litharge in acids. Lead acetate (Pb(C2H3O2)2·3H2O), called sugar of lead, and leadnitrate (Pb(NO3)2) are the most familiar examples. They are while crystalline solids and are poisonous in character.

Insoluble salts of lead; lead carbonate.While the normal carbonate of lead (PbCO3) is found to some extent, in nature and can be prepared in the laboratory, basic carbonates of varying composition are much more easy to obtain. One of the simplest of these has the composition 2PbCO3·Pb(OH)2. A mixture of such carbonates is called white lead. This is prepared on a large scale as a paint pigment and as a body for paints which are to be colored with other substances.

White lead.White lead is an amorphous white substance which, when mixed with oil, has great covering power, that is, it spreads out in an even waxy film, free from streaks and lumps, and covers the entire surface upon which it is spread. Its disadvantage as a pigment lies in the fact that it gradually blackens when exposed to sulphur compounds, which are often present in the air, forming black lead sulphide (PbS).Technical preparation of white lead.Different methods are used in the preparation of white lead, but the old one known as the Dutch process is still the principal one employed. In this process, earthenware pots about ten inches high and of the shape shown in Fig. 89 are used. In the bottomAis placed a 3% solution of acetic acid (vinegar answers the purpose very well). The space above this is filled with thin, perforated, circular pieces of lead, supported by the flangeBof the pot. These pots are placed close together on a bed of tan bark on the floor of a room known as the corroding room. They are covered over with boards, upon which tan bark is placed, and another row of pots is placed on this. In this way the room is filled. The white lead is formed by the fumes of the acetic acid, together with the carbon dioxide set free in the fermentation of the tan bark acting on the lead. About three months are required to complete the process.

Technical preparation of white lead.Different methods are used in the preparation of white lead, but the old one known as the Dutch process is still the principal one employed. In this process, earthenware pots about ten inches high and of the shape shown in Fig. 89 are used. In the bottomAis placed a 3% solution of acetic acid (vinegar answers the purpose very well). The space above this is filled with thin, perforated, circular pieces of lead, supported by the flangeBof the pot. These pots are placed close together on a bed of tan bark on the floor of a room known as the corroding room. They are covered over with boards, upon which tan bark is placed, and another row of pots is placed on this. In this way the room is filled. The white lead is formed by the fumes of the acetic acid, together with the carbon dioxide set free in the fermentation of the tan bark acting on the lead. About three months are required to complete the process.

Fig. 89Fig. 89

Lead sulphide(PbS). In nature this compound occurs in highly crystalline condition, the crystals having much the same luster as pure lead. It is readily prepared in the laboratory as a black precipitate, by the action of hydrosulphuric acid upon soluble lead salts:

Pb(NO3)2+ H2S = PbS + 2HNO3.

It is insoluble both in water and in dilute acids.

Other insoluble salts.Lead chromate (PbCrO4) is a yellow substance produced by the action of a soluble lead salt upon a soluble chromate, thus:

K2CrO4+ Pb(NO3)2= PbCrO4+ 2 KNO3.

It is used as a yellow pigment. Lead sulphate (PbSO4) is a white substance sometimes found in nature and easily prepared by precipitation. Lead chloride (PbCl2) is likewise a white substance nearly insoluble in cold water, but readily soluble in boiling water.

Thorium and cerium.These elements are found in a few rare minerals, especially in the monazite sand of the Carolinas and Brazil. The oxides of these elements are used in the preparation of the Welsbach mantles for gas lights, because of the intense light given out when a mixture of the oxides is heated. These mantles contain the oxides of cerium and thorium in the ratio of about 1% of the former to 99% of the latter. Compounds of thorium, like those of radium, are found to possess radio-activity, but in a less degree.

1.How could you detect lead if present in tin foil?

2.Stannous chloride reduces gold chloride (AuCl3) to gold. Give equation.

3.What are the products of hydrolysis when stannic chloride is used as a mordant?

4.How could you detect arsenic, antimony, or copper in lead?

5.Why is lead so extensively used for making water pipes?

6.What sulphates other than lead are insoluble?

7.Could lead nitrate be used in place of barium chloride in testing for sulphates?

8.How much lead peroxide could be obtained from 1 kg. of minium?

9.The purity of white lead is usually determined by observing the volume of carbon dioxide given off when it is treated with an acid. What acid should be used? On the supposition that it has the formula 2PbCO3·Pb(OH)2, how nearly pure was a sample if 1 g. gave 30 cc. of carbon dioxide at 20° and 750 mm.?

10.Silicon belongs in the same family with tin and lead. In what respects are these elements similar?

11.What weight of tin could be obtained by the reduction of 1 ton of cassiterite?

12.What reaction would you expect to take place when lead peroxide is treated with hydrochloric acid?

13.White lead is often adulterated with barytes. Suggest a method for detecting it, if present, in a given example of white lead.

SYMBOLATOMICWEIGHT DENSITYMELTING POINTFORMULAS OF ACIDSManganeseMn55.08.011900°H2MnO4and HMnO4ChromiumCr52.17.33000°H2CrO4and H2Cr2O7

General.Manganese and chromium, while belonging to different families, have so many features in common in their chemical conduct that they may be studied together with advantage. They differ from most of the elements so far studied in that they can act either as acid-forming or base-forming elements. As base-forming elements each of the metals forms two series of salts. In the one series, designated by the suffix "ous," the metal is divalent; in the other series, designated by the suffix "ic," the metal is trivalent. Only the manganous and the chromic salts, however, are of importance. The acids in which these elements play the part of a non-metal are unstable, but their salts are usually stable, and some of them are important compounds.

Occurrence.Manganese is found in nature chiefly as the dioxide MnO2, called pyrolusite. In smaller amounts it occurs as the oxides Mn2O3and Mn3O4, and as the carbonate MnCO3. Some iron ores also contain manganese.

Preparation and properties.The element is difficult to prepare in pure condition and has no commercial applications. It can be prepared, however, by reducing the oxide with aluminium powder or by the use of the electric furnace, with carbon as the reducing agent. The metal somewhat resembles iron in appearance, but is harder, less fusible, and more readily acted upon by air and moisture. Acids readily dissolve it, forming manganous salts.

Oxides of manganese.The following oxides of manganese are known: MnO, Mn2O3, Mn3O4, MnO2, and Mn2O7. Only one of these, the dioxide, needs special mention.

Manganese dioxide(pyrolusite) (MnO2). This substance is the most abundant manganese compound found in nature, and is the ore from which all other compounds of manganese are made. It is a hard, brittle, black substance which is valuable as an oxidizing agent. It will be recalled that it is used in the preparation of chlorine and oxygen, in decolorizing glass which contains iron, and in the manufacture of ferromanganese.

Compounds containing manganese as a base-forming element.As has been stated previously, manganese forms two series of salts. The most important of these salts, all of which belong to the manganous series, are the following:

Manganous chlorideMnCl2·4H2O.Manganous sulphideMnS.Manganous sulphateMnSO4·4H2O.Manganous carbonateMnCO3.Manganous hydroxideMn(OH)2.

The chloride and sulphate may be prepared by heating the dioxide with hydrochloric and sulphuric acids respectively:

MnO2+ 4HCl = MnCl2+ 2H2O + 2Cl,MnO2+ H2SO4= MnSO4+ H2O + O.

MnO2+ 4HCl = MnCl2+ 2H2O + 2Cl,

MnO2+ H2SO4= MnSO4+ H2O + O.

The sulphide, carbonate, and hydroxide, being insoluble, may be prepared from a solution of the chloride or sulphate by precipitation with the appropriate reagents. Most of the manganous salts are rose colored. They not only have formulas similar to the ferrous salts, but resemble them in many of their chemical properties.

Compounds containing manganese as an acid-forming element.Manganese forms two unstable acids, namely, manganic acid and permanganic acid. While these acids are of little interest, some of their salts, especially the permanganates, are important compounds.

Manganic acid and manganates.When manganese dioxide is fused with an alkali and an oxidizing agent a green compound is formed. The equation, when caustic potash is used, is as follows:

MnO2+ 2KOH + O = K2MnO4+ H2O.

The green compound (K2MnO4) is called potassium manganate, and is a salt of the unstable manganic acid (H2MnO4). The manganates are all very unstable.

Permanganic acid and the permanganates.When carbon dioxide is passed through a solution of a manganate a part of the manganese is changed into manganese dioxide, while the remainder forms a salt of the unstable acid HMnO4, called permanganic acid. The equation is

3K2MnO4+ 2CO2= MnO2+ 2KMnO4+ 2K2CO3.

Potassium permanganate (KMnO4) crystallizes in purple-black needles and is very soluble in water, forming an intensely purple solution. All other permanganates, as well as permanganic acid itself, give solutions of the same color.

Oxidizing properties of the permanganates.The permanganates are remarkable for their strong oxidizing properties. When used as an oxidizing agent the permanganate is itself reduced, the exact character of the products formed from it depending upon whether the oxidation takes place (1) in an alkaline or neutral solution, or (2) in an acid solution.

1.Oxidation in alkaline or neutral solution.When the solution is either alkaline or neutral the potassium and the manganese of the permanganate are both converted into hydroxides, as shown in the equation

2KMnO4+ 5H2O = 2Mn(OH)4+ 2KOH + 3O.

2.Oxidation in acid solution.When free acid such as sulphuric is present, the potassium and the manganese are both changed into salts of the acid:

2KMnO4+ 3H2SO4= K2SO4+ 2MnSO4+ 3H2O + 5O.

Under ordinary conditions, however, neither one of these reactions takes place except in the presence of a third substance which is capable of oxidation. The oxygen is not given off in the free state, as the equations show, but is used up in effecting oxidation.

Potassium permanganate is particularly valuable as an oxidizing agent not only because it acts readily either in acid or in alkaline solution, but also because the reaction takes place so easily that often it is not even necessary to heat the solution to secure action. The substance finds many uses in the laboratory, especially in analytical work. It is also used as an antiseptic as well as a disinfectant.

Occurrence.The ore from which all chromium compounds are made is chromite, or chrome iron ore (FeCr2O4). This is found most abundantly in New Caledonia and Turkey. The element also occurs in small quantities in many other minerals, especially in crocoisite (PbCrO4), in which mineral it was first discovered.

Preparation.Chromium, like manganese, is very hard to reduce from its ores, owing to its great affinity for oxygen. It can, however, be made by the same methods which have proved successful with manganese. Considerable quantities of an alloy of chromium with iron, called ferrochromium, are now produced for the steel industry.

Properties.Chromium is a very hard metal of about the same density as iron. It is one of the most infusible of the metals, requiring a temperature little short of 3000° for fusion. At ordinary temperatures air has little action on it; at higher temperatures, however, it burns brilliantly. Nitric acid has no action on it, but hydrochloric and dilute sulphuric acids dissolve it, liberating hydrogen.

Compounds containing chromium as a base-forming element.While chromium forms two series of salts, chromous salts are difficult to prepare and are of little importance. The most important of the chromic series are the following:

Chromic hydroxideCr(OH)3.Chromic chlorideCrCl3·6H2O.Chromic sulphateCr2(SO4)3.Chrome alums

Chromic hydroxide(Cr(OH)3). This substance, being insoluble, can be obtained by precipitating a solution of the chloride or sulphate with a soluble hydroxide. It is agreenish substance which, like aluminium hydroxide, dissolves in alkalis, forming soluble salts.

Dehydration of chromium hydroxide.When heated gently chromic hydroxide loses a part of its oxygen and hydrogen, forming the substance CrO·OH, which, like the corresponding aluminium compound, has more pronounced acid properties than the hydroxide. It forms a series of salts very similar to the spinels; chromite is the ferrous salt of this acid, having the formula Fe(CrO2)2. When heated to a higher temperature chromic hydroxide is completely dehydrated, forming the trioxide Cr2O3. This resembles the corresponding oxides of aluminium and iron in many respects. It is a bright green powder, and when ignited strongly becomes almost insoluble in acids, as is also the case with aluminium oxide.

Chromic sulphate(Cr2(SO4)3). This compound is a violet-colored solid which dissolves in water, forming a solution of the same color. This solution, however, turns green on heating, owing to the formation of basic salts. Chromic sulphate, like ferric and aluminium sulphates, unites with the sulphates of the alkali metals to form alums, of which the best known are potassium chrome alum (KCr(SO4)2·12H2O) and ammonium chrome alum (NH4Cr(SO4)2·12H2O).

These form beautiful dark purple crystals and have some practical uses in the tanning industry and in photography. A number of the salts of chromium are also used in the dyeing industry, for they hydrolyze like aluminium salts and the hydroxide forms a good mordant.

Hydrolysis of chromium salts.When ammonium sulphide is added to a solution of a chromium salt, such as the sulphate, chromium hydroxide precipitates instead of the sulphide. This is due to the fact that chromic sulphide, like aluminium sulphide, hydrolyzes in the presence of water, forming chromic hydroxide and hydrosulphuric acid. Similarly, a soluble carbonate precipitates a basic carbonate of chromium.

Compounds containing chromium as an acid-forming element.Like manganese, chromium forms two unstable acids, namely, chromic acid and dichromic acid. Their salts, the chromates and dichromates, are important compounds.

Chromates.When a chromium compound is fused with an alkali and an oxidizing agent a chromate is produced. When potassium hydroxide is used as the alkali the equation is

2Cr(OH)3+ 4KOH + 3O = 2K2CrO4+ 5H2O.

This reaction recalls the formation of a manganate under similar conditions.

Properties of chromates.The chromates are salts of the unstable chromic acid (H2CrO4), and as a rule are yellow in color. Lead chromate (PbCrO4) is the well-known pigment chrome yellow. Most of the chromates are insoluble and can therefore be prepared by precipitation. Thus, when a solution of potassium chromate is added to solutions of lead nitrate and barium nitrate respectively, the reactions expressed by the following equations occur:

Pb(NO3)2+ K2CrO4= PbCrO4+ 2KNO3,Ba(NO3)2+ K2CrO4= BaCrO4+ 2KNO3.

Pb(NO3)2+ K2CrO4= PbCrO4+ 2KNO3,

Ba(NO3)2+ K2CrO4= BaCrO4+ 2KNO3.

The chromates of lead and barium separate as yellow precipitates. The presence of either of these two metals can be detected by taking advantage of these reactions.

Dichromates.When potassium chromate is treated with an acid the potassium salt of the unstable dichromic acid (H2Cr2O7) is formed:

2K2CrO4+ H2SO4= K2Cr2O7+ K2SO4+ H2O.

The relation between the chromates and dichromates is the same as that between the phosphates and the pyrophosphates. Potassium dichromate might therefore be called potassium pyrochromate.

Potassium dichromate(K2Cr2O7). This is the best known dichromate, and is the most familiar chromium compound. It forms large crystals of a brilliant red color, and is rather sparingly soluble in water. When treated with potassium hydroxide it is converted into the chromate

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