CHAPTER XXX

MANGANESE AND CHROMIUM

==================================================================== SYMBOL ATOMIC DENSITY MELTING FORMULAS OF ACIDS WEIGHT POINT Manganese Mn 55.0 8.01 1900 deg. H{2}MnO{4} and HMnO{4} Chromium Cr 52.1 7.3 3000 deg. H{2}CrO{4} and H{2}Cr{2}O{7} ====================================================================

~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.

MANGANESE

~Occurrence.~ Manganese is found in nature chiefly as the dioxide MnO{2}, called pyrolusite. In smaller amounts it occurs as the oxides Mn{2}O{3} and Mn{3}O{4}, and as the carbonate MnCO{3}. 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, Mn{2}O{3}, Mn{3}O{4}, MnO{2}, and Mn{2}O{7}. Only one of these, the dioxide, needs special mention.

~Manganese dioxide~ (pyrolusite) (MnO{2}). 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 chloride MnCl{2}.4H{2}O. Manganous sulphide MnS. Manganous sulphate MnSO{4}.4H{2}O. Manganous carbonate MnCO{3}. Manganous hydroxide Mn(OH){2}.

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

MnO{2} + 4HCl = MnCl{2} + 2H{2}O + 2Cl,

MnO{2} + H{2}SO{4} = MnSO{4} + H{2}O + 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:

MnO{2} + 2KOH + O = K{2}MnO{4} + H{2}O.

The green compound (K{2}MnO{4}) is called potassium manganate, and is a salt of the unstable manganic acid (H{2}MnO{4}). 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 HMnO{4}, called permanganic acid. The equation is

3K{2}MnO{4} + 2CO{2} = MnO{2} + 2KMnO{4} + 2K{2}CO{3}.

Potassium permanganate (KMnO{4}) 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

2KMnO{4} + 5H{2}O = 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:

2KMnO{4} + 3H{2}SO{4} = K{2}SO{4} + 2MnSO{4} + 3H{2}O + 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.

CHROMIUM

~Occurrence.~ The ore from which all chromium compounds are made is chromite, or chrome iron ore (FeCr{2}O{4}). This is found most abundantly in New Caledonia and Turkey. The element also occurs in small quantities in many other minerals, especially in crocoisite (PbCrO{4}), 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 deg. 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 hydroxide Cr(OH){3}. Chromic chloride CrCl{3}.6H{2}O. Chromic sulphate Cr{2}(SO{4}){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 a greenish 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(CrO{2}){2}. When heated to a higher temperature chromic hydroxide is completely dehydrated, forming the trioxide Cr{2}O{3}. 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~ (Cr{2}(SO{4}){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(SO{4}){2}.12H{2}O) and ammonium chrome alum (NH{4}Cr(SO{4}){2}.12H{2}O).

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 = 2K{2}CrO{4} + 5H{2}O.

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

~Properties of chromates.~ The chromates are salts of the unstable chromic acid (H{2}CrO{4}), and as a rule are yellow in color. Lead chromate (PbCrO{4}) 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(NO{3}){2} + K{2}CrO{4} = PbCrO{4} + 2KNO{3},

Ba(NO{3}){2} + K{2}CrO{4} = BaCrO{4} + 2KNO{3}.

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 (H{2}Cr{2}O{7}) is formed:

2K{2}CrO{4} + H{2}SO{4} = K{2}Cr{2}O{7} + K{2}SO{4} + H{2}O.

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~ (K{2}Cr{2}O{7}). 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

K{2}Cr{2}O{7} + 2KOH = 2K{2}CrO{4} + H{2}O.

When added to a solution of lead or barium salt the corresponding chromates (not dichromates) are precipitated. With barium nitrate the equation is

2Ba(NO{3}){2} + K{2}Cr{2}O{7} + H{2}O = 2BaCrO{4} + 2KNO{3} + 2HNO{3}.

Potassium dichromate finds use in many industries as an oxidizing agent, especially in the preparation of organic substances, such as the dye alizarin, and in the construction of several varieties of electric batteries.

~Sodium chromates.~ The reason why the potassium salt rather than the sodium compound is used is that sodium chromate and dichromate are so soluble that it is hard to prepare them pure. This difficulty is being overcome now, and the sodium compounds are replacing the corresponding potassium salts. This is of advantage, since a sodium salt is cheaper than a potassium salt, so far as raw materials go.

~Oxidizing action of chromates and dichromates.~ When a dilute solution of a chromate or dichromate is acidified with an acid, such as sulphuric acid, no reaction apparently takes place. However, if there is present a third substance capable of oxidation, the chromium compound gives up a portion of its oxygen to this substance. Since the chromate changes into a dichromate in the presence of an acid, it will be sufficient to study the action of the dichromates alone. The reaction takes place in two steps. Thus, when a solution of ferrous sulphate is added to a solution of potassium dichromate acidified with sulphuric acid, the reaction is expressed by the following equations:

(1) K{2}Cr{2}O{7} + 4H{2}SO{4} = K{2}SO{4} + Cr{2}(SO{4}){3} + 4H{2}O + 3O,

(2) 6FeSO{4} + 3H{2}SO{4} + 3O = 3Fe{2}(SO{4}){3} + 3H{2}O.

The dichromate decomposes in very much the same way as a permanganate does, the potassium and chromium being both changed into salts in which they play the part of metals, while part of the oxygen of the dichromate is liberated.

By combining equations (1) and (2), the following is obtained:

K{2}Cr{2}O{7} + 7H{2}SO{4} + 6FeSO{4} = K{2}SO{4} + Cr{2}(SO{4}){3} + 3Fe{2}(SO{4}){3} + 7H{2}0.

This reaction is often employed in the estimation of iron in iron ores.

~Potassium chrome alum.~ It will be noticed that the oxidizing action of potassium dichromate leaves potassium sulphate and chromium sulphate as the products of the reaction. On evaporating the solution these substances crystallize out as potassium chrome alum, which substance is produced as a by-product in the industries using potassium dichromate for oxidizing purposes.

~Chromic anhydride~ (CrO{3}). When concentrated sulphuric acid is added to a strong solution of potassium dichromate, and the liquid allowed to stand, deep red needle-shaped crystals appear which have the formula CrO{3}.This oxide of chromium is called chromic anhydride, since it combines readily with water to form chromic acid:

CrO{3} + H{2}O = H{2}CrO{4}.

It is therefore analogous to sulphur trioxide which forms sulphuric acid in a similar way:

SO{3} + H{2}O = H{2}SO{4}.

Chromic anhydride is a very strong oxidizing agent, giving up oxygen and forming chromic oxide:

2CrO{3} = Cr{2}O{3} + 3O.

~Rare elements of the family.~ Molybdenum, tungsten, and uranium are three rather rare elements belonging in the same family with chromium, and form many compounds which are similar in formulas to the corresponding compounds of chromium. They can play the part of metals and also form acids resembling chromic acid in formula. Thus we have molybdic acid (H{2}MoO{4}), the ammonium salt of which is (NH{4}){2}MoO{4}. This salt has the property of combining with phosphoric acid to form a very complex substance which is insoluble in nitric acid. On this account molybdic acid is often used in the estimation of the phosphoric acid present in a substance. Like chromium, the metals are difficult to prepare in pure condition. Alloys with iron can be prepared by reducing the mixed oxides with carbon in an electric furnace; these alloys are used to some extent in preparing special kinds of steel.

EXERCISES

1. How does pyrolusite effect the decolorizing of glass containing iron?

2. Write the equations for the preparation of manganous chloride, carbonate, and hydroxide.

3. Write the equations representing the reactions which take place when ferrous sulphate is oxidized to ferric sulphate by potassium permanganate in the presence of sulphuric acid.

4. In the presence of sulphuric acid, oxalic acid is oxidized by potassium permanganate according to the equation

C{2}H{2}O{4} + O = 2CO{2} + H{2}O.

Write the complete equation.

5. 10 g. of iron were dissolved in sulphuric acid and oxidized to ferric sulphate by potassium permanganate. What weight of the permanganate was required?

6. What weight of ferrochromium containing 40% chromium must be added to a ton of steel to produce an alloy containing 1% of chromium?

7. Write the equation representing the action of ammonium sulphide upon chromium sulphate.

8. Potassium chromate oxidizes hydrochloric acid, forming chlorine. Write the complete equation.

9. Give the action of sulphuric acid on potassium dichromate (a) in the presence of a large amount of water; (b) in the presence of a small amount of water.