CHROMIUM
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 hydroxide | Cr(OH)3. |
| Chromic chloride | CrCl3·6H2O. |
| Chromic sulphate | Cr2(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 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(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.
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
K2Cr2O7 + 2KOH = 2K2CrO4 + H2O.
When added to a solution of lead or barium salt the corresponding chromates (not dichromates) are precipitated. With barium nitrate the equation is
2Ba(NO3)2 + K2Cr2O7 + H2O = 2BaCrO4 + 2KNO3 + 2HNO3.
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) K2Cr2O7 + 4H2SO4 = K2SO4 + Cr2(SO4)3 + 4H2O + 3O,
(2) 6FeSO4 + 3H2SO4 + 3O = 3Fe2(SO4)3 + 3H2O.
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:
K2Cr2O7 + 7H2SO4 + 6FeSO4 = K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H20.
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 (CrO3). 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 CrO3.This oxide of chromium is called chromic anhydride, since it combines readily with water to form chromic acid:
CrO3 + H2O = H2CrO4.
It is therefore analogous to sulphur trioxide which forms sulphuric acid in a similar way:
SO3 + H2O = H2SO4.
Chromic anhydride is a very strong oxidizing agent, giving up oxygen and forming chromic oxide:
2CrO3 = Cr2O3 + 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 (H2MoO4), the ammonium salt of which is (NH4)2MoO4. 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.