(q) Ferric salt and ammonium thiocyanate.

(r) Prussian and Turnbull's blue with caustic soda.

2. Write the equations for the reactions by which the following are prepared from cobalt nitrate: cobalt hydroxide, oxide, and cyanide; potassium cobaltocyanide, potassium cobalticyanide.

3. Write the equations for the preparation of the following from nickel nitrate: nickel oxide, hydroxide, sulphate, and sulphide; potassium nickel cyanide.

4. Give all the ways in which iron, nickel, and cobalt are alike. In what respects do they differ? How would you distinguish between ferrous, ferric, cobaltous, and nickelous ions in solution?

CHAPTER XXXVI.

MANGANESE AND CHROMIUM.

500. Manganese. Manganese occurs as the dioxide (MnO2), which is pyrolusite; also as hausmannite (MnO4) and manganese spar (MnCO3). The metal is made by reducing its oxides with aluminum (cf. § 308). It is hard, brittle, steel-gray, and nonmagnetic. Manganese melts at about 1300° C., rusts in moist air, and replaces the hydrogen of the ordinary dilute acids, giving manganous salts. Manganese is used in making steel (cf. § 484).

501. Manganese Compounds. The oxides of manganese have the formulas Mn"O, Mn2O3, Mn3O4, MnO2, MnO3, and Mn2O7. All are solids but the last, which is a dark liquid.

The only decidedly basic oxide is manganous oxide (MnO). This is a green, amorphous solid, formed when the carbonate (MnCO3) is heated in absence of air. The air oxidizes it to Mn304. Manganous hydroxide, Mn(OH)2, is precipitated as a white solid, insoluble in an excess of the strong bases; it has no acid properties. It is easily oxidized by the air to the dark hydrate of manganic oxide, Mn2O3. H2O. The stable manganese salts are all manganous. When the higher oxides of manganese are treated with acids, they lose all the oxygen they contain in excess of that represented by the formula MnO. The excess of

oxygen is either given off as such, as is the case when manganese dioxide is heated with concentrated sulphuric acid, or the oxygen is used in oxidizing the acid, as is the case with hydrochloric acid. The resulting salts are manganous (cf. §§ 115 and 138). The common manganous salts are pink, crystalline, soluble solids. Manganous sulphate crystallizes with different proportions of water (MnSO4. 7 H2O, 5 H2O, or 4 H2O). The chloride is MnCl2. 4 H2O. The carbonate is a white, insoluble solid. The sulphide (MnS) is precipitated as a light-pink solid by soluble metal sulphides, but not by hydrogen sulphide (cf. § 255). The oxide MnO4 is formed when the other oxides are heated in air (cf. § 22). The dioxide is the most important manganese compound. It is used to prepare manganese, chlorine, and oxygen, and to decolorize glass. It makes glass colorless by neutralizing the green color produced by the iron present in the materials used (cf. § 394). Manganese dioxide is the anhydride of the unstable compounds H.MnO4 and H2MnO3. These are the "ortho " and the "meta" form of manganous acid (cf. § 355). Calcium manganite is CaMnO3. Hausmannite is manganous manganite, Mn,MnO4. In the oxidizing flame manganese compounds form an amethyst-colored borax bead. In the reducing flame it is colorless.

502. Manganates and Permanganates. Manganese trioxide (MnO3) is the anhydride of manganic acid, H2MnO4, and the heptoxide (Mn2O7) is the anhydride of permanganic acid, HMnO4. Manganic acid is not known in the free state. Its salts, e. g., KMnO4, are hydrolyzed in solution. Manganate solutions, i. e., MnO4 ions, are green. Permanganic acid is a strong acid. It can be prepared (as a hydrate), but is unstable. Permanganate ions (MnO1) are purple.

MANGANATES AND PERMANGANATES.

457

A manganate is formed by fusing together a manganese compound, a base (or alkali carbonate), and an oxidizing agent, such as potassium nitrate or chlorate. If manganous sulphate is

used, the equations are:-

(1) MnSO4+K2CO3 (MnCO3)+K2SO4.
(2) (MnCO3)

(MnO)+CO2↑.

If MnO2 is used, equations (1) and (2) are omitted. Permanganates are formed by the oxidation of manganates. Thus ozone converts potassium manganate into the permanganate.

(1) K2MnO4+2 HOH

(2) 2 H2MnO4+03

(3) KOH+HMnO4

2 KOH+H2MnO4. 2 HMnO4+H2O+02. KMnO4+H2O.

The best way to represent the relation between manganic and permanganic acids is to consider them as hydrates of their anhydrides. It is thus made plain that the change is an oxidation of MnO3 to Mn2O7. Equation (2) then becomes:

2 (MnO3. H2O)+03 →→→→ Mn2O7. H2O+H2O+02.

Carbon dioxide also converts a manganate solution into one of permanganate; but some of the manganate is reduced to manganese dioxide to provide the necessary oxygen.

(1) 3 KMnO4+6 HOH 6 KOH+3 H2MnO4 [i. e., 3 (MnO3. H2O)].

(2) 3 (MnO3. H2O)

Mn2O7. H2O+MnO2 ↓ +2 H2O.

(3) 4 KOH+2 H¿CO3 (i. e., 2 H2O+2 CO2)

2 K2CO3+

4 H2O.

(4) 2 KOH+2 HMnO4 ·

› 2 KMnO4+2 H2O.

If we omit the formulas appearing on both sides of the equation, the " complete" equation is:

3 K2MnO4+2 H2CO2 · ·2 K2CO3+2 KMnO4+MnO2 ↓ + 2 H2O.

Other dilute acids may be used in place of the carbonic acid.

Potassium permanganate separates from solution as greenish-black prisms. Its solution is deep purple. When the solid is heated, it is decomposed.

2 KMnO4 →→→→ K2MnO4+MnO2+02 ↑.

A crude permanganate solution, made from sodium manganate and dilute sulphuric acid, is used as a disinfectant.

503. Oxidation by Potassium Permanganate. — Potassium permanganate is used for oxidizing purposes under acid, neutral, and alkaline conditions. Oxidation in acid solution is most common. As the oxygen is used up, the purple color disappears, because the resulting manganous salt solution is colorless. This fact makes it possible to use potassium permanganate solution in volumetric analysis. To determine the amount of some reducing agent, e. g., oxalic acid, in a solution we need only find out how much potassium permanganate solution, of known concentration, the oxalic acid can decolorize. The concentrations of solutions of formic acid, ferrous salts, sulphurous acid, hydrogen peroxide, etc., can be obtained in the same way. In acid