THIOSULPHATES.

249

Test for Sulphates. We test for a soluble sulphate, i. e., for SO, ions, by adding dilute hydrochloric acid and barium chloride solution, or dilute nitric acid and barium nitrate solution. If a white precipitate is formed, the solution taken probably contained a sulphate.

++

+

Ba+2 C1+2 Na+SO4

BaSO4+2 Ña+2 Cl.

The insoluble sulphates, besides barium sulphate, are those of lead and of strontium. Calcium sulphate is difficultly soluble (cf. § 79).

268. Thiosulphates. Thiosulphuric acid, H2S2O3, is sulphuric acid with one-fourth of its oxygen replaced by sulphur ("thion "= sulphur); its salts are called thiosulphates.

The most important thiosulphate is the sodium salt, Na2S2O3. This is made by boiling a solution of sodium sulphite with sulphur.

Na2SO3+S→→→→ Na2S2O3.

This reaction corresponds to the oxidation of sulphites to sulphates (cf. § 259).

When sodium thiosulphate is treated with dilute acids, it breaks down as represented by the equation,

Na2S2O3+2 HCl →→→→→ 2 NaCl+H2SO3+S.

It decomposes, therefore, like a sulphite plus sulphur.

Sodium thiosulphate is a reducing agent capable of converting chlorine and iodine into hydrochloric and hydriodic acids, respectively. It is therefore used to destroy the excess of chlorine in the process of bleaching. It has the power to dissolve silver chloride, bromide, iodide, etc., and is, therefore, used in "fixing" negatives in photography. Its technical name is "hypo," from its old chemical name, hyposulphite of soda.”

66

269. Exercises.

1. How many grams of sulphur dioxide can be made by roasting 75 g. of iron pyrites? How many liters at 20° C. and 740 mm.? 2. How many grams ferrous sulphide are needed to give, with dilute sulphuric acid, 25 1. of hydrogen sulphide at 10° C. and 600 mm.?

3. How much sulphuric acid was there in a solution from which an excess of barium chloride precipitated 2.836 g. of barium sulphate? If the sulphuric acid was produced by the oxidation of the sulphur in 4 g. of a sulphur ore, calculate the per cent of sulphur in the ore.

4. How could you distinguish between sodium sulphide, sulphite, and sulphate? Between sodium, ammonium, and barium sulphates?

5. Would you use sodium sulphide in a Kipp's apparatus to give hydrogen sulphide? Cupric sulphide? Lead sulphide? Why?

6. Write the molecular equation for the burning of hydrogen sulphide in air. Give the relative volumes of all the gases used and produced.

7. What acid is formed by heating alum, and collecting the product in water (cf. § 222)? Alum and salt? Alum and niter? Alum, salt, and niter?

8. What drying agents may be used for sulphur dioxide? For hydrogen sulphide? What ones may not?

9. Silver sulphate is soluble. Write the equation for its action with barium chloride solution. Would you use barium chloride or nitrate, to test for its sulphate ions? Why?

10. How would you show that "heavy spar is a sulphate (cf. §§ 252 and 254)?

II. Write the equation for the action of hot, concentrated sulphuric acid with mercury. With silver. With sugar (cf. § 543).

CHAPTER XXII.

CARBON AND ITS COMPOUNDS.

270. Carbon. The element carbon is found in a free and an almost pure state as diamond, graphite, and anthracite coal; it is found combined in all organic substances, in carbon dioxide, in carbonates, in coal, and in petroleum.

Like sulphur, carbon exists in several allotropic forms (cf. § 249), the forms usually distinguished being (1) diamond, (2) graphite, and (3) amorphous, i. e., non-crystalline, carbon.

These three modifications of carbon differ to such an extent that they were long considered different chemical individuals. Their identity is proved by burning them in oxygen; for equal parts by weight of each give equal amounts of carbon dioxide.

271. The Diamond. The diamond is prized for its luster, its strong refractive power, and its hardness. It is the hardest substance known.

The specific gravity of the diamond is 3.5. Acids and alkalies have practically no effect upon it; but when it is heated to about 700° C. in oxygen, it burns, forming carbon dioxide. When a diamond

AMORPHOUS CARBON.

253

is heated between the poles of a powerful battery it is changed into graphite.

Diamonds have been made artificially by crystallizing carbon from solution in melted iron. This process usually gives graphite, but if the melted iron cools under great pressure the carbon appears in the form of small diamonds. The necessary pressure is secured by chilling the exterior of the mass of iron; the contraction of the exterior thus causes great pressure upon the interior of the mass.

So far as known, no artificial diamonds yet made are large enough to have any commercial importance.

272. Graphite. - Graphite is sometimes found crystallized, but usually in an amorphous form. Its specific gravity is about 2.25. It owes its name, "black lead," to a confusion of names.

Graphite is a good conductor of heat and of electricity. Owing to its friability it is used to make "lead" pencils. Mixed with clay it is the material of graphite crucibles, which are used in making "crucible" steel. Other uses are: To protect iron from the air, as in stove polish, to coat grains of shot, and as a lubricant.

Graphite is produced artificially (cf. § 271) by crystallizing charcoal from molten iron and steel.

273. Amorphous Carbon. The amorphous forms of carbon include the several varieties of coal, gas carbon, coke, charcoal, and lampblack; all of these