CALCULATION OF FORMULE.

121

evaporating the liquid in a weighed dish placed over a steam bath, and weighing the hydrochlorate of ammonia, or more accurately by converting it into the double chloride of platinum and ammonium. Sometimes a solution of sulphuric acid of known strength is substituted for the hydrochloric acid in the bulbs, and the weight of the ammonia is ascertained by determining the quantity of acid which has been neutralised.

To illustrate the change which takes place when the organic substance is heated with the hydrates of soda and lime, let it be supposed that urea is the substance submitted to analysis.

C.H.N.O, + NaO.HO + CaO.HO = NaO.CO, + CaO.CO, + 2NH,

Urea.

The hydrate of soda alone would produce the same result, but would corrode the glass too rapidly.

In the analysis of an organic substance containing carbon, hydrogen, nitrogen, and oxygen, the proportions of carbon and hydrogen having been ascertained by the method described at p. 73, and that of nitrogen by the process given above, the sum of the carbon, hydrogen, and nitrogen is deducted from the entire weight of the substance to obtain the proportion of oxygen. The weights thus found are divided by the combining weights of the several elements to obtain the empirical formula, which is converted into a rational formula on the principle illustrated at p. 75.

For example, 10 grs. of urea were found to contain 2 grs. of carbon, 0·66 gr. of hydrogen, and 4.67 grs. of nitrogen.

10 grs. of urea minus 7·33 (carbon, hydrogen, and nitrogen) of oxygen.

=

2.67 grs.

Dividing each of these numbers by the combining weight of the element to which it refers, we have,

leading to the empirical formula CHNO3, or in its simplest form, CH,NO, for urea. But urea is an organic base, capable of uniting with acids to form salts, and it is found that to neutralise one combining weight (365 parts) of hydrochloric acid, 60 parts of urea are necessary. This quantity would contain 12 parts (two combining weights) of carbon, 4 parts (four combining weights) of hydrogen, 28 parts (two combining weights) of nitrogen, and 16 parts (two combining weights) of oxygen, so that the true formula for urea would be C2H,NO2.

90. Formation of ammonia in the rusting of iron.-Although free nitrogen and hydrogen cannot be made to form ammonia by direct combination, this compound is produced when the nitrogen meets with hydrogen in the nascent state; that is, at the instant of its liberation from a combined form. Thus, if a few iron filings be shaken with a little water in a bottle of air, so that they may cling round the sides of the bottle, and a piece of red litmus paper be suspended between the stopper and the neck, it will be found to have assumed a blue colour in the course of a few hours, and ammonia may be distinctly detected in the rust which is produced. It appears that the water is decomposed by the iron, in the presence of the carbonic acid of the air and water, and that the hydrogen liberated enters at once into combination with the nitrogen, held in solution by the water, to form ammonia.

122

OXIDATION OF AMMONIA.

91. Production of nitrous and nitric acids from ammonia.—If a few drops of a strong solution of ammonia be poured into a pint bottle, and ozonised air (from the tube for ozonising by induction, fig. 10) be passed into the bottle, thick white clouds will speedily be formed, consisting of nitrite of ammonia, the nitrous acid having been produced by the oxidation of the ammonia at the expense of the ozonised oxygen

If copper filings be shaken with solution of ammonia in a bottle of air, white fumes will also be produced, together with a deep blue solution containing oxide of copper and nitrite of ammonia; the act of oxidation of the copper appearing to have induced a simultaneous oxidation of the ammonia.

Fig. 135.

A coil of thin platinum wire made round a pencil, if heated to redness at the lower end and suspended in a flask (fig. 135) with a little strong ammonia at the bottom, will continue to glow for a great length of time, in consequence of the combination of the ammonia with the oxygen of the air taking place at its surface, attended with great evolution of heat. Thick white clouds of nitrite of ammonia are formed,

and frequently red vapour of nitrous acid (NO3) itself.

If a tube delivering oxygen gas be passed down to the bottom of the flask (fig. 136), the action will be far more energetic, the heat of the platinum rising to white

Fig. 136.

ness, when an explosion of the mixture of ammonia and oxygen will ensue. After the explosion the action will recommence, so that the explosion will repeat itself as often as may be wished. It is unattended with danger if the mouth of the flask be pretty large. By regulating the stream of oxygen, the bubbles of that gas may be made to burn as they pass through the ammonia at the bottom of the flask.

In the presence of strong bases, and of porous materials to favour oxidation, ammonia appears to be capable of suffering further oxidation and conversion into nitric acid, which combines with the base

to form a nitrate, thus

This formation of nitrates from ammonia is commonly referred to as nitrification, and appears to play an important part in the formation of the natural supplies of saltpetre which are of so great importance to the arts.*

COMPOUNDS OF NITROGEN AND OXYGEN.

92. Though these elements in their pure state exhibit no attraction for each other, five compounds, which contain them in different proportions, have been obtained by indirect processes. The relative proportions of

*The charcoal which has been used in the sewer ventilators (see p. 59) has been found to contain abundance of nitrates.

FORMATION OF NITRIC ACID.

123

oxygen in these compounds are exhibited in the following table, where the weight of nitrogen present is supposed to remain constant :

When a succession of strong electric sparks from the induction coil is passed through atmospheric air in a flask (especially if the air be mixed with oxygen), a red gas is formed in small quantity, which is either nitrous acid (NO) or nitric peroxide (NO).

If the experiment be made in a graduated eudiometer (fig. 137), standing over water coloured with blue litmus, the latter will very soon be reddened by the acid formed, and the air will be found to diminish very considerably in volume, eventually losing its power of supporting combustion, in consequence of the removal of oxygen.

When hydrogen gas, mixed with a small quantity of nitrogen, is burnt, the water collected from it is found to have an acid taste and reaction, due to the presence of a little nitric acid, resulting from the combination of the nitrogen with the oxygen of the air under the influence of the intense heat of the hydrogen flame.

Since all the compounds of nitrogen and oxygen are obtained, in practice, from hydrated nitric acid (HO. NO¿), the chemical history of that substance must precede that of the bodies enumerated in the above table.

NITRIC ACID.

Fig. 137.

93. This most important acid is obtained from saltpetre, which is found

as an incrustation upon the sur

face of the soil in hot and dry climates, as in some parts of India and Peru. The salt imported into this country from Bengal and Oude consists of nitrate of potash (KO. NO), whilst the Peruvian or Chilian saltpetre is nitrate of soda (NaO. NO). Either of these will serve for the preparation of nitric acid.

Fig. 138.-Preparation of nitric acid.

On the small scale, in the laboratory, nitric acid is prepared by distilling nitrate of potash with an equal weight of concentrated sulphuric acid.

124

PREPARATION OF NITRIC ACID.

In order to make the experiment, four ounces of powdered nitre, thoroughly dried, may be introduced into a pint-stoppered retort (fig. 138), and two and a half measured ounces of concentrated sulphuric acid poured upon it. As soon as the acid has soaked into the nitre, a gradually increasing heat may be applied by means of an Argand burner, when the acid will distil over. It must be preserved in a stoppered bottle.

When the acid has ceased distilling, the retort should be allowed to cool, and filled with water. On applying a moderate heat for some time, the saline residue will be dissolved. The solution may then be poured into an evaporating dish, and evaporated down to a small bulk. On allowing the concentrated solution to cool, crystals of bisulphate of potash (KO. HO.2SO,) are deposited. a salt which is very useful in many metallurgic and analytical operations.

The decomposition of nitrate of potash by an equal weight of concentrated sulphuric acid is explained by the equation

KO. NO, 2(HO. SO,) = HO. NO,
HO. NO, KO. SO, HO. SO,

Nitrate of

potash.

Hydrated sulphuric acid.

Hydrated nitric acid.

Bisulphate of potash.

It would appear at first sight that one-half of the sulphuric acid might be dispensed with, but it is found that when one equivalent only of sulphuric acid is employed, so high a temperature is required to effect the complete decomposition of the saltpetre (the above equation then representing only the first stage of the action), that much of the nitric acid is decomposed; and the neutral sulphate of potash (KO. SO,), which would

be the final result, is not nearly so easily dissolved out of the retort by water as the bisulphate.

For the preparation of large quantities of nitric acid, the nitrate of soda is substituted for nitrate of potash, being much cheaper, and furnishing a larger proportion of nitric

acid.

The nitrate of soda is introduced into an iron cylinder (A, fig. 139), lined with fireclay to protect it from the action of the acid, and half its weight of sulphuric acid (oil of vitriol) is poured upon it. Heat is then applied by a furnace, into which the cylinders are built, in pairs, when the hydrated nitric acid passes off in vapour, and is condensed in a series of stoneware bottles (B) surrounded with cold water.

The sulphate of soda left in the retort is useful in the manufacture of glass.

In the preparation of nitric acid, it will be observed at the beginning and towards the end of the operation, that the retort becomes filled with a red vapour. This is due to the decomposition of a portion of the colourless vapour of nitric acid by heat into water, oxygen, and nitric peroxideно + O + NO

HO. NO

=

PROPERTIES OF NITRIC ACID.

125

this last forming the red vapour, a portion of which is absorbed by the hydrated nitric acid, and gives it a yellow colour. The pure nitric acid is colourless, but if exposed to sunlight it becomes yellow, a portion suffering this decomposition. In consequence of the accumulation of the oxygen in the upper part of the bottle, the stopper is often forced out suddenly when the bottle is opened, and care must be taken that drops of this very corrosive acid be not spirted into the face.

The strongest nitric acid (obtained by distilling perfectly dry nitre with an equal weight of pure oil of vitriol, and collecting the middle portion of the acid separately from the first and last portions, which are somewhat weaker) emits very thick grey fumes when exposed to damp air, because its vapour, though itself transparent, absorbs water very readily from the air, and condenses into very minute drops of diluted nitric acid which compose the fumes. The weaker acids commonly sold in the shops do not fume so strongly. An exact criterion of the strength of any sample of the acid is afforded by the specific gravity, which may be ascertained by the methods described at page 115, using a hydrometer adapted for liquids heavier than water. Thus, the strongest acid (HO. NO) has the specific gravity 1·52, and contains 85.72 per cent., by weight, of NO,;* whilst the ordinary aquafortis or diluted nitric acid has the sp. gr. 1.29, and contains only 40 per cent. of NO. The concentrated nitric acid usually sold by the operative chemist (double aquafortis) has the sp. gr. 142, and contains 58 per cent. of NO.

A very characteristic property of nitric acid is that of staining the skin yellow. It produces the same effect upon most animal and vegetable matters, especially if they contain nitrogen. The application of this in dyeing silk of a fast yellow colour may be seen by dipping a skein of white silk in a warm mixture of concentrated nitric acid with an equal volume of water, and afterwards immersing it in dilute ammonia, which will convert the yellow colour into a brilliant orange. When sulphuric or hydrochloric acid is spilt upon the clothes, a red stain is produced, and a little ammonia restores the original colour; but nitric acid stains are yellow, and ammonia intensifies instead of removing them, though it prevents the cloth from being eaten into holes.

Nitric acid changes most organic colouring matters to yellow, but, unless very concentrated, it merely reddens litmus. If solutions of indigo and litmus are warmed in separate flasks, and a little nitric acid added to each, the indigo will become yellow and the litmus red. Here the indigo (CH.NO) acquires oxygen from the nitric acid, and is converted into isutine (CH,NO1).

When hydrated nitric acid is heated, it begins to boil at 184° F., but it cannot be distilled unchanged, for a considerable quantity is decomposed into nitric peroxide, oxygen, and water, the two first passing off in the gaseous form, whilst the water remains in the retort with the nitric acid, which thus becomes gradually more and more diluted, until it contains 68 per cent. of HO. NO,, when it passes over unchanged at the temperature of 248° F. The specific gravity of this acid is 1:42. If an acid weaker than this be submitted to distillation, water will pass off until acid of this strength is obtained, when it distils over unchanged. The facility with which hydrated nitric acid parts with a portion of its

It is extremely difficult to obtain the HO. NO, free from any extraneous water, as it undergoes decomposition not only when vaporised at the boiling point, but even at ordinary temperatures.