Go to National Library of New Zealand Te Puna Mātauranga o Aotearoa
Volume 44, 1911
Previous Article | Table of Contents | Next Article
This text is also available in PDF
(135 KB) Opens in new window
– 299 –

Art. XXXI.—Note on the Composition of Nitric Acid.

By H. T. M. Fathers.

Communicated by Professor Easterfield.

[Read before the Wellington Philosophical Society, 4th October, 1911.]

Though the composition and molecular weight of nitric acid are known to be represented by the formula HNO3, I have been unable to find in any text-book a method of showing to a class that the substance really has the formula assigned to it. I have therefore worked out the details of a comparatively simple method whereby the demonstration may without difficulty be carried out.

1. Strong nitric acid is prepared by distilling a mixture of sodium-nitrate with an excess of 98 per cent. sulphuric acid at as low a temperature as possible, and the resulting acid is further dehydrated by distilling at about 20 mm. pressure (vacuum of the water-ejector pump) with three times its volume of strong sulphuric acid, and condensation of the acid-vapours by means of a good freezing-mixture. Diffusion of the aqueous vapour from the pump into the distillate is prevented by means of a tube containing pumice moistened with sulphuric acid. An acid prepared in this way will be found to be practically colourless, and to have a specific gravity and titration value corresponding to over 99 per cent. of pure nitric acid.

2. As soon as the acid is prepared a number of thin-walled glass bulbs, each capable of holding about 0.2 grams of acid, and blown on capillary stems about 3 cm. in length, are filled with the acid by placing the bulbs with the open end of the stem downwards in a beaker containing the acid and placing the beaker in a desiccator, which is then evacuated with the aid of the filter pump. Upon readmitting the air the bulbs will be found to be completely filled with acid, except for the presence of a very minute air-bubble. The ends of the capillaries are now sealed over a small flame, and the weight of the contained acid ascertained.

3. The apparatus in which the analysis of the acid is actually carried out is as follows:—

a is a U tube in which one of the bulbs containing a weighed quantity of nitric acid is placed. b is a short and narrow piece of combustion tubing containing a 10 cm. spiral of copper gauze which has been heated to redness

– 300 –

in situ in a current of air and subsequently reduced and allowed to cool in a stream of dry hydrogen; after this treatment the hydrogen has been displaced by dry air, and the tube and its contents weighed. c is a weighed calcium-chloride tube. d is a Schiff's nitrometer containing strong potash, with a mercury trap below.

4. The tubes a, b, c, and d having been arranged in position as shown, the air is displaced from the apparatus by a current of dry carbon-dioxide, conveniently prepared by heating sodium-bicarbonate in a test-tube and passing the gas over pumice moistened with sulphuric acid. When all the air is displaced the copper in b is heated to redness by a group of three or four Bunsen burners, the current of gas being at the same time slackened. The U tube a is now surrounded with hot water, which causes the contained bulb to burst, owing to the high coefficient of expansion of the nitric acid. The stream of carbon-dioxide carries the vapour of nitric acid over the red-hot copper, where it is decomposed according to the equation—

  • 2HNO3 + 5Cu = 5CuO + H2O + N2

When no further increase in the volume of gas in d is observed, the nitrometer is disconnected and the rest of the apparatus allowed to cool in the current of carbon-dioxide. The carbon-dioxide is then displaced from b and c by a stream of dry air, and the increase in the weight of these tubes taken. The composition is then at once arrived at, for

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Hydrogen = 1/9 of the increase in weight of c.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Oxygen = 8/9 of the increase in weight of c + the increase in weight of b.

Nitrogen = Number of c.c. of gas in d (corrected) × 0.00125 gm.

The ratio of the number of atoms is then obtained in the usual way by dividing the weight of each of the elements by the atomic weight of the same element respectively. Two experiments carried out on separate preparations of nitric acid by the above method gave—

(1)

H: N : O = 1.02 : 1.00 : 2.92,
(2)

= 1.00 : 1.00 : 2.97, which are sufficiently near to the required ratio 1 : 1: 3. The time taken from the commencement of passing the carbon-dioxide to the disconnecting of the nitrometer need not exceed half an hour, so that with a little foresight the analysis can be conveniently carried out in a lecture of an hour's duration.

Since the density of the vapour of nitric acid diluted with air has been shown to correspond approximately to that required for the formula HNO3,* all the facts required by the student in the establishment of the formula are thus available.

[Footnote] * Playfair and Wanklyn, “Journal of the Chemical Society,” vol. 15, p. 142.

Previous Article | Table of Contents | Next Article