NITRIC ACID—COMPOUNDS OF AZOTE AND OXYGEN.
Since the account of nitric acid (Vol. 1, page 343) was printed, a change has universally taken place in estimating the weight of the nitric acid atom, and of the proportion of azote and oxygen in the same. This has been effected chiefly by a more correct analysis of nitre than existed at that time. Nitre is now found to consist nearly of 52 parts acid and 48 parts potash per cent. Hence if the atom of potash be 42, that of nitric acid must be 45; for, 48 ∶ 52 ∷ 42 ∶ 45, nearly. That is, the nitric acid atom consists of 10 azote + 35 oxygen by weight; or of 2 atoms of azote (according to my estimate) and 5 of oxygen. There appear to be two nitrous acids; namely, the one which I have designated by that name, which may now be called subnitrous, or as Gay Lussac terms it pernitrous; and the other what I considered as nitric acid in the former volume, composed of 1 atom azote, and 2 of oxygen.
Real nitric acid then is that combination which is effected by uniting oxygen with a minimum of nitrous gas; or 1 measure of oxygen with 1.3 nitrous gas, (See Vol. 1, page 328). The oxynitric acid, which I was led to infer from the last mentioned combination, (1 azote with 3 oxygen) does not appear to exist. The Table of nitric acid (Vol. 1, page 355) will require some correction. An increase of about 4 per cent. should be made, I apprehend, on the quantities of acid corresponding to the several specific gravities.
Since my former volume of Chemistry was printed, several essays on the compounds of azote and oxygen have been published, with some new and some additional experiments, the chief of which may be seen in Sir H. Davy’s Elements of Chemical Philosophy, the Annales de chimie et de physique, Vol. 1; Annals of philosophy, Vol. 9 and 10; and the Manchester Society’s Memoirs, Vol. 4, second series; also Dr. Thomson’s first principles of Chemistry. Notwithstanding all that has been written on the subject, there still appears uncertainty as to the number of combinations formed by these two elements, their relative weights, and the number of atoms in the several compounds.
The results of an experiment I lately made on the decomposition of nitrate of potash by heat seem to be worthy of record, as I am not acquainted with those of any other person who has pursued the experiment to the same extent.—I took an iron retort of 6 cubic inches capacity, and cleaned it as well as I could from carbonaceous matter which it had previously contained, first by heating nitre to redness for an hour or more in it, and then washing it repeatedly with water till the liquid came out tasteless, and only mixed with a little red rust; I then put in 480 grains of purified nitre, and having secured a copper tube to the retort so as to be air tight, the retort was put into a fire and gradually raised to a red heat, and the fire was occasionally urged with a pair of bellows, in order to keep up a glowing red on the retort for nearly two hours; the air was received over water in jars; the first 4 or 5 inches were thrown away, and the rest was preserved and transferred to a graduated jar; the products were examined in successive portions as under, namely,
| Inches. | |||||||
| 1 | produce, | 85 | cubic inches, | 83 | per cent. pure | = | 70.5 |
| 2 | 5 | 77 | = | 3.85 | |||
| 3 | 25 | 50 | = | 12.5 | |||
| 4 | 6 | 30 | = | 1.8 | |||
| Total | 121 | Oxygen | 88.65 | ||||
| Oxygen | 88.65 | = 30 grains. | |||||
| Residue | 32.35 | = 10 grains. | |||||
About 1 per cent. on the whole gas was carbonic acid, the rest oxygen and azote, the weights of which would be nearly as above.
Towards the last the gas came very slowly, and being of inferior quality, the operation was discontinued.
The remaining contents of the retort were diluted with water, and well washed till the water ceased to shew alkali; the liquid was then concentrated and gave 1600 water grain measures of the sp. gr. 1.153. There were obtained also 64 grains of red oxide of iron from the washing of the retort, containing 19 grains of oxygen.
The liquid was divided into portions and examined; the original nitre consisted of 250 grains of nitric acid united to 230 of potash = 480 grains. After the process there appeared to be,
| 10 | grains of carbonic acid united to | 21 | grains potash. |
| 62 | grains of subnitrous acid to | 84 | ”” |
| 134 | grains nitric acid to | 125 | ”” |
| 230 |
The quantity of carbonic acid was determined by lime water: the quantity of potash uncombined with nitric acid was found by precipitating it by tartaric acid, and manifested 105 grains of potash in the bitartrate = that combined with the carbonic and subnitrous acids; from which subtracting 21, it was inferred the remainder 84 must have been in union with subnitrous acid, or else with nitrous acid; the rest of the potash, not being acted upon by tartaric acid, was understood to be combined with nitric acid.
The quantity of subnitrous acid given above, appeared somewhat hypothetical, till it was confirmed by treating a portion of the liquid with oxymuriate of lime solution of known strength; it was found that 32 grains of oxygen were required to be combined with the subnitrous acid, in order to restore it to the state of nitric acid; that is, when oxymuriate of lime, containing that quantity of oxygen, was added to the liquid, and this was afterwards rendered acidulous by the addition of sulphuric acid, neither nitrous vapour nor oxymuriatic gas was perceptible; but a greater or less quantity of the oxymuriate being applied, and the liquid made acidulous, the fumes of the one or the other were abundantly manifest.
It remains to account for the oxygen. There were 250 grains of nitric acid at first in the nitre; of which 200 grains were oxygen and 50 azote, nearly. One-fifth part of the oxygen = 40 grains, corresponds to 1 atom of oxygen. Now the whole of the oxygen derived from the nitre in the course of the experiment, seems to be 30 grains in gas, 7 grains in the carbonic acid, and 19 grains in the iron oxide, together equal to 56 grains. Now the azote and oxygen in the gas collected, were very nearly in the proportion of those elements in nitric acid; so that a portion of the acid (about ⅙) might be considered as completely decomposed, whilst the rest was only losing a small part of its oxygen: this is remarkable, and I think indicates that the carbonic acid (formed from the carbon of the retort, or from the adhering carbon) unites to the potash, expelling the nitrous acid, which is immediately decomposed into its elements azote and oxygen. This would not however account for the whole of the azote: for, 40 grains of nitric acid would be united to 37 potash; whereas we find only 21 potash with carbonic acid; and I cannot believe that an error in the estimate of carbonate of potash could exist to that amount. The fact, however, was, that the elements of 40 grains of nitric acid were found in the evolved gas, and hence we have to account for the remainder 210 grains. From this there appears to have been expelled 26 grains of oxygen, nearly 19 and 7 as related above; of which the 19 grains cannot be correctly estimated by reason of the uncertainty as to the real quantity of oxide formed during the operation: there might be some left adhering to the retort, or on the other hand there might be more than the due share, derived from former experiments. Supposing then, that 26 grains of oxygen were extracted from the nitric acid, the remaining acid would require the same to be added to re-form the nitric; but by the experiments with oxymuriate of lime it seemed to require 32 grains of oxygen. This difference wants an explanation; I believe the greater error must belong to the 26 grains; perhaps the truth might be approximated best by supposing both to be 30 grains.
When the liquid decomposed nitre is treated with any acid, a gas is instantly expelled which produces red fumes in the air; it is pure nitrous gas, which joining with the oxygen of the atmosphere, generates nitrous acid vapour. At the same time, no doubt, the subnitrous acid is disengaged from the potash, but that part of it which is real nitrous acid (1 atom azote to 2 of oxygen) is retained by the water, whilst the other part, (1 atom azote and 1 of oxygen) assumes the gaseous form. In order to be satisfied respecting this point, I made several experiments with the liquid over mercury: taking a given portion of the liquid, and sending it to the top of a graduated tube filled with mercury, I passed up as much muriatic acid as was sufficient to engage the potash; immediately there was a disengagement of nitrous gas and carbonic acid gas, and afterwards a slow evolution of gas, evidently arising from the liquid in contact with the mercury. Wishing to ascertain the quantities, I sent up 25 grain measures of liquid, and to that nearly half its bulk of muriatic acid; in 2 or 3 minutes there was,
| 1.1 | cubic inch of gas. | H. | M. |
| 1.4 | in 0 | 45 | |
| 1.5 | 1 | 5 | |
| 1.7 | 2 | 45 | |
| 1.75 | 7 | 45 | |
| 1.78 | 9 | 45 |
The gas was washed in lime water, and lost .33 parts of an inch of carbonic acid; the rest, 1.45 cubic inch, was nitrous gas. It is obvious that ½ of the nitrous gas, together with the carbonic acid, was liberated instantly; the rest of the nitrous gas was due to the nitrous acid, slowly acting upon the mercury. At the end of the process, there was a little black oxide floating upon the mercury. Calculating from this, the whole quantity of nitrous gas would be 31 or 32 grains, whereas it ought to have been 48 grains to constitute 62 of subnitrous acid. It is probable that whilst a portion of the subnitrous acid is oxidizing the mercury, another portion may be forming nitric acid and dissolving the oxide.
From some trials, I have reason to think that even carbonic acid will expel nitrous gas from the liquid sub-nitrite of potash.
In the essay of Dr. Henry, already alluded to, published in the 4th Vol. of the Manchester Society’s Memoirs, a new and interesting discovery is made; namely, that a mixture of nitrous and olefiant gases, though not explosive by an electric spark, may still be exploded by the more powerful impetus of a shock from a charged jar. Dr. Henry has adduced the results obtained in this way, as corroboratory of those which shew the constitution of nitrous gas to be 1 volume of azote and 1 of oxygen united to form 2 volumes of nitrous gas. (See page 507 of the Memoirs.)
Some time ago in repeating these experiments of Dr. Henry, I found some extraordinary circumstances attending them. After determining that 1 volume of olefiant gas may be fired with from 6 to 10 volumes of nitrous, I found a shock from a jar sometimes inadequate to fire the mixture, which, however, when repeated a second or third time, succeeded. This is not a novelty; for, mixtures of olefiant gas as well as other gases and vapours, with a minimum of oxygen, frequently require several sparks before the explosion: but this case occurs at times with nitrous and olefiant gas, when they are mixed in the most favourable proportions for exploding. The most remarkable circumstance, however, was, that when a phial was filled with the mixture of the two gases in the proportion of 1 volume olefiant to 6 or 7 nitrous, (exclusive of small portions of azote), the decomposition of the nitrous gas and the combustion of the olefiant were scarcely ever perfect; and what increased the perplexity more, was, the results obtained from the same mixture scarcely ever agreed one with the other. After about 30 experiments, I was inclined to adopt the conclusion, that the uncertainty was occasioned by the oblong form of the eudiometer. The spark or shock, in my eudiometer, is imparted at one extremity of a column of air, which is often 10 times as much in length as in diameter: it mostly was found that the larger the quantity of mixture exploded at once, the more imperfect and incomplete was the combustion. I imagine the intensity of heat is not sufficient to carry on the combustion through the length of the column, owing, perhaps, to the cooling power of the sides of the tube. Hence it was, I apprehend, that in one or two instances, when a small quantity of gas was used, I got nearly complete results, as Dr. Henry reports his; but in the majority both gases were found in the residue after the explosion.
In pursuing this enquiry into the decomposition of nitrous gas by combustible gases, I found that it might be effected by any combustible gas or vapour: at least it succeeded in all I tried. The method I pursued, and which was suggested by the known properties of phosphuretted hydrogen, is this: it has been shewn ([page 181]) that a mixture of phosphuretted hydrogen and nitrous gas exploded by an electric spark, the former gas being completely burned in case the proportions are duly adjusted; now, it occurred to me, that as the above combustible gas is usually a mixture of pure phosphuretted hydrogen and of hydrogen, and that the latter of these is also burned as well as the former, the effect must be produced through the heat occasioned by the combustion of the former. Having some old phosphuretted hydrogen by me, at the time, which on examination, I found to be 91 per cent. combustible gas, and 9 azote; and the 91 combined with 156 of oxygen, consequently was 74 pure, and 17 hydrogen; I tried this mixture with nitrous gas, when it exploded by the spark, as usual; but on trying it with an excess or defect of nitrous gas, the spark was inefficient, but the shock instantly fired the mixture. As there did not appear to be any of the pure hydrogen left unburned in these experiments, I proceeded to mix the old phosphuretted hydrogen with hydrogen; and then this new mixture with nitrous gas. The first experiment was made with 4 parts of old phosphuretted hydrogen + 16 hydrogen + 36 nitrous gas = 56 total. On this mixture the spark, of course, had no effect; but it exploded the first trial by the jar, and left 20 measures, of which 2 were found to be oxygen, and the rest azote. This experiment succeeding so well, I next tried mixtures of phosphuretted hydrogen, with carbonic oxide, carburetted hydrogen, and ether vapour successively, along with nitrous gas; and found that all these mixtures refused combustion by the spark, but were instantly exploded by the shock, yielding carbonic acid and water, the same as if the combustion had been effected by free oxygen. In some instances the combustion was complete, leaving neither combustible gas nor nitrous gas; but generally there was a residue of one or both of the gases.
From these experiments it may be concluded that the heat, produced by the combustion of phosphuretted hydrogen and nitrous gas or oxygen gas, disposes other gases, accidentally in the mixture, to chemical changes. In conformity with this view, I mixed phosphuretted hydrogen and oxygen, in the proportion of mutual saturation; and taking a small proportion of this mixture, and as much ammoniacal gas as would saturate the phosphoric acid to be formed, I found that causing an explosion over mercury, the phosphoric acid combined with the ammonia, and nearly the whole gas disappeared. In this case, the heat was not sufficient to decompose the ammonia. But in another experiment, with a portion of the same explosive mixture and a less proportion of ammonia, after the firing a residue of azote and hydrogen was found, amounting nearly to the quantity due from the decomposition of the ammonia. Here the heat produced, had evidently decomposed the ammonia.