In accurate experiments in the laboratory, ammonia is usually WEIGHED either as chloride of ammonium (see above), or as ammonio-bichloride of platinum (NH4Cl, PtCl2); every gr. of the latter representing ·07614 gr. of pure ammonia. Sometimes, though rarely, the quantity of ammonia is determined from the volume of nitrogen eliminated from it, of which 14 gr. represent 17 gr. of ammonia.
Concluding remarks, Patents, &c. Whatever form or process may be adopted for the preparation of liquid ammonia, it is absolutely necessary to keep the receivers as cool as possible, by means of snow, ice, or a current of very cold water, for the purpose of promoting the absorption of the gas, and to prevent its loss. On the small scale, the glass receivers or bottles may be most conveniently surrounded with ice, or a freezing mixture, and two, or more of them, should be furnished with safety-tubes, to prevent accidents. On the large scale, a capacious oblong retort, usually of iron (but sometimes, though seldom, of lead), with a large opening or tubulature conveniently situated for inserting the ‘charges,’ and withdrawing the residuum of the distillation, is employed. The tubulature, or opening, is closed by means of a large and accurately ground iron stopper, or with a door secured by screws, as the case might be. The stopper is well greased before insertion, and is removed by means of a powerful lever. Should it become so firmly fixed that it cannot be displaced in the usual manner, a cloth moistened with cold water, and carefully wrapped round it, without touching the neck of the retort, will generally cause it to contract sufficiently to enable the operator to remove it with facility. Sometimes a large iron kettle, with a moveable and accurately fitting lid secured in its place like that of a ‘Papin’s digester,’ and having a large and long tubulature in its centre, is employed instead of a retort, over which it has the advantage of exposing a larger opening for the removal of the residuum of the process. In either case the distillatory
vessel is imbedded in sand supported by fire-brick, and is not exposed directly to the heat of the furnace. Before commencing the distillation the joints are all well luted, to avoid leakage. An excellent plan is to pass the gas, as it leaves the retort, through a silver or pewter ‘worm’ or ‘refrigerator’ set in a tub supplied with a stream of very cold water; by which it will be sufficiently cooled before it reaches the ‘receivers’ to obviate the necessity of any further attention to them than keeping the cloths wrapped round them constantly moistened with cold water. The lower end of the ‘worm’ should be connected, by means of a balloon-shaped ‘adopter,’ with the ‘still,’ and the upper end with the first ‘receiver,’ the use of the balloon being to intercept any volatilised ammonia-salt that might be accidentally driven over by the heat being too high, or too suddenly raised.
The heat should be gradually applied, and very gradually raised, to prevent any of the sal ammoniac or sulphate being volatilised undecomposed; and even towards the end of the process it should not even approach redness.
The lime is best ‘slaked’ and ‘papped’ with about 4 parts of water; as a lower heat is then required to expel the gas, and it passes over more easily and fully than when less water is employed. This is absolutely necessary when the sulphate is the ammonia-salt used; as otherwise the residuum of ‘sulphate of lime’ would become so hard that it could not be easily removed from the retort.
The gas being wholly expelled from the retort, or other distillatory vessel, it is disconnected from the receivers, and (when sal ammoniac has been employed) the heat is raised sufficiently high to fuse the residual chloride of calcium, which is then at once baled or poured out. Glass retorts often suffer fracture at this point; but if they escape now, it generally happens that they are broken when heat is applied for a second operation. Hence, according to Prof. Muspratt, it is rare to find a retort, even when carefully handled, that will stand two operations.
When crude sulphate of ammonia is employed it is advisable to have only a little water in the first receiver, which is placed there merely to purify the gas which passes through it, and to retain any traces of volatile empyreumatic or oily matter which may be carried over with it.
Pure solution of ammonia is most easily obtained from ‘sal ammoniac,’ but crystallised sulphate of ammonia, often crude, is more commonly employed, on account of its lower price.
The preparation of pure solution of ammonia admits of no other improvements than such as merely affect the form of the apparatus employed to produce it; and hence, unlike the ammonia-salts of commerce, has been little meddled with by inventors and patentees. Among the plans having for their object the production of an ammoniacal solution, more or less concentrated, fitted for many of the purposes of the arts, and for the preparation of salts, but not for chemical and medical use, besides those of Reece, Spence, Crane and Jullien, &c., already noticed, may be mentioned—
1. That of Watson (Patent dated 1838) in which gas-liquor mixed with a proper quantity of fresh-slaked lime is distilled from a spacious retort or still into a receiver containing cold water, until much steam passes over with the gas, when the strong alkaline liquor forming the distillate, and called the first portion, is drawn off. The distillation is then continued, when a weaker and impurer solution is obtained, called the second portion. The first portion is then reintroduced into a retort or still with a small quantity of fresh lime, and the distillation repeated. The product the patentee calls the first portion of the second distillation. The latter is a strong ammoniacal liquor sufficient for all the purposes of scouring, cleaning, conversion into commercial ammonia-salts, &c. It may be further purified by a third distillation; the second portion of each operation being transferred again to the still with the next fresh charge of gas-liquor.