The preparation of these salts and their application to the waterproofing of paper and textiles have been made the subject of much study by Dr. Alder Wright, to whom the following remarks are mainly due. The term "cuprammonium compound" is usually understood by chemists as indicating a member of the class of substances obtainable by the combination of ammonia with certain copper compounds, so as to give rise to a "metallo-ammonium" derivative containing copper. Salts of copper, e. g. copper sulphate, usually combine with 4 proportions of ammonia; thus cupro-tetrammonium sulphate is obtainable in crystals, by simply pouring a concentrated solution of copper sulphate into a solution of ammonia, in such proportions as to obtain a clear deep blue liquid, and then precipitating the crystallised salt by adding a considerable quantity of highly concentrated ammonia solution, or by shaking with alcohol; in a similar fashion numerous other cupro-tetrammonium salts can be obtained. A closely related compound, but possessing somewhat different properties, is cupro-ammonium hydroxide, prepared by dissolving cupric hydrate in ammonia solution, or by agitating together metallic copper and ammonia solution in presence of air, when the copper oxidises and dissolves in the ammoniacal liquor, forming a deep blue liquid, sometimes termed "copperised ammonia."
Most of these compounds are very unstable, breaking up under the influence of heat and water alone, or conjointly; thus cupro-tetrammonium sulphate treated with a large bulk of 3old water is partly decomposed, forming a basic insoluble copper sulphate, together with free ammonia and ammonium sulphate; cupro-ammonium hydroxide solution is decomposed by simple addition of alcohol to its ammoniacal solution, a blue substance essentially consisting of hydrated copper oxide being precipitated; the same result ensues on boiling, save that anhydrous black copper oxide is then formed, ammonia being driven off. In presence of a large excess of ammonia, the instability is less marked in all cases; the strongly ammoniacal fluids formed by dissolving copper salts or copper hydroxide in a considerable excess of ammonia water are the "cuprammonium solutions" referred to.
It has long been known that these solutions possess the power of apparently dissolving cellulose and various allied substances; thus paper, cottonwool, and similar materials, when digested with these fluids, disappear, and are apparently truly dissolved. It is held, however, by some chemists that these are not cases of true solution, but that the substances are simply gelatinised and disseminated through the fluid in a transparent form, as starch is in water. On the other hand, on neutralising the fluid by an acid, or better still, on adding potassium cyanide solution until the blue tint is discharged, the cellulose reappears as a gelatinous precipitate; this result would suggest that the reappearance of the cellulose is brought about by the destruction of the solvent in which it was truly dissolved, viz. the cuprammonium compound, by conversion into ammonia and cupro-cyanide (or into amnionic and cupric salts, if an acid be used). On evaporation to dryness of a cuprammonium solution in which cellulose has been dissolved, a more or less gummy mass is formed, containing the cellulose intermixed with copper oxide, and with ammonia and copper salts if a cuprammonium salt were used, but containing copper oxide and a green copper derivative or compound of cellulose if cuprammonium hydroxide were employed.
When the cellulose is in excess, e. g. when the solution is evaporated on the surface of paper or calico, just dipped in the solution, black copper oxide is often not formed at all; but a green varnish-like mass of cellulose conjoined with copper oxide, or of the copper salt of some feeble acid derived from and closely akin to cellulose, coats the surface of each filament of the fabric used, welding and cementing them together. This cement-like "cupro-cellulose," as it may be termed, being insoluble in water, communicates water-resisting properties to the material so treated; moreover, the presence of copper renders the dipped and dried substance less prone than before to the attacks of insects and mould, so that animal and vegetable life of a parasitic nature and fungoid growths are rarely, if ever, to be observed in the substances, even when kept under conditions where boring worms, ants, rot, and mould would be likely to attack them.
To produce the best results in this direction, solution of cuprammonium hydroxide is, for many reasons, preferable to solutions containing cuprammonium salts; not only is the action on cellulose more energetic for a given amount of copper and ammonia in solution, but various other advantages are gained. For example, if ammoniacal solution of cuprammonium sulphate be used, the dried treated fabrics will contain ammonium sulphate, and sometimes copper sulphate, soluble in water, rendering the material porous if exposed to the action of water in sufficient quantity to dissolve out the soluble matters, and causing more or less tendency to unsightly efflorescence under other conditions. Further, during the drying of materials treated with cuprammonium hydroxide solution, all the ammonia present is volatilised, and may be recovered by appropriate means; whereas, with cuprammonium sulphate solution, a considerable fraction of the ammonia is fixed in the fabric as sulphate, and so lost.
A peculiar property of cuprammonium solutions, and one most important from the manufacturing point of view, is that whereas iron is, as is well known, attacked and dissolved by solutions of ordinary copper salts (e. g. the sulphate, "blue vitriol"), an equivalent quantity of copper being precipitated during the operation, no such action is observable with cuprammonium hydrate solutions; so that cast-and wrought-iron tanks and baths for the reception of the liquor may be used with impunity, as may steel rollers and machinery of all kinds when employed in contact with the liquor, or with fabrics moistened therewith. On the other hand, copper and brass must be studiously avoided in the construction of such appliances, otherwise corrosion and injury are speedily brought about. This peculiarity, as regards the non-action of iron and steel, is the more remarkable in that it is not observed with zinc; this latter metal precipitating copper (and being itself dissolved) with about equal facility, whether the copper be in the form of an ordinary copper salt, or in that of a cuprammonium solution.