Substances which contain a great excess of carbon sometimes escape complete combustion by this process; when this is feared, all danger may be averted either by adopting the plan of Dumas, already mentioned, or by pulverizing some chlorate of potash finely, which is care fully dried, mixed with about four times its weight of oxide of copper, and the first portion of the retort is filled with it for about an inch ; the tail-like prolongation may in this case be dispensed with at the close of the operation; instead of sucking air through the apparatus, we very cautiously apply heat to the chlorate, oxygen is evolved, which burns the last traces of carbon and displaces the gas and aqueous vapour svhich the tubes contain. When the chlorate has been used, the last inch of oxide of copper must be kept separate, from the rest, as it will be mingled with chlOride of potassium, and must not be employed again until it has been washed from the salt, for as chloride of copper is slightly volatile it would be deposited in the drying tube and unduly increase its weight. lf the heat is too suddenly applied, a portion of the chlorate is apt to be carried forward me chanically, and this constitutes the chief ob jection to its use.
Sometimes chromate of lead is advantage ously substituted for oxide of copper with sub stances difficult of combustion, as by a bright red heat alone it gives off a portion of its oxy gen. It is easily prepared by precipitating the chromate or bichromate of potash with solution of acetate of lead. It should be well washed and heated to inCipient fusion before it is used for analysis. It has the advantage of being much less hygroscopic than the oxide of cop per. To ensure accuracy, when much gas (as in this case, and in instances where nitrogen is present,) passes through the potash apparatus during the whole experiment, it is best to con nect the open extremity with an additional drying tube, charged with solid hydrate of potash instead of chloride of calcium, the weight of which has been carefully noted, as a portion, very small but still susceptible, of aqueous vapour is carried off from the solution of potash by the gas, and would otherwise be lost, making the quantity of carbon appear sornevvhat too little.
3. 21nalysis of a body containing nitrogen.
Two separate analyses are in this case re quired ; the first, to discover the proportion of carbon and hydrogen; and the second expressly for the nitrogen. When bodies containing nitrogen are burned with oxide of copper, a variable proportion of the lower oxides of nitrogen is formed, which being retained by the chloride of calcium or potash would render the analyses incorrect. A precaution is there fore employed which renders it necessary to make use of a retort:Aube somewhat longer than common ; it is charged as usual to within four inches of the opening, and then filled up with clean copper turnings ; the apparatus is arranged as before directed, the copper turnings are brought to full redness, and the analysis proceeded with cautiously in the ordinary man ner. As the.oxides of nitrogen pass slowly over the ignited copper they are decomposed, the oxygen combining with the copper while pure nitrogen escapes ; the quantity of carbon and hydrogen is determined exactly as heretofore. To ascertain the proportion of nitrogen, the most accurate method is that recently devised by Varrentrapp and' Will, and suggested about the same time by Berzelius ; the fundamental fact consists in the observation of Gay Lussac, that when azotised matters are heated with a large excess of hydrate of potash (soda answers equally well), the whole of the nitrogen is ex pelled in the form of ammonia. In order to
render it available for the purposes of analysis the subjoined precautions are requisite.
A mixture of two parts of quicklime and one of hydrate of soda is prepared by slaking some well.burned lime with the necessary quantity of a solution of soda ; the whole is evaporated to dryness, ignited, the dry mass pulverized as quickly as possible, and then transferred to well-stopped bottles, in order to exclude carbonic acid and moisture. When an analysis is to be made we proceed as usual, making the mixture in a warm mortar, only substituting the alkalized lime for oxide of copper; the accidental presence of a little moisture, after the weight of material for ana lysis is accurately known, is of no consequence in this case.
Having introduced the mixture, it is better loosely to plug the aperture of the retort with a few fibres of asbestus (which has been ignited just before) to prevent any mechanical trans port of the mixture into the apparatus through which the gases are passed ; on applying heat to the combustion-tube in the ordinary way, and with the usual precautions, the substance is decomposed, and the whole of the nitrogen escapes as ammonia. The drying tube and potash apparatus are dispensed with, and the ammonia is collected by attaching a bulb-tube of the form represented (fig. 431, B), air-tight with a good cork, to the retort-tube, the apparatus having been previously charged with hydro chloric acid sp. gr. 1.1, as high as the lines in the figures indicate. Pure hydrochloric acid is easily procured for this purpose by diluting the ordinary acid of the shops till it has a sp. gr. of 1.1, and distilling in glass ves sels—the first eighth may be rejected. Dis tillation may be proceeded with until three quarters of the acid employed have passed over. It is better for the operator always to rectify his own acid, in order to be quite sure of the absence of any trace of am monia. The tube connecting the bulbs should be somewhat larger in diameter than that of the ordinary potash apparatus, in order to allow the liquid to be poured out readily. When the operation is complete, absorption will take place and the fluid rise in the bulb nearest the fire; at this moment we nip off the top of the combustion-tube and draw air carefully through the apparatus in the usual way. When the combustion is terminated, the contents of the bulb-tube are emptied into a small evaporating' disli, and the apparatus washed out first with a little alcohol and ether, and afterwards several times with water; some solution of bichloride of platinum is added, and the whole evaporated to dryness by a water-bath or chloride of cal cium bath ; when dry, it is digested with a mixture of two parts of alcohol, sp. gr. 833, and one of ether, which dissolres the excess of bichloride of platinum, and leaves the double chloride of platinum and ammonium in a crys talline form.