A text-book of assaying

by C. Beringer; J. J. Beringer

The assayer has only to take account of the organic matter when it is of commercial importance, so that in assays it is generally included under "loss on ignition."

In coals, shales, lignites, &c., the carbon compounds are, on heating, split up into oils and similar compounds. The products of distillation may be classified as water, gas, tars, coke, and ash. The assay of these bodies generally resolves itself into a distillation, and, in the case of the shales, an examination of the distillates for the useful oils, paraffin, creosote, &c., contained in them.

Elementary carbon is found in nature in three different forms, but these all re-act chemically in the same way. They combine with oxygen to form the dioxide.[116] The weight of oxygen required to burn a given weight of any form of carbon is the same, and the resulting product from all three has the same characteristic properties. Carbon dioxide is the common oxide of carbon. A lower oxide exists, but on burning it is converted into the dioxide. Wherever the oxidation of carbon takes place, if there is sufficient oxygen, carbon dioxide (carbonic acid) is formed; this re-action is the one used for the determination of carbon in bodies generally. The dioxide has acid properties, and combines with lime and other bases forming a series of salts called carbonates.

The carbon-compounds (other than carbonates, which will be subsequently considered) occurring in minerals are generally characterised by their sparing solubility in acids. The diamond is distinguished from other crystals by its hardness, lustre, and specific gravity. It may be subjected to a red heat without being apparently affected, but at a higher temperature it slowly burns away. Graphite, also, burns slowly, but at a lower temperature. The other bodies (coals, shales, &c.) differ considerably among themselves in the temperature at which they commence to burn. Some, such as anthracite, burn with little or no flame, but most give off gases, which burn with a luminous flame. They deflagrate when sprinkled on fused nitre, forming carbonate of potash. In making this test the student must remember that sulphur and, in fact, all oxidisable bodies similarly deflagrate, but it is only in the case of carbon compounds that carbonate of potash is formed. Carbon unites with iron and some of the metals to form carbides; combined carbon of this kind is detected by the odour of the carburetted hydrogen evolved when the metal is treated with hydrochloric acid; for example, on dissolving steel in acid.

The natural carbon compounds, although, speaking generally, insoluble in hydrochloric or nitric acids, are more or less attacked by aqua regia. The assayer seldom requires these compounds to be in solution. The presence of "organic matter"[117] interferes with most of the reactions which are used for the determination of the metals. Consequently, in such cases, it should be removed by calcination unless it is known that its presence will not interfere. When calcination is not admissible it may be destroyed by heating with strong sulphuric acid and bichromate or permanganate of potash or by fusion with nitre.

Carbon may be separated from other substances by conversion into carbon dioxide by burning. In most cases substances soluble in acids are first removed, and the insoluble residue dried, weighed, and then calcined or burned in a current of air. The quantity of "organic matter" may be determined indirectly by the loss the substance undergoes, but it is better to determine the "organic carbon" by confining the calcination in a tube, and collecting and weighing the carbon dioxide formed. Each gram of carbon dioxide is equivalent to 0.2727 gram of carbon.

Instead of a current of oxygen or air, oxide of copper may be more conveniently used. The operation is as follows:—Take a clean and dry piece of combustion tube drawn out and closed at one end, as shown in the figure (fig. 70), and about eighteen inches long. Fit it with a perforated cork connected with a U-tube (containing freshly-fused calcium chloride in coarse grains) and a set of potash bulbs (fig. 71) (containing a strong solution of potash), the exit of which last is provided with a small tube containing calcium chloride or a stick of potash. Both the U-tube and bulbs should have a loop of fine wire, by which they may be suspended on the hook of the balance for convenience in weighing. They must both be weighed before the combustion is commenced; to prevent absorption of moisture during weighing, &c., the ends are plugged with pieces of tube and glass rod.

Fill the combustion tube to a depth of about eight inches with some copper oxide, which has been recently ignited and cooled in a close vessel. Put in the weighed portion for assay and a little fresh copper oxide, and mix in the tube by means of an iron wire shaped at the end after the manner of a corkscrew. Put in some more oxide of copper, and clean the stirrer in it. Close loosely with a plug of recently ignited asbestos, place in the furnace, and connect the U-tube and bulbs in the way shown in the sketch (fig. 72).