Estimation of Carbon in a Sample of Graphite (Black-lead).—Weigh up 1 or 2 grams in a dish and calcine in the muffle till the carbon is burnt off. Weigh the residue, and calculate the carbon by difference.
Determination of Carbon in Iron.—The carbon exists in two states—free (graphite) and combined. The following process estimates the total carbon:—The carbon existing as graphite may be separately estimated in another portion by the same process, but using hydrochloric acid to dissolve the iron instead of the copper solution:—Weigh up 2 grams of the iron (or a larger quantity if very poor in carbon), and attack it with 30 grams of ammonic-cupric chloride[119] dissolved in 100 c.c. of water. Let the reaction proceed for a quarter-of-an-hour, and then warm until the copper is dissolved. Allow to settle, and filter through a filtering-tube. This is a piece of combustion tube drawn out and narrowed at one end, as shown in fig. 76. The narrow part is blocked with a pea of baked clay, and on this is placed half-an-inch of silica sand (previously calcined to remove organic matter), then a small plug of asbestos, and then a quarter-of-an-inch of sand. The tube is connected with a pump working at a gentle pressure, and the solution is filtered through the tube with the aid of a small funnel (fig. 77). The residue is washed, first with dilute hydrochloric acid, and then with distilled water. The tube is dried by aspirating air through it, and gently warming with a Bunsen burner. The tube is then placed in a small combustion-furnace, and connected with calcium chloride and potash bulbs, as shown in fig. 78. The potash bulb to the right of the figure must be weighed. A slow stream of air is drawn through the apparatus, and the heat gradually raised; in from thirty minutes to one hour the combustion will be complete. The potash bulbs are then disconnected and weighed, and the increase multiplied by 0.2727 gives the weight of carbon.
CARBONATES.
Carbon dioxide, which is formed by the complete oxidation of carbon, is a gas with a sweetish odour and taste, having a strong affinity for alkalies, and forming a series of compounds termed carbonates. The gas itself occurs in nature, and is sometimes met with in quantity in mining. The carbonates occur largely in nature, forming mountain masses of limestone, &c. Carbonates of many of the metals, such as carbonate of lead (cerussite), carbonate of iron (chalybite), carbonates of copper (malachite and chessylite), and carbonate of magnesia (magnesite), are common.
All the carbonates (those of the alkalies and alkaline earths excepted) are completely decomposed on ignition into the oxide of the metal and carbon dioxide; but the temperature required for this decomposition varies with the nature of the base. All carbonates are soluble with effervescence in dilute acids; some, such as chalybite and magnesite, require the aid of heat. The alkaline carbonates are soluble in water; the rest, with the exception of the bicarbonates, are insoluble therein.
Carbonates are recognised by their effervescence with acids—a stream of bubbles of gas are given off which collect in the tube, and possess the property of extinguishing a lighted match. The most characteristic test for the gas is a white precipitate, which is produced by passing it into lime or baryta-water, or into a solution of subacetate of lead.
The expulsion of carbon dioxide by the stronger acids serves for the separation of this body from the other acids and bases.