To understand this subject intelligently it will be necessary to possess some slight knowledge of chemistry and of the materials used by chemists, but any intelligent person can, by careful reading of these descriptions, test substances qualitatively without the aid of expensive apparatus or external assistance of any kind. For quantitative analysis a delicate chemical balance will, of course, be required. Take paints first. White lead is now very seldom sold in the dry state, but samples are occasionally met with. Its covering power being superior to that of any other known paint, it is very largely used, and it is frequently adulterated with substances of inferior quality. The most common adulterants are sulphate of baryta and chalk.
Treat a small quantity with dilute nitric acid, and heat on the sandbath. If any insoluble remains, it is either sulphate of baryta or insoluble silicates. Filter, take a portion of the insoluble on a piece of clean platinum wire moistened with hydrochloric acid, and test at the blowpipe. If the flame be colored green, the precipitate is sulphate of baryta. By moistening the wire in hydrochloric acid the green color is reproduced many times.
If no green color appears, the insoluble is a silicate. This may be proved by forming a bead with microcosmic salt on a loop at the end of the platinum wire, and taking some of the precipitate on this bead, fusing it again in the blowpipe flame. If small infusible particles whirl around within the bead while in the flame, the presence of silicates in the precipitate may be inferred.
Take 20 grains of the sample, treat with dilute nitric acid as before. The quantity taken for analysis may be weighed in a watch-glass or a small basin, but should be transferred to a beaker, and the glass or basin washed with distilled water before the acid is added. If this precaution be not taken, and the acid added directly in the watch-glass or basin, to be washed into the beaker afterwards, the violent effervescence which takes place on the addition of the acid will occasion considerable loss by spurting. If, after heating with nitric acid, an insoluble remains, a few crystals of chlorate of potash may be added to the boiling liquid to insure the solution of all soluble substances. The boiling is continued for a few minutes, then cold water is added, and the whole passed through a filter. The insoluble on the filter is washed with hot water until the water leaving the filter is no longer acid to litmus paper. It is then dried on the water-bath, ignited, and weighed. Test at blowpipe as before.
Sulphate of baryta and silicates, if both present, are not usually separated.
This general direction applies to almost all precipitates. If strong acids or acid and chlorate of potash are used, the liquid should invariably be diluted before filtering. In washing, allow the whole of the wash water to drop from the filter before adding more water. When thoroughly washed, fold the filter paper flat in the funnel, or, better, transfer the filter paper and its contents to a large watch-glass or flat basin, and dry on a water-bath. When dry, carefully unfold the filter over a quarto sheet of stiff glazed paper, with a feather brush off every particle of the precipitate adhering to the filter paper, collect the precipitate on the glazed paper, again using the feather, and cover over the precipitate with the funnel. Fold the filter paper till it assumes the appearance of a solid cylinder about one inch in length, and ignite with the Bunsen burner over a weighed platinum or porcelain capsule. After a while the filter paper becomes a charred mass of smaller dimensions, and drops from the wire into the capsule. The wire is cleaned into the capsule, by means of the feather, of any adhering particles, the charred paper is crushed with a glass rod, and ignited over a Bunsen until the ash is no longer black.
The capsule is then stood on a porcelain slab, and the precipitate carefully transferred from a glazed paper into the capsule, the feather being employed to remove the last traces. The capsule is then ignited over an Argand burner until the weight is constant. The filter paper must always be ignited before the bulk of the precipitate is transferred to the capsule.