When a solution of barium chloride is added to a neutral or slightly alkaline solution, a pre cipitate of the corresponding barium compounds is thrown down if the solution tested contains sulphates, phosphates, borates, oxalates, fluor ides, carbonates, silicates, tartrates, sulphites, hy posulphites, arseniates, arsenites or chromates. (If the original solution contains lead, silver or mercury, nitrate of barium is used as a reagent in the place of the chloride, because other wise a precipitate of the chlorides of those metals would also be obtained). The mixed pre cipitate just obtained is isolated by filtration, washed and treated with dilute hydrochloric acid. All of the compounds named pass into solution, with the single exception of the sul phate; and if a white, insoluble precipitate re mains after treatment with dilute hydrochloric acid, the presence of sulphates in the original solution is demonstrated. The hydrochloric acid solution is next made alkaline by ammonia, when a precipitate will again be thrown down, if the original solution contained oxalates or phosphates. Certain other salts also may be thrown down at this point ; and, therefore, if any precipitate is obtained, confirmatory tests for oxalic and phosphoric acids should be ap plied to the original solution before the pres ence of those substances can be regarded as really proved.
If the original solution contains a carbon ate, it will effervesce upon the addition of hy drochloric acid, owing to the liberation of car bon dioxide. If it contains a sulphite, the ad dition of hydrochloric acid will liberate sulphur dioxide, which is readily recognized by smell. If it contains a hyposulphite, sulphur dioxide will also be liberated, but the solution will at the same time become turbid from the liberation of free sulphur. If it contains an alkaline silicate (other silicates are not soluble in water) in considerable quantity, the addition of hydro chloric acid will cause the silicic acid to sep arate in the form of a gelatinous precipitate, which, when dried and ignited, is gritty. The presence of the arsenic acids may be estab lished by Marsh's test for arsenic. (See AR SENIC). If a tartrate is present, the original compound, when obtained in the solid state by evaporation or otherwise, is blackened by heat, with a characteristic odor suggestive of burnt sugar. The presence of a fluoride in the origi nal substance is established by the liberation of hydrofluoric acid (see FLUORINE) when that sub stance is heated gently with concentrated sul phuric acid. The presence of a chromate is indicated by the precipitate from the addition of barium chloride exhibiting a yellow color.
As a confirmatory test a solution of lead ace tate may be added to a neutralized sample of the liquid under examination. If a chromate is present, a yellow precipitate of chromate of lead is thrown down, which is soluble in caustic soda, but insoluble in acetic acid. To test for the presence of borates, a samplp of the original liquid is acidified with hydrochloric acid, and a piece of turmeric paper is wetted with it, thor oughly dried before a flame and finally moist ened with a solution of sodium carbonate. If a borate is present, the color of the paper, where wetted with the sodium carbonate, changes to a greenish black.
To test the chlorides and certain other salts, a fresh specimen of the original solution should be chosen, and if it is not already acid, it should be made so with nitric acid. A solution of nitrate of silver is then added, whereupon a precipitate will he thrown down if chlorides, bromides, iodides, cyanides or sulphides are pres ent. (Cyanide of mercury, however, gives no precipitate at this point). The precipitate should be removed by filtration, thoroughly washed and then treated with hot ammonia. The chloride, bromide and cyanide of silver dissolve, while the sulphide and iodide remain behind, un changed. The sulphide is black, while the iodide is yellowish. For methods of distinguishing be tween the chloride, broinide and cyanide precipi tates reference must be made to the works on chemical analysis. It may be said, however, that the cyanide, when heated, is reduced to me tallic silver, with the liberation of cyanogen gas. The presence of an iodide, when this is made doubtful by the black precipitate due to a sul phide may be further investigated by the iodide of starch test. See IODINE.
Nearly all of the nitrates and acetates are soluble, and hence precipitation tests are not commonly used for the detection of nitric and acids. Color tests are usually employed instead. To test for acetic acid, the original solution may be made neutral, and a few drops of ferric chloride adder,. If acetates are present, the solution takes on a dark red color, owing to the formation of ferric acetate. To test for nitrates, a portion of the original solution is mixed with a solution of ferrous sulphate in a test tube, and strong sulphuric acid is poured down the side of the inclined tube, in such a manner as not to mix with the contents, but to form a layer at the bottom of the tube. If nitrates are present, a purplish or reddish ring, changing to a dark brown, will be observed at the surface of separation of the two liquids.