5. CHLORIDE OF AMMONIUM. NH+ Cl.

Uses. For retaining certain salts and oxides in solution, when others are precipitated by ammonia or some such reagent, as magnesia, which is held in solution by ammonia and chloride of ammonium, whilst baryta, strontia, and calcia are precipitated by carbonate of ammonia; it is afterwards thrown down by phosphate of soda; this reagent is also used for precipitating from potassa solutions substances which are insoluble in ammonia and its salts; exempli gratia, alumina, which-in the separation of the third, or sulphide of ammonium group-after precipitation of sesquioxide of chromium by boiling, is thrown down by chloride of ammonium.

Tests. When heated upon platinum foil it must completely volatilize. It must not be affected by sulphide of ammonium, and its reaction on test-paper should be perfectly neutral.

c. Reagents used to separate, or otherwise characterize groups of substances.

1. REAGENT PAPERS.

a. Blue litmus paper. This is prepared by immersing slips of fine unsized paper in a decoction of litmus, for some time. When thoroughly saturated, they are suspended upon a thread to dry.

Uses. For detecting free acid in solution, its color being changed to red.

b. Reddened litmus paper.-A decoction of blue litmus is treated with sulphuric acid until it assumes a red tint. The slips of paper, which are then passed through this, should be distinctly red when dry.

Uses. For the detection of free alkali, its color being restored to blue.

c. Brazil wood paper.-If Brazil wood be boiled in water for a sufficient time, it communicates a fine red color to the fluid. It must then be filtered, and the slips of paper immersed in it, and dried as before.

Uses. It is an excellent reagent for detecting hydrofluoric

acid, being tinged straw-yellow, when immersed in a very dilute solution of this acid.

d. Turmeric paper.-Turmeric root is first reduced to powder, and then treated with boiling alcohol; the decoction must be filtered, and the paper saturated with it.

Uses. For detecting free alkalies: the change produced is very characteristic, its bright yellow color becoming dark brown.

2. SULPHURIC ACID. HO, SO3.

Uses. Sulphuric acid has a greater affinity for most bases than any other acid, and is, consequently, employed for liberating them, especially hydrochloric, nitric, acetic, boracic, and phosphoric acids from their combinations. It is used for preparing hydrogen, sulphide of hydrogen, and other gases. Many substances which cannot exist anhydrous, are decomposed when brought into contact with it, owing to its great affinity for water. It is a special test for the detection of baryta, strontia, and lead, with which it gives white precipitates; when used for this purpose it should be dilute. It also serves as a reagent for lime, when this earth is in combination with phosphoric acid, and shows no decided reaction before the Blowpipe.

Tests. It must not destroy the color of a solution of indigo when boiled with it. When mixed with pure zinc and water it will give hydrogen, which, on being passed through a tube heated to redness by the flame of a spirit-lamp, should not deposit

arsenic.

3. SULPHIDE OF HYDROGEN. HS.

Sulphide of hydrogen gas is prepared in the following manner : -Into the flask a, Fig. 43, is introduced a small portion of pure sulphide of iron, and it is then tightly corked up. Water is poured in till it is covered, and concentrated sulphuric acid added through the funnel tube b. The small bottle c contains distilled water for the purification of the gas, which is finally evolved through the tube d, into the solution intended to be saturated.

Sulphide of hydrogen water,-hydrosulphuric acid-may be prepared by completely saturating cold distilled water with the gas.

Uses. It is an invaluable reagent for separating metals into the principal groups. It precipitates cadmium, copper, lead, tin, mercury, antimony, arsenic, silver, gold, platinum, et cetera, from their acid solutions. It is frequently employed for reducing the oxides of metals as well at the ordinary as at a high heat, and also highly oxidized bodies to a lower degree of oxidation.

Tests. Sulphide of hydrogen water should be quite clear, possessing the odor of the gas in an eminent degree: addition of ammonia must not cause a black coloration : when treated with sesquichloride of iron it should yield a large precipitate of sulphur.

4. SULPHIDE OF AMMONIUM. NH+S.

Uses. This reagent is of great value, and divides those metals which are precipitated by sulphide of hydrogen as sulphides, into two groups, a few of them being soluble, while the remainder is insoluble in it. It is also the precipitant of the third group; alumina and sesquioxide of chromium being thrown down as bases, and nickel, cobalt, manganese, iron, and zinc, as sulphides. It also precipitates baryta, strontia, and lime, when in combination with phosphoric, oxalic, and boracic acids, and magnesia when united to phosphoric acid.

Tests. Sulphide of ammonium must be transparent, must completely volatilize when heated upon platinum foil, and ought not to precipitate sulphate of magnesia.

5. SOLUTION OF POTASSA. KO, HO.

Uses. As a precipitant for many oxides it is invaluable. Some of these dissolve in excess, as alumina, oxide of chromium; while others, as sesquioxide of iron, et cetera, remain undissolved. In consequence of this property, it is employed to separate the former from the latter. Potassa has a great solvent power for many salts, such as sulphides and antimonic acid. Many oxides precipitated by potassa have a peculiar hue, or possess other characteristics by which they are easily recognised. Potassa expels ammonia from its salts, when heated in contact with them, and may be detected by its odor.

Tests.-Solution of potassa must be colorless, and after neutralizing it with pure nitric acid, which should occasion no effervescence, it must yield no precipitate, neither with chloride of barium nor nitrate of silver. It ought to retain its transparency when heated with an equal portion of solution of chloride of ammonium. When evaporated to dryness, and redissolved in water, no residue of silicic acid should remain.

6. AMMONIA. NH O.

Uses. For neutralizing acids, and precipitating many oxides, some of which redissolve in excess of the reagent, whilst others

are insoluble in free ammonia, thus affording a ready means of of separating them from each other.

Tests.-Ammonia must be colorless, must not leave any residue when evaporated on platinum foil, nor should lime-water impart turbidity in the slightest degree. Neither chloride of barium nor nitrate of silver must cause the smallest precipitate after supersaturation with pure nitric acid.

7. CARBONATE OF AMMONIA.

NHO, CO2.

Uses. As a precipitant of the alkaline earths, baryta, strontia, calcia, and magnesia, it throws down many metallic oxides, some of which are soluble, and others insoluble. Complete precipitation of many of them only occurs on boiling. ployed for separating glucina from alumina, and the oxide of uranium from sesquioxide of iron.

It is also em

Tests. It must completely volatilize. Neither chloride of barium, nitrate of silver, nor sulphide of hydrogen should color or precipitate it, after complete neutralization with pure nitric acid.

8. CHLORIDE OF BARIUM. Ba Cl.

Uses. From its properties of forming soluble salts with certain acids, and insoluble with others, it is an invaluable reagent for subdividing the precipitable acids. As a special reagent for sulphuric acid, with which it gives a white precipitate insoluble in all acids, its worth cannot be estimated.

Tests. The solution of chloride of barium must not be colored, or rendered turbid by sulphides of hydrogen, or ammonium. The filtrate, after adding an excess of pure sulphuric acid, ought to leave no residue when evaporated to dryness on platinum foil.

9. NITRATE OF BARYTA. Ba O, N 05.

Uses.-The same as chloride of barium, and is substituted for that reagent where the presence of a metallic chloride would be deleterious.

Tests. The same as those for chloride of barium, except that nitrate of silver must produce no precipitate.