3. A portion of the solution is slightly acidified with hydrochloric acid (or in case oxide of silver or protoxide of mercury has been detected, with nitric acid), and chloride of barium (or nitrate of baryta) is added.

a. THE FLUID REMAINS CLEAR.

acid. Pass on to (70).

Absence of sulphuric

b. A WHITE FINELY PULVERULENT PRECIPITATE IS FORMED. SULPHURIC ACID. The precipitate must remain undissolved on addition of more hydrochloric (or nitric) acid.

4. Add solution of sulphate of lime to another portion of 70 the solution (which, if it has an acid reaction, must first be neutralized, or made slightly alkaline, by means of ammonia).

a. No PRECIPITATE IS FORMED: absence of phosphoric acid, silicic acid, oxalic acid, and fluorine Pass on to (73). b. A PRECIPITATE IS FORMED. Add acetic acid in excess. 71 a. The precipitate redissolves readily: PHOSPHORIC ACID OF SILICIC ACID. To a portion of the original solution hydrochloric acid is added in slight excess, the liquid is evaporated to dryness, and the residue treated with a little hydrochloric acid and water. If an insoluble residue remains it is SILICA. If no residue remains a portion of the original solution is mixed with chloride of ammonium, ammonia, and a little sulphate of magnesia. The formation of a crystalline precipitate is proof of the presence of PHOSPHORIC ACID (§ 145).

B. The precipitate remains undissolved or dissolves 72 with difficulty: OXALIC ACID OF FLUORINE. Oxalate of lime is pulverulent, fluoride of calcium flocculent and gelatinous. The reaction with binoxide of manganese and sulphuric acid (§ 148) will afford conclusive proof of the presence of oxalic acid; the reaction on glass (etching) of the presence of fluorine (§ 149).

5. Acidify a fresh portion of the original solution with nitric 73 acid, and add solution of nitrate of silver.

a. THE FLUID REMAINS CLEAR. This is a proof of the absence of chlorine, bromine, iodine, ferrocyanogen, and ferricyanogen; the absence of cyanogen (in simple cyanides) is also probable. (Of the soluble metallic cyanides, cyanide of mercury is not precipitated by nitrate of silver; if, therefore, in the analytical process for the detection of the bases, mercury has been found, cyanide of mercury may be present. For the manner of detecting the cyanogen in the latter, see § 158, 8.) Pass on to (76).

b. A PRECIPITATE IS Formed.

a. The precipitate is orange colored: FERRICYANOGEN; 74 the reaction with sulphate of protoxide of iron is resorted to as a confirmatory test (§ 158, appendix).

B. The precipitate is white or yellowish-white. Treat the precipitate with ammonia in excess-at once, if the base be of the 1st or 2d groups-after filtering and washing if a base of the 3d or subsequent groups be present.

aa. The precipitate is not dissolved: IODINE or · FERROCYANOGEN. In the former case the precipitate is pale-yellow, in the latter white and gelatinous. The reaction with starch and hyponitric acid (§ 157) will afford conclusive proof of the presence of iodine, the reaction with sesquichloride of iron of the presence of ferrocyanogen (§ 158, appendix).

66. The precipitate is dissolved: CHLORINE, BRO- 75 MINE, or CYANOGEN. If the original substance smells of hydrocyanic acid, and the silver precipitate dissolves with some difficulty in the ammonia, the precipitate may be assumed to consist of cyanide of silver, and, consequently, to indicate the presence of CYANOGEN. To remove all doubt on the point, add to the original solution sulphate of protoxide of iron, solution of soda, and hydrochloric acid (§ 158). If addition of chlorine water imparts a yellow tint to the original solution the precipitate may be held to consist of bromide of silver, and consequently indicates the presence of BROMINE; if the bromine is present only in very small proportion, chloroform or bisulphide of carbon must be used in conjunction with chlorine water to make the reaction distinctly apparent (156). In the proved absence of both bromine and cyanogen, the precipitate consists of chloride of silver, and consequently shows the presence of CHlorine.

6. Ada to a small portion of the aqueous solution hydro- 70 chloric acid, drop by drop, until a distinct acid reaction is just imparted to the fluid, then dip in a slip of turmeric paper, take it out, and dry it at 212°. If the dipped portion looks brownish-red, BORACIC ACID is present. To settle all doubt on the point, add sulphuric acid and alcohol, and set fire to the latter (§ 147).

7. With regard to NITRIC ACID and CHLORIC ACID, these are 77 usually discovered aiready in the course of the preliminary examination (6). The reaction with sulphate of protoxide of

iron and sulphuric acid (§ 162) will afford conclusive evidence of the presence of the former, treatment of the solid salt with concentrated sulphuric acid, of the presence of the latter acid (§ 163).

Simple Compounds.

A. SUBSTANCES SOLUBLE IN WATER. DETECTION OF THE ACID.

II. Detection of Organic Acids.*

§ 187.

The analyst should in the first place ascertain by reference. to the Index of solubilities (see Appendix IV.) what organic acids form soluble salts with the base which has been found in the substance under examination. Those which form insoluble salts, of course, cannot be present.

The following course implies that the organic acid is either in the free state or combined with an alkali or alkaline earth. If, therefore, any other base besides those belonging to groups. I. and II. is present it must be removed. If the base belongs to the 5th or 6th groups it may be separated by hydrosulphuric acid; if it belongs to the 4th group, by sulphide of ammonium. After the metallic sulphide is filtered off, and excess of sulphide of ammonium is disposed of by acidulating with hydrochloric acid, warming and filtering off the sulphur, the clear liquid is examined according to (78). In case alumina or sesquioxide of chromium is the base present, the attempt should be made to throw it down by boiling with carbonate of soda. If this plan does not succeed, as will be the case when a non-volatile acid is present, the acid itself is precipitated by neutral acetate of lead, the precipitate is washed, diffused in a little water and hydrosulphuric acid gas passed through until the lead salt is decomposed. The sulphide of lead is filtered off and the filtrate examined according to (78). Alumina may also be thrown down from its combinations with non-volatile organic acids, by means of a solution of silicate of soda, in the form of a silicate of alumina.

1. Add ammonia to a portion of the aqueous solution of the 78 compound under examination to slight alkaline reaction, then chloride of calcium. If the solution was neutral, or only

[The indications of the preliminary examination (10 and note) are always suff cient to prove the presence or absence of an organic acid in "simple compounds." If, from inexperience, the operator is uncertain whether an organic acid be present he will do well to go carefully through the following course, and then to repeat the preliminary examination (10), interpreting its results by the more positive proof of the actual analysis.]

slightly acid, add chloride of ammonium before adding the chloride of calcium.

a. NO PRECIPITATE IS FORMED, NOT EVEN AFTER SHAK

ING THE FLUID NOR AFTER THE LAPSE OF A FEW MINUTES : absence of oxalic acid and tartaric acid.* Pass on to (so).

b. A PRECIPITATE IS FORMED. Add lime-water in ex- 79 cess to a fresh portion of the original solution, and then add solution of chloride of ammonium to the precipitate formed.

a. The precipitate redissolves: TARTARIC ACID. The reaction with acetate of potassa may be resorted to as a confirmatory test; positive proof will also be afforded by the deportment which the precipitate produced by the chloride of calcium, and properly washed, exhibits with solution of soda or with ammonia and nitrate of silver (§ 166).

3. The precipitate does not redissolve: OXALIC acid. To remove all doubt, try the reaction with concentrated sulphuric acid (§ 148).

2. Heat the fluid of 1, a, to boiling, keep at that temperature so or some time, and add some more ammonia to the boiling fluid. a. IT REMAINS CLEAR: absence of citric acid.

to (81).

Pass on

b. IT BECOMES TURBID, AND DEPOSITS A PRECIPITATE: CITRIC ACID. To remove all doubt as to the nature of the acid, add solution of acetate of lead in excess, wash the precipitate formed, and see whether it dissolves readily in ammonia (§ 167).

3. Mix the fluid of 2, a, with alcohol.

a. IT REMAINS CLEAR: absence of malic acid. Pass on to (82).

b. A PRECIPITATE IS FORMED: MALIC ACID. To remove all doubt, it is invariably necessary to try the reaction with acetate of lead, to see whether the precipitate produced by that reagent dissolves with difficulty in ammonia, and to examine its deportment when the fluid in which it is suspended is heated to boiling (§ 168).

81

4. Neutralize a portion of the original solution exactly† (if not *[To be certain of the absence of tartaric acid, the solution must be concentrated.] [For this purpose, place a very small slip of blue litmus paper, and also one of red litmus or turmeric in the solution, and add, from the reagent bottle, either am monia or hydrochloric acid, as is needed, a single drop at a time, agitating, to mix the liquids thoroughly, until the reagent is in excess. Then dilute a drop of the othe reagent with enough water to fill a test-tube to the depth of half an inch, and apply this to the solution by means of a glass rod, until it is just in excess, when the point of neutralization is attained with sufficient accuracy.]

already absolutely neutral) with ammonia or with hydrochloric 82 acid, and add solution of sesquichloride of iron.

OR DIRTY YELLOW COLOR.

a. A BULKY PRECIPITATE FORMS, OF A CINNAMON BROWN, Wash the precipitate, heat it with ammonia, filter, concentrate the filtrate by evaporation to a small bulk, divide into two parts, and add to the one some hydrochloric acid, to the other alcohol and chloride of barium. The formation of a precipitate in the first portion indicates the presence of BENZOIC ACID, a precipitate in the second denotes the presence of succiNIC ACID. Compare § 171 and § 172.

b. THE LIQUID ACQUIRES A RATHER INTENSE DEEP RED 83 TINT, AND, UPON PROTRACTED BOILING, A LIGHT REDDISHBROWN PRECIPITATE SEPARATES: acetic acid or formic acid. Heat a portion of the solid salt under examination, or, if the substance is in the fluid state, of the residue left upon evaporating the fluid (which, if acid, you must neutralize first with soda), with sulphuric acid and alcohol (§ 174). The characteristic odor of acetic ether indicates the preesnce of ACETIC ACID.

If you do not detect acetic acid in the fluid, you may conclude that the substance under examination contains FORMIC ACID to remove all doubt, try the reactions with nitrate of silver and chloride of mercury (§ 175).

Simple Compounds.

B. SUBSTANCES INSOLUBLE OR SPARINGLY SOLUBLE IN WATER, BUT SOLUBLE IN HYDROCHLORIC ACID, NITRIC ACID, OR NITROHYDROCHLORIC ACID.

Detection of the Base.*
§ 188.

Dilute a portion of the solution in hydrochloric acid, nitric 84 acid, or nitrohydrochloric acid with water, and proceed to examine for bases of the 2d, 5th, and 6th groups exactly as directed § 185, beginning at (46), in cases where the substance is dissolved in nitric acid, and at (50), if the solution already contains hydrochloric acid.

In testing for bases of the 3d and 4th groups by means of sulphide of ammonium according to (56) the usual course of

* Regard is also had here to certain salts of the alkaline earths, as this course of examination leads directly to their detection.

If upon the addition of water the liquid becomes white and turbid or deposits a white precipitate, this indicates the presence of antimony or bismuth, possibly also of tin: compare § 124, 9, and 134, 4. Heat with hydrochloric acid until the fluid nas become clear again, and then begin at (50).