five or ten minutes: LIME. nia (§ 97) Confirm with oxalate of ammo B. The solution becomes turbid, but only after the lapse of some time: STRONTIA. Confirm by the flame-coloration (§ 96, 7 or 8). γ. A precipitate is immediately formed: BARYTA. with hydrofluosilicic acid (§ 95). Confirm 5. Mix that portion of the solution in which carbonate of am- 64 monia has, after previous addition of chloride of ammonium, failed to produce a precipitate (62), with phosphate of soda, add more ammonia, and rub the sides of the vessel with a glass rod gently. a. No PRECIPITATE IS FORMED: absence of magnesia. Pass on to 65. b. A CRYSTALLINE PRECIPITATE IS FORMED: MAGNESIA. 6. Evaporate a drop of the original solution on perfectly clean 65 platinum-foil as slowly as possible, and gently ignite the residue. a. THERE IS. NO FIXED RESIDUE LEFT. Test for AMMONIA, by adding to the original solution hydrate of lime, and observing the odor and reaction of the escaping gas, and the fumes which it forms with acetic acid (§ 91). b. THERE IS A FIXED RESIDUE LEFT: potassa or soda. 66 Add bichloride of platinum to a portion of the original solution (having first evaporated it to a very small bulk if dilute), and shake the mixture or rub with a glass rod. a. No precipitate is formed, not even after ten or fifteen minutes: SODA. Confirm with the flame coloration or with antimonate of potassa (§ 90). B. A yellow crystalline precipitate is formed: POTASSA. firm with tartaric acid or the flame coloration (§ 89). Simple Compounds. Con A. SUBSTANCES SOLUBLE IN WATER. DETECTION OF THE ACID. I. Detection of Inorganic Acids. § 183. Reflect in the first place which of the inorganic acids form soluble compounds with the detected base (compare Appendix IV.) and bear this in mind in your subsequent operations, giving due regard also to the results of the preliminary examination. 1. ARSENIOUS ACID and ARSENIC ACID have already been 67 found in the detection of the base. They are distinguished from each other by their reaction with nitrate of silver, or with potassa and sulphate of copper (see § 134, 9). 2. The presence of CARBONIC ACID in combination with bases, 68 SULPHUR in combination with metals, and CHROMIC ACID, has been also indicated already in the detection of the base. The two former betray their presence by effervescing upon the addition of hydrochloric acid; the escaping gases may be distinguished from one another by the smell. The presence of carbonic acid may be con firmed by lime water (see § 149), and that of hydrosulphuric acid by acetate of lead (§ 156). Free carbonic acid and free hydrosulphuric acid in aqueous solution may be detected by the same reagents. The presence of chromic acid is invariably indicated by the yellow or red tint of the solution, as well as by the transition of the red or yellow colour to green, and the separation of sulphur, upon the addition of hydrosulphuric acid water. Confirm with acetate of lead and nitrate of silver (§ 138). 3. Acidify a portion of the solution with hydrochloric acid, or 69 -if oxide of silver or suboxide of mercury has been found-with nitric acid, and add chloride of barium or nitrate of baryta as the case may be. Should a gelatinous precipitate of silicic acid form on addition of hydrochloric acid, repeat the experiment after having diluted the fluid considerably. In this case it is also best to add the acid all at once and not gradually, compare § 150, 2. a. THE FLUID REMAINS CLEAR. Absence of sulphuric acid. Pass on to 70. b. A PRECIPITATE IS PRODUCED, IN FORM OF A FINE WHITE POWDER: SULPHURIC ACID. The precipitate must remain undissolved even after further addition of dilute hydrochloric or nitric acid. 4. Add solution of sulphate of lime to another portion of the 70 solution (which, if it has an acid reaction, must first be neutralized, or made slightly alkaline, by ammonia, and if necessary, filtered). 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 B. The precipitate remains undissolved or dissolves with 72 5. Acidify a fresh portion of the solution with nitric acid, and 73 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 cyanides, cyanide of mercury is not precipitated by nitrate of silver; if, therefore, in detection of the base mercury has been found, cyanide of mercury may be present. For the manner of detecting the cyanogen in the latter see § 155, 11.) Pass on to 76. b. A PRECIPITATE IS FORMED. a The precipitate is orange: FERRICYANOgen. with sulphate of iron (§ 155, Appendix). Confirm 74 B. The precipitate is white or yellowish-white. Treat the precipitate with ammonia in excess-immediately, if the base was an alkali or an alkaline earth-after filtering and washing, if the base was an earth or the oxide of a heavy metal. aa. The precipitate is not dissolved: IODINE or FERROCYANOGEN. In the former case the precipitate is pale yellow, in the latter white and gelatinous. Confirm for iodine with starch and hyponitric acid (§ 154), for ferrocyanogen with sesquichloride of iron (§ 155, Appendix). 33. The precipitate is dissolved: CHLORINE, BROMINE, 75 or CYANOGEN. If the original substance smells of hydrocyanic acid, and the silver precipitate dissolves with some difficulty in ammonia, CYANOGEN is indicated. Confirm by adding to the original solution sulphate of iron, soda, and then hydrochloric acid (§ 155). If addition of chlorinewater imparts a yellow tint to the original solution, BROMINE is indicated; if the bromine is present only in small proportion, chloroform or bisulphide of carbon must be used in conjunction with the chlorine-water (§ 153). In the proved absence of both bromine and cyanogen the precipitate shows the presence of CHLORINE. 6. Add to a small portion of the aqueous solution hydrochloric 76 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 100°. If the dipped portion looks brownish-red, BORACIC ACID is present. Confirm by adding sulphuric acid and alcohol, and setting fire to the latter (§ 144). 7. With regard to NITRIC ACID and CHLORIC ACID, these are 77 usually discovered in the preliminary examination (6). Confirm for the former with sulphate of iron and sulphuric acid (§ 159); for the latter by treating the solid salt with concentrated sulphuric acid, (§ 160). Simple Compounds. A. SUBSTANCES SOLUBLE IN WATER. DETECTION OF THE ACID. II. Detection of Organic Acids. Consider, in the first place, which of the organic acids form soluble compounds with the detected base (compare Appendix IV.), and bear this in mind in your subsequent operations, giving due regard also to the results of the preliminary examination. The following course presupposes the organic acid to be present in the free state, or in combination with an alkali or an alkaline If the detected base belongs to another group, therefore, it must first be removed. Where the base belongs to group V. or group VI. the removal is effected by hydrosulphuric acid, where it belongs to group IV. by sulphide of ammonium. After filtering off the sulphides, and removing the excess of sulphide of ammonium by acidifying with hydrochloric acid, heating, and filtering off the sulphur, proceed to 78. Where the base is alumina or sesquioxide of chromium, try first to precipitate these substances by boiling with carbonate of soda; should this fail, as it will where the acid is non-volatile, precipitate the latter in a fresh portion of the solution with neutral acetate of lead, wash the precipitate, diffuse it through water, pass hydrosulphuric acid, filter off the sulphide of lead and treat the filtrate as directed below. Alumina may also be precipitated from its compounds with non-volatile organic acids by solution of soluble glass, as silicate of alumina. To separate acetic or formic acid from bases which lie in the way of their detection, you may also distil the salt with dilute sulphuric acid. 1. Add ammonia to a portion of the aqueous solution to slight 78 alkaline reaction, then chloride of calcium in sufficient quantity. If the solution was neutral, or only slightly acid, add chloride of ammonium before adding the chloride of calcium. a. NO PRECIPITATE IS FORMED, EVEN AFTER SHAKING THE FLUID AND AFTER THE LAPSE OF A FEW MINUTES: absence of oxalic acid and tartaric acid. Pass on to 80. a. The precipitate takes some time to form and is crystalline: TARTARIC ACID. Confirm by testing the behaviour of the washed precipitate with soda or with ammonia and nitrate of silver, or by testing the aqueous solution with acetate of potassa and acetic acid (§ 163). f. The precipitate forms immediately and is finely pulverulent: OXALIC ACID. Confirm by acidifying a fresh portion of the aqueous solution with acetic acid and adding solution of sulphate of lime (§ 145). 79 2. Heat the fluid of 1, a, to boiling, keep at that temperature for 80 some time, and add some more ammonia to the boiling fluid. a. IT REMAINS CLEAR: absence of citric acid. Pass on to 81. b. IT BECOMES TURBID, AND DEPOSITS A PRECIPITATE: CITRIC ACID.* Confirm by preparing the lead salt, washing it, and testing its deportment with ammonia, in which it readily dissolves; or preferably by preparing the characteristic baryta salt and examining it with the microscope (§ 164). 3. Mix the fluid of 2, a, with 2 volumes of alcohol. a. IT REMAINS CLEAR even after long standing: absence of malic and succinic acids. Pass on to 82. b. A PRECIPITATE is formed: MALIC or SUCCINIC ACID. Heat a portion of the original solid substance with nitric acid, evaporate to dryness. altogether, boil the residue with solution of carbonate of soda, filter if necessary, neutralize exactly with hydrochloric acid and test one portion of this solution with 81 If the ammonia contains carbonic acid, a precipitate of carbonato of lime may form here. Such a precipitate may be readily distinguished from citrate of lime by hydrochloric acid. sulphate of lime; another portion with sesquichloride of iron. 4. Neutralize a portion of the original solution completely (if not 82 already absolutely neutral) with ammonia or hydrochloric acid, and add sesquichloride of iron. B. a. A REDDISH YELLOW BULKY PRECIPITATE FORMS: BENZOIC ACID. Confirm by treating the original dry substance with hydrochloric acid (§ 169, 2). b. THE LIQUID ACQUIRES A RATHER INTENSE DEEP RED 83 TINT, AND, UPON BOILING, A LIGHT REDDISH-BROWN PRECIPITATE SEPARATES: acetic acid or formic acid. If the original substance is a solid, warm a portion with sulphuric acid and alcohol (§ 171). If the original substance is a fluid, having neutralized a portion with soda in case it is acid, evaporate it to dryness, and test the residue. The odor of acetic ether indicates ACETIC ACID. If you do not detect acetic acid, you may conclude that the substance contains FORMIC ACID : confirm with nitrate of silver and chloride of mercury (§ 172). Simple Compounds. SUBSTANCES INSOLUBLE OR SPARINGLY SOLUBLE IN WATER, SOLUBLE IN HYDROCHLORIC ACID, NITRIC ACID, OR NITRO-HYDROCHLORIC ACID. BUT Detection of the Base.† § 185. Dilute a portion of the solution in hydrochloric acid, nitric acid, 84 or nitro-hydrochloric acid with water, and proceed to examine for bases of the second, fifth, and sixth groups according to § 182, beginning at 46 if the solvent was nitric acid, at 50 if the solvent contained hydrochloric acid. It is to be noted here that hydrochloric acid when used as a solvent does not usually alter the state of oxidation of the base present, while nitric acid and nitro-hydrochloric acid generally have the effect of more or less oxidizing the base where this is possible. Hence when oxide of mercury, binoxide of tin, or sesquioxide of iron is found, and nitric or nitro-hydrochloric acid has been used for dissolving, you must examine the original substance specially to see in what state the metal is actually present, if this is not at once evident from the solubility. In the case of mercury salts for instance, treatment with soda would decide which * Compare end of § 165, 1. + 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 antimony or bismuth (possibly also tin). Compare § 121, 9, and § 131, 4. Heat with hydrochloric acid until the fluid has become clear again, then pass to 50. |