arsenious and arsenic acids, as well as of notable quantities b. A PRECIPITATE IS FORMED. Dilute the fluid, and add 174 hydrochloric acid or, as the case may be, nitric acid; if the precipitate does not redissolve, or at least not completely, SULPHURIC ACID is present. 3. Add nitrate of silver to a portion of the solution. If this 175 fails to produce a precipitate, test the reaction, and if acid, add to the fluid some dilute ammonia, taking care to add the reagent so gently and cautiously that the two fluids do not intermix; if the reaction is alkaline, on the other hand, add with the same care some dilute nitric acid instead of ammonia, and watch attentively whether a precipitate or a cloud will form at the junction of the two fluids. a. No PRECIPITATE IS FORMED AT THE JUNCTION OF 176 THE TWO FLUIDS, EITHER IMMEDIATELY OR AFTER SOME TIME. Pass on to 181; there is neither chlorine, bromine, b. A PRECIPITATE IS FORMED. Observe the color of it, 177 then add nitric acid, and shake the mixture. a. The precipitate dissolves completely: absence of chlorine, bromine, iodine, cyanogen, ferro- and ferricyanogen, and also of sulphur. Pass on to 181. B. A residue is left: chlorine, bromine, iodine, cyano- 178 gen, ferro- or ferricyanogen may be present; and if the residue is black or blackish, HYDROSULPHURIC ACID or a soluble METALLIC SULPHIDE. The presen c of sulphur may, if necessary, be readily confirmed, by mixing another portion of the solution with sulphate of copper, or with a solution of oxide of lead in soda. aa. Test another portion of the fluid for IODINE and subsequently for BROMINE, by the methods described in § 157. bb. Test a small portion of the fluid with sesqui- 179 chloride of iron for FERROCYANOGEN; and, if the color of the silver precipitate leads you to suspect the presence of FERRICYANOGEN, test another portion for this latter substance with sulphate of iron (freshly prepared, by warming wire with dilute sulphuric acid). If the original solution has an alkaline reaction, some hydrochloric acid must be added before the addition of the sesquichloride of iron, or of the sulphate of iron. * If the solution contains an ammoniacal salt in somewhat considerable proportion, the non-formation of a precipitate cannot be considered a conclusive proof of the absence of these acids, since the baryta salts of most of them (not the sulphate) are in presence of ammoniacal salts more or less soluble in water. + That the cyanogen in cyanide of mercury is not indicated by nitrate of silver has been mentioned (73). Chloride, bromide, cyanide, ferrocyanide, oxalate, silicate, and borate of silver are white; iodide, tribasic phosphate, and arsenite of silver are yellow; arsenate and ferricyanide of silver are brownish-red; chromate of silver is purple-red; sulphide of silver black. cc. CYANOGEN, if present in form of a simple cyanide of an alkali metal soluble in water, may usually be readily recognised by the smell of hydrocvanic acid which the substance emits, and which is rendered more strongly preceptible by addition of a little dilute sulphuric acid. If ferrocyanogen and ferricyanogen are absent, cyanogen may be detected by the method given in § 155, 6. If they are present see § 226. dd. Should bromine, iodine, cyanogen, ferrocyanogen, 180 ferricyanogen, and sulphur not be present, the precipitate which nitric acid has failed to dissolve consists of CHLORIDE of silver. But where one or other of these bodies is present, a special examination for chlorine may become necessary, particularly when the quantity of the precipitate does not afford a decided indication.* See § 157. 4. CHLORIC ACID is known by the yellow color produced 181 when a little of the solid substance is brought into contact with concentrated sulphuric acid in a watch glass (§ 160). 5. NITRIC ACID is tested for with sulphate of iron and sulphuric acid (§ 159). The presence of certain other acids (chloric, chromic, hydriodic) impedes this reaction. If such acids are present they must be destroyed or removed. Chloric acid is destroyed by ignition (§ 161, at the end), chromic acid is reduced by sulphurous acid, the sesquioxide of chromium being precipitated afterwards with ammonia; hydriodic acid is removed by sulphate of silver. You have still to test for phosphoric acid, boracic acid, silicic acid and oxalic acid, as well as for hydrofluoric acid. For the first four acids test only in cases where both chloride of barium and nitrate of silver have produced precipitates in neutral solutions. Compare also foot note to 173. 6. Test for PHOSPHORIC ACID, by adding to a portion of the 182 fluid ammonia in excess, then chloride of ammonium and sulphate of magnesia (§ 142, 7). Very minute quantities of phosphoric acid are detected most readily by means of molybdic acid (§ 142, 10). Arsenic acid, if present, must be first separated by sulphuretted hydrogen, the solution being acidified and kept at 70° during of the gas. the passage If 7. To detect OXALIC ACID and HYDROFLUORIC ACID, add chloride of calcium to a fresh portion of the solution. If the reaction of the fluid is acid, add ammonia to alkaline reaction. the chloride of calcium produces a precipitate which is not redissolved by addition of acetic acid, one or both bodies are present. Examine now a sample of the original substance for fluorine according to § 146, 5, another sample for oxalic acid according to § 145, 7. 8. Acidulate a portion of the fluid slightly with hydrochloric 183 acid, then test for BORACIC ACID, by means of turmeric paper (§ 144, 6). Chloric, chromic, and hydriodic acids impede the reaction. If present, they must be removed or destroyed as directed 181. * Supposing, for instance, the solution of nitrate of silver to have produced a copious precipitate insoluble in nitric acid, and the subsequent examination to have shown mere traces of iodine and bromine, the presence of chlorine may be held to be demonstrated, without requiring additional proof. 9. Should SILICIC ACID not yet have been found in the course of testing for the bases, acidulate a portion of the fluid with hydrochloric acid, evaporate to dryness, and treat the residue with hydrochloric acid (§ 150, 2). Complex Compounds. A, 1. SUBSTANCES SOLUBLE IN WATER. DETECTION OF ACIDS. II. In Presence of Organic Acids. § 200. 1. The examination for the INORGANIC ACIDS, including 184 oxalic acid, is made in the manner described § 199. As the tartrates and citrates of baryta and silver are insoluble, or at least difficultly soluble in water, tartaric acid and citric acid can be present only in cases where both chloride of barium and nitrate of silver have produced precipitates in the neutral fluid; still, in drawing a conclusion, you must bear in mind that these salts are slightly soluble in solutions of salts of ammonia. Before you commence testing for the ORGANIC ACIDS, you must remove all the bases of Groups III.-VI., as described in the beginning of § 184, since they might interfere. 2. Make a portion of the fluid feebly alkaline with ammonia, 185 add some chloride of ammonium, then a sufficient quantity of chloride of calcium, shake vigorously, and let the mixture stand from ten to twenty minutes. a. No PRECIPITATE IS FORMED, EVEN AFTER THE LAPSE OF SOME TIME. Absence of tartaric acid; pass on to 186. b. A PRECIPITATE IS FORMED, IMMEDIATELY, OR AFTER SOME TIME. Filter, and keep the filtrate for further examination according to 186. Wash the precipitate, digest and shake it with solution of soda, without applying heat, then dilute with a little water, filter, and boil the filtrate some time. If a precipitate separates, TARTARIC ACID is indicated. Filter hot, and test the precipitate with ammonia and nitrate of silver (§ 163, 8). 3. Mix the fluid in which chloride of calcium has failed to 186 produce a precipitate, or that which has been filtered from the precipitate formed-in which latter case some more chloride of calcium is to be added-with three measures of alcohol. a. NO PRECIPITATE IS FORMED. Absence of citric, malic 187 and succinic acids. Pass on to 190. b. A PRECIPITATE IS FORMED. Filter and treat the fil- 188 trate as directed 190. Treat the precipitate as follows: Wash with alcohol, dissolve on the filter in a little dilute hydrochloric acid, add ammonia to the filtrate to alkaline reaction, and boil for some time. a. IT REMAINS CLEAR. Absence of citric acid. Add more alcohol, filter off the precipitate, which may contain malate and succinate of lime, wash it a little with alcohol, dry, dissolve in a porcelain dish in a sufficient quantity of strong nitric acid, and evaporate to dryness on the water- Pre-189 B. A HEAVY WHITE PRECIPITATE IS FORMED. 4. Heat the filtrate of 188, or the fluid in which addition of 190 alcohol has failed to produce a precipitate (187), to expel the alcohol, neutralize exactly with hydrochloric acid, and add sesquichloride of iron. If this fails to produce a light brown flocculent precipitate, benzoic acid is absent. If a precipitate of the kind is formed, filter, and heat the washed precipitate with ammonia in excess; filter, evaporate the filtrate nearly to dryness, and test for BENZOIC ACID with hydrochloric acid (§ 169, 2). Benzoic acid may generally be readily detected in the original substance, by treating a small portion with dilute hydrochloric acid, which will leave the benzoic acid undissolved; it is then filtered off and heated on platinum foil (§ 169, 1). 5. Evaporate a portion of the solution to dryness-if acid, 191 after previous saturation with soda-introduce the residue or a portion of the original dry substance into a test tube, pour some alcohol over it, add about an equal volume of concentrated sulphuric acid, and heat to boiling. Evolution of the odor of acetic ether demonstrates the presence of ACETIC ACID. This odor is rendered more distinctly perceptible by shaking the cooling or cold mixture. 6. Test for FORMIC ACID by just acidifying a portion with 192 hydrochloric acid (if not acid already), adding chloride of mercury and heating. A white turbidity from the separation of subchloride of mercury indicates formic acid (§ 172, 6). Confirm by nitrate of silver and by nitrate of suboxide of mercury (§ 172).* * In the presence of chromic or chloric acid the reduction of the silver and mercury does not take place. If chromic acid is present, mix the original solution with sulphuric acid, add excess of oxide of lead, and shake, filter, mix the filtrate with excess of dilute sulphuric acid and distil. Test the distillate as above. If chloric acid is present, combine the acids with oxide of lead, and treat with alcohol; the Complex Compounds. A, 2. SUBSTANCES INSOLUBLE IN WATER, BUT SOLUBLE IN HYDROCHLORIC ACID, NITRIC ACID, OR NITRO-HYDROCHLORIC ACID. DETECTION OF THE ACIDS. I. In Absence of Organic Acids. In the examination of these compounds attention must be directed to all acids, with the exception of chloric acid. Cyanogen compounds and silicates are not examined by this method. (Compare § 204 and § 205.) 1. CARBONIC ACID, SULPHUR (in the form of metallic sul- 193 phides), ARSENIOUS ACID, ARSENIC ACID and CHROMIC ACID, if present, have been found already in the examination for bases; NITRIC ACID, if present, has been detected in the preliminary examination, by the ignition in a glass tube (8). 2. Mix a sample of the substance with 4 parts of pure carbon- 194 ate of soda and potassa, and, should a metallic sulphide be present, add some nitrate of soda; fuse the mixture in a platinum crucible if there are no reducible metallic oxides present, in a porcelain crucible if such oxides are present; boil the fused mass with water, and add a little nitric acid, leaving the reaction of the fluid, however, still alkaline; heat again, filter, and proceed with the filtrate according to § 199.* 3. As the phosphates of the alkaline earths are only incom- 195 pletely decomposed by fusion with carbonate of soda and potassa, it is always advisable in cases where alkaline earths are present, and phosphoric acid has not yet been detected, to dissolve a fresh sample of the substance in nitric acid, and test for PHOSPHORIC ACID with solution of molybdic acid (§ 142, 10). In the presence of silicic or arsenic acid, prepare a solution with hydrochloric acid, separate these acids, add nitric acid, evaporate nearly to dryness, dilute with water containing nitric acid and then test with molyb date of ammonia. 4. If in the examination for bases, alkaline earths have been found, it is also advisable to test a separate portion for FLUORINE, by § 146, 5. 5. That portion of the substance which has been treated as 196 directed in 194, can be tested for SILICIC ACID only in cases where the fusion has been effected in a platinum crucible; when a porcelain crucible has been used, examine a separate portion by evaporating the hydrochloric or nitric acid solution (150, 3). 6. Examine a separate portion of the substance for OXALIC ACID by boiling with carbonate of soda, see 198. Acidify the alka formate is insoluble, the chlorate soluble. If tartaric acid is present it will also be safer to mix the fluid with dilute sulphuric acid and distil off the formic acid. * In the presence of a metallic sulphide, a separate portion of it must be examined for sulphuric acid, by heating it with hydrochloric acid, filtering, diluting the filtrate, and adding chloride of barium. |