FERRICYANOGEN, test another portion for this latter cc. CYANOGEN, if present in form of a simple metallic 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. However, should the analytical process have revealed 4. Test another portion for NITRIC ACID, by means of sulphate of 181 iron and sulphuric acid (§ 159). 5. To ascertain whether CHLORIC ACID is present, pour a little concentrated sulphuric acid over a small sample of the solid body under examination: ensuing yellow coloration of the acid resolves the question in the affirmative (§ 160). You have still to test for phosphoric acid, boracic acid, silicic acid, oxalic acid, and chromic acid, as well as for hydrofluoric acid. For the first five 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 § 197, 2, a. 6. Test for PHOSPHORIC ACID, by adding to a portion of the fluid 182 ammonia in excess, then chloride of ammonium and sulphate of magnesia (§ 143, 7). Very minute quantities of phosphoric acid are detected most readily by means of molybdic acid (§ 143, 11). 7. To effect the detection of OXALIC ACID and HYDROFLUORIC ACID, add chloride of calcium to a portion of the solution. If this produces a precipitate which is not redissolved by addition of acetic acid, one or both bodies are present. Examine therefore now a sample of the original substance for fluorine, according to the directions of § 147, 5, another sample for oxalic acid, by the method given in § 146, 7. 8. Acidulate a portion of the fluid slightly with hydrochloric 183 * 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. acid, and then test for BORACIC ACID, by means of turmeric paper (§ 145). 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, 3). 10. CHROMIC ACID is readily recognised by the yellow or red color of the solution, and by the purple-red color of the precipitate produced by nitrate of silver. If there remains the least doubt on the point, test for chromic acid with acetate of lead and acetic acid (§ 138). Complex Compounds. A, 1. SUBSTANCES SOLUBLE IN WATER. DETECTION OF ACIDS. II. In Presence of Organic Acids. § 198. 1. The examination for inorganic acids, inclusive of oxalic acid, 184 is made in the manner described in § 197. As the tartrates and citrates of baryta and oxide of silver are insoluble 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 the said salts are slightly soluble in solutions of salts of ammonia. The examination for the organic acids is conducted as follows 2. Make a portion of the fluid feebly alkaline by addition of am- 185 monia, filter, if necessary, add some chloride of ammonium, then chloride of calcium, shake vigorously, and let the mixture stand at rest from ten to twenty minutes. a. No PRECIPITATE IS FORMED, NOT EVEN AFTER THE LAPSE OF SOME TIME. Absence of tartaric acid; pass on to 3. b. A PRECIPITATE IS FORMED IMMEDIATELY, OR AFTER SOME 186 Filter, wash, and keep the filtrate for further examination according to the directions of 3. TIME. Digest and shake the precipitate 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 may be assumed to be present. Filter hot, and subject the precipitate to the ammonia and nitrate of silver test described in § 162, 8. 3. Mix the fluid in which chloride of calcium has failed to produce a precipitate, or that which has been filtered from the precipitate-in which latter case some more chloride of calcium is to be added-with alcohol. a. No PRECIPITATE IS FORMED. malic acid. Pass on to 4. b. A PRECIPITATE IS FORMED. Absence of citric acid and 187 Filter and treat the filtrate 188 as directed in 4. As regards the precipitate, treat this as After washing with some alcohol, dissolve on the filter in a little dilute hydrochloric acid, add ammonia to the filtrate to feebly alkaline reaction, and then boil for some time. a. THE FILTRATE REMAINS CLEAR. Absence of citric acid. Probable presence of MALIC ACID. Add alcohol again to the fluid, and test the lime precipitate in the manner directed § 165, to make sure whether malic acid is really present or not. B. A HEAVY, WHITE PRECIPITATE IS FORMED. Presence 189 of CITRIC ACID. Filter boiling, and test the filtrate for malic acid in the same manner as in a. To remove all doubt as to whether the precipitate is citrate of lime or not, it is advisable to dissolve once more in some hydrochloric acid, to supersaturate again with ammonia, and to boil; if the precipitate really consisted of citrate of lime, it will now be thrown down again. (Compare § 164, 3.) 4. Heat the filtrate of 3, b (188) (or the fluid in which addition 190 of alcohol has failed to produce a precipitate, 3, a [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, neither succinic nor benzoic acid is present. If a precipitate of the kind is formed, filter, digest, and heat the washed. precipitate with ammonia in excess; filter, evaporate the filtrate nearly to dryness, and test a portion for SUCCINIC ACID with chloride of barium and alcohol; the remainder for BENZOIC ACID with hydrochloric acid (§ 169). Benzoic acid may generally be readily detected also in the original substance, by pouring some dilute hydrochloric acid over a small portion of the latter, which will leave the benzoic acid undissolved; it is then filtered and heated on platinum foil (§ 168, 1). 5. Evaporate a portion of the solution to dryness-if acid, after 191 previous saturation with soda-introduce the residue, or a portion of the original dry substance into a small 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. To effect the detection of FORMIC ACID, add to a portion of 192 the solution nitrate of silver in not too small a proportion, then soda until the fluid is exactly neutralized, and boil. If formic acid is present, reduction of the silver to the metallic state ensues (§ 171, 4). The reaction with nitrate of suboxide of mercury may be had recourse to as a conclusive test (§ 171, 4).* * In presence of chromic acid the reduction of oxide of silver and of suboxide of mercury is not a positive proof of the presence of formic acid. In cases where the two acids are present, the following method must be resorted to:-Mix the original solution with some nitric acid, add oxide of lead in excess, shake the mixture, filter, add to the filtrate dilute sulphuric acid in excess, and distil. Test the distillate as directed § 172. In presence of tartaric acid also it is the safest way to distil the formic acid first, with addition of dilute sulphuric acid. Complex Compounds. A, 2. SUBSTANCES INSOLUBLE IN WATER, BUT SOLUBLE IN HYDROCHLORIC ACID, NITRIC ACID, OR NITROHYDROCHLORIC ACID. DETECTION OF THE ACIDS. I. In the Absence of Organic Acids. $ 199. 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 § 202 and § 203.) 1. CARBONIC ACID, SULPHUR (in form of metallic sulphides), ARSE- 193 NIOUS ACID, ARSENIC ACID, and CHROMIC ACID, if present, have been found already in the course of the examination for bases; NITRIC ACID, if present, has been detected in the course of the preliminary examination, by the ignition of the powdered substance in a glass tube (§ 174). 2. Mix a sample of the substance with 4 parts of pure carbonate 194 of soda and potassa, and, should it contain a metallic sulphide, add some nitrate of soda; fuse the mixture in a platinum crucible if there are no reducible metals present, in a porcelain crucible if reducible metals 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 the directions of § 197, to effect the detection of all the acids which were combined with the bases.* 3. As the phosphates of the alkaline earths are only incompletely 195 decomposed by fusion in conjunction 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 body under examination in hydrochloric acid or nitric acid, and test the solution for PHOSPHORIC ACID with molybdic acid. 4. If in the course of the examination for bases, alkaline earths have been found, it is also advisable to test a separate portion of the body under examination for FLUORINE, by the method described in § 147, 5. 5. That portion of the substance under examination which is 196 treated of in § 199, 2 (194), can be tested for SILICIC ACID only in cases where the fusion has been effected in a platinum crucible; in cases where a porcelain crucible has been used, it is necessary to examine a separate portion of the body for silicic acid, by evaporating the hydrochloric or nitric acid solution. 6. Examine a separate sample of the body for OXALIC ACID as directed in § 200, 3. If the body examined has been found to contain a metallic sulphide, a separate portion of it must be examined for sulphuric acid, by heating it with hydrochloric acid, filtering, adding water to the filtrate, and then testing the fluid with chloride of barium. Complex Compounds. A, 2. SUBSTANCES INSOLUBLE IN WATER, BUT SOLUBLE IN HYDROCHLORIC ACID, NITRIC ACID, OR NITROHYDROCHLORIC ACID. DETECTION OF THE ACIDS. II. In Presence of Organic Acids. $ 200. 1. Conduct the examination for inorganic acids according to the 197 direction of § 199. 2. Test for ACETIC ACID as directed § 198, 5 (191). 3. Dissolve a portion of the compound under examination in the 198 least possible amount of hydrochloric acid, filter, if necessary, and test the undissolved residue which may be left, for BENZOIC ACID by application of heat; add to the filtrate solution of carbonate of soda in considerable excess, and, besides this, also a little solid carbonate of soda, boil the mixture for a few minutes, and then filter the fluid from the precipitate. In the filtrate you have now all the organic acids in solution, combined with soda. Acidify the filtrate with hydrochloric acid, heat, and proceed according to the direction of $198. Complex Compounds. B. SUBSTANCES INSOLUBLE OR SPARINGLY SOLUBLE BOTH IN WATER AND IN HYDROCHLORIC ACID, NITRIC ACID, OR NITROHYDROCHLORIC ACID. DETECTION OF THE BASES, ACIDS, AND NON-METALLIC ELEMENTS. § 201. To this class belong the following bodies and compounds. SULPHATE OF LEAD† and chloride of lead.‡ CHLORIDE OF SILVER, bromide of silver, iodide of silver, cyanide of silver,|| ferro- and ferricyanide of silver.T SILICIC ACID and many SILICATES. Native alumina, or alumina which has passed through a process of intense ignition, and many aluminates. Ignited sesquioxide of chromium and chrome-ironstone (a compound of sesquioxide of chromium and protoxide of iron). 199 * Sulphate of lime passes partially into the solution effected by water, and often completely into that effected by acids. Sulphate of lead may pass completely into the solution effected by acids. Chloride of lead can here only be found if the precipitate insoluble in acids has not been thoroughly washed with hot water. || Bromide, iodide, and cyanide of silver are decomposed by boiling with nitrohydrochloric acid, and converted into chloride of silver; they can accordingly be found here only in cases where the operator has to deal with a substance which-as nitrohydrochloric acid has failed to effect its solution-is examined directly by the method described in this paragraph (§ 201). With regard to the examination of these compounds, compare also § 202. |