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. α. 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 BARYTA, SULPHATE OF STRONTIA, and SULPHATE OF LIME.* 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. Ignited, and native binoxide of tin (tin-stone). Some metaphosphates and some arsenates. FLUORIDE OF CALCIUM and a few other compounds of fluorine. Of these compounds those printed in small capitals are more frequently met with. As the silicates perform a highly important part in mineral analysis, a special chapter (§ 203-§ 206) is devoted to them. The substance under examination which is insoluble in water and in acids is in the first place subjected to the preliminary experiments here described in a-e, if the quantity at your disposal is not absolutely too small to admit of this proceeding; in cases where the quantity is insufficient for the purpose, the operator must omit this preliminary examination, and at once pass on to 1, bearing in mind, however, that the body may contain all the aforesaid substances and compounds. a. Examine closely and attentively the physical state and con- 200 dition of the substance, to ascertain whether you have to deal with a homogeneous mass or with a mass composed of dissimilar particles; whether the body is sandy or pulverulent, whether it has the same color throughout, or is made up of variously-colored particles, &c. The microscope, or even a simple magnifying glass, will be found very useful at this stage of the examination. b. Heat a small sample in a glass tube sealed at one end. If 201 brown fumes arise, and SULPHUR sublimes, this is of course a proof of the presence of that substance. c. If the substance is black, this indicates, in most cases, the presence of carbon (wood-charcoal, pit-coal, bone-black, lamp-black, graphite, &c). Heat a small sample on platinum foil over the blowpipe flame; if the substance which blackens the fingers is consumed, this may be held to be a positive proof of the presence of CARBON in some shape or other. Graphite, which may be readily recognised by its property of communicating its blackish-gray color to the fingers, to paper, &c., requires the application of oxygen for its easy combustion. 202 d. Warm a small sample, together with a small lump of cyanide 203 of potassium and some water, for some time, filter, and test the filtrate with sulphide of ammonium. The formation of a brownishblack precipitate shows that the substance under examination contains a compound of SILVER. e. If an undissolved residue has been left in d, wash this tho- 204 roughly with water, and, if white, sprinkle a few drops of sulphide of ammonium over it; if it turns black, salts of LEAD are present. If, however, the residue left in d is black, heat it with some acetate of ammonia, adding a few drops of acetic acid, filter, and test the filtrate for LEAD, by means of sulphuric acid and hydrosulphuric acid.* The results obtained by these preliminary experiments serve to guide the operator now in his further course of proceeding. * The presence of lead in silicates, e. g. in glass containing lead, cannot be detected by this method. 206 1, a. SALTS OF LEAD ARE NOT PRESENT. Pass on to 2 (206). 205 b. SALTS OF LEAD ARE PRESENT. Heat the substance repeatedly with a concentrated solution of acetate of ammonia, until the salt of lead is completely dissolved out. Test a portion of the filtrate for CHLORINE, another for SULPHURIC ACID, and the remainder for LEAD, by addition of sulphuric acid in excess, and by hydrosulphuric acid. If acetate of ammonia has left a residue, wash this, and treat it as directed in 2. 2, a. SALTS OF SILVER ARE NOT PRESENT. Pass on to 3. b. SALTS OF SILVER ARE PRESENT. Digest the substance free from lead, or which has been freed from that metal by acetate of ammonia, repeatedly with cyanide of potassium and water, at a gentle heat (in presence of sulphur, in the cold), until all the salt of silver is removed. If an undissolved residue is left, wash this, and then proceed with it according to the directions of 3 (207). Of the filtrate, which contains cyanide of potassium, mix the larger portion with sulphide of ammonium, to precipitate the silver. Wash the precipitated sulphide of silver, then dissolve it in nitric acid, dilute the solution, and add hydrochloric acid, to ascertain whether the precipitate really consisted of sulphide of silver. Test another small portion of the filtrate for SULPHURIC ACID.* 3, a. SULPHUR IS NOT PRESENT. Pass on to 4. b. SULPHUR IS PRESENT. Heat the substance free from silver and lead in a covered porcelain crucible until all the sulphur is expelled, and, if a residue is left, treat this according to the directions of 4 (208). 207 4. Mix the substance free from silver, lead, and sulphur with 208 2 parts of carbonate of soda, 2 parts of carbonate of potassa, and 1 part of nitrate of potassa,† heat the mixture in a platinum crucible until the mass is in a state of calm fusion, place the red hot crucible on a thick, cold iron plate, and let it cool. By this means you will generally succeed in removing the fused mass from the crucible in an unbroken lump. Soak the mass now in water, boil, filter, and wash the residue until chloride of barium no longer produces a precipitate in the washings. (Add only the first washings to the filtrate.) a. The solution obtained contains the acids which were 209 present in the substance decomposed by fluxing (208). But it may, besides these acids, contain also such bases as are soluble in caustic alkalies. Proceed as follows: a. Test a small portion of the solution for SULPHURIC ACID. B. Test another portion with molybdic acid for PHOS- * As the carbonate of potassa contained in the cyanide of potassium may have produced a total or partial decomposition of any sulphates of the alkaline earths which happened to be present. Addition of nitrate of potassa is useful even in the case of white powders, as it counteracts the injurious action of silicate of lead, should any be present, upon the platinum crucible. In the case of black powders, the proportion of nitrate of potassa must be correspondingly increased, in order that carbon, if present, may be consumed as completely as possible, and that any chrome-ironstone existing in the compound, may be more thoroughly decomposed. |