Ignited, and native binoxide of tin (tin-stone). Some metaphosphates and some arsenates. FLUORIDE OF CALCIUM and a few other compounds of fluorine. CARBON. 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. 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). 206 207 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. forms, remove the arsenic acid which may be present with acid. Test another portion for FLUORINE (§ 147, 7). 8. If the solution is yellow, CHROMIC ACID is present. To remove all doubt on the point, acidify a portion of the solution with acetic acid, and test with acetate of lead. ε. Acidify the remainder of the solution with hydrochloric 210 acid, evaporate to dryness, and treat the residue with hydrochloric acid and water. If a residue is left which refuses to dissolve even in boiling water, this consists of SILICIC ACID. Test the hydrochloric acid solution now in the usual way for those bases which, being soluble in caustic alkalies, may be present. b. Dissolve the residue left in 4 (208) in hydrochloric acid 211 (effervescence indicates the presence of alkaline earths), and test the solution for the bases as directed in § 188. (If much silicic acid has been found in ε (210), it is advisable to evaporate the solution of the residue to dryness, and to treat the residuary mass with hydrochloric acid and water, in order that the silicic acid remaining may also be removed as completely as possible.) 5. If you have found in 4 that the residue insoluble in acids 212 contains a silicate, treat a separate portion of it according to the directions of § 205, 2 (228), to ascertain whether or not this sili cate contains alkalies. 6. If a residue is still left undissolved upon treating the 213 residue left in 4 with hydrochloric acid (211), this may consist either of silicic acid, which has separated, or of an undecomposed portion of sulphate of baryta; it may, however, also be fluoride of calcium, and if it is dark-colored, chrome-ironstone, as the lastnamed two compounds are only with difficulty decomposed by the method given in 4. I would therefore remind the student that fluoride of calcium may be readily decomposed by means of sulphuric acid; and, as regards the decomposition of chrome-ironstone, can recommend the following method, first proposed by Hart: Project the fine powder into 8 times the quantity of fused borax, stir the mixture frequently, and keep the crucible for halfan-hour at a bright red heat. Add now to the fusing mass carbonate of soda so long as effervescence continues, and then finally add 3 times the weight of the chrome-ironstone of a mixture of equal parts of carbonate of soda and nitrate of potassa, whilst actively stirring the mixture with a platinum wire. Let the mass cool, and, when cold, boil it with water. 7. If the residue insoluble in acids contained silver, you have 214 still to ascertain whether that metal was present in the original substance as chloride, bromide, iodide, &c., of silver, or whether it has been converted into the form of chloride of silver by the treatment employed to effect the solution of the original substance. For that purpose, treat a portion of the original substance with. boiling water until the soluble part is completely removed; then treat the residuary portion in the same way with dilute nitric acid, wash the undissolved residue with water, and test a small 226 ANALYSIS OF INSOLUBLE CYANIDES, FERROCYANIDES, ETC. [§ 202. sample of it for silver according to the directions of § 201, d (203). If silver is present, proceed to ascertain the salt-radical with which the metal is combined; this may easily be effected by boiling the remainder of the residue in the first place with rather dilute solution of soda, filtering, and testing the filtrate, after acidifying it, for ferro- and ferricyanogen. Digest the washed residue now with finely granulated zinc and water, with addition of some sulphuric acid, and filter after the lapse of ten minutes. You may now at once test the filtrate for chlorine, bromine, iodine, and cyanogen; or you may first throw down the zinc with carbonate of soda, in order to obtain the salt-radicals in combination with sodium. SECTION II. PRACTICAL COURSE IN PARTICULAR CASES. I. SPECIAL METHOD OF EFFECTING THE ANALYSIS OF CYANIDES, $202. THE analysis of ferrocyanides, ferricyanides, &c., by the common 215 method is often attended by the manifestation of such anomalous reactions as easily to mislead the analyst. Moreover, acids often fail to effect their complete solution. For these reasons it is advisable to analyze them, and mixtures containing such compounds, by the following special method: Treat the substance with water until the soluble parts are entirely removed, and boil the residue with strong solution of potassa or soda; after a few minutes ebullition add some carbonate of soda, and boil again for some time; filter, should a residue remain, and wash the latter. 1. The residue, if any has been left, is now free from cya- 216 nogen, unless the substance under examination contains cyanide of silver, in which case the residue would of course still contain cyanogen. Examine the residue now by the common method, beginning at § 178, 2 (34). 2. The solution or filtrate, which, if combinations of com- 217 pound cyanogen radicals were originally present, contains these combined with alkali metals, may also contain other acids, which have been separated from their bases by the process of boiling with carbonate of soda, and lastly also, such oxides as are soluble in caustic alkalies. * Before entering upon this course of analysis, consult the special remarks to the paragraph (§ 202), page 274, in the Third Section. Treat the solution as follows: a. Mix the alkaline fluid with a sufficient quantity of hydro- 218 sulphuric acid. a. No permanent precipitate is formed. Absence of zinc and lead. Pass on to b. B. A permanent precipitate is formed. Add to the fluid a little yellow sulphide of sodium, heat moderately, filter, wash the precipitate, and treat the filtrate as directed in b. Dissolve the washed precipitate in nitric acid, and examine the solution for copper, lead, zinc, and other metals of the fourth group, which may, in the same way as copper, have passed into the alkaline solution, by the agency of organic matters. 6. Mix the alkaline fluid, which contains now also sulphide 219 of an alkali metal, with nitric acid to acid reaction, and, if necessary, add once more hydrosulphuric acid. Absence of mercury and of a. No precipitate is formed. c. The fluid, acidified with nitric acid, and therefore abun- 220 Evaporate ẞ to dryness, and heat the residue to fusion. Pour the fused mass upon a piece of porcelain, boil with water, filter, and examine the residue for IRON, MANGANESE, COBALT, and ALUMINA. Test a portion of the filtrate (if yellow) for CHROMIC ACID, the remainder for ALUMINA-which may have passed partially or completely into the solution, through the agency of the caustic alkalies formed, in the process of fusion, from the nitrates of the alkalies present. II. ANALYSIS OF SILICATES. Whether the body to be analyzed is a silicate, or contains one, is 221 ascertained by the preliminary examination with phosphate of soda and ammonia before the blowpipe; since, in the process of fusion, the metallic oxides dissolve, whilst the separated silicic acid floats about in the liquid bead as a transparent, swollen mass. The analysis of the silicates differs, strictly speaking, from the common course only in so far as the preliminary treatment is concerned, which is required to effect the separation of the silicic acid from the bases, and to obtain the latter in solution. |