original solution; if a white precipitate is formed, bb. Add solution of soda to a portion of the origi- The presence of oxide of mercury is usually sufficiently indicated by the several changes of color through which the precipitate produced by the solution of hydrosulphuric acid in the fluid under examination is observed to pass; this precipitate is white at first, but changes upon the addition of an excess of the precipitant to yellow, then to orange, and finally to black (§ 122, 3). cc. Add ammonia in excess to a portion of the original solution; if a bluish precipitate is formed which redissolves in an excess of the precipitant, imparting an azure color to the fluid, this indicates COPPER. To remove all doubt, test with ferrocyanide of potassium (§ 123). d. If the precipitate produced by ammonia was white, and excess of ammonia has failed to redissolve it, filter the fluid off, wash the precipitate, dissolve it on a watch-glass in 1 or 2 drops of hydrochloric acid, with addition of 2 drops of water, and then add some more water. If the solution turns turbid and milky, this is caused by basic terchloride of bismuth the reaction consequently indicates BISMUTH. The blowpipe is resorted to as a conclusive test (§ 124). : ee. Add solution of sulphate of protoxide of iron to of GOLD. To remove all doubt as to the nature of ff. Add chloride of potassium and alcohol to a por- 3. Mix a small portion of the original solution with chloride 50 of ammonium,* add ammonia to alkaline reaction, and then, no matter whether the latter reagent has produced a precipitate or not, a little sulphide of ammonium, and apply heat, if a precipitate fail to separate in the cold. a. No PRECIPITATE IS FORMED; pass on to (62); for iron, cobalt, nickel, manganese, zinc, chromium, alumina, and silicic acid, are not present. b. A PRECIPITATE IS FORMED. a. The precipitate is black: protoxide of iron, nickel, 57 or cobalt. Mix a portion of the original solution with some solution of potassa or soda. aa. A dirty greenish-white precipitate is formed, which soon changes to reddish-brown, upon exposure to the air: PROTOXIDE OF IRON. To remove all doubt, test with ferricyanide of potassium (§ 113). bb. A precipitate of a light greenish tint is pro- cc. A sky-blue precipitate is formed, which is dis- COBALT. aa. If the precipitate is distinctly flesh-colored, it consists of sulphide of manganese, and is consequently indicative of the presence of PROTOXIDE OF MANGANESE. To remove all doubt, add soda to the original solution, or try before the blowpipe (§ 110). bb. If the precipitate is bluish-green, it consists of hydrated sesquioxide of chromium, and is consequently indicative of the presence of SESQUIOXIde of CHROMIUM. To dispel all doubt, test the original solution with soda, and apply the blowpipe tests (105). 58 cc. If the precipitate is white, it may consist of 59 hydrate of alumina, or hydrate of silicic acid, or sulphide of zinc, and may accordingly point to the presence of either alumina or oxide of zinc or silicic acid; the latter, in that case, is generally contained in the original solution as an alkaline silicate. To distinguish between these three bodies, add to a portion of The addition of chloride of ammonium is made to prevent the precipitation f maynesia by the ammonia. the original solution, solution of soda drop by drop, wait to see whether this produces a precipitate, and then add some more solution of soda, until the precipitate formed is dissolved. aa. If solution of soda fails to produce a precipi- 60 tate, there is reason to test for SILICIC ACID. For that purpose, evaporate a portion of the original solution with hydrochloric acid to dryness, and treat the residue with hydrochloric acid and water (§ 153, 2), when the silicic acid will be left undissolved. Determine the nature of the alkali which has been dissolved, as directed (66). 33. If solution of soda produces a precipitate, which redissolves in an excess of the precipitant, add to a portion of this alkaline fluid hydrosulphuric acid; the formation of a white precipitate indicates the presence of ZINC. The reaction with solution of nitrate of protoxide of cobalt before the blowpipe will afford conclusive proof (§ 109). If hydrosulphuric acid fails to produce a precipitate, add to the remaining portion of the alkaline fluid chloride of ammonium, and apply heat. The formation of a white precipitate indicates the presence of ALUMINA. The reaction with solution of nitrate of protoxide of cobalt before the blowpipe will afford conclusive proof (§ 104). Note to (58), and (59). As very slight contaminations may impair the distinctness of the tints exhibited by the precipitates considered in (58), and (59), it is advisable, in all cases where the least impurity is suspected, to adopt the following method for the detection of manganese, chromium, zine, alumina, and silicic acid. Add solution of soda to a portion of the original solution, first in small quantity, then in excess. aa. No precipitate is formed: SILICIC ACID may be 61 assumed to be present; proceed as directed (60). bb. A whitish precipitate is formed, which does not redissolve in an excess of the precipitant, and speedily turns blackish-brown upon exposure to the air: MANGANESE. The blowpipe is resorted to as a conclusive test (§ 110). cc. A precipitate is formed which redissolves in an excess of the precipitant: SESQUIOXIDE OF CHROMIUM, ALUMINA, OXIDE OF ZINC. aa. Add hydrosulphuric acid water to a portion of the alkaline solution. The formation of a white pre- 33. If the original or the alkaline solution is green, 77. Add chloride of ammonium to the alkaline solution. The formation of a white precipitate indicates the presence of ALUMINA. The reaction with solution of nitrate of protoxide of cobalt before the blowpipe will afford conclusive proof (§ 104). 4. Add to a portion of the original solution chloride of 62 aminonium and carbonate of ammonia, mixed with some caustic ammonia, and heat gently. a. NO PRECIPITATE IS FORMED: absence of baryta, strontia, and lime. Pass on to (64). b. A PRECIPITATE IS FORMED; presence of baryta, stron- 63 tia, or lime. Add a considerable quantity of solution of sulphate of lime to a portion of the original solution. a. The solution does not become turbid, not even after the lapse of from five to ten minutes: LIME. To remove all doubt, test with oxalate of ammonia (§ 100). B. The solution becomes turbid, but only after the lapse of some time: STRONTIA. This reaction is only conclusive when the solution is neutral or but slightly acid. The coloration of the flame is looked to for confirmation (§ 99, 6 or 7). 7. A precipitate is immediately formed: BARYTA. To remove all doubt, test with hydrofluosilicic acid (§ 98). 5. Mix that portion of the solution of (62) in which carbo- 61 nate of ammonia has, after previous addition of chloride of ammonium, failed to produce a precipitate, with phosphate of soda, add some more ammonia, and rub the sides of the vessel with a glass rod. 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 65 clean 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 (§ 94). b. THERE IS A FIXED RESIDUE LEFT: potassa or soda. 66 Add bichloride of platinum to a portion of the original solution, having first concentrated it by evaporation if dilute, and shake the mixture. a. No precipitate is formed, not even after the lapse of ten or fifteen minutes: SODA. The coloration of the flame is selected as a conclusive test, or the reaction with antimonate of potassa is resorted to for the purpose (§ 93). 6. A yellow crystalline precipitate is formed: POTASSA. The reaction with tartaric acid, and the coloration of the flame are selected as conclusive tests (§ 92). Simple Compounds. A. SUBSTANCES SOLUBLE IN WATER. DETECTION OF THE ACID. I. Detection of Inorganic Acids. § 186. 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. 1. ARSENIOUS ACID and ARSENIC ACID have already been 67 considered in the preceding paragraph (detection of the base). These two acids are distinguished from each other by their respective behavior with nitrate of silver, or with potassa and sulphate of copper (see § 137, 9). 2. The presence of CARBONIC ACID, HYDROSULphuric acid, and CHROMIC ACID, is also indicated already in the course of the process pursued for the detection of the bases. The two former betray their presence by effervescing upon the addition of hydrochloric acid; they may be distinguished from one another by their odor. Should additional proof be required, the presence of carbonic acid may be ascertained beyond a doubt by the reaction with lime-water (see § 152), and that of hydrosulphuric acid by the reaction with solution of acetate of lead (§ 159). 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 color to green, accompanied by the separation of sulphur, upon the addition of hydrosulphuric acid water. To remove all doubt, try the reactions with solutions of acetate of lead and of nitrate of silver (§ 141). |