has completely subsided, let the mixture cool, add water, and then proceed with the analytical process. Fr. Mohr has endeavoured to improve Gay-Lussac's method of deter mining silver. The following is the process which he recommends :— Dissolve the sample presumed to contain silver and copper, in nitric acid, in a flask, and add concentrated standard solution of chloride of sodium to decided predominance, best to a divisional line marking ten volumes. Heat the contents of the flask to boiling, add crystallized carbonate of soda in moderate excess, boil until the oxide of copper has turned black, transfer the fluid, together with the precipitate, to a cylinder or flask of 150 cubic cent. capacity, rinse the flask with water, which transfer also to the measuring flask or cylinder, until the fluid, when quite cold, reaches exactly to 0; cover with a greased plate of vulcanized india-rubber, and shake. Let the solid particles now subside, take 50 cub. cent. of the clear fluid, add 2 or 3 drops of solution of neutral chromate of potassa, and determine the quantity of chloride of sodium in the fluid, as directed § 141. Multiply the resulting number by 3, and deduct the quantity of chloride of sodium so found from that first added; the difference expresses the quantity equivalent to the silver contained in the analysed compound. This method, which is not likely, however, to supersede Gay-Lussac's at the Mint, may be useful in cases where the quantity of silver present is not approximately known. If you wish to employ it, make use of equivalent solutions: viz., a decimal solution of chloride of sodium, and a decimal solution of silver. To prepare the former, dissolve 5.846 grm. (0.1 eq.) of pure fused chloride of sodium to 1 litre of fluid; to prepare the latter, dissolve 10.797 grm. (0·1 eq.) of pure metallic silver, to 1 litre of fluid. There must be no excess of acid in the latter solution, which must consequently be treated as directed § 141. The calculation in this method is not absolutely correct, as the precipitates of chloride of silver and oxide of copper are in the fluid when the latter is diluted to 150 cub. centimetres, and the 50 cub. centimetres taken out do not accordingly represent an absolute third of the solution; but the error is very trifling indeed, and disappears altogether if only a slight excess of solution of chloride of sodium has been added at first. Few of the salts of lead are soluble in water. Metallic lead, oxide of lead, and most of the insoluble salts of lead, dissolve in dilute nitric acid. Concentrated nitric acid effects neither complete decomposition nor complete solution, since, owing to the insolubility of nitrate of lead in concentrated nitric acid, the first portions of nitrate formed protect the yet undecomposed parts of the salt from the action of the acid. For the solubility of chloride and sulphate of lead, see § 83. As we shall see below, the analysis of these compounds may be effected without dissolving them. Iodide of lead does not dissolve in cold dilute nitric acid; but upon application of heat it dissolves readily with separation of iodine. Solution of potassa is the only menstruum in which chromate of lead dissolves without decomposition; for the purpose of analysis, the chromate is best converted into the chloride (see below). Sulphide of lead is usually converted at once into sulphate (see § 116, 6, 8). b. Determination. Lead may be weighed as oxide, sulphate, chromate, sulphide, chloride, or as oxide of lead + lead; it may be determined also by volumetrical analysis. We may convert into a. By Precipitation. 1. OXIDE OF LEAD. All salts of lead soluble in water, and those of its salts which, insoluble in that menstruum, dissolve in nitric acid, with separation of their acid. b. By Ignition. u. Salts of oxide of lead with readily volatile or decomposable inorganic acids. Those of the insoluble salts of lead of which the acid cannot be removed from solution in nitric acid. b. By Evaporation. a. All the oxides of lead, and also the salts of oxide of lead with volatile acids. 3. Many of the organic compounds of lead. 3. CHROMATE OF LEAD. The compounds of lead soluble in water or in nitric acid. 4. CHLORIDE OF LEAD. Chromate, iodide, and bromide of lead. 5. OXIDE OF LEAD + LEAD. Many of the organic compounds of lead. 6. SULPHIDE OF LEAD. All salts of lead in solution. This method serves to separate The application of these several methods must not be understood to be rigorously confined to the compounds specially enumerated under their respective heads; on the contrary, all the compounds enumerated sub 1, may likewise be determined as sulphate of lead; and all those enumerated sub 1, 2, and 5, as sulphide of lead, &c. &c. Chloride, bromide, and iodide of lead are most conveniently reduced to the metallic state in a current of hydrogen gas, in the manner described § 115, 4 (reduction of chloride of silver), if not deemed preferable to dissolve them in water, or to decompose them by a boiling solution of carbonate of soda. If the reduction method is resorted to, the heat applied should not be too intense, since this might cause some chloride of lead to volatilize. The higher oxides of lead are reduced by ignition to the state of simple oxide, and may thus be readily dissolved and analysed. Should the operator wish to avoid having recourse to ignition, the most simple mode of dissolving the higher oxides of lead is to act upon them with dilute nitric acid, with the addition of alcohol. For the methods of analysing sulphate, chromate, iodide, and bromide of lead, I refer to the paragraphs treating of the corresponding acids, in the second part of this section To effect the quantitative estimation of lead in the oxide and in many salts of lead, especially also in the sulphate, the compound under examination may be fused with cyanide of potassium, and the metallic lead obtained well washed, dried, and weighed. From the sulphide also the greater portion of the lead may be separated by this method, but never the entire (H. Rose," Poggend. Annal.," 91, 104). 1. Determination of Lead as Oxide. a. By Precipitation. a. By Precipitation as Carbonate of Lead. Mix the moderately dilute solution of the compound to be analysed with carbonate of ammonia slightly in excess, add some caustic ammonia, apply a gentle heat, and, after some time, filter. Wash the precipitate with pure water, dry, and ignite in a porcelain crucible, having previously incinerated the filter on the lid. For the properties of the precipitate and residue, see § 83. The results are satisfactory, although generally a trifle too low, owing to carbonate of lead not being absolutely insoluble, particularly in fluids rich in ammoniacal salts. (Experiment No. 44). A small and thin filter should be used, and care taken to remove the precipitate as completely as practicable before proceeding to incineration; otherwise additional loss of substance might be incurred, from reduction of the adhering particles of the carbonate to metallic lead, by the carbon of the filter. B. By Precipitation as Oxalate of Lead. The solution of the compound under examination is mixed with oxalate of ammonia in excess, and ammonia added until it slightly predominates; the precipitated oxalate is then allowed to subside, the fluid filtered off, and the further process conducted as directed in a; the porcelain crucible is left uncovered during the ignition of the precipitate. Results as in a. b. By Ignition. Compounds like carbonate or nitrate of lead are cautiously ignited in a porcelain crucible, until the weight remains constant. For the manner of reducing salts of lead with organic acids to the state of oxide, see 5. 2. Determination as Sulphate of Lead a. By Precipitation. a. Mix the solution (which should not be over dilute) with moderately dilute pure sulphuric acid slightly in excess, and add to the mixture double its volume of spirit of wine; wait a few hours, to allow the precipitate to subside; filter, wash the precipitate with spirit of wine, dry, and ignite, after the method described in § 53. Though a careful operator may use a platinum crucible, still a thin porcelain crucible is preferable. A small and thin filter should be employed, and the adhering sulphate of lead carefully removed before proceeding to incineration (see 1, a, a). B. In cases where the addi'ion of spirit of wine is inadmissible, a greater excess of sulphuric acid must be used, and the precipitate, which is allowed some time to subside, filtered, and washed first with water acidulated with a few drops of sulphuric acid, then repeatedly with spirit of wine. The remainder of the process is conducted as in a. For the properties of the precipitate, see § 83. The method a gives very accurate results; those obtained by ẞ are nearly equally accurate, provided the proper excess of sulphuric acid be used, and the washing water properly acidulated with that acid. Neglect of the former condition will, in the presence, for instance, of ammoniacal salts, nitric acid, &c., cause a portion of the lead to remain in solution; neglect of the latter will lead to the solution of perceptible traces of the precipitate in the washing water, b. By Evaporation. Put the weighed substance into a weighed dish, dissolve in dilute nitric acid, add moderately dilute pure sulphuric acid slightly in excess, and evaporate, at a gentle heat, at last high over the lamp, until the excess of sulphuric acid is completely expelled. In the absence of organic substances, the evaporation may be effected in a platinum dish; but if organic substances are present, a light porcelain dish is preferable. With due care in the process of evaporation, the results are perfectly accurate. 3. Organic compounds of lead are converted into the sulphate by treating them, in a porcelain crucible, with pure concentrated sulphuric acid in excess, evaporating cautiously in the well-covered crucible, until the excess of sulphuric acid is completely expelled, and igniting the residue. Should the latter not look perfectly white, it must be moistened once more with sulphuric acid, and the operation repeated. The method gives accurate results; a trifling loss is, however, usually incurred, the escaping sulphurous and carbonic acid gas being liable to carry away traces of the salt. 3. Determination as Chromate of Lead. Mix the neutral or slightly acid solution with neutral chromate of potassa in excess, and, if free nitric acid has been present, add acetate of soda in sufficient quantity to replace the free nitric acid by free acetic acid; let the precipitate subside, at a gentle heat, and pass the fluid through a weighed filter dried at 212° F.; wash the precipitate with water, dry at 212° F., and weigh. For the properties of the precipitate, see § 93. The results are accurate. 4. Determination as Chloride of Lead. Mix the solution with hydrochloric acid in excess, concentrate in the water-bath, treat the residue with absolute alcohol, mixed with ether, let the precipitate subside, filter off the fluid, wash the precipitate with alcohol mixed with ether, dry, and expose to a gentle heat; were you to heat it to redness, some of the chloride of lead would volatilize. In other respects the operation is conducted as directed in 1, a, a. This method is had recourse to in certain separations of lead from other substances. 5. Determination as Oxide of Lead + Lead. Heat from one to two grms. of the organic compound of lead in a small weighed porcelain dish very gently, taking care to make the heat act first upon the rim of the dish, so that the ensuing decomposition may begin on one side, and proceed slowly. When the whole mass is perfectly decomposed, increase the heat a little, and continue until no more glowing particles are perceptible, and the residue forms a mixture of oxide of lead with globules of metallic lead, perfectly free from car, bon. Weigh the residue; then heat it with acetic acid until the oxide is completely dissolved, which does not take long; pour off the solution now from the metallic lead, and wash the latter by repeated decantation; remove the last traces of water by heat, and weigh the residuary metallic lead. By subtracting the weight of the metal from that of the original residue, you will find the quantity of oxide of lead which was present in that residue; and by calculating the proportion of metal contained in the oxide, and adding the resulting number to the weight of the metallic lead, you will obtain the total amount of that metal originally present in the analysed compound. This method is very convenient, and, if properly and carefully executed, gives very accurate results. The operator must take care to conduct the decomposition of the organic compound very slowly, since the rapid combustion of its carbon and hydrogen, at the expense of the oxygen of the oxide of lead, would produce so high a temperature as to volatilize a portion of the lead in visible fumes. If the residue is not perfectly free from carbon -which will always be detected in the heating of it with acetic acid— this will tend to give a higher number. This method was originally introduced by Berzelius. Dulk has recommended the following modification :-The compound to be analysed is gently heated, in a covered porcelain crucible, until the organic substance is completely carbonized; the lid is then removed, and the mass stirred with a piece of iron wire. Upon this, the mass begins to ignite, and a mixture of oxide of lead with metallic lead results, which may still contain unconsumed carbon. A few crystals of recently fused nitrate of ammonia are now thrown into the crucible, which has previously been removed from the flame, and the lid is again put on. The salt fuses, oxidizes the lead, and converts it partly into nitrate. The crucible is now exposed to a red heat, until no more fumes of hyponitric acid escape. The residuary oxide is then weighed. This modification of Berzelius' method gives very accurate results. It possesses this great advantage, that it ensures the complete combustion of the whole of the carbon; it saves also some trouble in weighing and calculating. 6. Determination as Sulphide of Lead. a. The same method which serves for the determination of silver as sulphide (§ 115, 2,) may also be resorted to for lead; with this exception, however, that the precipitation of the sulphide of lead from acid solutions must be effected without the application of heat; otherwise, a portion of the precipitate will redissolve. The operator should also, before proceeding to filtration, test a portion of this supernatant fluid with a comparatively large quantity of saturated sulphuretted hydrogen water, to see whether it will remain clear. This experiment is intended to guard against errors arising from incomplete precipitation of the lead, by hydrosulphuric acid, from acid solutions. Compare M. Martin, "Chem. Centralbl.," 1856, 501. For properties of the precipitate, see § 83. B. If the precipitate contains sulphur, it must be converted into sulphate of lead. To that end, it is dried on the filter, and put into a beaker, into which the filter is then also thrown; pure fuming nitric acid is now added, drop by drop, the beaker being kept covered with a glass plate. When the oxidation is effected, a gentle heat is applied for some time, and the contents of the beaker are then |