of cobalt. The cobalt in this solution is determined as described sub. a. For the properties of sulphuret of cobalt, vide § 54. This method, if properly executed, yields very accurate results, since it is free from all sources of error. c. By direct ignition. The same method as described § 81, 1 d. (Manganese.) The amount of cobalt in the oxide obtained by this method is determined by reduction with hydrogen, in the manner described sub. a. § 84. 5. PROTOXIDE OF IRON. Many of the protocompounds of iron are soluble in water. The pure protoxide of iron and those of its compounds which are insoluble in water, dissolve nearly without exception in hydrochloric acid. The solutions, if not prepared with perfect exclusion of the air, and with solvents absolutely free from air, contain invariably more or less peroxide. Some natural compounds of iron require fluxing with carbonate of soda, as an indispensable preliminary for their perfect solution in hydrochloric acid. Solutions prepared in this manner, contain nearly exclusively perchloride of iron, and but little protochloride, or even none. Metallic iron dissolves in hydrochloric acid and dilute sulphuric acid, as protochloride or protosulphate, with evolution of hydrogen; in hot nitric acid, it dissolves as pernitrate, and in aqua regia as perchloride. b. Determination. Protoxide of iron is invariably weighed in the form of peroxide, (§ 55.) To convert it into this form, the solution of the protoxide is first peroxidized, and the peroxide is then treated as directed § 85. In some instances, (vide § 85,) and especially for the purpose of separating the protoxide of iron from other oxides, the protoxide of iron is first precipitated as sulphuret of iron. The method applied for this purpose is the same as that which will be found described § 85, b. The following is the best method of peroxidizing the solution of a proto-salt of iron : The solution of the proto-salt of iron is mixed, in a flask, with a small amount of hydrochloric acid-(unless this acid be already present)-some nitric acid is added, and the mixture heated to incipient ebullition. The color of the solution will show whether a sufficiency of nitric acid has been added. In concentrated solutions, the addition of nitric acid produces a dark brown color, which disappears upon heating. This color is owing to the nitric oxide formed dissolving in the not yet decomposed solution of the proto-salt of iron. The conversion of the protoxide into peroxide may likewise be effected by transmitting chlorine gas through the solution, or by adding to it chlorine water in excess. With regard to the method of determining protoxide of iron from the amount of gold which it reduces from the perchloride of the latter metal, I refer to § 124, B., bb., since this method serves exclusively to separate the protoxide of iron from the peroxide. $ 85. PEROXIDE OF IRON. a. Solution. Pure Many of the percompounds of iron are soluble in water. peroxide of iron, and most of those of its compounds which are insoluble in water, dissolve in hydrochloric acid. In many instances, this solution proceeds with difficulty; it is necessary in such cases to use concentrated hydrochloric acid, and to reduce the compound to be analyzed to a state of the most minute division, previously to subjecting it to the action of the acid; this action is promoted by the application of a gentle heat; heating to ebullition is improper. Iron ores insoluble in hydrochloric acid are prepared for solution by fluxing with carbonate of soda. b. Determination. Peroxide of iron is invariably weighed as such. The compounds of iron are converted into this form either by precipitation as hydrated peroxide-preceded in some instances by precipitation as sulphuret or persuccinate of iron,-or by direct ignition. We may convert into PEROXIDE OF IRON. a. By precipitation as hydrated peroxide of iron. All persalts of iron with inorganic or volatile organic acids, which are soluble in water; and likewise those of the insoluble persalts, the acid of which may be removed upon their solution in hydrochloric acid. c. By precipitation as persuccinate of iron. All the compounds enumerated sub. a. b. By precipitation as sulphuret of iron. All compounds of iron without exception. d. By direct ignition. All persalts of iron with volatile oxygen acids. The method d. is very expeditious and accurate, and is therefore preferred to the other methods in all cases where its application is admissible. The method b. serves principally to separate peroxide of iron from other bases; this method is applied moreover in certain instances where a. is inapplicable, and thus especially in cases where sugar or other non-volatile organic substances are present; and likewise to determine peroxide of iron in its compounds with phosphoric acid and boracic acid. The method c. is exclusively used to separate iron from other bases. For the determination of the peroxide of iron in perchromate and persilicate of Iron, I refer to § 99 and § 106. Determination as peroxide of iron. a. By precipitation as hydrated peroxide. The iron compound under examination is dissolved in its appropriate menstruum, in a flask or beaker-glass, and ammonia is added in excess; the mixture is then heated nearly to ebullition, filtered, carefully washed with hot water, thoroughly dried, and ignited according to the method described § 32. The crucible should be kept closely covered at first, and exposed to a gentle heat; the lid should subsequently be removed, and placed against the crucible, in the manner illustrated by Plate XXIX., and a more intense heat should then be applied. This method, if properly and carefully executed, yields very accurate results, and is free from sources of error. Should the operator have reason to fear that part of the peroxide has suffered reduction by the charcoal of the filter, he need simply moisten the contents of the crucible with nitric acid, evaporate and ignite again; by this process he is sure to obtain all the iron present, in the form of peroxide. For the properties of the precipitate and residue, vide § 55. The precipitate must, under all circumstances, be carefully washed, since, should it retain any sal ammoniac, a portion of the iron would volatilize in the form of perchloride. b. By precipitation as sulphuret of iron. The iron compound under examination is dissolved in its appropriate menstruum, in a beaker-glass, and ammonia added to complete neutralization of the free acid present. (If organic nonvolatile substances be present, a little hydrated peroxide of iron will precipitate; however, this is not of any consequence.) Sulphuret of ammonium is then added in excess, and a gentle heat applied. The operator generally obtains, in this manner, a black precipitate in a colorless or yellowish fluid, in which case he may at once filter; but should the fluid exhibit a greenish color—which happens particularly with very dilute solutions, and is owing to extremely minute particles of sulphuret of iron remaining mechanically suspended in the fluid-the beaker-glass, covered with a glass plate, must be kept standing at rest on a moderately warm spot, until the fluid has acquired a yellow tint. The fluid may then be filtered. In either case, the precipitate is washed uninterruptedly with water, mixed with a small amount of sulphuret of ammonium, the funnel being kept well covered all the while. If any of these precautions be neglected, some loss of substance will arise from a portion of the sulphuret of iron oxidizing gradually in the air, and being carried into the filtrate as protosulphate. Since the protosulphate of iron which gets thus into the filtrate, is re-precipitated by the sulphuret of ammonium present, the fluid assumes in such cases a greenish color, and deposits gradually a black precipitate. After having completed the washing of the precipitated sulphuret of iron, this, together with the filter, is introduced into a beaker-glass, and some water poured over it; hydrochloric acid is added until all the sulphuret of iron present is re-dissolved. Heat is then applied until the solution smells no longer of sulphuretted hydrogen; the solution is filtered into a flask, the filter is carefully washed, and the filtrate peroxidized by heating with nitric acid, (vide § 84 ;) the peroxidized solution is finally treated as directed sub. a. c. By precipitation as persuccinate of iron. The persalt of iron under examination is dissolved in the appropriate menstruum, in a flask, and very dilute solution of ammonia is added drop by drop until a small portion of the iron precipitates in the form of hydrated peroxide; a gentle heat is then applied to ascertain whether the precipitate will re-dissolve or not. If it re-dissolves, the addition of dilute ammonia is to be continued, until the application of heat causes no longer this re-solution of the precipitate formed. If, on the contrary, it does not re-dissolve, and the fluid continues to exhibit a brown-red color, all the conditions requisite for precipitation with succinate of ammonia are fulfilled. But should the fluid appear colorless, this is to be considered as a sign that too much ammonia has been added; in that case, it will be necessary to add a small portion of hydrochloric acid, and then again some ammonia until the desired point is attained. A perfectly neutral solution of succinate of ammonia is to be added to the fluid thus prepared, and this addition is to be continued as long as a precipitate is formed; a gentle heat is then applied, and the fluid is subsequently allowed to cool; when perfectly cold, it is filtered |