A.--The Estimation. The binoxide of commerce, as taken from the mine, is well sampled, powdered, and dried at 100°C. 0.5 gramme of this is taken and placed in a 250 c.c. flask; in analysis the binoxide on the filter, from the treatments noted under separation is thoroughly washed with warm water; it is then washed down in a flask, as above, after breaking the filter paper; sufficient water is added to one-third fill the flask, and about twice the approximate weight of the binoxide in the flask of oxalate of potassa; these are agitated together. A twice perforated stopper is fitted to this flask, carrying through one opening a 25 c c. pipette nearly filled with sulphuric acid, sp. gr. 1.4, the lower point of which just dips below the mixture in the flask, and the upper end, carrying a rubber tube and pinch cock to control the flow of acid. Through the other opening passes a glass tube bent at an acute angle and connected by a short rubber tube to an adjoining flask, two-thirds filled with decinormal baryta solutions. These connections are all made air tight. Sulphuric acid is allowed in small portions at a time to flow into the mixture. Carbonic acid is evolved, and, passing into the adjoining flask, is absorbed by the baryta, precipitating it as carbonate. To prevent the precipitate forming around or choking up the entrance tube, the flask must be agitated at short intervals to break it off. The reaction so familiar to us in other determinations is expressed thus:
MnO2+KO,C2O3+2SO3 = MnO,SO3+KO.SO3+2CO2,
When no more carbonic acid is evolved, another tube from this last flask is connected with the aspirator, the pinch-cock of the pipette open, and air drawn through the apparatus for about half a minute, and thus all the carbonic acid evolved absorbed, or the flasks may be slightly heated. If danger of more carbonic acid being absorbed from the air is feared, and always in very accurate analysis, a potassa tube may be connected to the pipette before drawing the air through. The precipitate formed is allowed to settle, 50 c.c. of the supernatant solution is removed with a pipette and transferred to a beaker; 50 c.c. of decinormal nitric acid and some water is added with sufficient cochineal tincture. It is then titrated back with decinormal soda; from this is now readily deducted the amount of carbonic acid, and from that the MnO2, holding in view that 44 parts of carbonic acid is equivalent to 43.5 of MnO2 or 98.87 per cent, and that 1 c.c. of the N/10 baryta solution is equivalent to 0.0022 grm. of CO2.
If a carbonate, chloride, or nitrate, be present in the native binoxide, it must be removed with some sulphuric acid. This is afterward neutralized with a little caustic soda. This method yields the following results for its value in amount of manganese to 100: 99.91-99.902-99.895, and can be executed in about twenty minutes. Fifteen determinations can be carried on at once without loss of time, this, however, depending on the operator's skill. I have made many assays, and assays by this method with similarly excellent results.
Of the other methods, Bunsen's is acknowledged to be the most accurate, but is, of course, too troublesome to be used in technical work, although it is used in scientific analysis. Ordinary samples are not sufficiently accurate to allow the use of this method.
The methods of reducing with iron and titrating this with chromate of potassa, etc., have given a constant average of from 98.60-99.01. These results are fair, but hard to obtain expeditiously.
Of the methods of precipitating the compounds of the protoxide and estimating the acid, that of the phosphate is by far the most accurate, titrating with uranium solution; 99.82 is a nearly constant average with me, much depending on the operator's familiarity with the uranium process.
The methods of Lenssen, or ferricyanide of potassium method, yields very widely differing results. I have found the figures of Fresenius about the same as my own in this case; that is from 98.00-100.10.
B.--On the Separation. First, from its soluble simple combinations with the acids or bases containing no iron or cobalt; if they are present, it is treated as is noted later. If sulphuric acid is present it must be separated by treating the solution of the compound with barium chloride and filtering. A nearly neutral solution is prepared in water or hydrochloric acid and placed in a flask. Here it is treated with chlorine by passing a current of that gas through it as long as it causes a precipitate and for some time afterward. It is then discontinued, the mixture allowed to deposit for a few moments, and about two-thirds of the supernatant solution decanted; it is mixed with some more water, and these decantations repeated until they pass away without reaction, or by filtering it and washing on the filter; it is then dissolved in hot hydrochloric acid, this nearly neutralized, a solution of sesquichloride of iron is added, and again treated with an excess of chlorine. After washing it is transferred to the flasks of the apparatus mentioned in the first part of this paper, and estimated. Myself and several others have found this always to be a true MnO2, and not a varying mixture of protosesquioxide and binoxide, and will thus yield accurate results. This reprecipitation may sometimes be dispensed with by adding the iron salt before the first precipitation, but it of course depends upon the other elements present.
From Compounds containing Cobalt, Cobalt and Nickel, Iron and group III., together or with other elements.--Group III. and sesqui. iron are separated by agitation with baryta carbonate, some chloride of ammonia being added to prevent nickel and cobalt precipitation traces, and filtering. If cobalt is present we treat this filtrate with nitrite of potassa, etc., to separate it (that is, if it and nickel are to be separated and estimated in the same sample; but if they are to be estimated as one, or not separated, the treatment with nitrite, etc., is not used). The filtrate from this last is directly treated with chlorine. If nickel and cobalt are not to be estimated in this sample, the solution, as chlorides, is mixed with some chloride of ammonium and ammonia, then with a fair excess of phosphoric acid, a sufficient quantity more of ammonia to render the mixture alkaline. The precipitate formed is transferred to the filter and well washed with water containing NH3Cl and NH4O, then dissolved in hydrochloric acid and reprecipitated with ammonia, filtering and washing as before. It is again dissolved in HCl and titrated with uranium solution, or decomposed by tin, as noted below, and the manganese precipitated as binoxide with chlorine, and determined. The latter method is hardly practicable, and I never have time to use it, as the titration and all together yields a value of 99.80 in most cases, if accurately executed.