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The zinc must be present in excess of the quantity necessary to reduce the iron and is finally completely dissolved. If the solution turns brown, it is an evidence of insufficient acid, and more should be immediately added. The results are likely to be less accurate in this case, however, as a consequence of secondary reactions between the ferrous iron and the manganese dioxide thrown down.
When the oxidation is nearly completed only very small amounts of the ferrous compounds remain unoxidized and the reaction with the indicator is no longer instantaneous. It is necessary to allow a brief time to elapse before determining that no blue color is formed. Thirty seconds is a sufficient interval, and should be adopted throughout the analytical procedure.
Moreover, if the procedure is followed as prescribed, the concentration of unoxidized iron decreases very rapidly as the titration is carried out so that when the final tests are made, though large drops may be taken, the amount of ferrous iron is not sufficient to produce any appreciable error in results. !Selection of a Standard!
At the beginning of the titration, while much ferrous iron is still present, the end of the stirring rod need only be moist with the solution; but at the close of the titration drops of considerable size may properly be taken for the final tests. The stirring rod should be washed to prevent transfer of indicator to the main solution.
The two substances most commonly used as standards for a bichromate solution are ferrous ammonium sulphate and iron wire. A standard wire is to be purchased in the market which answers the purpose well, and its iron content may be determined for each lot purchased by a number of gravimetric determinations.
Use the following ointment for the pain in the back: Ext. Hyoscyamus, thirty grains; ext. Belladonna, thirty grains; Adipis, one ounce. Apply locally night and morning. EMMENAGOGUE: Ergotin, twenty grains; ext. cotton root bark, twenty grains; Purified Aloes, twenty grains; Dried Ferrous sulphate, twenty grains; ext. Savine, ten grains. Make twenty pills. Dose: One pill four times a day.
Third, In the case of an element forming two or more distinct series of compounds, e.g., ferrous and ferric salts, is the transition from one series to another necessarily connected with the addition or subtraction of an even number of hydrogenoid atoms?
What weight in grams of impure ferrous ammonium sulphate should be taken for analysis so that the number of centigrams of BaSO obtained will represent five times the percentage of sulphur in the sample? !Answer!: 0.6870 gram.
From the volumes of the solutions used, after applying corrections for burette readings, and, if need be, for the temperature of solutions, calculate the value of the ferrous solution in terms of the oxidizing solution.
Whatever the character of the compound may be, it has been shown that it loses sulphuric anhydride upon ignition, causing low results, even though the precipitate contains iron. The contamination of the barium sulphate by iron is much less in the presence of ferrous than ferric salts.
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