A system of instruction in quantitative chemical analysis / by C. Remigius Fresenius.
- Fresenius, C. Remigius, 1818-1897. Anleitung zur qualitativen chemischen Analyse. English
- Date:
- 1854
Licence: Public Domain Mark
Credit: A system of instruction in quantitative chemical analysis / by C. Remigius Fresenius. Source: Wellcome Collection.
85/680 page 65
![§ 31.] SEPARATION OF PRECIP1TATES. ÖÖ dips under the surface of the fluid. If the apparatus is well arranged, the water will now flow outof c in the same measure as it runs off througli the funnel. To substitute for these washing bottles, narrow-necked flasks, inverted directly into the funnel, is quite inadmissible in cases where we intend to determine the exact amount of the precipitated substance, since the ascending bubbles of air would invariably carry minute particles of the precipitate up into the flask. Care should be taken, wliilst washing the precipitate, no matter wliether with, or without a washing apparatus, to prevent the formation of channels in it, through which the water might flow off, without per- vading the whole mass of the substance. If such channels have formed, the precipitate must be carefully stirred with a glass rod, or a platinum spatula. The Operation of washing may be considered completed when all soluble matter has been removed; whether this end has been attained may generally be ascertained by evaporating a drop of the last washings upon a clean platinum knife, and observing whether or not this leaves any residue behind. But in cases where the precipitate is not altogether insoluble in water (sulphate of strontia, for instance), recourse must be had to more special tests, which we shall have occasion to point out in the course of the work. The Operation of washing precipitates requires the greatest care and attention at the hands of the analytical chemist; it is self-evident tliat the imperfect washing of a precipitate must materially interfere with the accuracy of the results. The Operation should, therefore, never be deemed concluded on the mere impression and judgment to that cffect of the operator, the Information elicited by the application of the appropriate test, or tests alone can safely be relied on. § 31. SEPARATION OF PRECIPITATES BY DEC ANTATION, AND FILTRATION COMBINED. In the case of precipitates which, from their gelatinous nature, or from an admixture of certain salts that have been thrown down together with them, appear to oppose insuperable, or, at all events, considerable obstacles to perfect washing on the Alter, the following method will be found to answer perfectly. Let the precipitate subside as far as practicable, pour the nearly clear supernatant liquid on the Alter, stir the precipitate up with the washing fluid (in certain cases, where such a course is indicated, the mixture of the washing fluid with the pre- cipitate may besides be heated to boiling), let it subside again, and repeat this Operation until the precipitate is nearly washed. Place](https://iiif.wellcomecollection.org/image/b28133742_0085.jp2/full/800%2C/0/default.jpg)
