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.
81/680 page 61
![§29.] RULES AND PRECAUTIONS. Gl § 29. b b. RULES TO BE OBSERVED IN THE PROCESS OF FILTRATION. In tlie case of curdy, flocculent, gelatinous, or crystalline precipitates there is no great danger of any particles of the precipitate passing through the filter. But, in cases where we have to deal with fine pulverulent preci- pitates, it is generally necessary, and always advisable, to let the preci- pitate subside, and then filter tlie supernatant liquid, before proceeding to place the precipitate upon the filter. Substances which have been pre- cipitated hot, are most properly filtered before cooling (provided always there be no objection to this course), since hot fluids run through the filter more speedily than cold ones. Some precipitates have a tendency to be carried though the filter with their menstruum; this may be pre- vented in some instances by modifying the latter. Thus sulphate of baryta, when filtered from an aqueous solution, passes rather easily through the filter—the additiou of chloride of ammonium prevents this in a great measure. Should the operator find, during the filtration of a precipitate, that he has taken too small a filter to hold the precipitate conveniently, and so as to admit of its being properly washed, he had always best use an additional filter, and thus distribute the precipitate over the two. The fluid ought never to be poured directly upon the filter, but always down a glass rod, as represented in Fig. 24 ; and the lip or brim of the vessel from which the fluid is poured, may be greased with tallow.* The stream ought invariably to be directed towards the sides of the filter, never to the centre, since this might occasion some loss by splashing. In cases where the fluid has to be filtered off, with the least possible dis- turbance of the precipitate, the glass rod must not be placed, during the intervals, into the vessel containing the precipitate; but it may con- veniently be put into a clean glass, which is finally rinsed with water upon the filter. The filtrate is received either in flasks, beaker-glasses, or dislies, ac- cording to the various purposes for which it may be intended. Strict care should be taken to conduct the filtrate down the side of the receiving vessel; it should never be allowed to fall into the centre of the eollecting fluid, since this again might occasion loss by splashing. The best method in this respect is that illustrated in Fig. 26, viz., to place the tube of the funnel against the upper part of the inner wall of the receiving vessel. If the process of filtration is couducted in a place perfectly free from * The tallow for this purpose may conveniently be kept in a small glass tube fitted with a piston, by means of which the tallow is pushed forward in proportion as it is required for use.](https://iiif.wellcomecollection.org/image/b28133742_0081.jp2/full/800%2C/0/default.jpg)
