File:Image from page 70 of "Practical methods of organic chemistry" (1901) (14771297592).jpg

Identifier: practicalmethods00gatt Title: Practical methods of organic chemistry Year: 1901 (1900s) Authors: Gattemann, Ludwig. [from old catalog] Subjects: Publisher: [n.p.] Contributing Library: The Library of Congress Digitizing Sponsor: The Library of Congress

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Text Appearing Before Image: rations it is much more desirable toconduct filtrations without employing pressure, the precipitatesobtained in organic preparation work are filtered with pressurewhenever it is possible. The method presents a number of ad-vantages : the filtration may be made in a much shorter time; theliquid may be much more completely sepa-rated from the precipitate, in consequence ofwhich the latter will dry more rapidly, etc. The student has already learned the meth-ods of filtering without pressure in the opera-tions of analytical chemistry, but he is advisedto reread the chapter on Crystallisation (seepage 1). Filtration with Suction. — For filtering un-der pressure (suction), a filtering flask a (suc-tion flask) with a side-tube b (Fig. 35) is used.An ordinary flask may be converted into asuction flask by fitting to it a two-hole rubberstopper ; through one hole is passed the stemof a funnel, through the other a glass tube,bent at a right angle, one end of which passes just through the cork,

Text Appearing After Image: 52 GENERAL PART while the other is attached to the suction. For this purpose flaskswith thick walls are selected, in order that they may not be crushedby the atmospheric pressure on exhaustion; if a thin-walled flaskis used, it must be exhausted but slightly. The funnel used is, in many cases, the ordinary conical glass form,in which is placed the filter. If the funnel is imperfect in construc-tion, and does not possess the correct angle (6o°), the filter is madenarrower or wider, as the case may be, to accommodate it to theangle of the funnel. In order that the point of the filter not sup-ported by the glass walls of the funnel, may not tear on exhaustion,a platinum cone c is previously placed in the funnel. If a platinum cone is not at hand, it may be replaced by a coni-cally folded piece of parchment paper or filter-cloth. The filteris moistened with the same liquid which is to be filtered, otherwiseit may happen that the filtration is prevented, or, at least, rendereddifficult; e

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