METHODS OF COLLOID RESEARCH 



97 



collodion, water is used. If gelatin is being used, the entire impreg- 

 nation trough must be placed in a bath of lukewarm water. Gelatin 

 niters are hardened by placing the niters, still moist and gelatinized 

 in the air, into a 2 to 4 per cent ice cold formaldehyde solution and 

 keeping them for a time in an ice box. 



The niters, however prepared, are washed several days in running 

 water and preserved in water to which a little chloroform has been 

 added to prevent the growth of mould. 



H. BECHHOLD generally uses glacial acetic acid collodion, a solu- 

 tion of soluble cotton in glacial acetic acid. 1 By diluting with glacial 

 acetic acid the solution may be reduced to the desired concentration. 



If non-aqueous solutions (e.g., benzol, ether, etc.) are to be ultra- 

 filtered, the water must be displaced by a series of solvents. (Water 

 is first displaced by acetone, the latter by benzol, and so on.) 



FIG. 16. Ultrafilter. (H. Bechhold.) 



The Ultrafiltration Apparatus. Very porous filters are permeable 

 under low pressure and can be used like any other filter. In by far the 

 largest number of cases, a pressure of from 1 to 20 atmospheres must 

 be exerted to obtain any filtrate at all. For this purpose H. BECH- 

 HOLD prepared an apparatus which in its simplest form is shown in 

 Fig. 16 2 ; Fig. 17 is more suitable for very high pressures. Fig. 16 

 consists of a cylindrical vessel H into which is inserted the funnel T. 

 Between the lower flanges of T and H, the disc of filter paper is 

 pressed. This is made tight by the two rubber rings G, G. 



To protect it from being torn, the filter lies on a nickel netting or 

 perforated nickeled plate N and is further protected from bulging un- 

 der pressure by the plate P, which has several holes in it. The upper 

 part of the funnel T is ground conically and is closed by means of 



1 The Chemischer Fabrik auf Aktien (vorm. Schering), Berlin, prepares to 

 order solutions of 10 per cent collodion and 2| per cent potassium carbonate, 

 which show only slight tendency to contract when they gelatinize. 



2 All this apparatus is manufactured by the Vereinigten Fabriken fur Labora- 

 toriumsbedarf , Berlin, Scharnhorst Str. 



