S. L. Jodidi and E. H. Kellogg 89 



for use. Unlike the asbestos filter, the paper pulp should in no case 

 be heated with strong acid (or alkali) since, by such treatment, it is 

 converted into a more or less sticky mass which is no longer fit as 

 a filtering medium. 



Preparation of paper-pulp FILTERS. A pulp filter can be 

 used either in connection with a perforated porcelain-plate in a fun- 

 nel or with a Gooch crucible. While the pulp filter can be prepared 

 in a variety of ways we have found the following convenient : The 

 Gooch crucible is connected with a suction flask, by means of rubber 

 tubing, funnel and rubber stopper, exactly in the same manner as in 

 the case of an asbestos filter. Care should be taken that the bottom 

 of the Gooch crucible is level. The crucible is now filled with paper 

 pulp and suction is gently started until all of the water is filtered 

 out, whereupon the suction valve is closed. The filling of the cru- 

 cible with paper pulp, etc., is repeated once or twice. The filter, 

 \Vhich is about 2 mm. thick, is now ready for use, namely, for the 

 filtration of precipitates to be estimated acidimetrically, as in the 

 case of the ammonium-phosphomolybdate precipitate. If it is de- 

 sired to use the paper-pulp filter for gravimetric analysis, the pulp 

 filter prepared as described above is washed with distilled water until 

 paper fibers no longer run through. 



Efficiency of the paper-pulp FILTER. The pulp filter com- 

 bines the rapidity of an asbestos filter with the accuracy of a good 

 paper-filter. In fact, the pulp filter is superior to both of them in 

 several respects. Tightly clinging, as it does, to the bottom and 

 walls of the Gooch crucible, the pulp filter easily retains fine pre- 

 cipitates, such as that of ammonium-phosphomolybdate, despite 

 rapid filtration. There is ample evidence in the literature that the 

 filtration of the yellow precipitate caused considerable difficulties. 

 Neumann'^ himself recommended that, prior to the filtration of the 

 yellow precipitate, the filter be filled with ice-cold water, which 

 causes the filter pores to contract and thus prevents the very fine 

 precipitate in the warm Solution from going through the filter. As 

 a matter of fact, in spite of the frequent use of ice-cold water, 

 the filtrate from the yellow precipitate is occasionally turbid, which 

 makes a second and even a third refiltration imperative. 



7 Neumann: Z. physiol. Chem., 37, 134 (1902-03). 



