90 A Simple, Efficienfj and Economic Filter 



Plimmer and Bayliss,^ who made phosphoric-acid estimations 

 according to Neumann's method, wrote as follows in this connec- 

 tion : " The greatest difficulty, however, occurred in the filtration of 

 the precipitate of ammonium phosphomolybdate and the washing of 

 it free from acid. This was found to be extremely slow. . . . We 

 have obtained this (a more speedy method) by employing a special 

 pattern of filtering tube." . . . Their apparatus, however, is far from 

 being simple and easy to handle. Wardlaw^ recently stated in this 

 connection, that an attempt made by him to wash the precipitate by 

 suction in the manner recommended by Plimmer and Bayliss, was 

 unsuccessful since " in every case it was found impossible to pre- 

 vent visible amounts of the precipitate from passing through the 

 filter." 



While the Gooch crucible in connection with the asbestos filter is 

 of excellent Service in analytical chemistry, the preparation of a 

 good asbestos filter requires some experience and is not satisfactory 

 in all cases. In our experience, for instance, the asbestos filter, 

 unless prepared with the iitmost care, permits small amounts of 

 ammonium-phosphomolybdate precipitate to pass through. To be 

 sure, the amount of yellow precipitate which may thus pass into the 

 filtrate is usually slight, but in cases where only a small amount of 

 phosphorus is involved the error is not quite negligible. The pulp 

 filter, on the other hand, so completely retains the yellow precipitate, 

 that out of ninety conseciitive analyses, in which the phosphorus was 

 precipitated as ammonium phosphomolybdate, all filtrates were abso- 

 lutely water-clear, not showing the slightest turbidity or the faintest 

 yellow color. By employing a few mgm. of phosphorus, in such 

 tests, the available yellow precipitate can be separated by filtra- 

 tion and washed in about five minutes. This enables one to 

 run conveniently three or four times as many phosphoric-acid esti- 

 mations as is possible by the filtration of the yellow precipitate 

 through a folded filter, which, by the way, is a very tedious Opera- 

 tion. The work with the pulp filter has still another advantage in 

 that the liquids to be titrated contain a considerably smaller amount 

 of paper pulp, and further that, because of the rapid filtration and 



8 Plimmer and Bayliss: Jour. of Physiol., 33, 441 (1905-06). 

 "Wardlaw: Jour. Proc. Roy. Soc. N. S. Wales, 48, 78 (1914). 



