PffOSPHOBUS FROM OASEINOGEN. 441 



Method. The method which we have employed for estimating the 

 phosphoric acid at the different stages of the digestions was that 

 recently described by A. Neumann (8) . It consists in oxidising the 

 organic matter with a mixture of equal parts of nitric and sulphuric 

 acids, precipitating the phosphoric acid as ammonium phospho-molybdate, 

 dissolving this precipitate after washing free from acid in excess of semi- 

 riormal alkali and titrating with semi -normal acid. The difference 

 multiplied by T268 gives the amount of P 2 O 5 in milligrammes. 



Neumann stated that not more than 40 c.c. of the nitric acid 

 mixture should be employed in the oxidation on account of the 

 hindering effect of the sulphuric acid on the precipitation of the 

 ammonium phospho-molybdate. In certain cases it was found 

 necessary to use more than this amount so that a modification had to 

 be made ; 10 c.c. of cone, sulphuric acid were added directly and nitric 

 acid, instead of the nitric acid mixture, was run in until the oxidation 

 was complete. The solution was allowed to cool before fresh quantities 

 of nitric acid, generally about 10 c.c., were added. For this amount of 

 sulphuric acid, complete precipitation always occurred, when 30 c.c. of 

 the 50 per cent, ammonium nitrate solution were added to the oxidised 

 residue dissolved in 150 c.c. of water. 



The greatest difficulty, however, occurred in the filtration of the 

 precipitate of ammonium phospho-molybdate and the washing of it free 

 from acid. This was found to be extremely slow, and, consequently, 

 for the number of estimations which were required, a more speedy 

 method had to be devised. We have obtained this by employing a 

 special pattern of filtering tube, which consists of a glass tube about 

 8 cm. long and 2-2*5 cm. in diameter; at one end a perforated platinum 

 plate is sealed into it and at this point the tube is drawn out like an 

 ordinary filtering funnel. The apparatus resembles a Buchner filtering 

 funnel but has the shape of a Soxhlet filtering tube. Either asbestos or 

 filter paper may be placed on the plate: in these experiments filter 

 paper was used exclusively. This apparatus was introduced to the 

 notice of one of us several years ago and has been employed for the 

 most various purposes. Unfortunately we are unable to find out who is 

 the inventor of this filtering tube, so that we have described it again in 

 some detail. 



By this means the filtration and the washing of the ammonium 

 phospho-molybdate can be completed in less than five minutes, and we 

 have been able to carry out over 400 determinations of phosphoric acid 

 during the course of the last eight months. With this rapid filtration 



292 



