MILK-SUGAR POLARISATION. 159 



phospho-tungstic acid is, however, very small, and it is usually 

 only in concentrated milks that it exceeds the experimental error. 

 For exact estimations add to a measured volume of the mercuric 

 nitrate 7>V of a 10 per cent, phospho-tungstic acid solution and 

 ~j of 1:1 sulphuric acid ; filter, polarise, and multiply the 

 readings by 1*1. 



Deniges' Method.- Deniges objects to the use of mercuric 

 nitrate because it necessitates the use of a glass polarimeter 

 tube, brass being attacked by the solution, and prefers the use 

 of meta-phosphoric acid to precipitate the proteins. His method 

 is as follows : Prepare sodium meta-phosphate by heating 

 sodium-ammonium-hydrogen phosphate (microcosmic salt) care- 

 fully in a platinum dish, till it has fused completely and no longer 

 evolves gas. Pour on a cool plate, break up, and preserve in 

 a stoppered bottle. Prepare a 5 per cent, aqueous solution by 

 boiling 5-7 grammes of the finely powdered salt with 50 c.c. of 

 water for five minutes, at the expiration of which time solution 

 should be complete. Add immediately 50 c.c. of cold water, 

 cool under a jet of water, and make up to 100 c.c. Twelve per 

 cent, of the meta-phosphate is converted into ortho-phosphate 

 by the boiling, and this is allowed for by taking 5-7 grammes 

 instead of 5 grammes. 



Add 25 c.c. of this freshly prepared solution to 10 c.c. of milk, 

 then 60 c.c. of water, and 0'3 c.c. of acetic acid ; make up to 

 100 c.c. and filter ; after rejecting the first few drops, fill a polari- 

 meter tube with the filtrate. A 500 mm. tube is to be used, if 

 possible, in preference to one of less length. It is hardly neces- 

 sary to make any correction for the volume of the precipitate on 

 account of the great dilution. As only 10 c.c. of milk are taken 

 and diluted to 100 c.c., a very good polariscope must be used 

 if accuracy is required. Unless glass polarimeter tubes are 

 unobtainable, the use of mercuric nitrate is preferable ; an 

 advantage of employing mercuric nitrate is that citric acid can 

 be estimated in the same solution. 



The proteins may also be precipitated by adding to milk an 

 equal volume of a saturated solution of picric acid containing 

 1 per cent, of acetic acid. 



Feder uses asaprol, and adds 75 c.c. of milk, and 6 c.c. of a 

 solution of 75 grammes of asaprol, and 75 grammes of citric 

 acid in 250 c.c., and makes up to 100 c.c., filters, and polarises. 



Fehling's Solution Method. Another method, which is 

 employed frequently for the estimation of milk-sugar, depends on 

 the oxidation of the sugar by alkaline cupric solution, and the 

 consequent reduction of the copper to the state of cuprous oxide. 



The alkaline cupric solution cannot be applied direct to milk, 

 as the proteins are attacked by the alkali to some extent. 



