ESTIMATION OF MILK-^UOAK. '.I.j 



pKcipitate is obtained by the addition of phospho-tungstic acid; 

 the difference in the percentage of milk-sugar found after adding 

 phospho-tuuK^fic acid is, however, very small, and it is usually- 

 only, in concentrated milks that it exceeds the experimental error. 

 Foi- ex.ict estimations add to a measured volume of the mercuric 

 filtrate ,j',, of a 10 per cent, phosplio-tun^stic acid solution and .r',-j 

 of 1 : 1 sulphuric acid; filter, polarise, M.nd multiplv the readings 

 by II. 



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

 nitrate because it necessitates the use of a ulass 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 1)\' carefully 

 heating sodium-ammonium-hydrogen phosphate (microcosmic 

 salt) in a platinum dish, till it is completely fused and no longer 

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

 a sto]>pered bottle. Prepare a 5 per cent. ai(ueous solution l)y 

 boiling .")-7 grammes of the finely powdered salt with -jO c.c. of 

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

 should )»' complete. Add immediately .50 c.<-. of cold wati-r, 

 cixjI under a jet of \>ii1er, and make up to liHj c.c. Twelve prr 

 ci'ut. of the meta-pliospjiate is converte(l into ortho-phfisphate 

 by the boiling, and this is allowed for by taking ")'7 uramnirs 

 instead of 5 grannnes. 



.\dd "25 e.c. of this freshly prepaied solution to 10 v.v. of milk, 

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

 inn c.c. and filter ; after rejecting the lirst few drops, fill a jiolari- 

 satioii tube with the filtrate. \ 500 mm. tube is to be used, if 

 possil)le, 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 ureat dilution. As only 10 c.c. of milk are taken 

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

 if accuiacy is reipiired. Unless glass polarisation tubes are 

 unobtainable, the use of mercuric nitrate is preferable ; an 

 advantage of employing mercuric nitrate is that citric acid can 

 l)e 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. 



Pehliiig's Solution Method. — Another method, which is 

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

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

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



The alkaline cupric solution cannot be applied direct to milk, 

 as the proteins are somewhat attacked by the alkali. 



The solution usually employed is Fehling's cupric tartrate 



