192 MILK 



11 Determination. The milk should be at constant temperature 

 and its specific gravity determined with a delicate hydrometer. 

 When greater accuracy is required, a pycnometer is used. 



"The quantities of the milk measured for polarization vary 

 with the specific gravity of the milk as well as with the polari- 

 scope used. The quantity to be measured in any case will be 

 found in the following table : 



VOLUME OF MILK TO BE USED 



"Place the quantity of milk indicated in the table in a flask 

 graduated at 102.4 c.c. for a Laurent, or 102.6 c.c. for a Ventzke 

 polariscope (Mohr cubic centimeter). Add 1 c.c. of mercuric 

 nitrate solution or 30 c.c. of mercuric iodid solution (an excess of 

 these reagents does no har,m), fill to the mark, agitate, filter through 

 a dry filter, and polarize. It is not necessary to heat before polar- 

 izing. In case a 200-mm. tube is used, divide the polariscope read- 

 ing by 3, when the sucrose normal weight for the instrument is 

 16.19 grams; or by 2, when the normal weight for the instrument 

 is 26.048. When a 400-mm.' tube is used these divisors become 6 

 and 4 respectively. For the calculation of the above table the 

 specific rotary power, of lactose is taken as 52.53, and the corre- 

 sponding number for sucrose 66.5. The lactose normal weight 

 to read 100 on the sugar scale for Laurent instruments is 20.496 

 grams, and for Ventzke instruments 32.975 grams. In case metric 

 flasks are used the weights here mentioned must be reduced to 

 16.160 and 26.000 grams respectively." 



Gravimetric Method. Preparation of the Milk Solution. 

 Dilute 25 c.c. of the milk with 400 c.c. of water and add 10 c.c. of 

 a copper sulphate solution prepared as follows: Dissolve 34.639 

 grams of CuSO 4 ,5H 2 O in water and make up to 500 c.c. Add to 

 the mixture about 7.5 c.c. of a solution of KOH of such strength 

 that one volume of it is just sufficient to completely precipitate 

 the copper as hydroxid from one volume of the solution of copper 

 sulphate. Instead of a solution of KOH of this strength, 8.8 c.c. 

 of a half -normal solution of NaOH may -be used. After the ad- 

 dition of the alkali solution the mixture must still have an acid 

 reaction and contain copper in solution. Fill the flask to the 500 

 c.c. mark, mix, and filter through a dry filter. 



