XXII] DAIRY PRODUCTS 289 



1 1 Gravimetric Method. — Official. 



Dilute 25 grams of the milk with 400 cc. of water in a 500 cc. graduated flask, 

 add 10 cc. of copper sulphate solution [VIII, 19 (a)] and about 7.5 cc. of a potas- 

 sium hydroxid solution of such strength that 1 volume is just sufficient to pre- 

 cipitate completely the copper as hydroxid from 1 volume of the copper sulphate 

 solution. Instead of potassium hydroxid solution of this strength, 8.8 cc. of N/2 

 sodium hydroxid solution may be used. After the addition of the alkali solution 

 the mixture must still have an acid reaction and contain copper in solution. Fill 

 the flask to the 500 cc. mark, mix, filter through a dry filter and determine lactose 

 in an aliquot of the filtrate as directed under VIII, 46 or 48. 



FAT. 



12 Roese-Gottlieb Method'^. — Official. 



Weigh 10-11 grams of the milk into a Rohrig tube or some similar apparatus, 

 add 1.25 cc. of concentrated ammonium hydroxid (2 cc. if the sample is sour) and 

 mix thoroughly. Add 10 cc. of 95% alcohol by volume and mix well. Then add 

 25 cc. of washed ether and shake vigorously for 30 seconds, then 25 cc. of petro- 

 leum ether (redistilled slowly at a temperature below 60°C.) and shake again for 

 30 seconds. Let stand 20 minutes, or until the upper liquid is practically clear. 

 Draw off as much as possible of the ether-fat solution (usually 0.5-0.8 cc. will be left) 

 into a weighed flask through a small, quick-acting filter. The flask should always 

 be weighed with a similar one as a counterpoise. Re-extract the liquid remain- 

 ing in the tube, this time with only 15 cc. of each ether, shake vigorously 30 seconds 

 with each and allow to settle. Draw off the clear solution through the small filter 

 into the same flask as before and wash the tip of spigot, the funnel and the filter 

 with a few cc. of a mixture of the 2 ethers in equal parts. For absolutely exact 

 results the re-extraction must be repeated. This third extraction yields usually 

 not more than about 1 mg. of fat (about 0.02% on a 4 gram charge) if the previous 

 ether-fat solutions have been drawn off closely. Evaporate the ethers slowly on a 

 •steam bath, then dry the fat in a boiling water oven to constant weight. 



Confirm the purity of the fat by dissolving in a little petroleum ether. Should 

 a residue remain, remove the fat completely with petroleum ether, dry the residue, 

 weigh and deduct the weight. Finally correct this weight b)^ a blank determina- 

 tion on the reagents used. 



Babcock Method. — Official. 



13 APPARATUS. 



(a) Standard Babcock test bottles. — The standard Babcock test bottles for milk 

 and cream shall be as follows: 



(1) 8%, 18 gram, 6 inch milk test bottle.— The total per cent graduation shall be 8. 

 The total height of the bottle shall be 150-165 mm. (5^-62 inches). The capac- 

 ity of the bulb up to the junction with the neck shall be not less than 45 cc. The 

 graduated portion of the neck shall have a length of not less than 63.5 mm. (2^ 

 inches) and the neck shall be cylindrical for at least 9 mm. below the lowest and 

 aljove the highest graduation marks. The graduations shall represent whole per 

 cents, halves and tenths of a per cent. 



(2) 50%, 9 gram, 8 inch cream test bottle.— The total per cent graduation shall 

 be 50. The total height' of the bottle shall be 150-165 mm. (5^61 inches). The 

 capacity of the bulb up to the junction with the neck shall be not less than 45 cc. The 

 graduated portion of the neck shall have a length of not less than 63.5 mm. (2^ 



