88 MILK AND DAIRY PRODUCTS PART 



has carefully investigated the sources of error, removed them, 

 and so brought the estimation of milk sugar by the polarimeter 

 again to the fore. Scheibe showed that the previously observed 

 differences were not due to any dextrin-like substance in the 

 milk, but to the use of unsuitable precipitants, and that very 

 satisfactory results could be obtained if the method, which he 

 describes below, was followed : 



Seventy-five c.c. of milk are treated with 7 '5 c.c. of sulphuric 

 acid (20 per cent, by weight) and 7'5 c.c. of a solution of mer- 

 curic iodide, prepared by dissolving 40 g. of potassium iodide in 

 200 c.c. of water, shaking with 55 g. of mercuric iodide, making 

 up to 500 c.c., and filtering off the unchanged mercuric iodide. 

 The milk which has been treated with the above clarifying 

 solution is made up to 100 c.c., and the filtrate polarised in a 

 4 dcm. tube at 17'5 C. If the Schmidt and Haensch half- 

 shadow polarimeter with double quartz wedge is used, then one 

 saccharimeter degree is equivalent to 0*16428 g. milk sugar in 

 100 c.c. of solution. With polarimeters having a graduated circle 

 and usingsodium light the determination has tobe made at 20 C. 

 and 1 degree in the 4 d.-m. tube = 0*4759 g. milk sugar in 100 

 c.c. milk. To eliminate the error due to the volume of the pre- 

 cipitate, either a correction has to be made or the volume of the 

 precipitate determined, and allowance made for it. The deter- 

 mination of the precipitate must never be forgotten in the case 

 of cream or colostrum. The correction to be applied, if the 

 volume of the precipitate is not determined, consists in multi- 

 plying the amount of milk sugar by 0*94 in the case of whole 

 milk (2-8-4-7 per cent, fat) or 0'97 in the case of separated 

 milk. 



The volume of the precipitate may be found in two ways : 



(a) The rotary power of 10 g. pure milk sugar dissolved in 

 75 c.c. of water treated with 7*5 c.c. of 20 per cent, sulphuric 

 acid and 7*5 c.c. of mercuric iodide solution, and then made up 

 to 100 c.c., is determined. The number of grams of milk sugar 

 which are found in this way correspond to M l in the formula 

 given below. 



(b) Ten g. milk sugar are dissolved, with the application of 

 heat, in 75 c.c. of the milk which is to be examined, and, after 

 cooling, 0'5 c.c. concentrated ammonia is added. When the 

 ammonia has acted for 10 minutes, 7-5 c.c. of 20 per cent. 



