CHAPTER XXII. 



THE OPTICAL ASSAY OP SUGARS. 



The optical assay of sugar is based on the direct proportionality of the 

 rotation of a ray of polarized light to the concentration of the solution, and to 

 the length of the layer through which the polarized ray passes. Hence if the 

 rotation of a ray of polarized light be measured for any known concentration and 

 length of layer, determination of the rotation and length of the layer of the 

 solution afford data to calculate the concentration ; further, in case the material 

 under examination is a solid, the solution of a known weight of the material 

 in a definite volume and determination of the rotation under known conditions, 

 give data to calculate the percentage of sugar in the material under 

 examination. 



The instruments used for this purpose are known as polariscopes, and are 

 discussed in Chapter XXI. ; in this Chapter the general principles of the 

 process are outlined ; their application to different products of the sugar 

 house is developed in Chapter XXIII. 



The Clearing Agents used. In general sugar solutions are 

 unfitted for polarimetric assay without the addition of some agent to precipitate 

 colouring and other bodies, which render the solution turbid. The materials 

 in general use are : 



1. Alumina Cream. Solutions of high grade sugars are often sufficiently 

 clear for polarimetric assay except fora slight turbidity which may be removed 

 by agitation of the solution with a little alumina cream and subsequent 

 filtration ; alumina cream is prepared thus : Precipitate a solution of an alum 

 with a slight excess of ammonia ; wash by decantation till all soluble matter 

 is removed, and preserve as a thick cream. 



2. Precipitation of alumina within the solution. 1 The precipitation of 

 alumina within the solution has a much greater decolourizing effect than the 

 addition of alumina cream, and the effect is increased by the addition of very 

 small quantities of hydrosulphites. As carried out by the writer the clarifi- 

 cation is thus made : to a measured quantity of the solution under analysis a 

 quantity of a solution of sodium aluminate is added, and after mixture 

 followed by an equivalent quantity of a saturated solution of aluminium 

 sulphate and about '05 grm. of sodium hydrosulphite. The aluminate and 

 alum solutions are previously standardized against each other, using phenol - 

 phthalein as an indicator. 



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