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CHAPTERXXII.THE OPTICAL ASSAY OF SUGARS.The optical assay of sugar is based on the direct proportionality of therotation of a ray of polarized light to the concentration of the solution, and tothe length of the layer through which the polarized ray passes. Hence if therotation of a ray of polarized light be measured for any known concentration andlength of layer, determination of the rotation and length of the layer of thesolution afford data to calculate the concentration; further, in case the materialunder examination is a solid, the solution of a known weight of the materialin a definite volume and determination of the rotation under known conditions,give data to calculate the percentage of sugar in the material underexamination.The instruments used for this purpose are known as polariscopes, and arediscussed in Chapter XXI.; in this Chapter the general principles of theprocess are outlined ; their application to different products of the sugarhouse is developed in Chapter XXIII.The Clearing Agents used.—In general sugar solutions areunfitted for polarimetric assay without the addition of some agent to precipitatecolouring and other bodies, which render the solution turbid. The materialsin general use are :—1. Alumina Cream.—Solutions of high grade sugars are often sufficientlyclear for polarimetric assay except for a slight turbidity which may be removedby agitation of the solution with a little alumina cream and subsequentfiltration ; alumina cream is prepared thus :—Precipitate a solution of an alumwith a slight excess of ammonia; wash by decantation till all soluble matteris removed, and preserve as a thick cream.2. Precipitation of alumina within the solution} — The precipitation ofalumina within the solution has a much greater decolourizing effect than theaddition of alumina cream, and the effect is increased by the addition of verysmall quantities of hydrosulphites. As carried out by the writer the clarificationis thus made : to a measured quantity of the solution under analysis aquantity of a solution of sodium aluminate is added, and after mixturefollowed by an equivalent quantity of a saturated solution of aluminiumsulphate and about .05 grm. of sodium hydrosulphite. The aluminate andalum solutions are previously standardized against each other, using phenol¬phthalein as an indicator.447