890 SUGAR.—Analysis. Estimation of Sugar.
To standardize this solution, prepare a solution of“ invert” sugar of known strength by dissolving 0-475grams, dry loaf sugar in about 10 c.c. water and 1 e.c.pure hydrochloric acid in a 100 c.c. flask, and heatingto 70° C. for five minutes, cooling, and making upto 100 c.c. 0-475 grams, cane sugar inverted = 0-5grams, glucose. When 10 c.c. of this solution( = 0-05 grams, glucose) are added slowly from aburette to 10 c.c. of Fehling’s solution, kept boilingas described below, the latter, if of right strength,should be completely, but only just, decolorized.
Estimation of Sugar and Glucose—Preparation ofthe Solution for Polarization. —First obtain a fairaverage sample, the doing which, though the mostdifficult part of the analysis, needs no description.Weigh off the normal weight (16-2—26 grams., accord-ing to the polariscope employed) of the sugar to beanalyzed; introduce it into a 100 c.c. flask by thehelp of a smooth wide-necked funnel, and waterenough to rinse all down. Rather more than half fillthe flask -with water, and shake occasionally till allthe sugar is in solution. This last point is most im-portant, as any grains left would become imbeddedin the precipitate next made, and might never getfairly dissolved in the bulk of the liquid. Accordingto the quality of the sugar the solution will be“colourless but cloudy,” “yellow,” “brown,” oralmost black.
In the first case add about 8 c.c. of the creamyalumina, and one drop of the basic lead acetate. Inthe second case the same, but with three to five dropsmore of the lead solution. In the third and fourthcases add about 2 c.c. of the solution of sodiumsulphite, and then basic lead acetate cautiously andwith constant shaking, till no more precipitate isformed. In either case shake well, and allow thewhole to stand ten minutes, to permit of the fullseparation of any precipitate that is to be formed;then fill up to within a short distance of the markwith water. The froth, which is almost alwayspresent, would take so long to break and allow offilling up exactly, that it is necessary to let a drop ortwo of spirit fall gently on the surface, which is bestdone by help of a small squirt bottle; this makesthe bubbles disappear at once. Complete the fillingto the mark with the utmost care ; close the mouthof the flask with the thumb, and shake vigorously,to insure all parts of the liquid having the samecomposition.
It is commonly advised to add 5 to 10 c.c. of thebasic lead acetate to the solution of a raw sugarfor the purpose of clearing it; but any excess of thisreagent dissolves some of the precipitate first formed,making an opalescent solution which filters withextreme difficulty. The above method is thereforemuch to be preferred. Sciieibler prefers the fol-lowing method of clarification, which is certainlysimple and good, for all ordinary qualities of beetand cane sugars:—He prepares two reagents, one ofbasic lead acetate, and the other of aluminium sulphate,of such a strength that equal volumes of each,when added together, leave no sulphate in solution.To the dissolved sugar he adds 5 c.c. of each of
these liquids, shakes, and makes up to 100 c.c. asabove.
Some exceptionally black, or rather deep red sugars,and most beet molasses, will not yield a solution socolourless as to admit of accurate reading in a colourinstrument, such as Soleil - Duboscq’s or Scheib-ler’s modification of it, by either of the abovemethods. In such a case weigh off double thenormal quantity; clarify by the method given fol-low sugars, only adding a slightly greater excess ofbasic lead acetate; filter; measure off 50 c.c. of thefiltrate into another 100 c.c. flask; add a solution ofsulphurous acid till the liquid smells strongly of it,then about 2 grammes of the purified animal charcoal,shake; make up to 100 c.c., and filter. The leadsulphite formed carries some of the colouring matterdown with it, the excess of sulphurous acid partiallybleaches the remainder, and the char exerts greatabsorbent power in the acid solution ; so that, as anend result, we obtain a beautifully bright lemon-yellow solution from the worst of molasses. In nocase resort to filtering through a column of animalcharcoal, as often advised. It is impossible to obtaincorrect results by such means ; the char absorbs andretains sugar as well as colouring matter as long asit retains any power of decolorizing at all.
The solutions having been thus obtained, proceedto fill the tubes. It is assumed that all these havebeen tested as to their exact length, and are knownto be correct. One cap being on, rinse out the tubewith a little of the liquid to be examined; then fillup till the liquid threatens to overflow, and slide onthe other glass and fasten with the cap. Loosenboth caps, so that the glass discs are not subjectedto any compression which might cause them also toexert a rotatory action on the ray, and then make thereadings. The direct reading thus obtained willbe sufficient in the case of all beet sugars, andof nearly all good cane sugar; but molasses,“concrete,” and some varieties of low syrupysugars, contain a glucose which is optically active,though less so than pure “invert sugar,” whichit is often supposed to be. We must in suchcases resort to “ inversion.” For effecting this takea flask marked for 50 c.c. at the lower part of itsneck and 55 c.c. in the upper part, fill to the lowermark with the liquid used for the direct polarization,drop in one or two crystals of sodium sulphite, fillto the upper mark with strong pure hydrochloricacid, transfer the liquid to a long-necked assay flask,and stand this in a bath of cold water, which is tobe slowly raised to 70° C. When this temperaturehas been maintained for two or three minutes, standthe flask in cold water till its contents have acquiredvery nearly the temperature of the air. Now fill atube (which must be of glass, provided with a lateralopening for the introduction of a small thermometer),which is -j-Jths of the length of the tube used for thedirect reading, and observe the rotation which isproduced, noting at the same time the temperatureof the liquid.
The direct reading gave the resultant of all therotations of the plane of polarized light effected by