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fore we may add besides, if necessary, some basic acetate of lead and a little alum-solution.
After introducing the clarifier, the contents of the flask are made to mix by a gentle motion, and then left undisturbed for five to ten minutes. The surface of the liquid is then brought to the mark by adding water from the wash-bottle. If foam forms on the surface of the solution and prevents the exact adjustment of the level, it can be removed by touching it with a drop of ether or by merely pouring upon it a small quantity of ether-vapour from the bottle (Scheibler). The flask is then closed with the thumb and shaken vigorously for some time, after which the mixture is to be filtered. For this
For this purpose round filters 13 to 14 centimetres (about 54 inches) diameter, which will easily take 50 cubic centimetres, should be used. Of course, they must not be wetted before use, and the funnel and receiver below must be perfectly dry. Evaporation must be prevented during filtration by covering the funnel and receiver with glass plates. Very often the first few drops of the filtrate are turbid, and should be received in another vessel. The clear solution should at once be put into the polariscopetube.
The mode of using flasks with double marks (as 50 and 55 cubic centimetres) is as follows :—The sugar-solution is made up to the 50 cubic centimetre mark, the acetate of lead, etc., added to the upper mark and the flask then shaken, etc., as before. The solution is then too dilute by one-tenth of its volume, and the angle of rotation observed in a 200 millimetre tube must be increased by one-tenth to get the correct value; or the rotation may be directly observed in a tube 220 millimetres long-a tube of this size being sometimes supplied with the instru
On the other hand, in using flasks with single marks only, an error occurs whenever the addition of basic acetate of lead produces a precipitate, from the diminution thereby caused in the volume of liquid contained by the flask, when the mixture stands at the 50 cubic centimetre or 100 cubic centimetre mark. The liquid will then be too concentrated, and its rotatory power too high. The error from this source will obviously vary in magnitude, with the amount of precipitate furnished by different saccharine products. Scheibler examined by a variety of methods the volume of the precipitate obtained in the decoloration of 100 cubic centimetres of beet-root liquors with 10 cubic centimetres of basic acetate of lead. His experiments showed a mean value of 1.3 cubic centimetres. Hence sugar-percentages, estimated from the observed rotation, appear 0.15 per cent. too high. According to Nebel and Sostmann, the error in beet-juices averages 0:17; in diffusionjuices 0.27 per cent. Pellet obtained the following error-values :For beet-juices 0·15 to 0-2 per cent. ; cane-sugar juices 0.1 per
1 Scheibler: Zeitsch. des Vereins für Rübenzuckerindustrie, 1875, 1051.
cent. ; thick syrups, 0-25 per cent.; sugars from the second and third crops, 0.25 per cent.; molasses, 0.63 per cent.
Accordingly, the results obtained by direct observation must be reduced by the above amounts to get the true sugar-percentages.
$ 94. Sometimes, indeed, as in the case of molasses and darkcoloured bye-products, the colour is so intense that even the basic acetate of lead fails to sufficiently decolorize the solution. In such cases an attempt should be made to observe the rotation by employing a 100 millimetre tube, or by diluting the solution to double its original volume, the observed rotation being, of course, doubled. Where this is found impracticable the solutions should be cleared with animal charcoal after preliminary treatment with acetate of lead. For this purpose, 30 to 40 cubic centimetres of the filtrate are placed in a flask, with 3 to 6 grammes of powdered and strongly dried bone-charcoal, and either shaken vigorously for some time or allowed to stand for twelve to twenty-four hours. In most cases, we shall then obtain, after filtration, a perfectly clear liquid.
The charcoal has, however, the disadvantage of abstracting not only colouring matter, but some sugar as well, so that the rotation results will be considerably too small. It will be necessary, therefore, in order to apply the proper correction, to make a preliminary determination of the absorptive power of the particular charcoal used, by a few experiments with sugar-solutions of known strength. Thus, Scheiblers found that, with 50 cubic centimetre solutions, containing 13:024 grammes of various natural sugars, cleared with basic acetate, after standing over 5-5 grammes of desiccated bone-charcoal for from twelve to twenty-four hours, the observed rotation gave sugarpercentages averaging 0:4 to 0.5 too low. He also showed that the amount of sugar absorbed is proportional to the quantity of charcoal used.
1 Nebel and Sostmann: Zeitsch, des Vereins für Rübenzuckerindustrie, 1876, 724. 2 Pellet : Idem., 1876, 730. 3 Scheibler: Idem., 1870, 218.
(f.) Determination of Cane-Sugar in the Presence of other Active
Substances. $ 95. Another source of inaccuracy met with in the saccharimetric analyses of beet-liquors, inferior natural sugars, and molasses, is the occurrence along with cane-sugar of a whole series of other substances, capable of affecting in different ways the plane of polarization of light. Of such substances the following have been found, viz. :—Malic acid (-), asparagin and aspartic acid (both + in acid solutions and – in alkaline), glutamic acid (+), invert-sugar (-), beet-gum (-), dextran (+), the two last-named possessing very high rotatory powers. In molasses these substances may be present in such quantity as to render the determination of the sugar in the highest degree inaccurate, and even with beet-liquors some uncertainty is thereby involved in the results. It is true that by clearing with basic acetate of lead such substances are precipitated in part; but a method of certainly removing them entirely or of optically neutralizing them is still a desideratum. Eisfeldt and Follenius? attempt this, by heating with a solution of copper sulphate and caustic soda, so as partly to precipitate and partly decompose them by oxidation. Sickelo has proposed a method, which consists in adding to 13:024 grammes of beet-juice 1 cubic centimetre of basic acetate of lead, and diluting to 50 cubic centimetres with absolute alcohol. In this way the asparagin, aspartic acid, malic acid, gum, and dextran are precipitated, and the rotatory power of the invert-sugar almost completely annulled by the presence of the alcohol. This method appears to be serviceable, but requires further confirmation.
$ 96. When invert-sugar alone accompanies the cane-sugar, the effect of which is to reduce the rotation, the correct percentage of the cane-sugar present can be determined by employing the so-called inversion method of Clerget. When a saccharimeter with Soleil scale is used, the method is as follows:—The usual normal solution, containing 16-35 grammes of the sugar is prepared in the ordinary way, cleared, if necessary, with basic acetate of lead, and the rotation determined. Then 50 cubic centimetres of the solution
1 Eisfeldt and Follenius: Zeitsch. des Vereins für Rübenzuckerindustrie, 1877, 728 and 794.
? Sickel : Idem., 1877, 779 and 800.
are heated with 5 cubic centimetres of concentrated hydrochloric acid for ten minutes, on a water-bath at a temperature of about 68° Cent. (154° Fahr.), whereby the whole of the cane-sugar is transformed into invert-sugar. After cooling, the rotation (in this case left-handed) is observed in a 220 millimetre tube, the temperature of the solution being also noted by introducing a thermometer.
The calculation of the percentage of cane-sugar from the above two observations is performed in the following manner :
According to Clerget's experiments, a solution containing 16.35 grammes of pure sugar in 100 cubic centimetres—which indicates a rotation of + 100° with a Soleil saccharimeter-will, after inversion, and observed at a temperature of 0° Cent., indicate a rotation of 44° on the scale, the total change of rotation indicated being 144o.
Moreover, it is found that the amount of lævo-rotation of a solution of invert-sugar varies very markedly with the temperature of observation. The above solution, for instance, would undergo, for every rise of 1o Cent., a reduction of 0.5 division on the Soleil scale, so that at a temperature to, its amount would be 144 - t. Putting S for the sum of the opposite saccharimetric readings (the readings before and after inversion), that is, for the total decrease of rotation, and t for the temperature at which the inverted solution is observed, the required percentage of cane-sugar R may be found from the proportion
(144 – 1 t): 100 = S:R, whence:
100 S R
If, for example, the amounts of rotation were :
then. S= 131:3° which gives
100 x 131:3
= 98:0 per cent. cane-sugar,
144 - 10 instead of the incorrect value, 94:1 per cent., directly obtained from the original solution.
Tuchschmid? has studied minutely the change of rotation produced by the inversion of sugar-solutions, as well as the influence of
1 Tuchschmid : Journ. für prakt. Chem. , 2, 235.
temperature, and gives the subjoined formula for the calculation of cane-sugar percentages :
100 S R
144.16 – 0·506 t If, instead of Soleil's saccharimeter, one with angular graduation be employed, Tuchschmid's formula becomes
R = 31-31 – 0:11 t To determine at the same time the percentage of invert-sugar originally present in the substance, the following method can be adopted :-As we have already said, a solution of 16-35 grammes of cane-sugar in 100 cubic centimetres gives, by inversion, a liquid, which at a given temperature, t°, rotates towards the left through (44 – 1 t) divisions of the scale. Now, since 171 parts of cane-sugar, when treated with acid, yield 180 parts invert-sugar, the above rotation corresponds to a percentage of 17:21 grammes invert-sugar in 100 cubic centimetres. Putting A for the result of direct observation, R for the sugar-percentage found after inversion, and J for the proportion of invert-sugar to be determined, we have the proportion
44 - 10:17:21 = R - A:J, whence,
17:21 (R – A) J
44 - 10 Taking, as in the former example, A = 94.1, R= 93.0, and
: assuming the rotation to have been observed, both before and after inversion, at 20° Cent., we get the result :
17.21 (980 - 94.1) JE
= 2:0 per cent.
10 Employing the more exact constants determined by Tuchschmid, we have :1. For Soleil's saccharimeter:
17.21 (R – A)
17.21 (R - A)
9.59 – 0:11 t This method will, of course, cease to furnish correct results when other optically-active substances are present in addition to invert