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sugar. As this generally is the case, the method is not of much practical utility.

Nevertheless, the inversion process is convenient for ascertaining whether a sugar is or is not contaminated with other active substances. It will be known to be free of such substances when the amounts of rotation before and after inversion correspond, whilst if they do not correspond we may reckon on the presence of impurities. Scheibler1 in this way detected the presence of dextrin in natural sugars. As the direction of the rotatory power is unaffected when that substance is treated with acids, the lævo-rotation indicated by his invert-solution, and, therefore, also the calculated percentage of saccharose, was found too low.

B. Determination of Glucose.

GRAPE SUGAR-DIABETIC SUGAR.

2

§ 97. The specific rotation of dextro-rotatory glucose has been determined by various observers, the earlier of whom assumed that it was independent of the strength of the solutions. We have already seen, however (§ 38), that, according to the careful and minute investigations of Tollens, the specific rotation rises with increase of concentration. Nevertheless, in dilute solutions not containing more than about 14 grammes of anhydrous grape-sugar in 100 cubic centimetres, the differences are of small account, and in such cases [a]D may be taken as constant. Putting this value in the equation

[a]

=

= 53.0°

100 a
we get the subjoined formula for determining the
"
l.c

concentration c, from the angle of rotation a, observed with a tube 7 decimetres in length:

α

c = 1.8868

We are supposing here that the angle of rotation is taken with a Mitscherlich, Wild, or Laurent polariscope, and with sodium flame. The temperature of the solution must not differ much from 20° Cent. Then, if a 2 decimetre tube be employed,

c = 0.9434 a.

Hence, 1° rotation represents 0·9434 gramme of anhydrous grapesugar in 100 cubic centimetres of solution. This constant will serve 1 Scheibler: Zeitsch. des Vereins für Rübenzuckerindustrie, 1871, 322.

2 Tollens: Ber. der deutsch. chem. Gesellsch. 1876, 1531.

for all degrees of rotation up to 15°, which is sufficient for most

purposes.

§ 98. With higher amounts of concentration (c greater than 14), the specific rotation of glucose undergoes a rather appreciable increase, as will be seen from the table given in § 38. In consequence of this variation, it was necessary to construct interpolation-formulæ for calculating the degrees of concentration corresponding to various angles of rotation. For this purpose the following observations of Tollens have been taken as a basis :—

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By means of these, the table given on page 192 has been prepared, showing the number of grammes of anhydrous glucose in (1) 100 grammes, and (2) 100 cubic centimetres, of solution, indicated by various angles of rotation observed for ray D, and in a tube 2 decimetres long. For angles under 10°, the constants given in § 97 have been employed.

1 The extent to which these formulæ agree with other observations made by Tollens, may be gathered from the annexed table :

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6

12

6

To express the result in terms of glucose-hydrate, CH12 O + H2O, since the molecular weights, C H12 O and C H12 О6 + H2O, are as 180: 198 or 1: 1·1, the values found for the anhydride must be increased by one-tenth.

§ 99. Determinations of glucose may also be made with a saccharimeter of quartz-wedge compensation form, as its dispersive power, according to Hoppe-Seyler's1 measurements, agrees approximately with that of quartz. In dealing with dilute solutions not containing more than about 10 grammes in 100 cubic centimetres, the specific rotations of cane- and grape-sugars may be assumed to stand in the constant proportion of 665 530. Hence, (1) with Ventzke's scale, in which the 100 point indicates 26-048 grammes of cane-sugar in 100 cubic centimetres, the same amount of rotation 66.5 will, in a solution of grape-sugar, indicate a concentration 26.048

53.0

= 32.683. This gives 0.3268 gramme of anhydrous glucose for

each division of the scale when the rotation is observed in a 2 decimetre tube. Or, (2) with a Soleil-Duboscq scale, 1 division of the . 0·1635 = 0·2051 gramme of anhy

scale will correspond with

66.5
53.0

drous glucose in 100 cubic centimetres.

Thus in the examination of grape-sugars a 100 cubic centimetre solution should be prepared, containing

for Ventzke's saccharimeter, 32.68 grammes,

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and observed in a 2 decimetre tube. The number of degrees recorded indicate directly the percentage of anhydrous glucose in the weighed samples.

Polariscopes are also constructed on Soleil's principle with scales expressly graduated for grape-sugar.2 In these so-called diabetometers the index reading gives the number of grammes of glucose in 100 cubic centimetres solution, observed in a tube 1 decimetre long. Where a 2 decimetre tube is used the reading must, of course, be

1 Hoppe-Seyler (Fresenius', Zeitsch. für analyt. Chem. 1866, 412) found:

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divided by 2. Usually these instruments have the graduation continued on the other side of the zero-point, so that the scale then serves. for determinations of albumen as well, which has a lævo-rotatory power equal in amount to the dextro-rotatory power of glucose.

§ 100. Determination of Grape-sugar in Diabetic Urine.-The urine should, if possible, be examined in its natural state, or if the colour interferes it may be diluted to twice its volume, or observations made with a tube of 1 decimetre length. Turbid urine must of course be filtered. If it be too dark in colour we may take 100 cubic centimetres, add to it 10 cubic centimetres basic acetate of lead, filter, and examine the filtrate, or we may attempt to decolorize it with animal charcoal. In either case, the urine may lose some of its grapesugar; this has been proved to occur when the basic acetate of lead1 is employed, and is probable in the case of charcoal (compare § 94).

The presence of albumens, exercising their lævo-rotatory power, may interfere by causing the sugar to appear too low. They must, therefore, be previously removed. For this purpose, 100 cubic centimetres of the urine are heated to boiling in a basin, and very dilute acetic acid added till the urine exhibits an acid reaction, and the albumen separates as a flocculent precipitate. It is then filtered, the filter washed, and the filtrate again made up to 100 cubic centimetres; or, having acidified with acetic acid a determinate volume of the urine, it is mixed with an equal volume of a concentrated solution of sodium sulphate. On boiling the mixture, the albumen separates completely, and can be removed by filtering.

Bile-acids, which are dextro-rotatory, are never present in such urine in sufficient quantity to cause error in the above processes.

In ordinary urines, and in general when the proportion of grape-sugar in a urine is less than about 0-2 gramme in 100 cubic centimetres, it can no longer be accurately determined by observing the rotation of the urine itself directly. In such cases, we must take from 1 to 2 litres for an analysis. To this we must add, first, some solution of ordinary acetate of lead, and then, after filtering, some basic acetate together with a little ammonia. The second precipitate will contain the whole of the grape-sugar. It must be filtered off, mixed with alcohol, and excess of lead removed by sulphuretted hydrogen. The solution separated from the lead sulphide by filtration 1 See Brücke: Sitzungber. der Wiener Akad. 39, 10.

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