quartz-wedges, Scheibler's method of double observation for the 100 point may likewise be adopted. In using Soleil's saccharimeter for the approximate determination of the specific rotation of other substances, the deviation of which has to be reckoned in angular measure, one has simply to bear in mind that, according to § 18, a quartz plate 1 millimetre thick rotates ray D through an angle of 21·67°, and mean yellow light (ray j) through 24.5°, or, 1° Soleil (ray j) 1° Soleil (ray j) = 0·2167 angular degrees (ray D), § 87. Correction for imperfect proportionality between deviation and concentration will also require to be made in using the SoleilDuboscq. The amount of correction necessary is given for every 10° in the table annexed :— As to the influence of temperature upon determinations by this instrument, Mategczek 1 gives a table from which the following is an extract : 1 Mategczek: Zeitsch. des Vereins für Rübenzuckerindustrie, 1875, 891. (c.) Wild's Polariscope, with Saccharimetric Scale. = § 88. Wild's polariscope, already described (§ 49), can be fitted with a scale for use as a saccharimeter. As supplied from the works of Hermann and Pfister, of Berne, these instruments have a scale divided into 400 equal parts, and their construction is based upon the specific rotation of cane-sugar, [a]D 66-417°, as determined by Wild1 in a solution containing 30-276 grammes of cane-sugar per 100 cubic centimetres. Then assuming the amounts of concentration and of rotation to be strictly proportional, he calculates the value of the angle of rotation a, which a solution containing 40 grammes of sugar in 100 cubic centimetres should give in a 2 decia × 100 2 × 40 metre tube. The equation = 66-417, gives a = 53·134°.· Accordingly, an angle of this amount, measured from one of the zeropoints of the instrument, is divided into 400 equal parts, so that, tak 1 Wild: Ueber ein neues Polaristrobometer u. eine neue Bestimmung der Drehungscontante des Zuckers. Berne: 1865. Also, Mélanges Phys. et Chim. Bull. de l'Acad. de St. Petersbourg, 8, 33. ing observations with a 2 decimetre tube, each division of the scale will represent 1 gramme of sugar in 1 litre of solution. The use of a sodium flame is here presupposed. According to this mode of graduating the scale, a solution containing 10 grammes of pure sugar in 100 cubic centimetres, observed in a 2 decimetre tube, should record 100 degrees, each divison of the scale indicating 1 per cent. of sugar. If a solution of 20 grammes of sugar in 100 cubic centimetres be used, the amount recorded should be 200 degrees, each division of the scale corresponding to one half per cent. of sugar, and so on for solutions containing up to 40 grammes of sugar. Thus any weight of sugar may be chosen as normal weight, but a solution of 20 grammes in 100 cubic centimetres, or 10 grammes in 50 cubic centimetres, will be found most convenient. The observed rotation must then be divided by 2, and as with a little practice the scale can be read to one-fifth of a division, we can bring out values to 0.1 per cent. For example, if a solution of 20 grammes of a crude sugar in a 2 decimetre tube recorded 184.6 degrees, the percentage weight of sugar would be 92.3. In dealing with substances of low sugar-percentage, such as beet-root juice, it is more convenient to weigh out as much as 60 or 80 grammes, and dilute to 100 cubic centimetres. The degrees recorded must then, of course, be divided by 6 or 8 to get the correct sugar-percentage. The greater the weight of substance taken for solution, the greater will be the accuracy of the determination. In other respects, the mode of observation is similar to that described in § 51. The lamp, figured in § 46, with a bead of salt or soda, can be employed as the source of light. To convert the readings on the saccharimetric scale into angular measure, we have only to remember that as an angle of 53·134° was divided into 400 equal parts, 1° of the scale = 0·1328 angular degrees. § 89. The effect of inconstancy of specific rotation on the saccharimetric scale is shown in the annexed table of corrections, calculated by Schmitz,1 and based on the assumption that a solution containing 20 grammes of pure sugar records exactly 200 degrees. 1 Schmitz Zeitsch. des Vereins für Rübenzuckerindustrie, 1878, 48. N For per It will be seen from the two last columns that no correction is needed for percentages under 20 or over 80, the error in such cases not affecting the results to the amount of a tenth per cent. centages between 25 and 75, the indications directly observed are from 0.05 to 0.06 too high. (d.) Saccharimeter with Angular Graduation on Mitscherlich's, Wild's or Laurent's Principle. § 90. Any one of the forms of polariscope described in §§ 45, 49 and 57 may be used for determining the concentration c of a given sugar-solution by observing the angle of rotation a for a column / decimetres long, and substituting values in the equation :— Disregarding the variation of specific rotation [a] with the concentration of the solutions, a mean value may be assigned to it, which will serve for most of the sugar-solutions met with in practice. For example, by adopting, in analyses of natural sugars and their refined products, a normal solution of 15 grammes in 100 cubic centimetres, we shall obtain percentages ranging between 80 and 100, and the mean concentration c = 14 grammes. But according to the observations of Schmitz and Tollens recorded in § 91, c = 14 should yield an angle, which by the above equation corresponds to a specific rotation [a] = 66.50. Introducing this value for [a] in the equation, we get the annexed formula for determining the number of grammes of sugar in 100 cubic centimetres solution : in which a is the angle of rotation observed with the sodium light, and 7 the length of tube employed. With a 2 decimetre tube c = 0.752 a. In many cases this may be simplified to 0.75. Thus a solution of 15 grammes of pure sugar should give a rotation of 20° exactly. When, therefore, 15 grammes of any saccharine (natural sugar) are made into a 100 cubic centimetre solution, and the angle of rotation observed in a 2 decimetre tube, the sugar percentages may be obtained by simply multiplying by 5. In weighing out any other number of grammes, P, of substance the percentage can be calculated from the observed deviation a from the proportion P: 0.752 a = 100: x. § 91. Here, again, in exact determinations the imperfect proportionality between the angle of rotation and the concentration must be taken into account. Schmitz has prepared a table based upon the following observations, partly his own and partly those of Tollens, at a temperature of 20° Cent.3 2 1 Schmitz: Zeitsch. des Vereins für Rübenzuckerindustrie, 1878, 53 and 58. 2 Tollens: Ber. der deutsch, chem. Gesellsch. 10, 1403. 3 The angles of rotation were all observed at 20° Cent.; the concentrations by Schmitz at 20° Cent., those by Tollens at 17.5° Cent. The latter trifling difference does not affect the results. |