Handbook of the polariscope and its pracitcal applications / adapted from the German editon of H. Landolt, by D.C. Robb and V.H. Veley.

  • Hans Heinrich Landolt
Date:
1882
Catalogue details

Licence: Public Domain Mark

Credit: Handbook of the polariscope and its pracitcal applications / adapted from the German editon of H. Landolt, by D.C. Robb and V.H. Veley. Source: Wellcome Collection.

    79/320 (page 59)
    formula [a] = A + B q + C q2 or some other similar expression, will represent the true specific rotation of the pure substance, the more imperfectly the shorter the length of the experimental curve. How far a correct determination is attainable in such cases will depend on the solubility of the active substance itself. If indeed only dilute solutions can be prepared, and if, moreover, the increase or decrease in the values of [a] is not constantly proportional to qf an estima- tion of the true specific rotation of the pure substance is quite impracticable. § 25. If we replace q in formulae I. and II. by p—i.e., the pro- portion of active substance in 100 parts by weight of solution—the •constant A will then represent the specific rotation of the active substance in a state of infinite dilution, and that of the pure sub- stance will be obtained when p — 100. But the employment of q (or, adopting Biot’s notation e,—i.ethe proportion of inactive substance per unit-weight of solution) as above is preferable. In estimating rotatory power by the formula [a] aV _a . 100 T7k~~T77~ (see § 21), no determination of the specific gravity of the solutions is required but merely the concentration c, determined by means of a flask of known capacity. Modifying the foregoing equations I. and II. accordingly, we have [a] = A + B c, and [a] = A -f B c + C c2, and putting c = 100, we get the specific rotation of a solution con- taining 100 grammes of active substance in 100 cubic centimetres of solution. But this would only represent the pure substance if it had a density cl = 1. If, as is always the case, d has some other value, say 6, we must give c the value 100 5. This, however, is a condition which can be but rarely satisfied, and never with certainty, as it pre- supposes a knowledge of the specific gravity of the active substance in an unknown amorphous condition. Hence, no specific rotation data where only the concentration of the solutions is taken account of, and specific gravity or weight per cent, composition is neglected, can be employed for the determination of the rotatory powers of the pure substances. § 26. When an active substance is influenced by inactive mole- cules of two different kinds, as wEen some other substance is dissolved along with it, or the active substance is dissolved in a mixture of two different liquids, the case becomes much more complicated. Each of the inactive substances exerts its own influence on the true specific