Page:Outlines of Physical Chemistry - 1899.djvu/103

 For solid substances dissolved in an inactive solvent, the

specific rotation is — r-£, where d is the density of the

p Id

solution, and p the percentage of dissolved substance.

No matter how the rotatory power is determined, it is always more or less influenced by the temperature.

And furthermore, when we are concerned with the rotation of a solution, we find that it varies according to the nature of the solvent, the concentration 1 and the time which has elapsed between the dissolving and the

Data concerning [a] of a solution must always be accom- panied by details of the temperature, concentration, &c.

The expression for the molecular rotation is

M[«]

where [a] is the specific rotation and m the molecular weight of the substance. The division by 100 is merely to reduce the size of the values.

[Note by Translator — We have learned two con- stants of rotation which can be used for comparing the effects of different substances. Ouye has introduced a third which he terms the molecular deviation^ and which he represents by [£]. This is calculated from the formula

��?-\A.

��ra-r**

��1 By taking polarimetric observations of solutions of different concentrations of the same substance we can obtain a formula or a graphic curve expressing the variations of the rotation in terms of the concentration ; and from this it is easy to calculate or extra- polate what the true rotatory power would be for a solution of maximum concentration, i.e. for the substance taken in the pure state.

2 Bi-rotation (extra great activity) of dextrose and lactose freshly dissolved ; hemi-rotation (reduced activity) of 7- lactose freshly dis- solved. In order to obtain normal and constant rotations, the solution must either be allowed to stand for 24 hours, or be boiled for some time.

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