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PROCESS OF DETERMINING SPECIFIC ROTATION.
§ 42. In calculating specific rotations by the formulæ given in §§ 20, 21, viz.,
the following data must be obtained by direct experiment:
1. The measurement of the angle of rotation a for a given ray. 2. The measurement of the length of the experimental-tube, in millimetres.
3. The weight p of active substance in 100 parts by weight of solution.
4. The specific gravity d of the active liquid.
5. The concentration c-i.e., the number of grammes of active substance in 100 cubic centimetres of solution.
If the object of determining the specific rotation of a solution of a solid substance, is merely to obtain a characteristic of its presence in solution, formula III., based on the knowledge of its concentration c, will suffice. But if, on the contrary, it is desired to ascertain the actual specific rotation of the substance itself, from observations on a number of different solutions, it is necessary (see § 25) to employ formula II., involving a knowledge of the percentage composition, and specific gravity of the several solutions.
A. Determination of the Angle of Rotation.
POLARISCOPIC APPARATUS. § 43. Apparatus for the Qualitative Examination of Rotatory Power.—To determine merely whether a given substance is or is not optically-active, and, if active, the direction in which the
rotation takes place, the instrument here represented (Fig. 20), which is delicate enough to detect even feeble degrees of rotatory power, may be used.2
A brass trough a b, of semi-circular section, fitted with a cover c so as to form a tube, carries at the extremity a, in a fixed case, a polarizing Nicol d. In front of the latter is placed the convex lens e, and on the other side of the polarizer at f, a so-called Soleil double-plate, formed of two plates, one of dextro-rotatory, the other of lævo-rotatory quartz, fitted vertically together and ground to a uniform thickness of either 3.75 or 7.5 millimetres.
The opposite end of the brass tube holds the movable Nicol g,
? [f is given apparently out of proper section, representing a front view, whilst the rest of the figure shows a longitudinal section.-D.C.R.]
2 The instrument shown above is manufactured by F. Schmidt and Haensch, Stallschreiberstrasse 4, Berlin. Instruments on the same optical principle, but of simpler construction (described by C. Neubauer in Fresenius' Zeitschr. für analyt. Chem. 16, 213) intended for determining grape-sugar in wine, but equally applicable for all other active substances, are procurable from the Optical Instrument Works of Dr. Steeg and Reuter, Homburg v. d. Höhe.
besides a small Galilean telescope, consisting of an object glass h, and an ocular i. The Nicol is turned by the handle k, which moves round the face of a small graduated disc 1, so as to allow the amount of rotation to be determined, at least approximately. The brass trough receives the glass tube p p (the ends of which may be closed by glass plates fixed with brass screw-caps) containing the liquid to be examined. The whole rests on a stand 0. As a considerable depth of liquid is requisite for the detection of feeble rotatory power, the brass case is so constructed as to take glass tubes 5 or 6 decimetres in length. It is to this, and the introduction
. of the Soleil double-plate, that the sensitiveness of this instrument is due. ,
In using the instrument, the glass tube is at first left out whilst the extremity is directed towards a bright flame, for which purpose the gas-lamp, shown in Fig. 25, will be found best. The eyepiece of the telescope is then adjusted so that the vertical division of the double-plate appears sharply defined. By turning the analyzer 9, a certain position will readily be found in which the two halves of the field of vision exhibit a perfectly uniform purplish tint, which the least turn of the Nicol to the right or left changes, one half becoming red, the other blue. Further particulars of this so-called sensitive tint will be given later on ($78) in speaking of the Soleil saccharimeter. Having thus established perfect uniformity of colour in the two halves of the field of vision, with the index standing at the zero-point on the scale of the analyzer, the glass tube containing the liquid to be tested is laid in the trough, when its optical activity will at once be declared by inequality of tint in the field of vision. To know whether the rotation be right-handed or left-handed, it is requisite, in the first place, to determine in the instrument, once for all, what relative positions the red and blue take up when some substance of known rotatory power, such as a (dextro-rotatory) solution of canesugar, is inserted. If the substance under examination shows the colours in the same relative order in which they are shown by the sugar, it likewise is dextro-rotatory; if the positions are interchanged, it must be lævo-rotatory. Further, with dextro-rotatory substances uniformity of tint in both halves of the field of vision is restored by turning the analyzer to the right, or in the direction of the hands of a watch, and with lævo-rotatory substances, to the left. The position of the index on the graduated disc of the analyzer shows the angle of rotation in each case.
Instead of this instrument any of the forms of polariscope described further on may be used. The advantage of the above instrument lies in its sensitiveness and the facility with which with it the direction of the rotation can be determined.
§ 44. For the accurate measurement of the angle of rotation, a variety of instruments have been devised, which may be divided into two classes, according to their objects :
1. The so-called polaristrobometers, what in England are known as polariscopes, which indicate the amount of rotation in angular measure, and are applicable to all optically-active substances.
2. The saccharimeters, which are specially intended for the analysis of solutions of cane sugar, the angular measurement being replaced in them by an empirical scale.
(a.) Mitscherlich's Instrument.?
45. This simplest of all forms of polariscope consists, a.s already stated ($ 5), of a pair of Nicol prisms, placed one at each end of a brass or wooden rod or bar d, Fig. 21. The polarizer a is provided with a brass case, by means of which it can be turned if required, and then clamped with the small screw e. The circular movement of the analyzer b is effected with the handle c, the angle through which it is revolved being read off on a fixed graduated disc by means of opposite index arms, with or without verniers. The
[At this point something requires to be said as to the nomenclature of various polarizing instruments. The word polariscope, commonly used in English for any instrument the essential parts of which consist of a polarizer and analyzer, and which may or may not be applicable for showing the rotation phenomena with which this work deals, has no such range of meaning in German, as will be seen by reference to the use of the word in describing a special contrivance, the Savart polariscope forming part of the instrument described in § 49. On the other hand, we have in English no word limited to describe what the Germans call rather clumsily a polaristrobometer, and the French a polarimètre, a polariscope that is of special form, suited to observe and to measure the rotatory power of substances. If we deemed it advisable to introduce an expression for the purpose, it seems that “rotation polarimeter” would, as nearly as possible, represent the German polaristrobometer ; but since the word polariscope has become so familiar to practical people it seemed better to retain it and make the needed explanation.-D.C.R.]
2 Mitscherlich : Lehrbuch der Chem. 4 Aufl. Bd. 3, 361 (1844).
graduation is in degrees, and the reading is taken in degrees and tenths. The experimental tube f is laid between the prisms, and usually has a length of 2 decimetres (7.6 in.). For increasing the illuminating power and giving a circular field of vision, a small convex lens is inserted in the case of the polarizer.
chimney, having a side aperture, b. In the movable pillar dis inserted horizontally a small cross-bar, carrying at its extremity a bundle of fine platinum wires, arranged so as to form a small pointed spoon c, the hollow being filled with well-dried common salt; the
Laurent: Dingler's Folyt. Journ. 223, 608. This lamp may be obtained of Schmidt and Haensch, Berlin.