§ 105. The constants given by Hesse and Oudemans serve for testing the purity of commercial samples. For this purpose it is necessary, of course, that the conditions in respect of solvent and concentration should be the same as in the determination of these normal values, and the temperature should not differ very much. Moreover, it is necessary, in dealing with compounds containing water of crystallization, to determine directly the amount of the latter, in order either to establish the identity of the same with the formula assigned, or, if it disagrees, to be able to find the percentage of anhydrous substance, viz., of alkaloid present. To give some idea of the amount of variation that may be expected, a few determinations of specific rotations obtained by Hesse and Oudemans respectively, for the same substances are given below. This is only practicable in the experiments made on aqueous solutions, as the other solutions employed by these observers are not comparable. Employing the constants given in §§ 103, 104, the specific rotation for the pure alkaloid, in a solution of 16 grammes in 100 cubic centimetres, appears for the undermentioned salts as follows: It thus appears, that with samples equally pure, different observers may obtain values differing by seven degrees, or even more. The differences, however, which occur when impurities are present, are much larger in amount when another alkaloid is present, the specific rotations of the four cinchona bases differing very considerably from one another. This is shown in a number of experiments by Hesse, of which the results are as follows:-A solution of a specimen of neutral diquinine sulphate, which a separate analysis showed to contain 15 per cent. of water of crystallization was prepared with hydrochloric acid having a concentration c = 2 grammes anhydrous base. To prepare 50 cubic centimetres of this solution, since 85 parts of anhydrous base were equivalent to 1 × 100 100 parts of the hydrated substances, 85 24 =1·176 grammes, was required. This amount was put in a 50 cubic centimetre flask, together with so much standard hydrochloric acid to give for 1 molecule (C20 H21 N2 O2)2. H2SO4 four molecules H Cl, and the resulting solution diluted up to the mark with water. Observed in a Wild's polariscope with a 2 decimetre tube, the solution gave an angle of rotation ap 9.58°, whence the specific rotation of the anhydrous 9.58 × 100 substance = = 239.5°. Now the figures in § 103 (8), show that perfectly pure sulphate of quinine in such a solution gives [a]D= 239-2° (as a mean of three observations which gave respectively 239-1°; 239 1°; 239-3°) and hence the preparation under examination must have been free from admixture. Three other samples of commercial sulphate of quinine, examined under like conditions, gave the following specific rotations respectively : Of these the two first agree so nearly with the normal value that no admixture of foreign substance, at least in any appreciable quantity, could have been present; the third, on the contrary, shows such a marked rotation to the right, that it must contain a notable amount of quinidine and cinchonine. In this way Hesse showed that a so-called quinidine disulphate of English houses is an essentially different product from pure quinidine (conchinine) sulphate as prepared by the firm of F. Jobst, of Stuttgart. These preparations exhibited the following differences :Quinidine (conchinine) sulphate, with 4 mols. H Cl c = (anhydrous salt) c = ? } [a] = + 286°. English quinidine sulphate, with 4 mols. H Cl, c = 2 (anhydrous salt) Quinidine (conchinine) sulphate, with 4 mols. H2 SO, c = 2 (anhydrous salt) English quinidine sulphate, with 4 mols. H, SO4, c = 2 (anhydrous salt) [a] = + 264°. [a], = 280° to 282°. [a] = +247·5°. The direct optical analysis of alkaloids in cinchona-bark extracts is, as Hesse remarks, beset with difficulty from the solutions containing a yellow colouring matter, which cannot be removed alone, and which impedes accurate observation of rotation. Hence the polariscope cannot serve for the direct valuation of cinchona-barks, although it provides a useful check on the results obtained by other modes of analysis. § 106. Optical Analysis of Mixtures of Cinchona Alkaloids.-The quantitative composition of a mixture of two alkaloids may be deduced from its specific rotation with the aid of the values given in § 103. To test this method Hesse1 determined the specific rotations of a number of mixtures of known composition, to ascertain whether the former could be deduced from the rotatory powers of the constituents. This was found to be sufficiently exact, and there Hesse: Liebig's Ann. 182, 146. fore the association of alkaloids in solution does not materially interfere with their individual optical properties. Hence a quantitative analysis by this method is practicable.1 1 The mode of calculating the specific rotation of a mixture from those of its components will be best understood by taking an example. Let us suppose a mixture of 2 grammes of quinine hydrochloride with 1 gramme of cinchonine hydrochloride dissolved in water to 100 cubic centimetres. The specific rotation of the separate salts is given by the formulæ in § 103 :-- = 3.15 c), (144.98 - 138.68, For quinine hydrochloride (Form. 2) [a] And for cinchonine hydrochloride (Form. 42) [a]D Whence for quinine hydrochloride, when c = and for cinchonine hydrochloride when c Introducing these values for [a] in the equation rotation α, which solutions of these strengths ought to give, when observed in a tube whose length 100 we obtain the angles of = 1 decimetre: For quinine hydrochloride when = 2.774° + 1.631° 1.143°. For the mixture when c = 3, angle a = And hence the specific rotation of the mixture should be In dealing, however, with mixtures of unknown composition, we are unable to arrive at the true specific rotations corresponding to the amounts of the separate substances present; and we are forced to take for c in each case the total weight of mixture employed. But even so we obtain numbers closely agreeing with the results of actual experiment. Thus, suppose in the example given above we had taken c in each case as 3, and introduced into the equation the values for specific rotation corresponding thereto, viz: cinchonine hydrochloride c = 3. [a]v we should have obtained from as the angle of rotation for the mixture in a 1 decimetre tube, whence the specific rotation This latter mode of calculation, although not strictly accurate, is found to yield fairly good results, so long as the 100 cubic centimetre mixture does not contain more than a few grammes of substance. In such cases the true values of c for the components do not differ greatly from the value of c for the whole mixture, and so the departure from the true specific rotations is small. с с The mode in which the percentage composition of a mixture of two alkaloids is deduced from the observation of its specific rotation, will be seen from the following examples given by Hesse: I. Four grammes of a mixture of quinine hydrate and cinchonidine were dissolved in alcohol of 97 per cent. by volume, to form 100 cubic centimetres of solution. Examined in a Wild's polariscope with a 2 decimetre tube, the solution gave an angle of rotation ap = 9.95°, and thus the specific rotation of the mixture The specific rotation of the individual alkaloids is, according to § 103, when c = 4, α Quinine hydrate, according to Form. (1): [a]D D Putting for the required percentage of quinine hydrate, whereby that of cinchonidine = 100x, we get the equation whence 142.57 106 29 (100) = 124.37 × 100, - x According to this, the mixture consists of Quinine hydrate 49.8 parts. The real composition of the mixture actually consisted of equal parts of the alkaloids, with which the results of optical analysis coincide almost exactly. II. A mixture of 3 parts quinine sulphate and 1 part quinidine sulphate gave in an aqueous solution containing 4 grammes in 100 cubic centimetres, the specific rotation [a]D: = 71.87°: For quinine sulphate, according to Form. (9), when c = 4: [a] 163.61°. = For quinidine sulphate, according to Form. (37), when c = 4: [a]D=208-80°. Indicating by a the percentage of quinine sulphate we have: |