2 III. A mixture of 1.714 grammes quinine and 1.756 grammes quinidine was dissolved in so much standard hydrochloric acid to give for 1 molecule of the two alkaloids (C20 H24 N2 O2 = 324), 3 molecules hydrochloric acid, and diluted with water to 100 cubic centimetres. This solution, of which the concentration c = 3.470, gave the specific rotation [a]D = +27·92°. In hydrochloric acid solution, when c347, the specific rotation is For quinine, according to Form. (5): [a]D=269.29°. (33): [a]D= + 323·73°. Putting ≈ for the proportion of quinidine, we have : 323.73 x 269.29 (100 — x) = 27.92 × 100. This gives x the values 50-1, the results appearing as hereunder : IV. 1.878 grammes of a mixture of quinidine (0.878 gramme) and cinchonine (1 gramme) made into a 100 cubic centimetre solution, containing 3 molecules hydrochloric acid for 1 molecule alkaloid (316), showed a specific rotation [a]P = + 291·80°. With concentration c = 1.878, we have— For quinidine, according to Form. (33): [a] = + 330·44°. Putting x for the percentage of quinidine, we have— 330-44x+259-44 (100 - x) =291.80 × 100, To analyze a mixture of three cinchona alkaloids in a similar manner, the specific rotation must be determined by means of two different solvents. Thus, Hesse' has investigated weighed mixtures of cinchonidine, quinidine, and cinchonine, in the following solutions :— I. 0.5 gramme of the mixture dissolved in alcohol of 97 per cent. by volume, into a 25 cubic centimetre solution, so that the concentration c = 2 gave in a 2 decimetre tube an angle of rotation a = +2.78°, whence [a] = + 69.5°. II. 0.5 gramme of the same mixture dissolved in hydrochloric acid to a 25 cubic centimetre solution, containing 3 molecules H Cl to 1 molecule alkaloid, gave in a 2 decimetre tube a = + 2·82°, whence [alp = + 64·09°. Now the specific rotations of the three alkaloids, with a concentration c = 2, are as follows: : Putting x, y, z, for the several required proportions, cinchonidine = x, quinidine = y, and cinchoninez, we get the equations : - 106.89 x 261·77 y + 226·13 ≈ = 100 × 69.50 177.47 x + 329.94 y + 259·12 ≈ 100 × 64.09 From the solution of which we obtain : While the values for cinchonidine agree, there are considerable 1 Hesse: Liebig's Ann. 182, 152. differences between those for each of the other two alkaloids. These differences disappear when in the analysis of solution I. the angle of rotation 2.80° is substituted for 278°, and in that of solution II. a 2.80° instead of 2.82°. Errors of observation thus exert considerable influence over the results, and, consequently, the optical analysis of a mixture of three alkaloids is attended with some uncertainty. § 107. Other experiments for determining by the polariscope the composition of various mixtures of cinchona bases have been made by Oudemans,1 jun., and with fairly satisfactory results. The following special method for determining the quinine in a mixture of quinine and cinchonidine, has been given by Oudemans: If to any solution (e. g., bark-extract) containing the above alkaloids, as sulphate or chloride, we add a solution of neutral tartrate of potash or Rochelle salt, the tartrates, being insoluble in water, are precipitated : 6 6 Quinine tartrate (C20 H24 N2 O2)2. C4 H ̧ O ̧ + H2 0. 6 6 2 Each of these compounds is readily soluble in dilute hydrochloric acid, and Oudemans took the rotations in three solutions of different concentrations. Each of the solutions contained for every 04 gramme of the tartrates, 3 cubic centimetres normal hydrochloric acid (containing 364 grammes H Cl per litre), and was made up with water to 20 cubic centimetres. At a temperature of 17° Cent. the following rotation-constants were observed: 0.4 tartrate + 3 cub. cent. Quinine tartrate. Cinchonidine tartrate. + water to 20 cub. cent. [a] = -215·8. [a] = −131·3. solution 0.8 grm. tartrate + 6 cub. cent. normal hydrochloric acid D [a]=— + water to 20 cub. cent. [a]D=-211·5. [a] = −129.6. solution 1.2 grm. tartrate + 9 cub. cent. normal hydrochloric acid + water to 20 cub. cent. [a] = -2078. [a]p=-128·1. solution 1 Oudemans: Liebig's Ann. 182, 63, 65. By means of these figures the percentage composition may be determined of a mixture of the two tartrates, obtained as evaporated extract from a solution. For this purpose we dissolve 0.4, 0.8, or 1-2 gramme of the dried extract, in 3, 6, or 9 cubic centimetres normal hydrochloric acid, dilute the solution to 20 cubic centimetres with water, and observe the specific rotation at 17° Cent. Denoting this by M, and putting ≈ for the required percentage of quinine tartrate, we can calculate its value from one of the following formulæ : A number of experiments by Oudemans showed, however, that the proportion of quinine calculated as above is, in most cases, too high. This is seen in the following results: VII. ROTATION CONSTANTS OF ACTIVE SUBSTANCES. § 108. The following collection of rotation values embraces only such substances as have been properly examined-that is to say, those respecting which information has been recorded on the following points :—1 1. Name of ray used (generally D or j). 2. Nature of the solvent. 3. Concentration or percentage composition of solutions. 4. Temperature at which the angle of rotation has been observed. (In some cases this has been omitted through want of data.) 5. In the case of interpolation formulæ, the limits within which they are correct. Accompanied with such data, specific rotation values, as marked in § 40, afford, when the given experimental conditions are strictly observed, striking characters for the substances, useful in determining the nature of the substances as well as in examining the purity of particular samples. Moreover, they may serve other purposes, as follows:: (a) For obtaining at least an approximate estimation of the percentage composition of solutions of active substance, in cases. where the specific rotation of the substance does not alter very much with the concentration of its solutions. Thus given, the angle of rotation a of any solution in a tube 7 decimetre long, the number of 1 The list has no pretension to absolute completeness. Other substances which have been adequately examined may have escaped notice. |