SPECIFIC ROTATION. 



199 



COH 

 HCOH 

 HOCH 

 HOCH 

 HCOH 

 CH 2 OH 



d-Galactoae 



COH 

 HOCH 

 HCOH 

 HCOH 

 HOCH 

 CH 2 OH 



J-Galactose 



d-Fructose 



J-Fructose 



We designate the optical activity of the carbohydrates with the 

 letter I- for levogyrate, d- for dextrogyrate, and r- for the racemic. 

 These are only partly indicative. Thus dextrorotatory glucose is 

 designated d-glucose, levorotatory Z-glucose, but EMIL FISCHER has 

 used these signs in another sense. He designates by these signs the 

 mutual relationship of the various kinds of sugars instead of their 

 optical activity. For example, he does not designate the levorotatory 

 fructose Z-fructose, but d-fructose, showing its close relation in stereometric 

 structure to dextrorotatory d-glucose. This designation is generally 

 accepted, and the above-mentioned signs only show the optical proper- 

 ties in certain cases. v 



Specific rotation means the rotation in degrees produced by 1 gm. substance 

 dissolved in 1 cc. liquid placed in a tube 1 dcm. long. The reading is ordinarily 

 made at 20 C. and with the monochromatic sodium light. The specific rotation 

 with this light is represented by ()D, and is expressed by the following formula: 



(a) D = ~T, in which a represents the reading of degrees, 1 the length of the 



tube in decimeters, and p the weight of substance in 1 cc. of the liquid. Inversely 

 the per cent P of substance can be calculated, when the specific rotation is known, 

 lOOa 



by the formula P 



-, in which s represents the known specific rotation. 



In the determination of the change in specific rotation with various concen- 

 trations we must know also the amount of substance in grams in 1 gram of the 

 solution (p) and the specific gravity of the solution (d) at 20. The rotation 



is calculated according to the formula (a) D = 



A freshly prepared solution of a substance often shows a different rotation 

 from one that has been allowed, to stand for some time (multirotation). The 

 correct values which are found on allowing the solution to stand for a sufficiently 

 long time can be obtained immediately by boiling or on the addition of very 

 little ammonia. 



