SOLVENT EFFECT 



041 



wave peak apiiears as a lower liiimp separated from the main peak by a 

 trough. 



Wakkie (1935), too, divided solvents into classes and gave four 

 separate X^ax. = /(^) curves — one for nonpolar solvents, one for weakly 

 polar solvents (ethers, ketones), one for alcohols and one for colloidal solu- 

 tions in water and glycerol. However, the last curve is not directly com- 

 parable with the other three, since, as we shall see later, the position of the 



X 



»- 

 o 



z 



Ijj 



I 



(/7^-|)/(/7^-h2)' 



Fig. 21.25. Wave lengths of absorption maxima of green pigment extracts in 

 different organic solvents, in relation to their refractive indices. (1) Carbon di- 

 sulfide, (2) pyridine, (3) chloroform, (4) l)enzene, (5) carbon tetrachloride, (6) 

 ethanol, (7) ether, (8) methanol, (9) acetone (after Katz and Wassink 1939). 

 Top scale: extracts from purple sulfur bacteria (strain D) (O). Bottom scale: 

 extracts from ChlorcUa (•). 



absorption bands of colloidal chlorophyll solutions depends not only on the 

 solvent, but also on the degree of dispersion of the sol. 



Wakkie's curves (as well as our two curves in fig. 21.24) are displaced 

 toward longer waves with increasing dipole moment of the solvent — a rela- 

 tion that can be explained by the superposition of attraction forces be- 

 tween solvent and solute caused by permanent 'polarization upon forces 

 due to polarizabiliiy. This dipole effect is stronger for chlorophyll b than 



