SOLUBILITY AND ACID-BASE CHARACTER 69 



acid as hydrochlorides and by alkali as carboxylic acid salts. Por- 

 phin octacarboxylic acids (uroporphyrins) cannot be extracted by 

 ether, unless their carboxylic acid groups are esterified. 



The extraction of porphyrins from ether by strong acids is a 

 property of great importance for their purification. Willstatter 

 found that different porphyrins require hydrochloric acid of different 

 concentration for extraction from ether and defined the HCl number 

 as the concentration of hydrochloric acid in per cent which from an 

 equal volume of ether solution of a porphyrin extracts two-thirds 

 of the porphyrin (3088). Less frequently used is the distribution 

 number ("^^erteilungszahl"), which is the percentage of porphyrin 

 extracted by 100 ml. hydrochloric acid of stated concentration from 

 one liter of ether solution containing 3 mg. of porphyrin (c/. 1523a). 



The HCl number depends not only on the dissociation constant of the 

 porphyrin as base, but also on the distribution coefficients of free porphyrin 

 and its hydrochloride in water and organic solvent {3170). Hence the esters, 

 which are certainly not weaker bases than the free porphyrins, have a 

 higher HCl number, and porphyrins with hydrophilic side chains {e.g., 

 coproporphyrin with four carboxylic acid groups and hematoporphyrin with 

 two alcoholic groups in addition to two carboxylic acid groups) have a low 

 HCl number. The pK of the basic groups of porphyrins cannot be measured 

 in water on account of the insolubility of the free porphyrins; however, by 

 titration with perchloric acid in glacial acetic acid solution Conant (j^7.4) 

 found two basic groups of pK '-2. .5 in etioporphyrin and mesoporphyrin. 



The HCl numbers of some of the most important porphyrins are 

 collected in Table V. The "pH number" corresponds on the alkaline 



TABLE V 

 HCl Numbers of Some Porphyrins 



side to the "HCl number" on the acid side (2827). It is defined as 

 the pH of a buffer solution which extracts half the porphyrin from 



