Studies on Bile Acid Salt Solutions. II. The Solubility of Cholic Acid in Sodium Cholate Solutions and that of Deoxycholic Acid in Sodium Deoxycholate Solutions.

Abstract

The soly. of cholic acid (I) in Na cholate solns. and of deoxycholic acid (II) in Na deoxycholate solns. is suitably detd. by potentiometric titration of dil. solns. of their Na salts, where no assocn. has occurred; for solns. contg. micellar substance the soly. detn. more appropriately involves measuring the changes in refractive indices when the solns. are satd. with the acids. The soly. product exponents are [long dash] 8.72 and [long dash]9.15 for I and II, resp. From bile salt solns. satd. with the corres-ponding acids, I and II sep. in pure form; no acid salts are detected in the ppts. From an activity of 1.8 X l0-4 M for undissocd. I in Na cholate solns., the activity of the cholate anion from the pH values of the solns. is calcd.; these values indicate no assocn. below a chelate concn. of 0.01M a partial assocn. at concns. from 0.01 to 0.03M, and complete assocn of all added chelate ions at concns. above 0.03 M. The activity of undissocd. II in solns. of its Na salt satd. with II is 105 X 10-4 M; no assocn. occurs up to a deoxycholate concn. of 0.005-0.006M, above which a partial assocn. occurs which seems complete at least when the deoxycholate concn. exceeds 0.01M. As soon as assocn. sets in, the bile salt solns. begin to dissolve the resp. bile acids. The solubilizing power increases stepwise, showing that the micelles have different properties in different concn. ranges. The concns. limiting these ranges in solns. satd. with bile acids are detd.