Solubility and Activity Coefficients of Acidic and Basic Nonelectrolytes in Aqueous Salt Solutions. 1. Solubility and Activity Coefficients of o -Phthalic Acid and l -Cystine in NaCl(aq), (CH 3 ) 4 NCl(aq), and (C 2 H 5 ) 4 NI(aq) at Different Ionic Strengths and at t = 25 °C