1. The solubility in water of purified, uncombined casein has previously been reported to be 0.11 gm. in 1 liter at 25°C. This solubility represents the sum of the concentrations of the casein molecule and of the soluble ions into which it dissociates.

2. The solubility of casein has now been studied in systems containing the protein and varying amounts of sodium hydroxide. It was found that casein forms a well defined soluble disodium compound, and that solubility was completely determined by (a) the solubility of the casein molecule, and (b) the concentration of the disodium casein compound.

3. In our experiments each mol of sodium hydroxide combined with approximately 2,100 gm. of casein.

4. The equivalent combining weight of casein for this base is just half the minimal molecular weight as calculated from the sulfur and phosphorus content, and one-sixth the minimal molecular weight calculated from the tryptophane content of casein.

5. From the study of systems containing the protein and very small amounts of sodium hydroxide it was possible to determine the solubility of the casein molecule, and also the degree to which it dissociated as a divalent acid and combined with base.

6. Solubility in such systems increased in direct proportion to the amount of sodium hydroxide they contained.

7. The concentration of the soluble casein compound varied inversely as the square of the hydrogen ion concentration, directly as the solubility of the casein molecule, Su, and as the constants Ka1 and Ka2 defining its acid dissociation.

8. The product of the solubility of the casein molecule and its acid dissociation constants yields the solubility product constant, Su·Ka1·Ka2 = 2.2 x 10–12 gm. casein per liter at 25°C.

9. The solubility of the casein molecule has been estimated from this constant, and also from the relation between the solubility of the casein and the sodium hydroxide concentration, to be approximately 0.09 gm. per liter at 25°C.

10. The product of the acid dissociation constants, Ka1 and Ka2, must therefore be 24 x 10–12N.

11. It is believed that these constants completely characterize the solubility of casein in systems containing the protein and small amounts of sodium hydroxide.

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