2.2.1. Solubility Studies for Rabeprazole Sodium and Sodium Bicarbonate

The solubility of each component in deionized water, solvents (ethanol, methanol), citric acid/sodium citrate buffer, and pH buffer was evaluated [29]. To assess the stability of rabeprazole sodium and sodium bicarbonate at various pH, color morphology was observed after solubility test completion for 24 h.

Rabeprazole sodium solubility was evaluated by the apparent and equilibrium solubility test methods and analyzed by HPLC (Waters 1529; UV/Vis 2707; Waters, Milford, MA, USA). Each vial contained 10 mg rabeprazole sodium, and 200 µL of each solvent was slowly added. Dissolution was visually confirmed and apparent drug solubility was calculated from the time when all active ingredients were dissolved and no particles were visible. Excess rabeprazole sodium (400 mg) was then added, and 1 mL aliquots were collected at 1 h, 5 h, and 24 h. The solutions were filtered and then diluted 2000× to measure the quantity of rabeprazole sodium dissolved in the solution. Equilibrium or final rabeprazole sodium solubility was calculated from the HPLC peak.

The concentration of the standard solution = 100 μg/mL.

Sodium bicarbonate could not be analyzed using HPLC and was evaluated via the apparent solubility test method [30]. Each vial contained 10 mg sodium bicarbonate, and 200 μL of each solvent was slowly added. Dissolution was visually confirmed, and apparent drug solubility was calculated from the time when all active ingredients were dissolved and no particles were visible. If the sodium bicarbonate did not dissolve in ≤100 mL, it was deemed nearly insoluble (Solubility Study Method <1236> in the U.S. Pharmacopeia) [31].