Comprehensive Analytical Investigation into the Chemical Incompatibility, Ion-Exchange Kinetics, and Assay Degradation of Diclofenac Sodium in Calcium Carbonate and Di-Calcium Phosphate-Based Wet Granulation Systems

During the routine Quality Control (QC) analysis and stability evaluation of Diclofenac Sodium Tablets 50 mg (Batch Number: Test-I, Average Weight: 900 mg), a persistent, severe, and reproducible deviation in assay recovery was documented. Standard analytical extraction methods prescribed by the Indian Pharmacopoeia (IP 2010) repeatedly yielded sub-potent recoveries ranging strictly between 60.0% and 70.0% of the target label claim. Initial systematic root-cause investigations definitively ruled out physical material losses during rapid mixer granulation (RMG), inadequate blending uniformity, and thermal degradation during fluid bed drying (FBD). Consequently, this research investigates the hypothesis of a profound drug-excipient incompatibility—specifically, a thermodynamically driven ion-exchange chemical reaction occurring in the localized aqueous phase of wet granulation between the highly soluble Diclofenac Sodium and the divalent calcium ions (Ca2+) sourced from the primary diluents, Calcium Carbonate and Di-Calcium Phosphate (DCP). To validate this chemical conversion hypothesis, an investigational acidification extraction methodology utilizing 0.1 N Hydrochloric Acid (HCl) was developed to chemically sever the suspected ionic bond and reverse the precipitation of the insoluble Diclofenac Calcium complex. The acidified extraction successfully recovered 98.39% of the label claim, definitively proving the integrity of the formulation's mass balance and confirming that the active pharmaceutical ingredient (API) was chemically locked within the excipient matrix rather than physically lost. In response to these findings, a rigorous process modification was engineered and executed, shifting the Diclofenac Sodium addition entirely from the intra-granular wet massing phase to the extra-granular dry blending phase. This structural process modification effectively isolated the API from the aqueous calcium micro-environment, preventing the ion-exchange cascade. The subsequent trial batch demonstrated a robust, compliant standard assay recovery of 99.52%. This paper provides an exhaustive analysis of the chemical thermodynamic mechanisms, analytical methodology, Quality Risk Management (QRM) assessments, and formulation process corrections necessitated to permanently resolve this specific pharmaceutical incompatibility across all calcium-based formulations.