Computer simulation of the effect of temperature on pH

Area of Science:

• Physical Chemistry
• Pharmaceutical Sciences
• Computational Chemistry

Background:

• Temperature fluctuations can alter the acid-base equilibria of formulation components.
• Changes in pKa values due to temperature shifts can influence solution pH and accelerate product decomposition.
• Understanding these thermodynamic effects is critical for pharmaceutical formulation stability.

Purpose of the Study:

• To simulate and quantify the effect of temperature on solution pH using computational methods.
• To assess the impact of temperature-induced pH changes on the stability of pharmaceutical formulations.
• To investigate the relationship between enthalpy of dissociation, pH, and temperature effects.

Main Methods:

• Computer simulation using the PHTEMP program to model pH changes from 25°C to 60°C.
• Analysis of various compounds including aliphatic amines, carboxylic acids, and mineral acids.
• Experimental verification of simulation predictions using glycine and amino acid mixtures.

Main Results:

• Substantial pH shifts (delta pH) were observed for compounds with high enthalpies of acid dissociation, like triethylamine.
• Temperature-induced pH changes were dependent on component concentrations, enthalpies, and initial pH.
• Simulations predicted smaller delta pH at lower pH (< 4) and larger changes at higher pH (> 9).

Conclusions:

• Temperature-induced shifts in acid-base equilibria significantly affect solution pH, impacting formulation stability.
• The PHTEMP program accurately predicts these pH changes, validated by experimental data.
• Formulation strategies must consider thermodynamic effects on pH alongside kinetic factors for optimal product stability.