Near-IR spectroscopy as a method for studying the formation of calcium naphthenate

A method for studying the precipitation of calcium naphthenate particles by means of near-IR spectroscopy is presented. Naphthenic or fatty acids were dissolved in water at high pH (11.2–11.5). Upon addition of a Ca²⁺ solution the nucleation period and particle growth were monitored. The near-IR spectra experience a baseline elevation owing to the formation and growth of calcium naphthenate particles. The resulting change in optical density over time is discussed on the basis of supersaturation, particle sizes, agglomeration, Ca²⁺-to-carboxylic acid ratio and nucleation process. Solubility products, defined as the ion concentration products where no particle growth was detected, were estimated for the calcium soaps. The method showed some quantitative limitations since the particle sizes changed with supersaturation. Smaller particles will have less influence on the optical density and the larger particle will dominate the resulting scattering contribution. However, it is obvious that the method has qualitative value, for example, to study the efficiency of different calcium naphthenate inhibitors.