Understanding the relationship between trace mineral forms and digestive pH conditions. Part I—Copper

Abstract

Understanding the chemical nature of trace-metal elements in an aqueous solution is pivotal to interpreting absorption and nutritional studies in production animals. To characterize copper(II)-ion solution chemistry, a series of aqueous titrations with strong base were performed on 1:1- and 1:2-molar-ratio mixtures of copper(II) nitrate and 5 weak organic acids commonly found in commercial trace-metal products. Three amino acids, glycine, lysine, and methionine, and 2 weak organic acids, propionic and 2-hydroxy-4-(methyl-thio)-butyric, were included. Based on the volume of NaOH and the pH measured after each addition, descriptions of the chemical forms of the copper(II) ions in solutions of varying acidities were developed. The forms fell into 4 groupings: free copper(II) ions and organic-acid molecules; mixtures of copper(II) ions, organic-acid molecules, and copper–organic acid complexes; mixtures of organic-acid complexes and hydroxyl complexes; and copper(II) precipitates. As the various copper and ligand solution combinations changed from an acid (stomach conditions) to a basic (intestine conditions) environment, there was either the formation of insoluble metal ion precipitates that indicated a loss of the soluble metal ions available for absorption, or there was the formation of stable and soluble copper(II) complexes that prevented precipitation but also reduced the free metal ion available for absorption.