Retention of mineral acid by humic substances: Measurement and influence on acidic properties of humic acids.

摘要

Humic acids are extracted from soils, peats, and coals with dilute alkali and precipitated when the alkaline extract is acidified (HCl is usually used). Evidence was obtained indicating that not all of the HCl is removed by the common methods used for recovery of humic acids, thereby leading to high values for acidic functional groups by methods based on ion-exchange. Also, errors are introduced in the determination of pK's for carboxyl groups, and the accompanying anion (i.e., Cl{dollar}sp{lcub}-{rcub}{dollar}) interferes with metal ion binding measurements due to the formation of metal-anion complexes (e.g., MCl{dollar}sp{lcub}-{rcub}{dollar} and MCl{dollar}sb2{dollar}). Retention of HCl by humic acids was found to be due to: (1) entrapment within the fiber-like structure of humic acids at low pH, and (2) binding at positively charged sites on the humic acid molecule, the most likely possibly being at NH{dollar}sb{lcub}rm x{rcub}sp{lcub}+{rcub}{dollar} groups. A method was developed for the determination of free and bound forms of Cl{dollar}sp{lcub}-{rcub}{dollar} and other inorganic anions in humic acid preparations. Some peats and mucks were found to contain anions in organically bound forms.; Some commonly used methods were examined and evaluated for eliminating residual HCl in humic acid preparations. Whereas free HCl could be removed by batch or flow-through dialysis, prolonged dialysis decreased titratable acidity to a greater extent than could be explained by the decrease in residual HCl. The preferred method for the preparation of humic acids is by dialysis of the Na{dollar}sp{lcub}+{rcub}{dollar}-humates (i.e., dialysis at neutral pH), under which condition the molecules are extended due to repulsion of charged groups, thereby facilitating removal of electrolytes. Following dialysis, the Na{dollar}sp{lcub}+{rcub}{dollar}-humates can be converted to the H{dollar}sp{lcub}+{rcub}{dollar}-form by passage through a cation-exchange resin.