Determination of Free Acid Content of Solutions Containing Fe(III)

Importance of the Topic

Analysing free acid content in metal-containing solutions is critical for quality control in mining, refining, and industrial processes. Accurate determination of sulphuric acid levels ensures process efficiency, product consistency, and environmental compliance.

Objectives and Study Overview

This application note describes an approach for quantifying free acid in Fe(III)-bearing solutions using thermometric titration in an alcoholic medium. The study aims to demonstrate separation of acid and metal salt endpoints and evaluate method precision using synthetic copper ore leach and concentrate solutions.

Methodology and Instrumentation

An aliquot of test solution is pipetted into a titration vessel and diluted with isopropanol. Titration is performed with 1 mol/L potassium hydroxide in isopropanol at a delivery rate of 2 mL/min, data acquisition at 10 Hz, and a smoothing factor of 50. The alcoholic environment precipitates metal salts and shifts acid–metal equilibria, enabling a single clear exothermic endpoint. Synthetic samples were prepared from sulphuric acid, ammonium iron(III) sulfate, iron(II) sulfate and copper(II) sulfate. Positive displacement pipettes can be used for routine analyses.

Main Results and Discussion

Analysis of synthetic solutions yielded mean free acid values of 22.9 g/L (±0.05) for leach solutions and 196.7 g/L (±0.18) for concentrate solutions (n=5). The precision demonstrates excellent repeatability under the described conditions. Thermometric plots show distinct temperature and second-derivative curves at the acid endpoint, confirming reliable endpoint detection.

Benefits and Practical Applications

• Rapid and precise free acid determination in high-metal-content matrices.
• Minimal interference from metal ions due to precipitation in alcohol.
• Suitable for routine QA/QC in refineries and laboratories handling metal-sulfate solutions.

Future Trends and Possibilities

Advances may include automated titration platforms with enhanced software for endpoint detection, expansion to other metal–acid systems, and integration with process control in continuous flow operations. Miniaturised sensor technology could further reduce reagent consumption and analysis time.

Conclusion

Thermometric titration in isopropanol provides a robust, precise method for measuring free sulphuric acid in iron- and copper-rich solutions. The approach effectively separates metal and acid effects, offering a valuable tool for industrial and laboratory settings.