Determination of Hydrofluoric Acid by Aluminium Titration

Importance of the topic

Hydrofluoric acid is a critical etching reagent in semiconductor and materials processing. Accurate quantification of HF in industrial etchant mixtures ensures process safety, product quality and regulatory compliance. Thermometric titration using aluminum nitrate provides a rapid, sensitive and selective approach for HF determination in complex acidic matrices.

Study objectives and overview

This application note presents a method for measuring HF concentration in mixed acid etchant solutions. The primary goals are to:

• Develop a reliable thermometric titration protocol using standard Al(NO₃)₃ titrant.
• Establish operating parameters that yield precise and reproducible endpoints.
• Demonstrate method performance on a synthetic etch solution.

Methodology and instrumentation

The technique relies on an exothermic precipitation reaction at pH 4.5 where Al³⁺ reacts with F⁻ in the presence of Na⁺ and K⁺ to form insoluble NaK₂AlF₆. Endpoint detection is achieved thermometrically by monitoring solution temperature change.

Instrumentation used

• Thermometric titrator head (Model 859 Titrotherm)
• Magnetic stirrer unit (Model 802)

Reagents and operating parameters

Reagents:

• 1 mol/L Al(NO₃)₃ titrant, standardized against anhydrous NaF.
• Ionic strength and buffer reagent: mixture of potassium acetate, sodium acetate and glacial acetic acid.
• 5 mol/L NaOH for sample neutralization.
• 0.1 % phenolphthalein indicator in ethanol.

Key titration settings:

• Titrant flow rate: 2 mL/min
• Single exothermic endpoint
• Data smoothing factor: 45
• Stirring speed: setting 10 on 802 stirrer
• Equilibration delay before titration: 60 s

Sample preparation involves diluting approximately 30 mL of etchant, neutralizing to faint pink with NaOH, adding buffer reagent and initiating the automated titration.

Key results and discussion

Analysis of a synthetic etch solution yielded an average HF concentration of 1.78 % w/w with a standard deviation of 0.01 % (n=8). The exothermic temperature rise and its second derivative provided a clear and reproducible endpoint. The precipitation of NaK₂AlF₆ ensured high selectivity for fluoride in the presence of strong acids.

Benefits and practical applications

• Rapid and automated analysis reduces operator time and potential errors.
• High selectivity avoids interference from other acid components.
• Suitable for routine QA/QC in semiconductor etching and metal finishing industries.
• Minimal sample preparation and robust buffer system ensure reproducibility.

Future trends and potential applications

Advances in sensor miniaturization and data analytics may further improve thermometric endpoint detection. Integration with online process monitoring could enable real-time HF control. Adaptation of this method to continuous flow or microfluidic platforms may expand its use in high-throughput laboratories.

Conclusion

The thermometric titration method using Al(NO₃)₃ titrant provides a precise, selective and efficient approach for determining hydrofluoric acid in mixed etchant solutions. Its robust performance and ease of automation make it well suited for industrial QA/QC applications.

References

Thermo Fisher Scientific. Application Note No. H-087: Determination of Hydrofluoric Acid by Aluminium Titration.