Quality Control of Mixed Acids

Importance of the Topic

Accurate quantification of phosphoric, sulfuric, nitric and hydrofluoric acids in mixed solutions is critical for quality control in etching processes and other industrial applications. Traditional titration methods can be laborious and time-intensive when handling multi-acid mixtures, creating a demand for faster, reliable alternatives.

Objectives and Study Overview

This study aimed to develop a near-infrared (NIR) spectroscopy method to predict the concentrations of four acids (H3PO4, H2SO4, HNO3, HF) in mixed solutions within one minute, providing an efficient substitute to thermometric titration for routine quality control.

Methodology

A total of 27 mixed acid samples were measured in transmission mode across the 400–2500 nm range. Spectra were recorded with a 2 mm pathlength using disposable vials. Calibration models were built and validated with Vision Air Complete software, correlating NIR spectral data to reference values obtained by primary methods.

Used Instrumentation

• DS2500 Liquid Analyzer covering 400–2500 nm with temperature control up to 80 °C
• Disposable glass vials, 2 mm pathlength, transmission mode
• Vision Air Complete spectroscopy software for data acquisition and model development

Main Results and Discussion

High correlation coefficients (R²) and low prediction errors were achieved for all acids:

• H3PO4: R² = 0.969; SEC = 0.29 %; SECV = 0.41 %
• H2SO4: R² = 0.945; SEC = 0.24 %; SECV = 0.30 %
• HNO3: R² = 0.901; SEC = 0.28 %; SECV = 0.35 %
• HF: R² = 0.936; SEC = 0.21 %; SECV = 0.28 %

These figures of merit demonstrate the models’ precision and robustness. NIR predictions closely matched reference titration values, confirming the method’s suitability for rapid routine analysis.

Benefits and Practical Applications

Compared to thermometric titration—which requires approximately 37 minutes for sample preparation and analysis—the NIR approach delivers results in under one minute. Key advantages include minimal sample preparation, non-destructive measurement, high throughput, and suitability for harsh production environments due to the instrument’s robust design.

Future Trends and Potential Uses

Integration of NIR spectroscopy with automated sampling systems and flow-through cells could further enhance productivity. Advances in chemometric algorithms and real-time process monitoring will expand applications to other complex mixtures and inline quality control in manufacturing lines.

Conclusion

The DS2500 Liquid Analyzer combined with NIR spectroscopy provides a fast, accurate and user-friendly alternative to conventional titration for mixed acid analysis. This method meets industrial demands for rapid decision-making and reliable quality control.