Quality Control of Mixed Acids

Importance of the topic

Accurate determination of individual acid concentrations in mixed acid solutions is essential for quality control in industrial etching and cleaning processes. Conventional titrimetric methods become time‐consuming and complex when multiple acids are present. Near‐infrared (NIR) spectroscopy offers a rapid, reagent‐free alternative that can deliver results in under a minute, improving efficiency and throughput.

Study objectives and overview

This work evaluates a NIR‐based approach to quantify phosphoric (H3PO4), sulfuric (H2SO4), nitric (HNO3) and hydrofluoric (HF) acids in a single measurement. A prediction model was developed using Vis‐NIR spectra of mixed acid solutions, aiming to match or exceed the accuracy of thermometric titration while drastically reducing analysis time.

Methodology and instrumentation

Measurements were carried out in transmission mode over 400–2500 nm using a DS2500 Liquid Analyzer with disposable 2 mm vials. A data set of 27 spectra with known acid concentrations was used to calibrate and validate multivariate prediction models. Vision Air Complete software managed data acquisition, spectral preprocessing and model building. Key steps included:

• Sample handling in 2 mm pathlength disposable glass vials
• Full‐range Vis-NIR acquisition on DS2500
• Chemometric calibration and cross‐validation with Vision Air

Main results and discussion

High coefficients of determination (R2) and low prediction errors demonstrate the method’s reliability:

• H3PO4: R2=0.969; SEC=0.29 % acid; 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 %

The NIR method yielded complete acid profiles in approximately one minute, compared to over 37 minutes for triple thermometric titration (including titrant standardization).

Benefits and practical applications

NIR spectroscopy allows real‐time monitoring of mixed acid formulations without chemical reagents or extensive sample preparation. Its robustness and speed make it ideal for on‐line or at‐line quality control in semiconductor etching, metal pickling and other industrial processes requiring precise acid blend management.

Future trends and applications

Advances in miniaturized NIR sensors and integration with automated sampling systems will further streamline industrial quality control. Expanding calibration libraries to include additional acid types or contaminants can broaden applicability. Real‐time feedback loops can optimize process stability and reduce chemical consumption.

Conclusion

The DS2500 Liquid Analyzer combined with chemometric modeling provides a fast, accurate and user‐friendly alternative to traditional titration for mixed acid analysis. This approach enhances laboratory productivity and supports continuous process monitoring in harsh production environments.

Used instrumentation

• DS2500 Liquid Analyzer (400–2500 nm)
• DS2500 Holder for 2 mm vials
• Disposable borosilicate vials, 2 mm pathlength
• Vision Air Complete software for method development and routine analysis