Direct Determination of the Concentration of Nitric Acid Using Spatially Offset Raman Spectroscopy

Importance of the Topic

Nitric acid is a widely used strong mineral acid and powerful oxidizing agent in industrial, laboratory, and commercial settings. Its hazard level rises sharply with concentration, creating a need for rapid, in situ identification of acid strength to protect personnel and equipment. Handheld Raman spectroscopy offers a non-invasive route to determine concentration directly through containers, improving safety and decision-making in quality control and hazard assessment.

Objectives and Study Overview

The primary goal of this work was to develop and validate a user library for direct concentration determination of nitric acid using Spatially Offset Raman Spectroscopy (SORS) on the Agilent Resolve handheld Raman analyzer. Eight nitric acid samples spanning 28–98% by volume were measured, and a custom spectral library was generated and deployed via Agilent Resolve Command fleet management software. Three verification samples (32.5%, 65%, 95%) were then tested as unknowns to assess match quality and concentration discrimination.

Methodology and Used Instrumentation

Samples: A stock 98% HNO3 solution was serially diluted with deionized water to prepare eight standards (28–98%, 6.28–21.98 M). Standards were stored in clear glass vials; verification samples were placed in amber bottles to simulate field conditions.

Instrument: Agilent Resolve handheld Raman analyzer operated in SORS through-barrier mode with default acquisition settings and full laser power.

Workflow:

• Acquire Raman spectra of each nitric acid standard in SORS mode.
• Export spectral data to PC via USB stick.
• Use Agilent Resolve Command software to assemble a custom “Nitric dilutions” library with metadata and hazard classifications.
• Deploy the library file back to the handheld analyzer for on-site testing.

Main Results and Discussion

Normalized SORS spectra exhibited systematic changes in band intensities and peak positions as nitric acid concentration varied from 28% to 98%. Key vibrational features related to nitrate and water species shifted predictably with dilution. The custom library generated on the workstation enabled the handheld device to correctly identify and quantify unknown samples at 32.5%, 65%, and 95% HNO3 with match quality exceeding 90%, even through amber glass barriers.

Benefits and Practical Applications

• Rapid, non-destructive concentration assessment directly through containers.
• Enhanced operator safety by avoiding direct handling of corrosive fumes.
• Fleet management of spectral libraries via centralized software ensures consistent identification across multiple devices.
• Applicable to hazard classification and on-site quality control in chemical production, transport, and storage facilities.

Future Trends and Potential Applications

Advances in handheld Raman sensitivity and chemometric algorithms will broaden applicability to more complex mixtures and formulations. Integration with cloud-based spectral databases could enable real-time updates and remote library sharing. Expansion to other strong acids, bases, and oxidizers, as well as multicomponent analysis in environmental monitoring and process control, represents promising directions.

Conclusion

This study demonstrates that SORS-based handheld Raman spectroscopy, combined with user-generated libraries via Agilent Resolve Command software, provides a reliable, rapid, and safe method to determine nitric acid concentration through containers. The approach supports improved hazard assessment and streamlined quality control workflows.

Reference

Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann, 1997; pp 465–471.