In-situ detection of nitrite impurities in molten solar salt via cyclic voltammetry

Molten nitrate salts, particularly solar salt, are widely used in commercial concentrated solar power (CSP) plants for thermal energy storage. During long-term operation, partial thermal decomposition generates nitrite (NO 2 − ), which is commonly assessed through periodic off-line analysis and controlled through operational measures such as temperature regulation and salt chemistry management. However, these approaches do not provide continuous information on salt composition. This study presents an in situ electrochemical method for quantifying NO 2 − impurities in molten solar salt at 290 °C using cyclic voltammetry (CV). A platinum three-electrode system integrated into a custom high-temperature cell enabled stable measurement without salt extraction or thermal quenching. NaNO 2 addition induced a new anodic feature at 1.2–1.5 V. The corresponding peak current density in the early-warning regime (0.1–1.0 mol% added NaNO 2 , R 2 = 0.9789) and the rising-edge slope in the critical-warning regime (0.5–5.0 mol% added NaNO 2 , R 2 = 0.9792) provided a dual-regime calibration framework. Scan-rate analysis indicated diffusion-influenced behavior of the nitrite-related electrochemical response. The diagnostic anodic feature showed no significant masking by common co-impurities, including NaOH, NaCl, Na 2 SO 4 , and Na 2 CrO 4 . Cross-validation with mass spectrometric analysis of oxygen evolution from thermally aged salts showed deviations below 5%. These results demonstrate that high-temperature CV can provide a quantitative and selective approach for monitoring nitrite formation in molten solar salt, offering a basis for future online salt-aging diagnostics in CSP thermal energy storage systems.