Analytical Pitfalls in Electrocatalytic C–N Coupling: Limitations of Indirect Assays for Urea Quantification

Abstract Accurate quantification of urea is essential for catalyst evaluation in urea electrocatalysis, as even minor analytical errors can severely distort Faradaic efficiency, activity, and yield calculations. In practice, however, most studies rely on indirect protocols that are not rigorously validated under the complex electrolyte and product matrices characteristic of electrocatalytic urea systems. Here, we systematically assess three widely used indirect methods—diacetyl monoxime–thiosemicarbazide (DAMO–TSC), urease–indophenol blue (Urease–IB), and urease–ion chromatography (Urease–IC)—under common realistic conditions involving nitrate (NO3–), nitrite (NO2–), bicarbonate (HCO3–), and ammonia (NH3)-containing electrolytes and compare their performance to 1H nuclear magnetic resonance (NMR) as a direct urea quantification technique. We show that DAMO–TSC is highly susceptible to nitrite interference and that literature-reported sulfamic acid/hydrochloric acid pretreatments fail to provide robust or reproducible correction across nitrite concentrations. Urease–IB and Urease–IC exhibit good precision for pure urea standards, but in the presence of pre-existing ammonia, their accuracy rapidly degrades due to cumulative dilution, ionic overlap, and ammonia losses, leading to errors that can exceed 100% and yield false positives for urea formation. By contrast, 1H NMR provides direct, interference-resistant detection of urea with a practical detection limit on the order of a few ppm under optimized conditions and reveals that several promising urea-producing electrocatalysts in fact generate no detectable urea. Collectively, these results demonstrate that indirect colorimetric and enzymatic assays cannot be used as standalone proof of urea electrosynthesis and that NMR or similar robust techniques should be regarded as either confirmation techniques to support indirect assays or used directly to verify urea formation. We further recommend that researchers systematically test, document, and report the accuracy and limitations of any urea quantification protocol employed to ensure that reported urea yields and Faradaic efficiencies in this field are both accurate and reproducible.