A pH-based method for measuring gaseous ammonia

Cumulative emission or gaseous concentrations of ammonia (NH₃) are commonly measured by trapping gaseous NH₃ in an acidic solution that is later analyzed for total ammonia content. This traditional acid trap method is inexpensive, reliable, and accurate, but it is labor-intensive and inconvenient for high-frequency sampling. This paper describes a new acid trap method in which total ammonia concentration in a citric acid solution is calculated from measured pH by applying a speciation model. With this new method, the quantity of captured ammonia in a single acid trap can be determined repeatedly over time. Testing through titrations, laboratory emission measurements, and field measurement of gaseous NH₃ showed that the method is accurate and reasonably precise. For the most sensitive case standard deviation was 8 μmol kg⁻¹ in titration results, and 10−20 μmol kg⁻¹ in the field trials. The lower quantitation limits from emission measurements and field trials were around 100 and 300 μmol kg⁻¹, respectively. Speciation modeling was used to assess the importance of interferences. Results showed that error due to minor contamination of chemicals and evaporation of water can be kept at low levels. Acidic gases may be a significant interference, but their presence is easy to recognize, and significant error can be avoided by selecting an appropriate acid concentration.