All chemicals were of analytical reagent grade. Deionized water (18.2 MΩ cm, Millipore, France) was used for the preparation of all solutions. The acid–base indicator stock solution contained 0.05% and 0.15% (w/v) cresol red (Sigma-Aldrich) and thymol blue (Sigma-Aldrich), respectively [29]. The acceptor solution was made daily by 50 fold dilution of the acid–base indicator stock solution and its pH was adjusted by drop-wise addition of 0.01 M NaOH (Chem Supply) solution. The ammonium stock solution (100 mg NH4+ L1) was prepared by dissolving the corresponding amount of NH4Cl (Sigma-Aldrich) in 0.1 M HCl solution. The ammonium working solutions were prepared immediately before the experiments by appropriate dilution of the ammonium stock solution. Stock solutions (100 mg L1) of methylamine (Sigma-Aldrich) and ethylamine (Sigma-Aldrich) contained 0.1 M HCl and were stored refrigerated (4 °C). Sulfuric acid (Chem Supply) was used for cleaning-up of the nitrogen carrier gas from traces of gases with basic properties such
as ammonia