1. General Discussion
a. Principle: Aqueous ammonia nitrogen in samples is converted to dissolved ammonia gas by raising sample pH above 11 with alkaline solution. The sample/alkaline solution is placed in one cell of a two-cell diffusion chamber with a buffered absorbing solution in the second cell. A hydrophobic membrane, through which the dissolved ammonia gas diffuses, separates the cell. The pH of the absorbing solution is neutral, facilitating conversion of the dissolved ammonia gas to the ammonium ion from. Change inconductivity of the absorbing solution is determined in a conductivity cell. The method is best applied using a continuous-flow conductimetric system such as shown in Figure 4500-N:2. In the continuous-flow system, samples are injected into a continuously fed stream of alkaline solution at selected time intervals, usually 1 to 2 min, and pass through the system as plugs. Response time usually is less than 1 min. total oxidized inorganic nitrogen (nitrate and/or nitrite) is measured by reducing to ammonia with zinc after mixing sample with the alkaline solution.1The reaction is
NO_3^- + 4Zn +〖7OH〗^-→ NH3 + 〖4ZnO〗_2^(2-) + 2H2O
In the continuous-flow system, a zinc reduction cartridge is placed in the flow stream, as indicated in Figure 4500-N:2. The reduction reaction is catalyzed by copper; addition of copper sulfate to the alkaline solution isbeneficial. When both ammonia and oxidized forms of nitrogen are present in a sample, the total nitrogen and the ammonia nitrogen concentrations are determined separately and the oxidized nitrogen is calculated by difference. Because the reaction with zinc