Combining submerged membrane technology with anaerobic and ...

N-NO 3-(mg·L -1 )Chapter 2interfere. The determination should be promptly made on fresh samples in order to avoidbacterial conversions of NO 2- . At least filtration of the samples should be done immediatelyafter collection. For longer storage of unchlorinated samples (more than two days),preserve with 2 mL conc. H 2SO 4·L -1 and store at 4 ºC. When sample is preserved withacid, NO 3-and NO 2-cannot be determined as single species.543y = 5.04xR² = 1.002100.0 0.2 0.4 0.6 0.8 1.0absorbance 220nm- 2·absorbance 275nmFigure 2.5. Example of a calibration curve for nitrate concentration determination.2.1.2.4. Dissolved total nitrogen (DTN), dissolved organic nitrogen (DON) anddissolved inorganic nitrogen (DIN)DTN was determined in a total organic nitrogen analyzer (Rosemount-DohrmannDN-1900) equipped with a quimioluminiscence detector with two channels. One channeldetermines the DTN, by oxidation at high temperature, and the other determines the DIN,by a chemical reduction. DON is determined as the difference between DTN and DIN.All the nitrogen present in the water is catalytically oxidised to nitrous oxide (NO).The process for DTN determination occurs in two steps. The first step is a catalytic (Cu ascatalyst) oxidation in the combustion tube at 850°C and with pure oxygen (1 atm) ascarrier gas. The second one is the chemical reduction of residual NO 2 with H 2SO 4 at 80ºCand catalyzed by VaCl 3. For the DIN determination, only the second step (chemicalreduction) is used. The NO obtained in the two steps is dried and forced to react with O 3producing an unstable excited state NO 2*. The change back of this oxide to itsfundamental state releases a proton, from which the determination of DTN and DIN iscarried out by quimioluminiscence, using a multiplicator tube. The instrument is calibratedwith a certified standard solution (KNO 3, 20 mg N·L -1 ) using a response factor method.70