Combining submerged membrane technology with anaerobic and ...

Materials and methodsb) Sulphuric acid reagent: 10.7 g of Ag 2SO 4 are added to 1 L ofconcentrated H 2SO 4. The solution is used after 2 days of preparation.c) Ferroin indicator solution: 1.485 g of C 18H 8N 2·H 2O (phenanthrolinemonohydrate) and 0.695 g of SO 4Fe·7H 2O are dissolved in 100 mL of distilledwater.d) Standard potassium dichromate solution 0.05 N. 1.226 g of K 2Cr 2O 7,previously dried at 105ºC for 2 hours, are dissolved in 500 mL of distilled water.e) Standard ferrous ammonium sulphate titrant (FAS) 0.035 N: 13.72 g ofFe(NH) 4(SO) 2·6H 2O are dissolved in distilled water. Then, 20 mL of concentratedH 2SO 4 are added and, finally, the solution is cooled and diluted to 1000 mL. AFAS solution concentration of 0.016 N was used for COD concentrationdetermination below 100 mg·L -1 .Determination procedureThis procedure is applicable to samples with COD concentrations between 90-900mg·L -1 . COD values of 100 mg·L -1 or less can be determined by using a more dilutedichromate digestion solution and a more dilute FAS titrant. Place 2.5 mL of sample in 10-mL Pyrex tubes. Add 1.5 mL of digestion solution and 3.5 mL of sulphuric acid reagentslowly on the wall of the tube slightly inclined (to avoid mixing). A blank sample usingdistilled water is prepared in the same way. This blank acts as “reference”, representingthe COD of the distilled water. After being sealed with Teflon and tightly capped, the tubesare finally mixed and placed in the block digester (HACH 16500-100) preheated to 150ºC.The duration of the digestion period is 2 h.After digestion, the tubes are cooled to room temperature. Then, the content of thetubes is transferred to a beaker and, once added 1-2 drops of ferroin indicator, the solutionis titrated under rapid stirring with standard FAS. The FAS solution is standardised daily asfollows: Put 5 mL of distilled water into a small beaker. Add 3.5 mL of sulphuric acidreagent. Cool to room temperature and add 5 mL of standard potassium dichromatesolution (0.05 N). Add 1-2 drops of ferroin indicator and titrate with FAS titrant. The endpointis a sharp colour change from blue-green to reddish brown. Molarity of FAS solutionis calculated with the following equation 2.1:eq. 2.163