Analysis of dissolved nitrite (NO 2- )

Transcription

1 Analysis of dissolved nitrite (NO 2 ) Table of content 1. PURPOSE PRINCIPLE REQUIREMENTS EQUIPMENT AND MATERIALS REAGENTS PROCEDURE SAMPLE PREPARATION SAMPLE ANALYSIS CALCULATION OF RESULTS QUALITY CONTROL ERRORS, CALIBRATION AND INTERFERENCES WASTE STREAM AND PROPER DISPOSAL HAZARDS AND PRECAUTIONARY STATEMENTS REFERENCES CONTRIBUTIONS

2 1. PURPOSE The purpose of this analysis is to determine the dissolved nitrite (NO 2 ) concentration in water and wastewater. 2. PRINCIPLE Samples are filtered at 0.2 µm to remove suspended solids. Nitrite ions in the filtered water sample react with primary aromatic amines in acidic solution to form diazonium salts. These combine with aromatic compounds that contain an amino group or a hydroxyl group to form intensively colored azo dyes. The absorbance of the colored azo dyes is measured spectrophotometrically at 520 nm. Fig. 1: Chemical reactions of the determination of nitrite in filtered water samples 2

3 3. REQUIREMENTS 3.1. EQUIPMENT AND MATERIALS a syringe filter with 0.2 µm pore size (Chromafil RC20125, Germany) and syringe a glass beaker of 100 ml a Hach Lange kit LCK 341, mg NO 2 N L 1, mg NO 2 L 1 a spectrophotometer (Hach Lange DR 2800, Belgium) 1 centrifuge tube of 10 ml (VWR, Belgium) a micropipette of 15 ml and tips a micropipette µl and tips 3.2. REAGENTS 20 ml of sample (2 ml of filtered sample is needed for the analysis) 4. PROCEDURE 4.1. SAMPLE PREPARATION The raw sample should be filtered as soon as possible after taking it from the reactor in order not to change its nitrite concentration. Pour around 20 ml of sample in a glass beaker of 100 ml. Take with a syringe 10 ml of sample, add the syringe filter and pour minimum 3 ml of sample through the syringe filter in a labeled centrifuge tube. This filtered sample can be stored for 2 weeks at 4 C or longer at 18 C SAMPLE ANALYSIS Here under follows the procedure for filtered samples with concentration of mg NO 2 N L 1, mg NO 2 L 1. For samples with a higher concentration, dilutions should be made with demineralized water. 1. Carefully remove the foil from the screwedon DosiCap Zip. 2. Unscrew the DosiCap Zip. 3

4 3. Pipette 2.0 ml filtered sample from the centrifuge tube in the cuvette. 4. Immediately screw the DosiCap Zip back; fluting at the top. 5. Shake firmly until the freezedried contents are completely dissolved. 6. Wait exactly 10 min. Clean the outside of the cuvette thoroughly with paper while waiting. 7. Put the cleaned cuvette in the spectrophotometer. The mode to determine (LCK 341) will be automatically selected by the bar code reader. The spectrophotometer shows the amount of NO 2 in mg NO 2 L 1 on the display. 5. CALCULATION OF RESULTS The conversion of absorbance to NO 2 or NO 2 N is done by the spectrophotometer. For diluted samples, the used dilution factor should be taken into account. 4

5 6. QUALITY CONTROL Validity range: mg NO 2 N L 1, mg NO 2 L 1 ph: 310 Temperature: 1525 C The measurements results must be subjected to plausibility checks (dilute and/or spike the sample). 7. ERRORS, CALIBRATION AND INTERFERENCES Chromium (VI) ions, amongst others, interfere with the determination (Table 1). Copper (II) ions interfere with the determination even at concentrations below 1 mg L 1. Confidence interval of mg NO 2N L 1, mg NO 2 L 1 : (95%): ± 0.87 mg L 1. No more than 3 hours should elapse between sampling and filtering the sample. Table 1. Substances which can interfere with nitrite measurements Interfering substance Interference level (mg L 1 ) Tin (Sn 4+ ), iron (Fe 3+ ) 5 Iron (Fe 2+ ), silver (Ag + ) 10 Nickel (Ni 2+ ) 12 Cobalt (Co 2+ ), zinc (Zn 2+ ), cadmium (Cd 2+ ), manganese (Mn 2+ ), mercury 25 (Hg 2+ ) Chromium (Cr 3+ ) 50 Magnesium (Mg 2+ ) Ammonium (NH 4+ ), phosphate (PO 4 ), calcium (Ca 2+ ) 500 Potassium (K + ), nitrate (NO 3 ) 1000 Chloride (Cl 2 ), sulfate (SO 4 ) WASTE STREAM AND PROPER DISPOSAL Hach Lange local agencies will accept used cuvettes to ensure their proper disposal in accordance with local and national regulations. Used syringe filters should be disposed in the correct waste stream disposal in the lab ( General waste in ). 5

6 9. HAZARDS AND PRECAUTIONARY STATEMENTS Requirements for storage rooms and vessels: keep tightly closed in a dry, cool and wellventilated place. Always wear a laboratory coat, eye protection and laboratory gloves. Act carefully and take appropriate measure concerning the hazardous reagents. Reagent Hazard statements Precautionary statements If in eyes: rinse cautiously with water for several minutes. Citric acid (C6H8O7) Remove contact lenses, if present Causes serious eye irritation. and easy to do. Continue rinsing. 10. REFERENCES APHA, Awwa, WEF, Standard Methods for the Examination of Water and Wastewater, 21st ed. American Public Health Association, Washington DC. Beelen V., Van Den Hende S., Verbal communication. Ghent University, Campus Kortrijk. Hach Lange, Safety Data Sheet LCK 341. Available on (01/06/2014): SigmaAldrich, Citric acid. Available on (25/06/2014): CONTRIBUTIONS SOP written by: Alexandra Lefoulon (internship student EnAlgae project). SOP approved by: Sofie Van Den Hende (staff EnAlgae project). 6