W6 – DETERMINATION OF NITRATE IN WATER BY …

1 W6 DETERMINATION OF NITRATE IN WATER BY UV- visible . spectrometry , ION chromatography . Introduction NITRATE and nitrite ions are important indicators of pollution by organic materials as nitrogen from decomposing organic substances often ends up as NITRATE or nitrite ions. The DETERMINATION of NITRATE is often difficult because of the low levels found , and the distinct possibility of interfering materials being present. When determining NITRATE it is important to choose a method which suits both the interferences which are present and the level of analyte in the solution. The UV- visible method of determining NITRATE is a useful method for screening of a solution to make a rough DETERMINATION of the level of NITRATE . It determines the absorption f the NITRATE ion at 220nm. Good results are obtained if the WATER contains little or no organic matter (which also absorbs at 220nm).

2 A correction for organic matter can be performed by taking absorbance readings at 275nm (where NITRATE does not absorb). and estimating the organic content of the sample. After screening the sample and determining the amount of NITRATE other methods may be chosen and used to give more accurate estimates of NITRATE content of the WATER sample. For example the NITRATE ion selective electrode is suitable from . 1400mg/L, and ion chromatography from 10mg/L upwards. In this practical you will determine the NITRATE ion level of an unknown by first screening your sample with the UV- visible method, then undertaking analysis by ion chromatography . Procedure Part 1 Screening by UV- visible Spectrophotometry 1. Prepare 100 mL of NITRATE calibration standards for 2, 4, 6, 8 and 10mg/L by serially diluting appropriate amounts of the 100mg/L stock solution, and making them up to the mark with HCl (1mL) and modulab WATER .

3 2. Obtain an unknown NITRATE sample and analyse this in parallel with a freshly obtained creek WATER sample. 3. Filter a portion of the sample through m Millipore paper, then take a 25. mL aliquot and transfer to a 50mL volumetric flask, add HCl (1mL) and make up to the mark with modulab WATER . 4. Read the absorbance of the samples and standards at both 220nm and 275nm. Use modulab WATER containing HCl as a blank 5. Prepare a calibration graph and determine the concentration of NITRATE ion in the unknown and the creek sample Part 2 Ion Chromatographic Method (Do not use the IC for creek WATER or high chloride content samples). 1. Prepare calibration standards (1, 2, 5 and 10 mg /L) for the NITRATE ion by serial dilution of the stock solution. 2. Run each of the standards on the ion chromatograph as instructed by the teacher. The following conditions should be recorded Column Flow rate Eluent solution Detector type/setting Sample Loop size 3.

4 Determine the retention time of the NITRATE ion from standards and plot a calibration graph of peak area versus concentration. 4. Filter and de-gas the sample solution through m millpore paper. 5. Inject unknown sample into chromatograph and calculate the concentration of NITRATE in your unknown by reference to the calibration graph. Questions 1. Describe the origins and importance of NITRATE as a WATER pollutant. 2. Why is it important that the levels of NITRATE in our WATER systems are kept to very low levels? 3. Describe at least one other method for the DETERMINATION of NITRATE in WATER or waste WATER . References 1. Rump, & Krist, H., Laboratory Manual for the Examination of WATER , Wastewater & Soil, VCH, 1988. ISBN 0-89573-851-1. 2. Rand , , Greenberg, , and Taras, ed., Standard Methods For Examination of WATER and Wastewater , 18th edition, APHA, AWWA, WPCF, Chapman & Hall, 1992.

5 ISBN 0-87553-207-1. 3. Australian Standard 2383, Standards Association of Australia, 1986. 4. Barker, D., Electroanalytical techniques Theory & Practical notes, Hunter Institute of Technology Students Union, 1994. 5. Barker, D., Advanced chromatography Theory & Practical notes, Hunter Institute of Technology Students Union, 1995.