1. Field Methods of Monitoring Aquatic Systems Unit 8 – UV-Vis Spectrometry Copyright © 2006 by DBS

2. Ions in Natural Waters (mg L -1 ) Ca 2+ Mg 2+ Fe 2+ K+K+ Na + Mn 2+ Zn 2+ Cl - SO 4 2- HCO 3 - NO 3 - PO 4 3- NO 2 - 0 - 100 0 - 1 0 - 25 0 – 0.1 M n+

3. Nitrate, Phosphate and Sulfate Nitrate and Phosphate (Plant nutrients) Sources: –Fertilizer run-off (Ammonium nitrate, potassium nitrate and ammonium dihydrogen phosphate fertilizers) –NOx from cars and industry Effects –Enrichment of lakes and streams – eutrophication –Acidification –NO 3 - at > 10 mg L -1 is a health hazard Sulfate Sources: –Weathering of minerals –Combustion of coal and oil Effects –Acidification –Boiler scale –Taste

4. Question From your knowledge of this spectroscopic technique, describe the law on which the analytical method is based At sufficiently low concentrations, the Beer-Lambert law is followed: A = εcl Where A = absorbance of radiation at a particular wavelength (= log(I 0 /I), ε = proportionality constant (molar absoptivity (1 mol -1 cm -1 )), c = concentration of absorbing species (mol L -1 ), and l = pathlength of the light-beam (cm)

5. Ultraviolet and Visible Spectrometry Range from sophisticated instruments covering whole UV-VIS spectrum to portable colorimeters (visible)

6. Question None of the common ions in water absorb in the visible region (water is colorless!) The only ions that absorb in UV are NO 3 - and NO 2 - …how do we measure the others? Analyze light-absorbing derviatives

7. Quantification With UV-Vis spectrometry there is a simple linear relationship between instrument response (absorbance) and concentration Calibration by external standards

8. Quantification e.g. prepare a solution 1000 ppm SO 4 2- using sodium sulfate Calculate % weight SO 4 2- in this compound: M(Na 2 SO 4 ) = 142.04 g mol -1 119.04 / 142.04 = 83.81 % sulfate Divide desired concentration by the % sulfate in this compound: 1000 mg / 0.8381 = 1193 mg Label should read 1000 ppm Na 2 SO 4 as SO 4 2-

9. Question Plot a standard curve (calibration graph) from the following data and determine the concentration of phosphorus in the sample Absorbance = 0.00281(concentration) + 0.00104 Unknown = 151 μ g L -1 P Concentration (μg L -1 P) 2550125250375Unknown Absorbance0.0580.1490.3700.6831.0600.426

10. Field Instruments Un-manned stations or portable instruments Analog (colored discs or stepped cubes) and digital colorimeters

11. Nitrate: Colorimetric Method Zinc-reduction method H 2 O (l) + NO 3 - (aq) + Zn (s) → Zn 2+ (aq) + NO 2 - (aq) + 2OH - (aq) Addition of 1-napthylamine forms a pink azo dye NO 2 - reacts with HCl and sulfanilic acid to form diazonium salt

12. Procedure 1.Using 50 ppm nitrate standard prepare 100 mL of 2.0, 5.0, 10.0 and 15.0 ppm standards. 2.Measure 50 mL of standard (or sample) into a 250 mL conical flask. Add 1.0 mL 3M HCl and 1.0 mL sulfanilic acid solution and mix thoroughly. Add 1 mL Zn/NaCl mixture. 3.Mix for exactly 7 minutes. 4. Vacuum filter the solution. 5.Add 1.0 mL napthylamine hydrochloride solution and 1.0 mL 2 M sodium acetate solution to the filtered sample and mix well. Allow 5 minutes for color development. 6.Set the spectrometer to 520 nm. Zero with a blank and measure the solutions. Dilute the samples where necessary. 7. Construct a calibration graph of absorbance vs. concentration. 8.Convert nitrate in mg L -1 to nitrogen mg L -1 (divide by 4.4)

13. Standards 2 ppm NO 3 - 5 ppm NO 3 - 10 ppm NO 3 - 15 ppm NO 3 - 150.01 mg NaNO 3 in 1L = 50 ppm NO 3 - 1:25 or 4 mL in 100 mL30 mL in 100 mL1:10 or 10 mL in 100 mL1 :5 or 20 mL in 100 mL C 1 V 1 = C 2 V 2

14. Phosphate: Colorimetric Method Orthophosphate (H 3 PO 4, H 2 PO 4 -, and PO 4 3- ) Reaction with ammonium molybdate (NH 4 ) 6 Mo 7 O 24.4H 2 O in acid forms molybdophosphoric acid Reaction of molybdophosphoric acid with ammonium metavanadate (NH 4 VO 3 ) forms yellow acid complex Measured at 420 nm ~ orthophosphate in sample 4-144

15. Procedure 1.Using 20 ppm phosphate standard prepare 100 mL of 2.0, 5.0, 8.0 and 10.0 ppm standards. 2.Measure 25 mL of standard (or sample) into a 50 mL conical flask. Solution must be acidic. Add one drop of phenolpthalein indicator. If solution is pink (pH > 8.5) add 6 M HCl dropwise until colorless. Dilute to 50 mL. 3.Put 25 mL of this solution into a 50 mL volumetric flask. Add 10.0 mL vanadate-molybdate reagent and bring the volume up to 50 mL with DI water. Allow 10 minutes for color development. 4.Set the spectrometer to 420 nm. Zero with a blank and measure the solutions. Dilute the samples where necessary. 5.Construct a calibration graph of absorbance vs. concentration.

16. Sulfate: Turbidimetric Method Precipitation of BaSO 4 SO 4 2- (aq) + Ba 2+ (aq) → BaSO 4(S) Measured using spectrometer or turbidity (light scattering) meter (nephelometer) 4-178

17. Procedure 1.Using 1000 ppm sulfate standard prepare 100 mL of 10.0, 20.0, 40.0 and 80.0 ppm standards. 2.Measure 10 mL of standard (or sample) amd 10 mL of DI water into a 250 mL conical flask with magnetic stirrer. 3.Add 6.0 mL acetate buffer and stir gently. 4.Add 0.1-0.2 g 20-30 mesh BaCl 2 crystals and stir for 1 minute. 5.Transfer to the spectrometer cell. 4.Set the spectrometer to 420 nm. Zero with a blank. After 5 minutes measure the solutions. Dilute the samples where necessary. 5.Construct a calibration graph of absorbance vs. concentration.

18. Standards 10 ppm SO 4 2- 20 ppm SO 4 2- 40 ppm SO 4 2- 80 ppm SO 4 2 - 1.193 g Na 2 SO 4 in 1L = 1000 ppm SO 4 2- 1:100 or 1 mL in 100 mL8:100 or 8 mL in 100 mL2:100 or 2 mL in 100 mL4:100 or 4 mL in 100 mL C 1 V 1 = C 2 V 2

19. Note on Units Sometimes expressed as concentration of major element within the ion e.g. mg L -1 NO 3 -, NO 2 - and NH 4 + sometimes expressed as mg L -1 N Easier to compare relative concentrations of species without using moles e.g. 50 mg L -1 NO 3 - expressed as mg L -1 N 50 mg x 1 mol = 8 x 10 -4 mol NO 3 - L 62 g 8 x 10 -4 mol x 14 g mol -1 = 0.0113 g L -1 = 11.3 mg L -1 N

20. Text Books Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil. Wiley-VCH. Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker. Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods. CRC Press. Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis Publishers. Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books. Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative methods. Taylor & Francis. Reeve, R.N. (2002) Introduction to environmental analysis. Wiley. Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the Examination of Water and Wastewater, 20th Edition. Published by American Public Health Association, American Water Works Association and Water Environment Federation.