Field Methods of Monitoring Aquatic Systems

Unit 8 – UV-Vis Spectrometry

Copyright © 2006 by DBS

Ions in Natural Waters (mg L-1)

Ca2+

Mg2+

Fe2+

K+

Na+

Mn2+ Zn2+

Cl- SO42- HCO3

-

NO3-

PO43-

NO2-

0 - 100

0 - 1

0 - 25

0 – 0.1Mn+

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– NO3

- at > 10 mg L-1 is a health hazardSulfate• Sources:

– Weathering of minerals– Combustion of coal and oil

• Effects– Acidification– Boiler scale– Taste

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(I0/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)

Ultraviolet and Visible Spectrometry

Range from sophisticated instruments covering whole UV-VIS spectrum to portable colorimeters (visible)

Question

None of the common ions in water absorb in the visible region (water is colorless!)

The only ions that absorb in UV are NO3- and NO2

-…how do we measure the others?

Analyze light-absorbing derviatives

Quantification

• With UV-Vis spectrometry there is a simple linear relationship between instrument response (absorbance) and concentration

• Calibration by external standards

Quantification

e.g. prepare a solution 1000 ppm SO42- using sodium sulfate

Calculate % weight SO42- in this compound:

M(Na2SO4) = 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 Na2SO4 as SO42-

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)

25 50 125 250 375 Unknown

Absorbance 0.058 0.149 0.370 0.683 1.060 0.426

Field Instruments

• Un-manned stations or portable instruments

• Analog (colored discs or stepped cubes) and digital colorimeters

Nitrate: Colorimetric Method

Zinc-reduction method

H2O(l) + NO3-(aq) + Zn(s) → Zn2+

(aq) + NO2-(aq) + 2OH-

(aq)

Addition of 1-napthylamine forms a pink azo dye

NO2- reacts with HCl and sulfanilic acid to form diazonium salt

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)

Standards

2 ppm NO3- 5 ppm NO3

- 10 ppm NO3- 15 ppm NO3

-

150.01 mg NaNO3 in 1L

= 50 ppm NO3-

1:25 or 4 mL in 100 mL 30 mL in 100 mL1:10 or 10 mL in 100 mL 1 :5 or 20 mL in 100 mL

C1V1 = C2V2

Phosphate: Colorimetric Method

• Orthophosphate (H3PO4, H2PO4-, and PO4

3-)

• Reaction with ammonium molybdate (NH4)6Mo7O24.4H2O in acid forms molybdophosphoric acid

• Reaction of molybdophosphoric acid with ammonium metavanadate (NH4VO3) forms yellow acid complex

• Measured at 420 nm ~ orthophosphate in sample

4-144

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.

Sulfate: Turbidimetric Method

• Precipitation of BaSO4

SO42-

(aq) + Ba2+(aq) → BaSO4(S)

• Measured using spectrometer or turbidity (light scattering) meter (nephelometer)

4-178

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 BaCl2 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.

Standards

10 ppm SO42- 20 ppm SO4

2- 40 ppm SO42- 80 ppm SO4

2-

1.193 g Na2SO4 in 1L

= 1000 ppm SO42-

1:100 or 1 mL in 100 mL 8:100 or 8 mL in 100 mL2:100 or 2 mL in 100 mL 4:100 or 4 mL in 100 mL

C1V1 = C2V2

Note on Units

• Sometimes expressed as concentration of major element within the ion

e.g. mg L-1 NO3-, NO2

- and NH4+ sometimes expressed as mg L-1 N

• Easier to compare relative concentrations of species without using moles

e.g. 50 mg L-1 NO3- expressed as mg L-1 N

50 mg x 1 mol = 8 x 10-4 mol NO3-

L 62 g

8 x 10-4 mol x 14 g mol-1 = 0.0113 g L-1 = 11.3 mg L-1 N

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.