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Measuring Total Arsenic in Drinking Water using Arsenomolybdate
This Standard Operating Procedure (SOP) was adapted from Frisbie SH, Mitchell EJ, Yusuf AZ, Siddiq MY, Sanchez RE, Ortega R, Maynard DM, Sarkar B. The development and use of an innovative laboratory method for measuring arsenic in drinking water from western Bangladesh. Environmental Health Perspectives 113(9):1196-1204 (2005).
- The Arsine Generator, Scrubber, and Absorber are shown in Figure 1. The Arsine Generator is a 125-milliliter (mL) Erlenmeyer flask. The Scrubber is made from a 10-mL volumetric pipette and a number 23 rubber stopper. The
Absorber is made from a 20-mL volumetric pipette, a number 11 rubber stopper, and a size 16/16 polyethylene cap. This cap has 4 grooves cut along its side to vent hydrogen (H2) gas without the loss of liquid
during arsine (AsH3) generation.
|Figure 1. The Arsine Generator, Scrubber, and Absorber. (Reproduced from Frisbie SH, Mitchell EJ, Yusuf AZ, Siddiq MY, Sanchez RE, Ortega R, Maynard DM, Sarkar B. The development and use
of an innovative laboratory method for measuring arsenic in drinking water from western Bangladesh. Environmental Health Perspectives 113(9):1196-1204 (2005).)
- An improved Absorber is shown in Figure 2. The size 16/16 polyethylene cap is replaced with Tygon® tubing and a Nalgene® powder funnel. This Tygon® tubing has a 16 millimeter (mm) inside diameter. The
stem of this Nalgene® funnel has a 16 mm outside diameter.
|Figure 2. The Arsine Generator, Scrubber, and improved Absorber.
- The spectrophotometer was set at 835 nanometers (nm) and used a 1.0-centimeter (cm) glass cell.
- All glassware was acid washed in 1.00 molar (M) hydrochloric acid (HCl).
- Stock As. Deliver 4.0 grams (g) of sodium hydroxide (NaOH) to a 1-liter (L) volumetric flask. Dissolve the NaOH with 10 mL of distilled water. Dissolve 1.320 g of arsenic(III) oxide
(As2O3) in the volumetric flask. Dilute to 1000 mL with distilled water.
- Intermediate As. Dilute 5.00 mL of stock As solution to 500 mL with distilled water.
- Standard As. Dilute 10.00 mL of intermediate As solution to 100 mL with distilled water.
- 50.0% (weight/volume) KI. Deliver 50.0 g of potassium iodide (KI) to a 100-mL volumetric flask. Dilute to 100 mL with distilled water. Store protected from light.
- 40.0% (weight/volume) SnCl2.2H2O. Deliver 40.0 g of stannous chloride dihydrate (SnCl2.2H2O) to a
100-mL volumetric flask. Dilute to 100 mL with concentrated HCl.
- 10.0% (weight/volume) Pb(COOCH3)2.3H2O. Deliver 10.0 g of lead(II) acetate trihydrate
(Pb(COOCH3)2.3H2O) to a 100-mL volumetric flask. Dilute to 100 mL with distilled water.
- I2/KI. Deliver 20.0 g of KI to a 500-mL volumetric flask. Dissolve the KI with approximately 250 mL of distilled water. Add 12.5 g of iodine (I2) and a Teflon®-coated
magnetic stir bar to the volumetric flask. Mix until all the I2 dissolves, this may require several hours. Remove the stir bar. Dilute to 500 mL with distilled water. Store protected from light. Prepare fresh weekly.
- 1.00 M NaHCO3. Deliver 8.40 g of sodium bicarbonate (NaHCO3) to a 100-mL volumetric flask. Add approximately 90 mL of distilled water to the volumetric flask. Mix
until all the NaHCO3 dissolves, this may require several hours. Dilute to 100 mL with distilled water.
- Concentrated H2SO4.
- Concentrated HCl.
- Zn. 20-mesh granules.
- H2SO4/(NH4)6Mo7O24.4H2O. First, prepare 100 mL of 6.50 M sulfuric
acid (H2SO4) in distilled water. Second, dilute 6.9 g of ammonium molybdate tetrahydrate
((NH4)6Mo7O24.4H2O) to 100 mL with distilled water. Finally, mix the 100 mL of 6.50 M H2SO4 and
the 100 mL of 6.9% (weight/volume) (NH4)6Mo7O24.4H2O together.
- 6.00% (weight/volume) Na2S2O5. Deliver 6.00 g of sodium metabisulfite (Na2S2O5) to a 100-mL
volumetric flask. Dilute to 100 mL with distilled water. Prepare fresh daily.
- 0.20% (weight/volume) SnCl2.2H2O. Dilute 0.50 mL of 40.0% (weight/volume) SnCl2.2H2O to 100 mL
with distilled water. Prepare fresh daily.
- Sample Collection, Preservation, and Storage
- All samples must be collected in either glass or plastic bottles.
- Immediately after collection, all samples must be preserved by acidification to pH <2 with ≥5.0 M HCl. Do NOT use nitric acid (HNO3) to preserve samples.
- The temperature of all samples must be maintained at 0° to 4° Celsius (C) until immediately before analysis.
- Sample Treatment
- Up to 30 samples and standards may be treated at the same time.
- Deliver 35.0 mL of sample or standard to an 125-mL Erlenmeyer flask.
- Add 0.35 mL of 50.0% (weight/volume) KI and mix.
- Add 0.35 mL of 40.0% (weight/volume) SnCl2.2H2O and mix.
- Boil this mixture vigorously for 1.0 minute to reduce As(V) to As(III).
- Use a water bath to cool the mixture to room temperature. This water bath should initially contain no more than 2.5 cm of tap water and should hold no more than 10 Erlenmeyer flasks.
- Hereafter, the samples and standards are typically processed in batches of up to 10.
- Scrubber Preparation
- Up to 10 Scrubbers may be prepared at the same time for immediate use.
- Place 0.17±0.03 g of glass wool onto a piece of filter paper.
- Deliver 10 drops of 10.0% (weight/volume) Pb(COOCH3)2.3H2O to this piece of glass wool.
- Squeeze the glass wool in the filter paper to remove excess solution.
- Fluff the glass wool and place it in the Scrubber (see Figures 1 and 2).
- Absorber Preparation
- Up to 10 Absorbers may be prepared at the same time for immediate use.
- Determine the mass of an empty 20-mL test tube. A unique test tube is used for each sample or standard throughout the "Absorber Preparation"
and "Arsine Generation, Color Development, and Spectrophotometry" procedures.
- Deliver 2.50 mL of I2/KI solution to this test tube.
- Add 0.50 mL of 1.00 M NaHCO3 and mix.
- Pour this mixture into the Absorber.
- Affix the cap to the Absorber (see Figure 1).
- Arsine Generation, Color Development, and Spectrophotometry
- The amount of time for each step of this procedure, from adding concentrated H2SO4 to measuring the absorbance, must be consistent for all samples and all standards. These samples and
standards typically begin this procedure at 3-minute intervals.
- Add 2.0 mL of concentrated H2SO4 to the treated sample or treated standard in the 125-mL Erlenmeyer flask and mix.
- Add 10.0 mL of concentrated HCl and mix.
- Add 1.0 mL of 40.0% (weight/volume) SnCl2.2H2O and mix.
- Add 5.0 g of zinc (Zn). Immediately connect the Scrubber and Absorber to the Erlenmeyer flask (see Figures 1 and 2).
- Allow 30 minutes for the complete evolution of AsH3 from the Erlenmeyer flask to the Absorber. Do NOT let the absorbate get sucked into the Scrubber. If the absorbate gets sucked to the part of the Absorber
shown in Figure 3, use a hot plate to gently heat the Arsine Generator. Stop heating the Arsine Generator when 1 bubble goes through the Absorber.
|Figure 3. Do NOT let absorbate get sucked beyond this part of the Absorber.
- Pour the liquid from the Absorber back into its unique test tube. Use 0.50 mL of distilled water to rinse the residual liquid from the Absorber to the test tube.
- Add 1.00 mL of H2SO4/(NH4)6Mo7O24.4H2O solution to the test tube and mix.
- Add 0.50 mL of 6.00% (weight/volume) Na2S2O5 solution to the test tube and mix. The Na2S2O5 should change the mixture from
deep reddish-brown to faint yellow. The brown color must be eliminated.
- If necessary, add distilled water to the test tube until its total volume of liquid is 5.00 mL or the total mass of liquid is 5.37±0.04 g (5.33 g to 5.41 g), and mix.
- Add 0.50 mL of 0.20% (weight/volume) SnCl2.2H2O to the test tube, mix, and wait 30 minutes for the bluish-green arsenomolybdate color to develop.
- Measure the absorbance at 835 nm.
- Deliver 0, 0.50, 1.00, 2.00, 4.00 and 8.00 mL of standard As solution into 6 separate 125-mL Erlenmeyer flasks.
- Add distilled water until the total volume of liquid in each Erlenmeyer flask equals 35.0 mL (see Figures 1 and 2). The resulting standards contain 0, 0.50, 1.00, 2.00, 4.00 and 8.00 micrograms (µg) As or 0, 14, 28.6, 57.1, 114,
and 229 µg As/L, respectively.
- Use the "Sample Treatment", "Scrubber Preparation", "Absorber Preparation",
and "Arsine Generation, Color Development, and Spectrophotometry" procedures to analyses these standards.
- Calibrate daily using 0, 14, 28.6, 57.1, 114, and 229 µg As/L standards. Confirm that the calibration results obey Beer’s law; that is, the plot of absorbance versus As concentration is linear and the y-intercept goes
through the origin. A test for a higher order polynomial relationship should be done for each daily calibration to see if this plot of absorbance versus As concentration is linear at the 95% confidence level. Similarly, a test of the null
hypothesis that this y-intercept goes through the origin should be done for each daily calibration to see if the y-intercept is equivalent to 0, 0 at the 95% confidence level. If Beer’s law is obeyed, linear regression through the origin is used
to calculate the calibration curve. If Beer’s law is NOT obeyed, 1 or more systematic errors may need to be corrected.
- Recommended Quality Controls
- All routine analytical methods must use a quality control plan that will identify and correct systematic errors as soon as they arise. The extreme toxicity of As in drinking water demands that such a quality control plan be strictly
implemented. Any quality control plan for this method should include daily calibrations (see the "Calibration" procedure) and the frequent analysis of externally supplied standards,
calibration check standards, blanks, duplicate samples, and known additions of standard to samples.
- An externally supplied standard should be analyzed against each freshly prepared standard As solution. These externally supplied standards are used to assess the accuracy of the calibration
standards (0, 14, 28.6, 57.1, 114, and 229 µg As/L) and the calibration check standards (114 µg As/L).
- A calibration check standard (114 µg As/L) and a blank (0 µg As/L) should be analyzed after every 20 samples and as the last 2 analyses of each day. These calibration check standards and blanks are used to assess
the accuracy of each batch of 20 or less samples.
- A duplicate sample should be analyzed after every 20 samples. These duplicates are used to assess precision.
- A known addition of standard to a sample should be analyzed after every 20 samples. These known additions of standard to samples are used to identify interferences.
Last updated November 26, 2008
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