A Comparison of Soil Types via a Quantitative Determination of the Chromium Content Using Visible Spectrophotometry and flame atomic absorption spectroscopy or inductively coupled plasma-optical emission spectrometry

5.13.1 Background and Summary of Method

Chromium exists in three oxidation states, of which Cr (III) and Cr (VI) are the most stable. Hexavalent Cr is classified as a known human carcinogen via inhalation, and Cr(III) is an essential dietary element for humans and other animals! Certain soils that exhibit a strong chemically reducing potential have been shown to convert Cr(VI) to Cr(III). It is possible for the analysis of soils from a hazardous waste site to reveal little to no Cr(VI) via the colorimetric method because this method is selective for Cr(VI) only. Because atomic absorption spectrophotometric methods yield total Cr, the difference between analytical results from both methods should be indicative of the Cr(III) content of a given soil type. Both methods will be implemented in this laboratory exercise and applied to one or more soil types.

This exercise affords students the opportunity to use several instrumental techniques to which they have previously been introduced in the laboratory in order to conduct a comparison of soil types with respect to determining the ratio of Cr(III) to Cr(VI) in terms of their respective concentration in an environmental soil matrix. The exercise includes pH measurement, calibration of a UV-vis spectrophotometer, calibration of an atomic absorption spectrophotometer in the flame mode (FLAA), and sample preparation techniques.

Cr(VI) in its dichromate form, Cr2Of2, reacts selectively with diphenyl carbazide, in acidic media to form a red-violet color of unknown composition. The molecular structure for diphenyl carbazide is shown below:

This selectivity for Cr occurs in the absence of interferences such as molybdenum, vanadium, and mercury. The colored complex has a very high molar absorptivity at 540 nm. This gives the method a very low detection limit (MDL) for Cr(VI) using a UV-vis spectrophotometer. Flame atomic absorption spectroscopy is also a very sensitive technique for determining total Cr, with instrument detection limits (IDLs) as low as 3 ppb. Inductively coupled plasma- (atomic) emission spectrometry (ICP-AES) may also be available. Your instructors may choose whether FLAA or ICP-AES is available.

A modification to EPA Method 7196 has been published and will be implemented in this lab exercise (refer to Suggested Readings below). The method uses a hot alkaline solution (pH 12) to solubilize chromates that are to be found in soils obtained from hazardous waste sites. One portion of the aqueous sample would then be aspirated into the FI AA for a determination of total Cr, whereas diphenyl carbazide dissolved in acetone will be added to another portion, and the absorbance of the red-violet complex will be measured at 540 nm using a visible spectrometer. In this manner, both total Cr and Cr(VI) can be determined on the same sample. Thus, the ratio of the concentration of Cr(III) to the concentration of Cr(VI) in a soil sample can be calculated from the data generated in this experiment.

  • 5.13.2 Experimental
  • Chemical Reagents Needed per Student or Group

Note: All reagents used in this analytical method contain hazardous chemicals. Wear appropriate eye protection, gloves, and protective attire. Use of concentrated acids and bases should be done in the fume hood.

Potassium dichromate stock solution: Dissolve 141.4 mg of dried K,Cr,07 in distilled deionized water (DDI) and dilute to 1 L (1 mL = 50 pg of Cr).

Potassium dichromate standard solution: Dilute 10.00 mL of stock solution to 100 mL (1 mL = 5 pg of Cr).

Sulfuric acid. 10% (v/v): Dilute 10 mL of concentrated H,S04 to 100 mL with DDL Also, 1.8 M H,S04 is needed. An aliquot of 10% H,S04 could be used to prepare this solution.

Diphenyl carbazide (DPC) solution: Dissolve 250 mg of 1,5-diphenyl-carbazide in 50 mL of acetone. Store in an amber bottle. Discard when the solution becomes discolored.

Acetone, CH,COCH,: Use the highest purity available.

Alkaline digestion reagent: 0.28 M Na,CO, / 0.5 M NaOH: Use your knowledge of chemical stoichiometry to calculate the amount of each base needed to prepare a solution of the desired molarity. Recall that a J_M solution contains one mole of a pure chemical substance per liter of solution. Recall that one mole of a pure chemical substance is its formula or molecular weight in grams.

Concentrated nitric acid, HNOr

  • Procedure for Alkaline Digestion
  • 1. Place 2.5 g of a given soil type into a 250 mL beaker. Add 50 mL of the alkaline digestion reagent. Stir at room temperature for at least 5 min and then heat on a hot plate to maintain the suspensions at 90 to 95°C, with constant stirring for about 1 h. Repeat for all other soil types to be studied whose Cr content is to be determined in this experiment. Note: Heating on a hot plate can cause bumping and lead to splatter, and thus loss of analyte.
  • 2. Cool the digestates to room temperature, then filter through 0.45 pm cellulosic or polycarbonate membrane filters.
  • 3. Adjust the pH to 7.5 using concentrated HNO, and dilute with DDI to a final volume of 100 mL. You now have 100 mL of digestate.
  • Procedure for Conducting Visible Spectrophotometric Analysis
  • 1. Prepare six working standards from careful dilutions of the 5 pg/mL Cr standard. The range of concentrations should be from 0 to 2 pg/mL Cr. You should prepare 100 mL of each standard.
  • 2. Prepare an initial calibration verification (ICV) standard, which should have its concentration approximately near the mid-range of the calibration. You should prepare 100 mL of the ICV.
  • 3. Prepare a matrix spike and a matrix spike duplicate. The amount of spike should double the concentration found in the original sample. The spike recovery must be between 85 and 115% in order to verify the method.
  • 4. To 45 mL of DDI (this is the method blank), standard, ICV, and digestate, add 1 mL of DPC solution, followed by the addition of 1.8 M H,S04 until the pH reaches approximately 2. This should be done in a 125 mL beaker with stirring and immersion of the glass electrode until the desired pH is attained. After cessation of effervescence, dilute the mixture with DDI to 50 mL. Allow the solution to stand from 5 to 10 min. If the solution appears turbid after the addition of DPC, filter through a 0.45 pm membrane. Store the remaining samples and standards in properly labeled bottles. Use if it is necessary to repeat this analysis.
  • 5. Set the spectrophotometer at 540 nm; correctly set the 0 and 100% transmittance settings. Transfer an aliquot of the 50 mL sample to a cuvette. Measure the absorbance of all blanks, standards, and samples. Construct a table in your notebook to facilitate the entry of data. Procedure for Atomic Absorption Spectrophotometric Analysis or ICP-AES

Refer to SW-846 Methods 7000A (“Atomic Absorption Methods”) and 7190 (“Chromium, Atomic Absorption, Direct Aspiration”) or Method 6010D (“Inductively Coupled Plasma-Atomic Emission Spectrometry”) (ICP-AES) for the quantitative determination of chromium at total Cr. In the lab, proceed to prepare calibration standards and ICVs and aspirate these into the flame using the Model 3110® (PerkinElmer) atomic absorption spectrophotometer. Your instructors may also have the ICP-AES available for you to use such as a Model 2000® (PerkinElmer) ICP- AES. Use the remaining digestates from step 3 and determine total Cr. The F1AA may need to be set up from its present configuration. Refer to previous exercises and training manuals for the necessary information.

5.13.3 For the Report

Include all calibration data, ICVs, and sample unknowns for both instrumental methods. Perform a statistical evaluation in a manner that is similar to that in previous experiments. Use Excel or an alternative to conduct a least squares regression analysis of the calibration data. Calculate the accuracy (expressed as a percent relative error for the ICV) and the precision (relative standard deviation for the ICV) from both instrumental methods. Calculate the percent recovery for the matrix spike and matrix spike duplicate. Report on the concentration of Cr in the unknown soil samples. Be aware of all dilution factors and concentrations as you perform calculations.

Find the ratio of the concentration of Cr (III) to that of Cr (VI) in each of the soil samples analyzed and present this value at the end of your report.

5.13.4 Suggested Readings

To develop this experiment, the author consulted the following resources:

  • Standard Methods for the Examination of Water and Wastewater. 16th ed. Washington, DC: Association of Public Health Association. 1988, pp. 201-204.
  • • Method 7196A. Chromium. Hexavalent (Colorimetric). In Test Methods for Evaluating Solid Waste Physical/Chemical Methods. SW-846. Revision 1. Washington, DC. Environmental Protection Agency, July. 1992.
  • • Vitale R. et al. Am Environ Lab 7:1, 8-10, 1995. This paper discusses a modification to SW-846 Method 7196.
  • • Method 7000B. Flame Atomic Absorption Spectrophotometry. In Test Methods for Evaluating Solid Waste, Physical/Chemical. SW-846. Revision 2. Washington, DC: Environmental Protection Agency, 2007.
  • • Method 6010D Inductively Coupled Plasma Optical Emission Spectrometry. In Test Methods for Evaluating Solid Waste, Physical/Chemical. SW-846. Revision 5. Washington, DC: Environmental Protection Agency, July, 2018.
  • • Budde W. Mass Spectrometry!: Strategies for Environmental and Related Applications, Washington, D.C.: American Chemical Society, Oxford University Press, 2001, Chapter 7 pp. 328-347 provides an excellent introduction to trace environmental elemental analysis.
  • • Skoog D., J. Leary. Principles of Instrumental Analysis, 4th ed. Philadelphia: WB Saunders, 1992, Chapter 11, pp. 233-251. This textbook and certainly subsequent editions provide the student with an excellent introduction to the principles of ICP-AES.
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