edited by
filtrates
&
JamesO. Schreck
residues
University of Northern Colorado Greeley, CO 80639
Testing the Waters for Chromium Mary S. Herrmann University of Cincinnati-Raymond Walters College, Cincinnati, OH 45236
Frequently, metal ions are introduced into waterways by as waste from various processes. Many of the metal ions are toxic to humans, and their release must be monitored and controlled carefully. A metal ion that can be a pollutant is the hexavalent chromium ion. There are two natural forms of ionic chromium, the hexavalent ion, Cr(VI) and the trivalent ion, Cr(III). Cr(III) is much less toxic than Cr(VI) and seldom is found in potable waters. Cr(VI), however, is toxic to humans and is found in water. It has been shown to be toxic when in aerosol form causing damage to the skin and upper respiratory system and causing lung cancer (1). The toxic effects from Cr(VI) in drinking water are not well documented, but it is a suspected carcinogen. There are many industries that use chromic acid and other forms of Cr(VI) and are possible sources of Cr(VT) pollution in either water or air or both. One industry that pollutes water with Cr(VI) is the chrome-plating industry (for the plating of such things as automobile bumpers). Chromic acid is used in the electroplating process and can be present in industrial waste waters. Cr(VI) also can enter water supplies from industrial cooling towers where chromic acid is added to the water to inhibit metal corrosion. The Environmental Protection Agency recently banned Cr(VI) from use in 37,500 building roof cooling towers (that leak coolant into the air) in the United States that had caused an estimated 20 cancer deaths (2). Some other products that contain Cr(VI) are paints, pigments, tanning agents, inks, fungicides, and wood preservatives
Downloaded via UNIV OF SUSSEX on June 24, 2018 at 11:50:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
(3).
The maximum permissible level of Cr(VI) allowed to be released into waterways is 50 gg/L. Its level in drinking water normally is much lower than this and a level higher than 3 gg/L is suggestive of industrial pollution. The experiment outlined here is a test for the presence of Cr(VI) in water that uses a sensitive colorimetric reagent. Students determine the level of Cr( VI) in both the local tap water and some polluted “industrial” waste water. The experiment also investigates some methods by which industry can lower Cr(VT) concentrations prior to releasing their waste water.
Materials •
•
•
• •
Chromium(VI) solution [1.27 mg/L Cr(VI)] To prepare place 3.6 mg of K2Cr207 and 10 mL of concentrated sulfuric acid into about 500 mL distilled water in a volumetric flask. Dissolve and then add distilled water to a final volume of 1.0 L. Polluted water (dilute 100 mL of chromium(VI) solution to 1.0 L with distilled water) Diphenylcarbazide solution (0.50 g in 200 mL acetone) Ascorbic acid solution (0.2 g in 100 mL distilled water) 0.18 M Sulfuric acid solution
mL of concentrated sulfuric acid to about 500 mL distilled water in a volumetric flask. Mix and then add distilled water to a final volume of 1.0 L. 3.0 M Sulfuric acid solution To prepare, add 42 mL sulfuric acid to about 150 mL of distilled water in a 250-mL volumetric flask. Mix and then add distilled water to make a final volume of 250 mL. Pipet, 0.5 mL Graduated cylinder, 10 mL Visible spectrophotometer (Spec 20) and cells, if available To prepare, add 10
industry
•
• • •
Student Safety and Disposal Goggles should be worn throughout the experiment. Although low concentrations and small volumes are used, all solutions must be disposed of by local guidelines. Procedure Preparation of Standards 1.
Obtain six test tubes capable of holding 15-20 mL and label them 0, 1, 2, 3, 4, and 5. Add to these test tubes the quantities of Cr(VI) solution and the 0.18 M sulfuric acid according to the table below using separate 10-mL graduated cylinders. Stopper and mix the contents of each test tube by shaking.
Preparation of Standards Test tube
0
1
2
3
4
Cr(VI) solution, mL
0.0
0.4
1.0
2.0
4.0
10.0
0.18 M Sulfuric Acid, mL
10.0
9.6
9.0
8.0
6.0
0.0
5
test tube pipet 0.5 mL of diphenylcarbazide solution. Mix the contents of the test tubes, and let them stand for 5 min for color development. 3. If a spectrophotometer is available, measure the absorbance of each sample at 540 nm, and plot a standard curve. For the blank, use test tube 0. The absorptivity for the diphenylcarbazide-Cr(VI) complex is 40,000 L g_1 cm-1 at 540 nm (4). If no spectrophotometer is available, save the standard solutions for color comparison in the determination of chromium in water samples. 2. To each
Determination of Chromium in Water Samples 1.
2. 3.
For each water sample to be tested obtain a test tube and label it. Place 10.0 mL of the water sample into the test tube. The “polluted water” should be tested as well as any other water samples available. To each test tube add 12 drops of 3 M sulfuric acid. To each test tube, pipet 0.5 mL of diphenylcarbazide solution and allow 5 min for color development.
Volume 71
Number 4
April 1994
323
ABC Steel Corp.
river water at the various sites indicated in order to locate the source of the pollution.
Materials for Variation The above materials will be used except that the following solutions will be substituted for the polluted water. Label six jars (mayonnaise jars or similar containers) with the numbers 1 through 6. Place the following solutions into the appropriate jar. and 2: 500 mL of unpolluted water (distilled water or tap water known to be free of contamination with Cr(VD. 3: 250 mL Cr(VI) solution and 250 mL of unpolluted water. 4: 150 mL Cr(VI) solution and 350 mL of unpolluted water. 5 and 6; 100 mL Cr(VI) solution and 400 mL of unpolluted 1
Happy Acres Farm
water. ©
Chemicals To Go ©
Cover-Me Paint Co. ©
200 pg/L Cr (VI) found
Map of Anytown and surrounding area—site of Cr(VI) pollution.
4.
Determine the amount of Cr(VD present either by absorbance at 540 nm or by a visual comparison with the standard solutions.
Reducing Chromium(VI) Levels for Disposal
Industries use a variety of methods to reduce the Cr(VI) concentration to levels permissible for disposal. This section describes two methods for reducing the concentration of the polluted water. Students may wish to try other methods as well.
Dilution Method The maximum permissible level of Cr(VI) allowed to be released is 50 pg/L. Assume an industry has 100 L of Cr(VT) polluted water at the same concentration as the polluted water from the determination of chromium in water samples. Calculate how many liters of chromium-free water must be mixed with the polluted water so that it can be released. (Answer—add around 150 L of chromium-free water.) Reduction Method
Cr(VI) is reduced easily to Cr(III) that can be released at the much higher level of 1,000 pg/L. Take a sample of polluted water and add 5 drops of ascorbic acid solution (a mild reducing agent). Swirl to mix and then determine the Cr(VI) concentration as you did in the part above. Many other methods of reduction are possible (5). Variation to Experiment A variation in the above procedure that teachers may choose to use involves a bit more preparation time but will be more meaningful to students. The variation presents students with a Cr(VI) pollution mystery that they are to solve. Students are given the map shown in the figure prior to performing the experiment and told that at location seven on the map an unusually high level of Cr(VI) was discovered in the river water (200 pg/L). The map is of a hypothetical town, Anytown, and some surrounding industries. The students will be testing Cr(VI) levels in the
324
Journal of Chemical Education
Procedure for Variation The procedure is identical to that described above except that solutions 1-6 are substituted for the “polluted water” in the determination of chromium in water samples,in the first part of this experiment. Teachers can have different student groups do different solutions, 1-6, and then the class can pool their results to determine the polluting culprit. Alternatively, students can choose one solution, determine its Cr(VI) value, and then choose another one to test, based on the results of the first test. In this way students will arrive at their own answer to the mystery. Conclusions The experiment described has the sensitivity to detect Cr(VI) at levels that might be found in polluted water. Most drinking water or river water will not be exposed to Cr(VT) pollution and so will have a level below the sensitivity of the test. By using the synthetically polluted water, students can discover and determine the Cr(VI) level that might be found in chromium-polluted water. The experimental results are highly reproducible and should result in an excellent standard curve, making this experiment a possible candidate for introduction of speetrophotometric techniques. The experiment works well without a spectrophotometer because the colorful solutions can be referenced quite well visually. Instructors may wish to combine the experiment with other metal determinations such as Fe(III) (6) or with other water tests such as pH. Extremely low levels of Cr(VT) are used thus minimizing human risk and water contamination. The variation described gives an added dimension to the experiment, one that students will find more enjoyable. It presents students with the type of problem and means to solve it similar to that faced by the Environmental Protection Agency. The results should show that the most likely polluter is the Shiny Plating Co. Instructors may have students do some research on the types of industry that could pollute waterways with Cr(VI). Overall the experiment is appealing to students because they are dealing with water pollution, a realistic problem and timely issue. Furthermore, students have the opportunity to investigate methods to reduce Cr(VI) pollution, thereby realizing the usefulness of a knowledge of chemistry. Literature Cited 1.
Varma, M. M.; Serdahely, S. G.; Katz, H. M. J. Envir. Health 1976. 39 (Sept./OctJ, pp 90-100.
2. Cooper, M. NCI Cancer Weekly Jan. 15, 1990, p 12. 3. Chromium; National Academy of Sciences: Washington, DC, 1974. 4. Standard Methods for the Examination of Water and Wastewater, 17th ed,, American Public Health Association: Washington, DC, 1989. 5. Lunn, G.; Sansone, E. B. J. Chem. Educ. 1989, 66,443. 6. ChemCom; The American Chemical Society, Kendall/Hunt: Dubuque, 1988, p 31.
