The original practical.
This has been reproduced from The Journal of Chemical Education, Volume 71, Number 4, April 1994 pgs 323-324. The text has only been changed to be in English (rather than American English) and for the inclusion of S.I. units. All other text remains the same. The superscript numbers are for the text and not for the main body of this project.
TESTING THE WATERS FOR CHROMIUM
MARY S. HERRMANN, University of Cincinnati - Raymond Walters College.
Frequently, metal ions are introduced into waterways by industry 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 Cr(III). Cr(III) is much less toxic than Cr(VI) and seldom found in potable waters. Cr(VI), however, is toxic to humans and is found in water. It has been shown to toxic when in aerosol form causing damage to the skin and upper respiratory system and causing lung cancer1. 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(VI) 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 car 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 deaths2. Some other products that contain Cr(VI) are paints, pigments, tanning agents, inks, fungicides and wood preservatives3.
The maximum permissible level or Cr(VI) allowed to be released into the waterways is 50mg dm-3. It level in drinking water normally is much lower and a lever higher than 3mg dm-3 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(VI) concentrations prior to releasing their waste water.
Chromium (VI) solution (1.27mg dm-3 Cr(VI)
To prepare place 3.6mg of K2Cr2O7 and 10cm3 of conc. sulphuric acid into about 500cm3 distilled water in a volumetric flask. Dissolve and then add distilled water to a final volume of 1dm3.
Polluted water (dilute 100cm3 Cr(VI) solution to 1dm3 with distilled water)
Diphenyl carbazide solution (0.50g in 200cm3 propanone)
Ascorbic acid solution (0.2g in 100cm3 distilled water)
0.18mol dm-3 sulphuric acid solution
To prepare, add 10cm3 of conc. sulphuric acid to about 500cm3 with distilled water in a volumetric flask. Mix and add distilled water to a final volume of 1dm3.
3.0mol dm-3 sulphuric acid solution
Add 42cm3 conc. sulphuric acid to about 150cm3 of distilled water in a 250cm3 volumetric flask. Mix and add distilled water to make a final volume of 250cm3.
Graduated cylinder, 10cm3
Visible spectrophotometer and cells, if available.
GOGGLES MUST BE WORN THROUGHOUT THE EXPERIMENT. Although low concentrations and small volumes are used, all disposal must be disposed of by local guidelines.
|H2SO4, 0.18M cm3||10.0||9.6||9.0||8.0||6.0||0.0|
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.
The maximum permissible level of Cr(VI) allowed to be released is 50 m g dm-3. Assume an industry has 100dm3 of Cr(VI) polluted water at the same concentration as the polluted water from the determination of chromium in water samples. Calculate how many litres of chromium free water must be mixed with the polluted water so that it can be released (ans - add around 150dm3 of Cr-free water.)
Cr(VI) is reduced easily to Cr(III) that can be released at the much higher level of 1000 m g dm-3. Take a sample of polluted water and add 5 drops of ascorbic acid solution (a mild reducing agent). Swirl to mix and determine the Cr(VI) concentration as you did in the part above. Many other methods of reduction are possible5.
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 a map 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 m g dm-3). The map in of a hypothetical town, Anytown, and some surrounding industries. The students will be testing Cr(VI) levels in the river water at the various sites indicated in order to locate sources of the pollution.
The above materials will be used except that the solutions will be substituted for the polluted water.
Label six jars (mayonnaise jars or similar) with the numbers 1 through to 6. Place the following solutions into the appropriate jar.
1 and 2 : 500cm3 of unpolluted water (distilled water or tap water known to be free of Cr(VI))
3 : 250cm3 Cr(VI) solution and 250cm3 unpolluted water.
4 : 150cm3 Cr(VI) solution and 350cm3 unpolluted water.
5 and 6 : 100cm3 Cr(VI) solution and 400cm3 unpolluted water.
The procedure is identical to above except that solutions 1-6 are substituted for "polluted water" in the determination of chromium water samples in the first part of the experiment.
1 Varma, M. M.; Serdahely, S.G.; Katz H.M. J. Envir Health 1976, 39 (Sept/Oct.). 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 Assoc., Washington DC, 1989
5 Lunn, G.; Sansone, E.B. J. Chem. Educ. 1989 66, 443
ANALYSIS OF CHROMIUM IN WATER9
Chromium (VI) is a toxic pollutant in our waterways. The experiments that follow are to be carried out and from the results, the effectiveness of the method can be determined and the suitability of the test.
For the purpose of the major part of the first part of the experiments, the method of detection is by colorimetric techniques. The chromium is first to be complexed with a solution of 1,5 diphenyl carbazide (DiPC) in propanone, acidified and then, tested in a spectrophotometer set at 540nm.
The DiPC will not complex with Cr(III). This test is therefore specific to Cr(VI).
Q. Why is it specific to Cr(VI) and not Cr(III)?
A. Cr (III) is a very stable oxidation state for chromium. In this state, the chrome is labile and kinetically very slow to react or form complexes. It is not a strong oxidising agent. Cr (VI) is a different story. To begin with, Cr (VI) is not a very stable state when compared to (III). The Cr (VI) is a very strong oxidising agent (therefore very fast in reacting, unlike Cr (III) and likely to form complexes).
PREPARATION OF THE Cr(VI) STANDARD SOLUTION (1.27mg dm-3)
NOTE : Cr(VI) AND IT'S REDUCTION PRODUCT (Cr(V)) ARE BOTH VERY HAZARDOUS TO HEALTH. THEY HAVE BEEN SHOWN TO BE CARCINOGENIC BY INHALATION, SKIN ABSORPTION AND IF TAKEN ORALLY. ALL SAFETY MEASURES MUST BE OBSERVED (GLOVES, EYE PROTECTION AND LAB COATS MUST BE WORN.)
1. Carefully weigh out 0.36g of AR potassium dichromate into a 100cm3 beaker. Dissolve this in about 75cm3 of deionised water. Transfer this to 1dm3 volumetric flask, washing the beaker as many times as required to ensure that all of the dichromate has been transferred to the flask. The stirrer used, must also be rinsed into the flask.
2. Fill the flask to just beneath the graduation mark with deionised water. Stopper the flask and invert six or seven times. Unstopper the flask and leave to stand for a couple of minutes. Fill to the graduation mark, stopper, invert six or seven times. Let the solution settle.
3. Pipette a 10cm3 portion into a clean, fresh 1dm3 flask, followed by 10cm3 of 0.18mol dm-3 sulphuric acid. Repeat instruction 2 and label as "Cr(VI) standard solution"
The sulphuric acid solutions (0.18mol dm-3 and 3.0mol dm-3) are supplied.
DiPC has to be made up freshly and stored in an amber bottle as it is very light sensitive. To prepare the stock solution, accurately weigh 0.625g of the solid and dissolve in the minimum amount of propanone. Transfer the solution to a 250cm3 volumetric flask and dilute to the mark with propanone. The solution must be kept stoppered when not in use. Propanone will readily evaporate and therefore the concentration of the solution will alter.
Construction of a calibration curve with a spectrophotometer.
1. Obtain six test tubes and label 0 to 5. Pipette into each tube the quantities of solutions outlined below. Mix the solutions well. DO NOT COVER THE MOUTH OF THE TUBE WITH THE FINGER. (This will make the concentration of each tube different to the experiment and therefore effect the end result. Cr(VI) is also a known carcinogen).
|H2SO4 0.18M cm3||10.0||9.6||9.0||8.0||6.0||0.0|
Table 2. Cr(VI)-DiPC & acid standards.
2. To each tube, pipette 0.5cm3 of DiPC solution. Mix the contents and let them stand out of direct light for 5 minutes to develop. It is vital that they are left for 5 minutes as the solutions will continue to complex for this length of time (ie maximum complexation takes place by 5 minutes).
3. With the spectrophotometer set at 540nm, fill a cuvette with each solution and measure the absorbtivity. The blank should be from tube 0. The absorbtivity for the complex is 40000 dm3 g-1 cm-1. On the machine, the control should be turned to "Absorbance" or "A". If it only has Transmission, the following formula should be employed to calculate the absorbance:
Equation 1 - Conversion of absorbance from transmission.
4. From the above results, plot a graph of absorption against concentration. This is the calibration curve for the complex and should be made before carrying on with the experiment.
THE EFFECT OF INTERFERING METAL IONS
Water will not solely contain Cr(VI). It is known to contain (amongst other things) such as lead (II), chromium (III), cobalt, iron (II), iron (III), mercury and barium. The second set of experiment is to determine if these will have any effect on the absorbtion. As in all cases, the elements should only be in trace amounts, for the purposes of this test, the concentrations will be same as for the chromium (VI).
PREPARING THE SOLUTIONS
Following the method outlined for the Cr(VI) solution above, weigh out the masses of the following salts :
CoSO4 (0.2242g), HgCl2 (0.1212g) (WARNING : this is a schedule 1 poison. Great care must be employed when handling), Ammonium iron(II) sulphate (0.2003g), Iron (III) chloride (0.2321g), BaCl2 (0.2358g) and KCrO3 (0.4170g).
Accurately make these up to a final volume of 500cm3. Prepare the final solution as before except only use 5cm3 instead of 10cm3 for each solution.
DETERMINATION OF ION INTERFERENCE.
This will require another 9 tubes set up as follows:
|Mx+||0.0||5.0 Hg||5.0 FeII||5.0 FeIII||5.0 Ba||5.0 Co||2.5 FeII||2.5 Hg||2.5 Co|
|Mx+||2.5 FeIII||2.5 CrIII||2.5 Ba|
Table 3 - Formulation of interfering ion solutions with Cr(VI)
To each of the tubes, add 0.5cm3 of the DiPC solution. Leave for five minutes and then pour into a cuvette. Measure the absorbtion at 540nm.
DETERMINATION OF CHROMIUM IN WATER SAMPLES
You have been given a set of water samples to test for the presence of Cr(VI). This can be carried out essentially as for the calibration curve but with a number of differences:
1. 0.5cm3 of 3.0mol dm-3 H2SO4 must be added to the water sample.
2. One of the water samples given is very murky. This is not unusual as most streams and canals will have bits of plants and also micro-organisms in it. To eliminate these, perform the following:
(1) Take 50cm3 of the water sample and filter under pressure using a Buchner set up
(2) To remove any waste organic materials and to acidify the water, add 10cm3 of conc. sulphuric acid. All organic solids and organic miscible liquids will now have been oxidised.
(3) Re-filter under pressure to remove any carbonised material.
The experiment can now proceed. Measure the absorbance of the test solution and from comparison of the calibration curve, calculate the concentration of each water sample.
REDUCTION OF CHROMIUM LEVELS
As previously stated, Cr(VI) will have an adverse environmental impact on marine life as well as our own. The following 3 experiments will show how to reduce the levels.
REDUCTION METHOD 1. DILUTION METHOD
The maximum permissible level of Cr(VI) allowed to be released is 50mg dm-3. Assume an industry has 100dm3 of Cr(VI) polluted water at the same concentration as the polluted water from the determination of chromium in water samples. Calculate how many litres of chromium free water must be mixed with the polluted water so that it can be released.
REDUCTION METHOD 2. CHEMICAL REDUCTION METHOD 1.
As stated above, the maximum permissible level of Cr(VI) allowed to be released is only 50mg dm-3. However, by reacting with 0.2cm3 of l-ascorbic acid (0.2g in 100cm3 of distilled water) BEFORE complexation, the Cr(VI) will be reduced to Cr(III) which can be released at the far higher level of 1000mg dm-3.
To verify this, take tube 5 from the calibration curve experiment. From this, pipette of 3cm3 and add the ascorbic acid and mix. Pipette in 0.5cm3 DiPC and leave for five minutes.
While this is developing, take a second 3cm3 from the same tube, add the DiPC and mix. Now add the ascorbic acid and leave to develop.
To verify, record the observed absorbtion of both and compare from the calibration curve data.
REDUCTION METHOD 3. CHEMICAL REDUCTION METHOD 2.(10)
Take a 50cm3 aliquot of the Cr(VI) solution and add to this 10cm3 of 10% sodium metabisulphite (Na2S2O5). Neutralise with magnesium hydroxide solid until all of the effervescing ceases.
Take a 5cm3 aliquot of the neutralised solution and transfer to a clean test tube. Add 0.5cm3 of DiPC and test at 540nm for an absorbtion peak. No addition of the 3M sulphuric acid is needed.
CALCULATION OF RESULTS
The following are required from you for the results :
i. A calibration curve of [Cr(VI)] v absorbtion.
ii. Using the linear regression program (least mean squares), find the error in the calibration curve absorbtion.
iii. Calculate the molar absorption value, e, for the complex at 540nm.
iv. Did any of the other metal ions cause any significant variation of absorbance?
v. Calculate the [Cr(VI)] in the water samples by comparison with the calibration curve.
vi. Out of the three methods of reduction, which will be the most environmentally unsound and why?
vii. Besides the methods outlined above, how else could the [Cr(VI)] be calculated?
viii. What will be the shape of the complexed molecule and would the colour be due to d - d orbital shift or electron charge transfer?
ix. What will be the final oxidation state of the Cr(VI) complex and how is this deduced?
x. How were the figures arrived at for the concentrations of the interfering ions (i.e. 0.4484g Co2+). Calculate for all of the ions used (inc. the dichromate).
xi. A full COSHH assessment MUST be given with the written script. Included in this should be reasons why Cr(VI) is so much more toxic than Cr(III).
xii. All bibliographical references should be given.
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