1.i : Why is chromium so important?
Chromium metal is used in this country for a large variety of applications ranging from an additive in the manufacture of stainless steel to chromium plating (chrome plating) used for motorcycle exhausts and some older types of car bumpers to the colourisation of Rubies and Emeralds. Chromium metal has a more important use, it is a very hard transition metal and is normally amalgamated with titanium (another transition element) to make replacement hips in the USA and UK.
Chromium compounds (such as Chromic acid [a mixture of H2SO4 with sodium dichromate]) are used in the electroplating industry as both an additive and (in the case of chromic acid) as a highly powerful oxidising agent (chromic acid is roughly 3 times as powerful an oxidising agent as sulphuric acid due to the oxidising power of the Cr(VI) itself).
In the North West, there are only three chromium using plants. Two on Merseyside (both on the Wirral) and one in Manchester. As with any ore which comes in from other countries, the sightings of the plants reflect all the elements required for a successful plant; water (for raw materials processing and waste), a fine roads and rail infrastructure (transportation of other materials required as well as for sales of product) and cheaper power (Fiddler's Ferry and Salford Power generators are both near to the plants).
With all industrial processes, a waste product is inevitably formed. In the chromium industries (plating and manufacturing), it is normally the chromium (VI) compound (such as chromic acid and other high oxidising Cr(VI) cleaners). A smaller amount of the reduced Cr(III) and Cr(s) are released. The maximum permitted Cr(VI) in the UK is currently set at 50mg dm-3, with Cr(III) set at 1000mg dm-3. (1)
1.ii : Complex theory
Cr(VI) is an element which will form octahedral complexes with ligands. In this experiment, the ligand is 1,5 diphenyl carbazide (DiPC). By constructing a molecular model of the ligand and positioning it around a suitable 6 branched element, the following complex is made (fig. 1). Notice that three of the DiPC ligands can be joined to the central Cr. This complex is extremely stable (see experimental details for further information). A second structure has also been proposed(2) (shown in fig. 2) with the Cr being 'sandwiched' between the delocalised rings on the primary benzene rings. The apparent conflicts between the two theories for the structure are due to one version being borne out of models (as in fig. 1) while the second is as a result of the use of x-ray diffraction techniques which gave the second result.
fig.1. Complexation model
fig. 2 X-Ray diffraction of the DiPC-Cr(VI) complex
The colour of the compound is as a result of electron transfer, rather than d orbital shifts (see Complex colour - d-d shift or electron transfer? for explanation.)
1.iii : Final oxidation state of the complex
As described, the DiPC will not complex with the Cr(III) to form a colour. The reason is due to the stability of the Cr(III) ion and it's subsequent chemical inertness (as described previously). This therefore means that the final oxidation state of the Cr complex must be Cr(VI)(3).
1.iv : Chromium detection
In it's most stable form (Cr (III)), chromium can be detected by AA, by gravimetric analysis with a number of substances (such as hydrolysis of potassium cyanate to form the insoluble hydroxide(4)).
Cr (VI) can also be detected by AA and also by titration with standard Na2S2O4 with I2(4).
If the solutions to be tested were concentrated enough (above 0.01M), then analysis by titration or gravimetric techniques could be considered. AA cannot be considered as this will only determine Cr (any oxidation state).
In water samples, the maximum permitted level in this country is 50mg dm-3 for Cr(VI). As this is far below 0.01M, only one of two methods can be considered.
The first is rather impracticable. X-Ray Crystal photography. This would be very expensive and very long to perform.
A quicker and easier method will be the complexation of Cr(VI) with 1,5 - Diphenyl carbazide and the determination of concentration colorimetrically. The carbazide will form a very strongly coloured compound.
Cr(VI) will absorb best at 540nm(5). The absorbtivity with the complex is 40000 dm3 g-1 cm-1 at 540nm(5). Even without a colorimeter, the complex is strongly enough coloured for a relatively able person to compare one concentration to another.
1.v : Reasons for toxicity of Cr (VI) compared to Cr (III)
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 oxidiser and the human's natural body acidity is enough for the chrome to keep to this Cr (III) state.
Cr (VI) is a different story.
Cr (VI) is not a very stable state when compared to Cr(III). The Cr (VI) is a very strong oxidising agent (therefore very fast in reacting, unlike Cr (III) and likely to form complexes). This is not why Cr (VI) is toxic.
One of the reduction products of Cr (VI) is Cr (V). Chrome (V) is a known carcinogen(6) and will lodge in any tissue to form cancerous growths. There are reports that chromium (V) is also a factor leading to premature senility in parts of Russia(7). This has not been substantiated by the UN or any other academic group.
In the body, the acidity and action of enzymes on Cr (VI) will promote the formation in small quantities of Cr (V)(8). However, as the size of this is normally too large to be adopted by a tissue, the Cr (V) will pass out. The only place where the Cr (V) is likely to lodge is in some of the fine capillaries in either the kidneys, intestines or lungs.
During the passage out, Cr (VI) will continue to oxidise anything it can, leaving deposits of the relatively safe Cr (III) and completely unsafe Cr (V) behind.
Even at the concentrations used for this experiment, the levels of Cr(VI) will pose a health risk and so all protective methods MUST be employed (see hazard sheets for details).
Tube number | 0 | 1 | 2 | 3 | 4 | 5 |
CrVI cm3 | 0.0 | 0.4 | 1.0 | 2.0 | 4.0 | 10.0 |
H2SO4, 0.18M cm3 | 10.0 | 9.6 | 9.0 | 8.0 | 6.0 | 0.0 |
2. To each test tube, pipette 0.5cm3 of diphenyl carbazide solution. Mix the contents of the test tubes, and let them stand for five minutes for colour development.
3. If a spectrophotometer is available, measure the absorbtivity of each sample at 540nm, and plot a standard curve. For the blank, use tube 0. The absorbtivity for the diphenyl carbazide-Cr(VI) solution is 40,000 dm3 g-1 cm-1 at 540nm4. If no spectrophotometer is available, save the standard solutions for colour comparison in the determination of chromium in water samples.
Determination of Chromium in water samples
1. For each sample to be tested, obtain a test tube and label it. Place 10cm3 of the water sample in the test tube. The "polluted water" should be tested as well as any other samples available.
2. To each test tube, add 12 drops of 3M sulphuric acid.
3. To each tube, pipette 0.5cm3 of diphenyl carbazide solution and allow 5 minutes for colour development.
4. Determine the amount of Cr(VI) present either by absorbance at 540nm or by visual comparison with 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 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 (ans - add around 150dm3 of Cr-free water.)
Reduction Method
Cr(VI) is reduced easily to Cr(III) that can be released at the much higher level of 1000mg 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.
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 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 (200mg 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.
Materials for variation
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.
Procedure for variation
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.
Literature Cited
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
Compound | RMM | 1M soln | 1ppm | 0.1ppm | 0.01ppm | 1.27ppm | Total |
Cobalt Sulphate | 281.0972 | (g) | (g) | (g) | (g) | (g dm-3) | |
Co | 58.9332 | 58.9332 | 0.001697 | 0.000170 | 0.000017 | 0.002115 | |
SO4 | 96.0576 | 96.0576 | 0.001041 | 0.000104 | 0.000010 | 0.001322 | |
7 Water | 126.1064 | 126.1064 | 0.000793 | 0.000079 | 0.000008 | 0.001007 | 0.004484 |
Mercury Chloride | 271.4960 | ||||||
Hg | 200.59 | 200.59 | 0.000499 | 0.000050 | 0.000005 | 0.000633 | |
Cl2 | 70.906 | 70.906 | 0.001410 | 0.000141 | 0.000014 | 0.001791 | 0.002424 |
Barium Chloride | 294.1844 | ||||||
Ba | 137.33 | 137.33 | 0.000728 | 0.000073 | 0.000007 | 0.000925 | |
Cl2 | 70.906 | 70.906 | 0.001410 | 0.000141 | 0.000014 | 0.001791 | 0.002716 |
Potassium Dichromate | 294.1844 | ||||||
K | 78.1966 | 78.1966 | 0.001279 | 0.000128 | 0.000013 | 0.001624 | |
Cr | 103.992 | 103.992 | 0.000962 | 0.000096 | 0.000010 | 0.001221 | |
O | 111.9958 | 111.9958 | 0.000893 | 0.000089 | 0.000009 | 0.001134 | 0.003979 |
Iron (III) Chloride | 270.2972 | ||||||
Fe | 55.847 | 55.847 | 0.001791 | 0.000179 | 0.000018 | 0.002274 | |
Cl3 | 106.359 | 106.359 | 0.000940 | 0.000094 | 0.000009 | 0.001194 | |
6 Water | 108.0912 | 108.0912 | 0.000925 | 0.000093 | 0.000009 | 0.001175 | 0.004643 |
Amm. Iron (II) Chloride | 392.130 | ||||||
Fe | 55.847 | 55.847 | 0.001791 | 0.000179 | 0.000018 | 0.002274 | |
Amm. Sulphate | 228.1918 | 228.1918 | 0.000438 | 0.000044 | 0.000004 | 0.000557 | |
6 Water | 108.0912 | 108.0912 | 0.000925 | 0.000093 | 0.000009 | 0.001175 | 0.004006 |
The required Cr(VI) in acid concentration is 1.27mg dm-3 (or 1.27 ppm). The sulphuric acid has only been added to stabilise the Cr in the +6 oxidation state. Therefore, if the method was followed but with 0.360g of Cr(VI) used and all the rest added when this had been diluted down by a factor of 100 (10cm3 of the Cr(VI) solution to a 1dm3 flask will result in 3.6mg), the required solution can be made with a great deal more accuracy and ease.
A Cr(III) solution is also required as well as a mixed metal ion solution to test if these will interfere with the actual determination of Cr(VI). These can be made up as above with the following chemicals. If the concentrations are all kept the same, then the testing can be a fairer tests (will equal concns of Mx+ interfere with the Cr(VI)?).
CHEMICALS : Mercury (II) Chloride (S1 poison - care!), Iron (II) Chloride, Iron (III) Chloride, Barium Chloride, Cobalt (II) Sulphate, Potassium Chromate.
These have been chosen as they are the most likely ions to be found in British Waterways. They are also coloured (except for barium and mercury salts).
PROCEDURAL CHANGES.
Preparation of standards.
This will remain unchanged for the Cr(VI) curve.
To determine if the hypothesis of metal ion interference is valid or not, the following tests should be performed.
A further 9 tubes are set up as in the table below. The procedure is then the same as before.
Tube No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
CrVI | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 |
H2SO4 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 |
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 |
Determination of Chromium in water samples
To 2. in the published method, a volume should be inserted to replace the phrase "a drop". This will be x cm3 where x is the correct amount. A drop is a rather haphazard method of addition as a drop can vary greatly from the method of dropping (a Pasteur pipette will dispense 0.1cm3 per drop, while a plastic disposable will dispense up to 0.19cm3 with every possible inbetween!.)
As most of the water samples are from natural sources (rather than, say, out of a tap), it may be necessary to prepare the water. This can be done quickly by performing the following:
(1) Take 50cm3 of the water sample and filter under pressure using a Buchner set up (this is more for speed than anything else!).
(2) To remove any waste organic materials and to acidify the water, add 10cm3 of conc. sulphuric acid. All organic solids and miscible liquids will now have been oxidised. Any alkalinity (which will favour the Cr(III) state more hence the addition of the 3mol dm-3 to all of the water samples to ensure the Cr(VI) state) will have been removed.
(3) Re-filter under pressure to remove any carbonised/oxidised material.
The chromium sample is now ready for complexation with the DiPC. Further addition of the 3M sulphuric acid is not needed as the solution will now be sufficiently acidic to keep the Cr(VI) state.