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)

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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.)

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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).

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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.

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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).

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1.vi : The complex colour - d-d shift or electron transfer?

By calculating the molar absorption, A, it is possible to calculate if it is a d-d shift. However, it should be noted that Cr has the configuration of 3d5 4s1 and so Cr(VI) will have the d0 configuration. Without anything in the d orbital to shift, the colour has to be due to electron transfer, therefore, the following may seem to be a bit redundant!.

The complex contains 0.00127g of Cr(VI). Convert this into molarity by division with the RMM of Cr.

Ans : 2.4 x 10-5 M. This is c.

We know the path length is 1 and that e = 40000 dm3 g-1 cm-1 (5)

Feeding this into the formula : A = ecl, A turns out to be 0.48.

With e being 40000 (4 x 104) and this value of A = 0.48, combined with the maximum absorption of the complex being just below 1, all of the indicators point to charge transfer giving rise to the intense colour of the complex.

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1.vii : 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.



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.

Pipette, 0.5cm3

Graduated cylinder, 10cm3

Visible spectrophotometer and cells, if available.

Student safety and disposal

GOGGLES MUST BE WORN THROUGHOUT THE EXPERIMENT. Although low concentrations and small volumes are used, all disposal must be disposed of by local guidelines.


Preparation of standards

1. Obtain six test tubes capable of holding 15-20cm3 and label them 0, 1, 2, 3, 4 and 5. Add to these test tubes the quantities of Cr(VI) and the 0.18mol dm-3 sulphuric acid according to the table below using separate 10cm3 graduated cylinders. Stopper and mix the contents of each test tube by shaking.

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

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1.viii : Changes to published practical

Due to the fact that the experiment outlined is a very much 'bare bones' experiment, a number of changes are required.


3.6mg of K2Cr2O7 is a minute amount (0.0036g) and would be very difficult to accurately weigh (even with the best will in the world!!.). However, it would be inpractible to multiply the quantities of all reagents by 100 to achieve 0.36g of solid, 1dm3 conc. sulphuric acid to a final volume of 10dm3.

A better solution to this problem would be to use a serial dilution method. This may introduce errors, but at HC2/BSc 2 level, these should be negligible.

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).


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

Table 1. Formulation of interfering ion solutions with chromium (VI)

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.

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