Recommendations for the use of dichromate salts by students
It is important to consider the risks involved prior to using chemicals. Science ASSIST recommends that the use of potassium dichromate or sodium dichromate solution (≤ 0.1 M) should be restricted for Year 12 students to only small-scale qualitative purposes where only a few drops of solution or micro quantities are used, so that exposure to the chemical is kept at a minimum. For example, the use of dichromate salts as solutions (~ 0.1 M) are used to demonstrate the oxidation of alcohols and to illustrate the application of Le Chatelier’s Principle in aqueous solutions.
Hazardous properties of dichromate salts
Dichromate salts are toxic, irritants and corrosive. They are also classified as Group 1 carcinogens by the IARC,1 i.e., they are carcinogenic to humans. Dichromate salts can affect the respiratory tract, gastrointestinal tract and immune system and can also damage fertility or the unborn child. They are sensitisers, i.e., they can cause an allergic reaction, and are toxic for the environment.
Exposure to dichromate salts can be minimised by using the reagents in micro quantities, as solutions of low concentration, and by handling the solutions for only a short period of time.
Preparation of solutions of dichromate salts
To avoid the handling of the very hazardous solid chemical, and particularly to minimise laboratory technicians’ exposure to potassium dichromate, Science ASSIST recommends the purchase of commercially available dilute (e.g., 0.1 M) potassium dichromate solution, rather than the solid chemical.
However, if you are considering using the pure chemical to prepare solutions, then Science ASSIST recommends the following guidelines should be adhered to.
- A site-specific risk assessment should be conducted before using chromium (VI) salts. See the Science ASSIST Risk Assessment Template.
- Suitable PPE such as protective clothing, nitrile gloves, closed-in shoes and safety glasses should be worn.
- The chemical should only be used under an operating fume cupboard.
- Always wash hands immediately after handling the chemical.
- Pregnant women should carefully consider the risks before proceeding.
- Stocks of sodium and potassium dichromate should be kept to a minimum and only one bottle of either the sodium or potassium salt should be stored at any one time.
- Store the chemical in a cool, dry, well-ventilated area in suitable tightly closed containers protected from direct sunlight, moisture, heat and all sources of ignition.
- Keep the container away from incompatible chemicals (See SDS for details).
- Prepare 0.1 M solution of potassium/sodium dichromate in distilled water and keep in a properly labeled and tightly closed bottle. This solution can then be used to prepare solutions of lower concentrations.
- See previous questions related to chromates:
Use of dichromate solutions for larger scale activities
The use of potassium and sodium dichromate solutions for larger-scale, analytical activities such as titrations or for preparative-scale oxidations is not recommended as the risks associated with the high degree of exposure (large volume and time), coupled with the logistics and cost of waste disposal and clean-up outweigh the educational benefits. The drawbacks of using dichromate salt solutions for titrations are:
- the titration requires fairly large quantities of dichromate solution;
- titration techniques need skills and practice and students will need to exhibit much care to avoid skin contact during the titration procedure (A high standard of clean up would also be required to remove any spilt solution.);
- the activity would generate large quantities of contaminated glassware which would then need to be carefully rinsed to remove the hazardous chromium residues;
- the activity would also generate large volumes of waste, and as dichromate solutions are not permitted to be flushed down the sink, the employment of a licensed chemical waste disposal contractor would be required to dispose of the waste generated.
Determination of the alcohol content of wine
In a school laboratory, the alcohol content in wine can be determined by a distillation process followed by the use of a hydrometer.
The wine is first distilled to give a distillate of ethanol and water, leaving behind sugar and other impurities. Water is then added to make up the ethanol–water solution to the original volume of wine. A hydrometer is then used to determine the density of the ethanol–water mixture. Using reference tables of ethanol–water mixture densities, the percentage alcohol can then be calculated and expressed as a percentage volume per volume. For a procedure,2 see Alcohol content by distillation and using hydrometers.
Determination of the tannin content in wine
A suitable titration exercise using wine would be to determine the tannin content in red wine using potassium permanganate solution in the presence of indigo carmine solution as indicator. The amount of potassium permanganate used in the titration can be used to determine the amount of tannin in wine. For a procedure,3 see Tannin Content in Wine.
Note: Tannins are the naturally occurring polyphenols found in fruit skins, seeds, leaves and plants which give wines their characteristics bitterness, dryness and astringency. Red wines contain more tannin, although white wines can gain tannin from being aged in wooden barrels. Tannins act as a natural antioxidant to protect the wine and that is why some wines age so well.
1 International Agency for Research on Cancer, Monograph ‘Chromium (VI) compounds’, Volume 100C, 2012 https://monographs.iarc.fr/wp-content/uploads/2018/06/mono100C-9.pdf (Updated July 2018)
2 ‘Determination of Alcohol Content of Wine by Distillation followed by Density Determination by Hydrometry’. Sirromet, Senior Chemistry website http://seniorchem.com/1.%20Alcohol-Content-by-distillation.pdf (Accessed April 2016)
3 ‘Chapter 20. Vintage Titrations: Tannin in Wine’ in The Royal Society of Chemistry. 1995. In Search of more Solutions: More Ideas for Problem Solving Activities, RSC: London. Learn Chemistry website http://www.rsc.org/learn-chemistry/resource/res00000585/vintage-titrations-tannin-in-wine (Accessed April 2016)
‘Alcohol Determination by Distillation Process’. Monash Scientific website, http://www.monashscientific.com.au/AlcoholDeterminationlDistillationProcedure.htm (Accessed April 2016)
‘Potassium dichromate’ Material Safety Data Sheet, Chem Supply website, https://www.chemsupply.com.au/documents/PL0181CH5A.pdf (Aug 2011)
‘Potassium dichromate’ Safety Data Sheet, Agros Organics, Fisher Scientific website, https://www.fishersci.com/msds?productName=AC197760050 (April 2014)
‘The Differences Between Red and White Wines: Methods and Materials’. St Lawrence College website, http://web2.slc.qc.ca/jmc/w05/Wine/method.htm (Accessed April 2016)
‘What Is A Tannin? A Guide To Tannins’. Vinepair website, https://vinepair.com/wine-101/guide-to-tannins/ (Accessed April 2016)