Practical investigations
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What are the molecules commonly found in malt vinegar that give it its dark brown colour and why do they have a greater affinity for absorption by activated charcoal than other molecules?
Igloo writes
The compounds in malt vinegar which cause it to have colour include complex carbohydrates and phenols. Small amounts of caramel are sometimes added to malt vinegar to “improve” its colour.
Through a process known as adsorption, (absorption at the surface) activated charcoal has a natural affinity for many different types of molecule which bind to its surface. Charcoal is “activated” by heating certain forms of carbon with steam to very high temperatures. In this process the charcoal forms little pores and pockets that increase its surface area. The molecules to be adsorbed slide into the holes of this honeycomb and hold fast, mostly because they “fit” in the holes but partly through some sort of chemical attraction to the carbon atoms. Many types of activated charcoal exist, and these differ in density and pore size. They are also available in powdered and pellet form, for example. The type which is appropriate depends on what you're using it for: an activated carbon with “large” holes would be best at adsorbing relatively complex organic molecules, while smaller pores would adsorb the simpler ones.
Presumably your school or college has supplies of activated charcoal appropriate for sixth-form students who wish to remove large complex molecules, such as those which cause the malt vinegar to be brown. Other molecules, e.g. water and ethanoic acid and other relatively simple components of the vinegar will not be adsorbed by this particular brand of activated charcoal.
It is worth noting that the process of adsorption is not discriminating by colour, but by size of molecule more than anything else.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 30 March 2006
