The Halogens and the Sea

There are a few references to lab work in this post – I do need to stress that any labs you carry out need to be under the supervision of one of your teachers (and risk assessed by the teacher) and carried out in school.

I have always been fascinated by the link with the number of substances dissolved in the sea. For example, gold ions are dissolved in the sea and it is estimated that there are getting on for 20 million tonnes of it in the sea! [That’s 15 trillion dollars]. But I digress, this article will focus not on an element but a group of elements.

The group in question is Group 17 or the Halogens. Unfortunately, the top element Fluorine and bottom element, Astatine are not included. Fluorine, because it is too reactive and too dangerous and Astatine because it is too radioactive and rare.

Chloride ions are one of the key components of seawater (ie, salt water, sodium chloride) and electrolyzing the seawater could produce chlorine gas.

That said, the sea does also contain a lot of bromide ions as well. Bromine is commercially extracted from sea water and I found a great link to a website explaining this process in some detail here.

Essentially, a redox reaction is carried out. Chlorine gas (possibly made from the method above) is bubbled through seawater. The chlorine molecules are reduced to chloride ions and in doing so; bromide ions are oxidized to bromine molecules – so one would imagine that a brown vapour is observed (I don’t believe bromine is very water soluble).

Finally iodine. There is not much iodine in seawater but there is lots of it in seaweed. The iodine can be extracted by heating the seaweed so it turns into an ash, adding boiling water to remove the iodide ions and forming iodine through a redox reaction involving the reduction of hydrogen peroxide (to water; H₂O₂ + 2H⁺ + 2e –> 2H₂O) and the oxidation of iodide ions to iodine in acidic conditions. The iodine will turn the water brown but the characteristic color of iodine can still be observed by the addition of cyclohexane and then by evapourating it off.

I have never carried out this lab but it is on my list of ones to try. You can find much more information on this lab from the RSC (Royal Society of Chemistry) here.

Have you carried out any of these experiments? If so, please share your findings with us below. We would love to read them!

Image taken from: “Ascophyllum nodosum” by Dozen. Licensed under CC BY 2.5 via Commons.