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Chemistry


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Friday Chemistry Fix – Contact Lens Cleaner & Potato

Storm in a shot glass.

Storm in a shot glass.

It seems that any kind of scheduling of blog posts has gone out of the window this year, which is due to a number of smaller reasons rather than 1 big & juicy one, and I can only apologise and ask you to bear with us. However, even I get itchy fingers for some practical chemistry in large intervals, and it is Friday, so here is another Friday Chemistry Fix.

Things you need.

Things you need.

You will need:

A couple of glass vessels – shot glasses or jam jars will work

A kitchen knife

A grater

A suitable work surface (stainless steel sink or a glass surface protector; I tend to use my glass hob, which is of course turned off and cold)

Washing-up liquid

Contact lens cleaner which contains peroxide (others will not work, but these are usually marked clearly)

A potato

(I’m not endorsing the shops from which I have bought these things and many other brands are available…)

You should think about:

Safety: You will only use things you are likely to find around the house already or can buy without restrictions, but that should not make you complacent.

Hydrogen peroxide is classed as harmful, irritant to skin and eyes (can cause burns) and corrosive. It can also cause respiratory irritation. The type of contact lens cleaner solution we use here is a 3 % solution, so it contains only a small amount of hydrogen peroxide, which helps to minimise these risks. Nevertheless, you should carefully read the information provided with the solution and especially avoid contact with skin and especially eyes, washing with plenty of water if contact occurs. If the irritation persists, seek medical advice.

Safety information on lens cleaner kit.

Safety information on lens cleaner kit.

Washing-up liquid is also a mild irritant and you should avoid eye and prolonged skin contact. And the potato becomes unfit for human consumption once it has been exposed to the hydrogen peroxide and washing-up liquid, so should be thrown away to general household waste. The worms in your compost bin will not like it any more, either.

Ideally, wear gloves and safety specs (people will take you more seriously that way, too), and work on a sturdy surface, but at the very least use small amounts and wash immediately if you get anything on your skin or face.

Work space and activity: Hydrogen peroxide is oxidising, meaning that it releases oxygen when it decomposes, which is flammable, so keep this away from open flames. Its decomposition can cause a buildup of pressure, so do no seal the reaction vessel or leave the contact lens cleaner solution in contact with any catalysts (metals, potatoes etc.). In addition, hydrogen peroxide can bleach fabrics and carpets, so you should work on a suitable surface – I used my glass kitchen hob (turned off, of course) mainly to make the photos stand out, but a stainless steel sink would have been better in case the reaction vessel is too small.  I certainly would avoid working over a carpeted area or on unfinished wood, and have an old cloth or kitchen roll to hand to wipe up any spillage.

You will need to use a kitchen knife and grater, so there is a risk of cuts and scrapes. Get help if you are not confident with sharp objects, children should be supervised by an adult, and if you cut yourself, wash out the wound and dress as required.

And finally, if you break anything, take care not to cut yourself while picking up the pieces and wrap them in newspaper before disposal.

Good to go:

Step 1: Put some contact lens cleaner solution into each of 2 glass vessels. You will not need a lot, but it should cover the bottom and be at least a few millimeters high. If the glass is clean, you should not see any reaction (bubbles) yet.

Just lens cleaner solution

Just lens cleaner solution

Step 2: Add a bit of washing-up liquid to each of these glasses. This will be more viscous (gloopy, for want of a better description) than the contact lens cleaner, but you should not see any vigorous bubbling.

Lens cleaner (peroxide) and washing-up liquid.

Lens cleaner (peroxide) and washing-up liquid.

Step 3: Cut a small piece off your potato. Carefully drop it into one of the glasses and observe what happens.

Potato pieces

Potato pieces

Reaction with sliced potato piece

Reaction with sliced potato piece.

You should see small trails of bubbles forming all over the cut surfaces (although almost none in areas where there is still potato skin). Over time, you will also see a layer of foam on top of the liquid, where the bubbles have become trapped in the washing up liquid.

Step 4: Carefully grate a fresh small piece of potato and put the grated potato in the other glass. Again, observe whether there is any reaction and compare the amount of foam formed with the first reaction.

Grated potato.

Grated potato.

Reaction with grated potato.

Reaction with grated potato.

You should see a much more vigorous reaction occuring, producing more foam than with the cut potato.

Step 5 (optional): If your contact lens cleaning kit comes with a special “neutralising” lens case, prepare another glass vessel with lens cleaner solution and washing-up liquid and immerse the grey disc of the lens case into this mixture. Once more, observe the reaction.

Reaction on "neutralising disk" of lens cleaner case.

Reaction on “neutralising disc” of lens cleaner case.

Step 6 (also optional): Do your best scientist laugh, cackle or snigger because you have just observed some cool chemistry.

<Mwhahahahaha!>

Step 7: Wash your solution down the sink with lots of water and clean up after yourself. Throw away the potato pieces.

What just happened?

As indicated on the packet, the contact lens cleaning solution contains 3 % hydrogen peroxide. The molecular formula of hydrogen peroxide is H2O2, which you might think looks quite a lot like the molecular formula of water (H2O). At room temperature and low concentrations, hydrogen peroxide decomposes very slowly to form water and oxygen. The reaction equation for this process is:

2 H2O2 (aq) -> 2 H2O (l) + O2 (g)

At higher concentrations of hydrogen peroxide, this reaction also becomes warm, and we call it exothermic because it releases heat. This indicates that the products (water and oxygen) are more favourable than the starting material (hydrogen peroxide). You might now be wondering why there is any hydrogen peroxide at all, given that it decomposes, albeit slowly when it has no help. The answer is that there is a barrier to the reaction (an activation energy), which slows down the reaction and so prevents complete decomposition.

However, in the experiments here the reaction was sped up when potato was added. Potato (along with celery, liver, yeast and many other foods and living organisms) contains an enzyme called catalase, which catalyses the decomposition of hydrogen peroxide (quite damaging) to water and oxygen. Catalysis is important to many areas of chemistry (it is also my main area of research), as it changes the speed of a reaction (reaction rate). This is achieved by following a different reaction pathway, which has a lower activation energy. In addition, the catalyst is not used up by the reaction and left unchanged, so it can be used many times, or, in another way of thinking about this, you do not have to add very much. Catalysts can be destroyed or deactivated by other reactions, though, so even this good thing will not last forever. In addition, where multiple reaction pathways exist in competition, catalysts can be used to favour one possible outcome over many others, making reactions selective.

Comparing the two reactions (sliced (left) and grated (right) potato).

Comparing the two reactions (sliced (left) and grated (right) potato).

Here, we released a little bit of catalse on the freshly cut surfaces of the potato, while grating it increased the surface area substantially, so more catalase became available to decompose the hydrogen peroxide. While the same weight of fresh potato should contain about the same amount of catalase, I am not sure the cut potato would catch up with respect to the foam produced – it is likely that the surface will become clogged up with inactive catalase, so the fresh one inside the potato piece will not become used up in the same way as when using grated potato.

Catalysis by lens case (well, the metal disc at the bottom).

Catalysis by lens case (well, the metal disc at the bottom).

If you have also tried the grey disc at the bottom of the special lens case, it is likely that this was made from a noble metal. Metal surfaces can also catalyse the decomposition of hydrogen peroxide and this is a useful way of using up any hydrogen peroxide when cleaning your contacts, thus preventing any leftovers from reaching your eyes.

Metal catalysts.

Metal catalysts.

And just to show how much faith I have in my research specialism (and I have to say, even some of my colleagues though this was brave), I repeated the reaction with my wedding ring. Gold is a good catalyst for this reaction, even when it is plated with rhodium as is likely the case for my white gold ring (although after 15 years of wear, perhaps not much rhodium is left), and it should not be consumed by the reaction. As you can see in the photo, it does work, producing trails of little bubbles and foam, and after a thorough wash, the ring is back in its accustomed place.

Catalysis by wedding ring.

Catalysis by wedding ring.

There are many extensions to this experiment, testing peroxide solutions of different concentrations (hydrogen peroxide is also present in some hair dyes and sold to clean wounds), as well as a range of catalysts, such as celery and uncooked liver. You can make it quantitative by measuring a standard amount of hydrogen peroxide solution in a small measuring jug or measuring spoon and then observe with a stopwatch how long it takes to produce a given amount of foam – this will allow comparison of different experimental setups. And if you do a Google search for “elephant toothpaste”, there will be further ideas online. Happy experimenting.

Contributor: Natalie Fey

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