An acid trip

I would like to say that I have a good excuse for not posting anything for so long; however, I would be kidding myself. New Year’s resolution therefore, more blog posts!

To start the year I am going to write about something everyone will have heard of, but may not know the details, pH. If your secondary school was anything like mine, you probably encountered this at around the second science lesson of year 7 (age 11/12). The previous week you had boiled water with a Bunsen burner, while, of course, maintaining safe practice and possibly even obtaining a ‘Bunsen burner licence’. At no point did you melt biros and burn the desk… me neither. This week’s (equally exciting) practical lesson involved dripping a greenish slime-coloured substance into various liquids and looking to see if there was a colour change. Of course, you were measuring their pH.

But what is pH? Most people will tell you it is a number from 1 to 14, with low numbers meaning acid, high numbers meaning alkali and 7 meaning neutral. However, what is an acid? What is an alkali? And what do these numbers actually mean? This isn’t properly addressed until A-level chemistry (6 years after that second science lesson). The correct (but complicated sounding) answer is that this number, the pH value, is the negative logarithm of the concentration of hydrogen ions in a solution. A statement that raises more questions than it answers.

A solution, by definition is a mixture of a solute dissolved in a solvent. For example, sea water is a solution of salt (solute) dissolved in water (solvent). It is common knowledge that the chemical formula of water is H2O, which means that two hydrogen atoms are bound to one oxygen atom. Every atom is made up of a nucleus (positively charged) surrounded by orbiting electrons (negatively charged). When hydrogen binds to oxygen (forming water), they share some of these negative electrons, which hold the atoms together. This is called a covalent bond.

However, in water, these bonds continually break and re-form, meaning water shifts between H2O and a mixture of separated H and OH. When this happens, the OH ‘steals’ an electron from hydrogen (H), meaning OH now has more negative charge and H becomes positively charged. Hence, they are written as H+ and OH. These charged molecules are called ions. H+ is the hydrogen ion referred to in the above definition of pH.

In a solution dissolved in water, the concentration of H+ multiplied by the concentration of OH ions is always equal to 10-14 moles/litre (1 mole = 6.02 x 1023 atoms). This is called the ionic product of water. Therefore, the concentration of H+ ions can range from 100 to 10-14 moles/litre. The pH of a solution is equal to the negative logarithm of this concentration of H+ ions. In water, the concentrations of H+ and OH ions are both equal to 10-7 moles/litre (10-7 x 10-7 = 10-14 moles/litre). Hence, the pH is equal to the negative logarithm of 10-7, which equals 7, or neutral. As the concentration of H+ ions increases, the pH drops. For example, a strong acid that has a H+ concentration of 10-1 moles/litre will have a pH of 1. Phew.

So that’s the theory, now what about the fun part. All of this leads to some very simple experiments that can be done at home, to re-create that sense of wonder that you had back in that early science lesson. Acids and alkalis are common place in the home, as are pH indicators; you just need to know where to look. Examples of household acids include vinegar (acetic acid) and lemon juice (citric acid). Household alkalis are substances such as bicarbonate of soda (used in baking – weak alkali) and bleach (strong alkali). Various indicators also exist around the house, the one I recommend using, simply because it is the easiest, is the Indian spice turmeric. Simply mix about a teaspoon of turmeric powder with a little water and add it to various acids and alkalis. At a neutral pH and below, turmeric solution remains yellow; however, at a pH of 8.6 or higher, it turns reddish brown. An alternative is to crush red cabbage with a strong alcohol solution (50% ethanol). The resulting liquid gives a much better range of colours over the entire pH scale than turmeric, but the requirement of using ethanol makes this harder to obtain.

Both of these substances work as indicators due to the pigments the give them their vivid colours. In turmeric, this is called curcumin, and in red cabbage it is a mixture of pigments, hence the broader range of colours. I really like these experiments as they use readily available, everyday items and give an immediate and obvious result. Furthermore, the theory behind the topic of pH demonstrates the complexities that lie within these seemingly simple colour changes.

Using turmeric as an indicator
Using turmeric as an indicator

4 thoughts on “An acid trip”

  1. Liked the blog Ben!

    Question: What is the reaction that takes place when the tumeric is added to turn the liquid a reddish brownish colour?

  2. Hi Hannah,

    It only took a year to respond! Sorry, I had a break from the blog but have decided to start it back up again!

    The whole reason that pH indicators work is because of the fact that they will absorb reflect different parts of the visible light spectrum at different pH levels, hence the change in colour. It is the curcumin in turmeric that gives it its yellow colour at a neutral pH. When the pH rises, the increase in the concentration of OH- ions causes a change in the wavelength of light that is reflected by curcumin, hence the change in colour from yellow to reddish-brown.

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