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What makes things acid: The pH scale

I remember learning about acids and bases (or acids and alkalis) fairly early on at school. Acids were sharp vinegary substances like lemon juice, while alkalis were soapy substances, like limewater or caustic soda. We also learnt about the pH scale which measures the acidity or alkalinity of a substance. The pH scale goes from 1-14, with pH7 being completely neutral, i.e water. Anything with a very low pH is acidic, which substances with a high pH are alkaline.

It wasn’t until later that I found out what the pH scale actually meant, and when I did, it blew my mind completely. I’d always assumed that it either stood for two words or was named after someone. I did sometimes wonder why it was written like that (small p, capital H), but never enough to seriously ask about it.

For starters, that H isn’t a H. The reason it’s capitalised is because it’s the symbol for hydrogen.

The p isn’t a p either, it’s a letter used as a shorthand for a mathematical operation. To be specific, the operation “-log10″. What I’d been assuming were the letters pH was actually a scientific formula, -log10 of the concentration of hydrogen ions:

pH = – log10[H+]

With that in mind, the pH scale made a lot more sense as a measure of acidity. Acids have a few different definitions, but overall they are substances that can generate hydrogen ions when in a solution. pH is a way of showing how many hydrogen ions they release without having to count up all the ions. pH also has no units, which is very useful for people like me who find units confusing.

In order to be acidic then, a substance must contain hydrogen, in a form that can be released into water. Substances such as CH4 (methane) are not acidic as all four hydrogens are bound very tightly to the carbon and are not going anywhere. CH4 has a neutral pH, around 7. On the other hand, substances such as hydrochloric acid, HCl, are held together by polar ionic bonds and when placed into water the hydrogen will break away to form hydrogen ions, making the liquid acidic. HCl therefore has a very low pH and is a very strong acid.

Weak acids, with pH 5 or 6 are slightly more complex. These are formed when a compound can release hydrogen ions, but only very weakly. Examples are often organic compounds, which while they have many hydrogens in their chemical structure, very few of these can break away in solution. Acetic acid (HC2H3O2) is a weak acid, as only the hydrogen at the front of the equation that can dissociate, and it is not hugely energetically favourable for it to do so.

The pH scale, with a list of substances at each pH. Acids at the top, bases at the bottom. Image credit below.

I do like the fact that “pH” has an actual chemical and mathematical meaning, rather than just being a random set of letters. Although the mathematics of acidity can get a little complex the further into chemistry you go, the -log10[H+] is still the major definition of both acidity and alkalinity.

Credit link for image 2

HC2H3O2
About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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1 Comment

1. 1. Michelle 7:49 pm 12/3/2012

Its also helpful to think of pH as the amount of free protons. Afterall, that is all that a hydrogen ion is. So acids can release protons; bases can capture them. Since this is a negative log, the lower the number the higher the concentration.

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