PH acidity/alkalinity etc

[Loosehead]

Now I'm not usually too bad on sciencey things but I have never really understood the whole acid/alkaline/pH thing. There are lots of explanations on the web of course but none so far have rang the bell. I know all about the ph scale etc etc but what is happenning at a molecular level? what is actually measured? What actuallyh is pH, what does it stand for? what happens at a chemical level etc etc. Thanks

[Cheeris]

There are different ways of describing pH or acidity/alkalinity at a molecular level, even at university degree level. Probably the simplest way is to describe it is in terms of the levels of positively charged ions in a solution.

Chemicals in solution frequently dissassociate into charged ions For example, as everybody knows the chemical formula of common salt is NaCl. In water, however, it breaks up into Na+ and Cl-. An acidic chemical, such as hydrochloric acid HCl breaks down into H+ and Cl- in solution, and it is the positively charged hydrogen ions (H+) that cause acidity (acidity really being nothing more a name than certain chemical's ability to cause a particular type of chemical reaction). The higher the level of H+ ions, the the more acidic the solution and lower the pH.

H+ ions can be counteracted and cancelled out by negatively charged 'OH-' ions, however. A chemical with high levels of OH- ions is called alkaline, as it tends to cause the opposite type of reactions to an acid.

This is why there is a separation between 'acid' and 'alkaline', even though they are just different parts of a spectrum of chemical activity. An acidic solution has more H+ ions than OH- ions while an alkaline solution would have more OH- ions than H+ ions. Water is exactly neutral because it has exactly equal numbers of H+ and OH- ions (H+ plus OH- = H2O!).

Hope this helps; it is a bit wordy and simplistic at the same time! Perhaps other more knowledgable users can step in and clarify things?

[Loosehead]

That's great Cheeris, just one thing, presumably the H in pH stands for hydrogen, what does the small p stand for?

[squarebear]

potential.

[Cheeris]

Indeed yes, sorry! Meant to put that in my original post, but pH is short for 'potential of hydrogen'.

[Loosehead]

So what is happenning when a strong acid for example is eating through metal ?

[Teddio]

Ph is defined as the negative logarithm to base 10 of the hydrogen ion concentration.
In simple terms, a solution with a high concentration of hydrogen ions is acidic and a solution with a very low concentration of hydrogen ions is alkaline.
When an acid "eats" away at a metal such as zinc, the positively charged hydrogen ions remove electrons from the zinc atoms. Positively charged zinc ions are formed which go into solution (the zinc metal is seen to dissolve) and hydrogen gas is given off.

[Loosehead]

Thanks all, I now have a much better understanding

[newtron]

Actually, p stands for "power" http://antoine.frostburg.edu/chem/senese/101/a cidbase/faq/what-is-pH.shtml

In chemistry a small p is used in many different terms and it always stands for the negative log of what ever it is in front of. For example pKa is the negative log of an acid equilibrium constant (Ka).