In my post on magical properties of water I tried to explain why Essentia water could not have a high concentration of “active” hydrogen while also having a high pH. I thought it might be useful to write a post explaining pH. This could be considered to be part of the on-going basic concepts series.
So pH is defined as the negative log of hydrogen ion activity:
pH = -log aH+
where aH+ is the hydrogen ion activity. Where activity is the effective concentration of hydrogen ions rather than the actual concentration. This is because in solutions that contain other ions, activity and concentration are not the same. Generally the activity is equal to concentration so the equation above is typically written as:
pH = -log[H+]
Where the square brackets are used to signify concentration; one of those chemistry jargon things. Like in math you have + for adding etc.
The fact that hydrogen activity = hydrogen concentration (aH+ = [H+]) is only true in dilute solutions. It is not true if there is a lot of ions present as these alter the ionic strength, which in turn alters the aH+. Additionally, the measurement of pH is not so accurate in a concentrated solution as the additional ions interfere with the workings of pH meters.
So assuming that our solution has low ionic strength and therefore a valid pH value, what does pH mean?
pH is an indication of how many hydrogen ions are present. The lower the pH the higher the hydrogen ion concentration. Hydrogen ion concentration is an indication of acidity. So a low pH is associated with acidity. Conversely a high pH is associated with alkalinity and is an indication of a low concentration of hydrogen ions.
Conveniently, water has a pH 7 and is considered to neutral – that is water is neither acid or base as it dissociates into equal concentrations of H+* and OH− ions which just happens to be 1×10−7 mol/L giving a -log10[H+] of 7!**
pH can be used to compare the acidity of foods. So orange juice (pH 3.3-4.2) is more acidic than milk (pH 6.4-6.8). pH is also important because bacteria and other micro-organisms cannot grow or survive in an acidic environment. So if food is treated with acid will be preserved. For example, converting lactose into lactic acid reduces the pH, so yoghurt is a way of preserving milk. Sauerkraut (pH 3.3-3.6), chutneys (3.5-4.5), pickles (pH 2.6-3.8), etc. are other foods that are preserved in this way.
Foods with a low pH have a separate legal definition because of their high resistance to micro-organisms, Clostridium botulinum in particular. Acid foods are defined as foods that have a natural pH of 4.6 or less and acidified foods are defined as low-acid foods to which acids or acid foods have been added.
* It is more accurately to put H3O+ but H+ is easier to understand.
**It is interesting writing these posts as I realize that I know need to do a post on the self-ionization of water and on acids and bases. I also want to explain acidity and buffers as these concepts are important in food science.
The FDA has a great list of foods and their pH.