Soap & pH
What is pH?
pH stands for the "potential of hydrogen". The pH scale is a log scale, which means for every 1 unit change there is a 10 fold change in the number of hydrogen atoms present. pH is determined by the balance between the acid making H+ and alkali forming OH-. Combine these two together and you get H2O, water, which is neutral when pure.
- pH 4 is quite acid (lots of free / available hydrogen atoms )
- pH 5.5 is normal skin, which is caused by a combination of sweat and sebum.
- pH 6 is slightly acid (some free hydrogen)
- pH 7 is neutral: the acid and alkali components balance out: in a perfect world skin should be about pH 7.0 but does vary from individual to individual.
- pH 10 is quite alkaline (opposite to acid).
Because soap is made by reacting a strong alkali with oil, there is often residual alkali left behind, hence soap tends to be alkaline in nature. Neutral soap can be made by adding a bit of acid to neutralize the alkalinitiy (eg Citric acid such as lemon juice) or by particular care in the soap making process and addition of other materials.
As soap is usually alkali, it removes the "acid mantle" of the skin, making the skin initially feel clean and fresh. Over-cleaning is not good though, as this "acid mantle" is part of the body's natural defence mechanism against fungal or bacterial attack
So a soap with an absolute minimum of additives may not necessarily be the best for your skin (as different skin types respond to different types of soap): A largely natural soap may not be the optimal pH for your skin type. But it is very likely that many sensitive skins do react to the commercial additives in mass produced soap. These additives are used to improve hardness, shelf life, are often preservative in nature and alter texture and skin "feel" properties.
It is always best to find something that suits your particular skin type / individual tastes and stick with that. This usually involves a bit of trial and error.