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The pH scale

Concentration of H+ ions impacts most of chemical reactions. Depending on their concentration hydrogen peroxide can behave as AN oxidizing or A reducing agent. Pepsine - one of the enzymes used for digestion - works best in strongly acidic conditions and is inactive in neutral solutions. There are flowers which are either pink or blue, depending on the acidity of the soil they grow in. Even tea changes its color when you add a slice of lemon.

Acidity of the solution is so important it was convenient to create a special pH scale for its measurements. This pH scale uses Sørensen's pH definition. Concentration of H+ is usually confined to 1-10-14M range. Thus pH scale contains values falling between 0 and 14. In some rare cases you may see pH lower than 0 or higher than 14, when the concentration of H+ take some extreme values.

On the pH scale pure water at 35°C has pH 7.00, although you will probably never see water pure enough for such a pH value. Air always contains small amounts of carbon dioxide which dissolves in water making it slightly acidic - with pH of about 5.7. All values on the pH scale lower than 7 denote solutions that are acidic - the lower the pH, the more acidic the solution. On the contrary solutions with pH above 7 are basic - the higher the pH the more basic the solution is.

The pH scale is used very extensively in the chemistry, biochemistry and biology. Please try to remember several pH scale values for reference:

pH scale - example values
substancepH scale value
battery acidpH below 1
gastric juicepH about 2
orange juicepH between 3 and 4
milkpH about 6.5
bloodpH between 7.34 and 7.45, very precisely kept
in this range, as the correct pH is crucial for the survival
normal soappH about 9-10
bleachpH about 12.5

There are two things worth of remembering about the pH scale. First, as the pH scale is logarithmic 1 unit pH change means tenfold change in the H+ ion concentration. Second, while only solution with pH=7.00 is strictly neutral (see water ion product page for more information on how neutral solutions pH depends on the temperature), all solutions with pH in the range 4-10 have concentration of H+ and OH- lower than 10-4M - which can be easily disturbed with small additions of acid and base (unless pH is kept constant by some buffer).

The pH scale as described above is called sometimes the "concentration pH scale" as opposed to the "thermodynamic pH scale". Main difference between both scales is that in the theromodynamic pH scale we are interested not in H+ concentration, but in H+ activity (check out ionic strength and activity coefficients lecture). In fact what we measure in the solution - for example using pH electrodes - is the activity, not the concentration. Thus it is the thermodynamic pH scale that describes real solutions, not the concentration one.

Finally it is worth of knowing that thermodynamic pH scale can be defined not only for water solutions, but also for some other solvents, like methanol, ammonia, acetic acid and so on. Note that we are talking about pH scale defined for solutions in which given substance is the solvent, not about water solutions. Range of pH for such solvents depends on their ion product (similar to the water ion product) - and for example the pH scale for acetic acid ranges from 0 to 15.2, while the pH scale for methanol ranges from 0-16.7. You will most likely never use these pH scales in practice.

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