What is the pH of a neutral solution

A medicine or skin lotion is often described as pH neutral as though it was obviously a good thing. A solution is defined as neutral if it contains neither an excess of solvated protons nor an excess of hydroxide ions. Equation (6.4) tells us the autoprotolysis constant Kw of super-pure water (water containing no additional solute) is 10-14 (moldm-3)2. Furthermore, we saw in Worked Example 6.1 how the concentration of the solvated protons was 10-7 mol dm-3 at 298 K. 

By considering both the definition of pH in Equation (6.20) and the concentration of the solvated protons from Worked Example 6.1, we see how a sample of super-pure water - which is neutral - has a pH of 7 at 298 K. We now go further and say all neutral solutions have a pH of 7. By corollary, we need to appreciate how an acidic solution always has a pH less than 7. If the pH is exactly 7, then the solution is neutral. 

The pH of a Lowry-Brons led acid decreases as its concentration increases. Bench nitric acid of concentration 1 mol dm-3 has a pH = 0. An acid of higher concentration will, therefore, have a negative pH (the occasions when we need to employ such solutions are, thankfully, rare). 

The maximum pH of an acid will be just less than 7, at 298 K. 

The pH of a Lowry-Bronsted acid DEcre-ases as its concentration INcreases. 

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