The pH is measurement of concentration of hydrogen ions in a solution or fluid. Acids generate hydrogen ions whereas Alkali removes Hydrogen ions from a solution. The quantity of hydrogen ions (H⁺)  or hydroxide ions (OH^-) in a solution determines whether the solution is acid or alkaline. When a solution has more hydrogen ions (H⁺) than hydroxide ions (OH^-), then it is acidic. When there are more hydroxide (OH^-) ions than hydrogen ions (H⁺) then the solution is alkaline.

The concentration of hydrogen ions in a solution is expressed in terms of pH scale. The pH does not have a unit, it is expressed  as a number. The pH level is expressed on a logarithmic scale with a  range from 0 (strongly acidic) to 14 (strongly alkaline).  

pH is measure of degree of acidity or alkalinity of a solution. If the pH value of a solution is less than 7 then it is acidic.  The less pH denotes that solution is more acidic.  If the pH value of  a solution is more than 7 then it is alkaline or basic. The more pH, the solution is more basic or alkaline.  

The number of hydrogen ions (H⁺) and hydroxide ions (OH^- ) is always equal in pure or distilled water  ( H₂O <--> H⁺ + OH^- ), the pH of pure or distilled water is 7 i.e. it is neutral, neither acidic nor alkaline.

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7EUTRAL

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Increasing Alkaline    Increasing Hydroxide ions (OH- )

Inceasing Acidic                            Increasisng ions  (H+)

                   Acidic            Neutral                   Alkaline

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pH scale is logarithmic expression. The small “p” denotes – log (base 10) so  pH = - log [H⁺] i.e. negative log (in base 10) of the concentration of H⁺ ions.  Thus a change of one pH unit implies a tenfold shift in the concentration of hydrogen ions.

A change of “1” pH unit is a factor of ten change in acidity/ Alkalinity.

The pH scale is logarithmic. This means numbers on the pH scale are based on powers of ten For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. The same holds good for pH values above 7, each of which is ten times more alkaline or basic than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8. pH 7 means the solution is 10 times more acidic than pH 6.

Increasing pH means the H⁺ ions are decreasing. Decreasing pH means H⁺ ions are increasing.