Hydrogen Ions, pH, and Indicators

 Background: 

            The hydrogen ion concentration [H⁺] in water solutions can vary over a wide range of values.  These values may be over 1 Molar [H⁺] or less than 1 E – 14 Molar [H⁺].  To deal with such large possible variations a mathematical conversion of H⁺ to a logarithmic value has been used to express the H⁺  concentration as a pH.

                                     pH  =  -Log₁₀[H⁺]

             As you know perhaps, a logarithm is the exponent of a base number which makes the exponential form equal to another  natural number.  For example [10² = 100], if the base number 10 has an exponent of 2, the expression is equal to 100.  The exponent 2 is the logarithm of 100.  The logarithm of 1000 would be 3  [10³ = 1000].  In the base 10 system, a difference of one in two logarithmic values would mean a 10 fold difference in the values of the two natural numbers which they represent, i.e., the logarithms of 2 and 3 represent 100 and 1000.  There are logarithm tables which will give decimal logarithmic values so that any natural number can be represented.  [10^1.699  =  50; the logarithm of 50 is 1.699].

             The variation of the value of the [H⁺] of 1 to 10^-14 molar can be expressed as pH values of 0 to 14.  Neutral water has a [H⁺] of 1 E –7 M and therefore has a pH of 7.  Acidic solutions will have pH values less than 7 and basic solutions will have pH values greater than 7.

             The pH of a solution can be used to describe the acidic or basic qualities of soil samples, cosmetics, food, beverages, etc.

             The pH can be determined by electrical devices which have electrodes which are sensitive to the hydrogen ion concentration (pH meter) or by certain compounds which change color over certain pH ranges.

             Indicators are weak organic acids or bases which change color (and structure) with a change in pH.