The pH Scale
●​ Scientists identify indicators and they could determine the acidic and alkaline nature of
materials.
●​ Then they wanted to check the strength of acids and alkalis.
●​ Scientist could identify that acidic and alkaline nature of substance is determine by presence of
hydrogen ion in the solution and level of acid and alkali is related to the strength of acids and
alkalis.
●​ A Danish scientist S∅rons S∅rensen develop a method to determine strength of acidic and
alkaline nature of a solution. It’s known as pH scale and letter p(simple) and H(capital) stands for
the ‘power of Hydrogen.
❖​ pH scale runs from 0 to 14.
❖​ pH 7 is neutral.
❖​ pH Bellow 7 acidic and acid strength increase as pH value decreases. Therefore pH values
from 6- 3 are weak acids and 0-2 are strong acids.
❖​ pH value above 7 are alkaline. Alkalinity increases as pH value increases. Therefore pH
values from 8-11 are referred as weak alkalis and 12-14 are considered as strong alkalis.
●​ pH of a solution can be determine by measuring with pH meter.
●​ For general laboratory uses, pH papers or Universal indicator can be used to determine pH value
of solutions.
When a piece of pH paper is placed in solution , pH paper changes the colour (when few drops universal
indicator solution is added to a solution, it changes the colour of solution)
By comparing colour obtained with colour chart of indicator provided, pH value of the solution can be
determined. But this not very precise value like pH meter.