Søren Peter Lauritz Sørensen was a Danish chemist, known for the introduction of the concept of pH, a scale for measuring acidity and alkalinity.
S. P. L. Sørensen
In chemistry, pH, also referred to as acidity or basicity, historically denotes "potential of hydrogen". It is a logarithmic scale used to specify the acidity or basicity of aqueous solutions. Acidic solutions are measured to have lower pH values than basic or alkaline solutions.
Test tubes containing solutions of pH 1–10 colored with an indicator
Lemon juice tastes sour because it contains 5% to 6% citric acid and has a pH of 2.2 (high acidity).