pH, an acronym for Potential of Hydrogen (i.e., the presence of positive ions or cations), is the index that indicates the acidity, alkalinity or neutrality of any liquid solution in which the solvent is water, which is the same as saying “aqueous”. The scale that measures it varies between 0 and 14, with the neutral point being 7. If the pH value is equal to 7, the solution is neutral; if less than 7, it is acidic; if it is greater than this value, it is base or alkaline.
However, simply increasing or decreasing one unit on the scale does not mean that the solution is only one time more alkaline or acidic. pH is measured according to a logarithmic scale, i.e. a decrease or increase of one unit is always multiplied by ten for each unit. For example, a pH of 5 is ten times more acidic than a pH of 6, and a hundred times more acidic than a pH of 7.
As for measurement methods, although in an industrial context it is common to use highly sophisticated devices that provide electronic readings, the most common way is the use of acid-base indicators, i.e., substances that change colour to indicate the result.
Among the most commonly used is phenolphthalein, which becomes colourless when added to an acidic medium and pink in an alkaline medium. All one has to do is to compare the shade with a scale with different colours to obtain the result; another is litmus paper (a mixture of different organic pigments extracted from lichens), which turns red in the presence of acids and blue in the presence of bases.
Obtaining pH values is also possible from natural elements. The spontaneous emergence of plant species in the soil is a good indicator of its acidity or alkalinity, as these species only develop under certain conditions. Another good example provided by botany is that of the hydrangea, which, when it has a blue flower, indicates an acidic soil, and when pink, an alkaline soil.