Arrhenius defined acids as substances that, when dissolved in water dissociate into ions. One of these ions is positive (cation) and is always the H+. The other ion is negative (anion) and depends on the nature of the substance.
Example:
HCl → H+ + Cl-
Moreover, he defined the bases as substances that, when dissolved in water, dissociate into ions. One of these ions is negative and is always OH-. The other ion is positive and depends on the nature of the substance.
Example:
NaOH → Na+ + OH-
There exist more definitions and theories about the acid-base behavior of substances (such as the theory proposed in 1923 by Bronsted and Lowry).
A practical way to obtain a measure of the acidity or basicity of a substance is using the pH scale (the French "pouvoir hydrogenate", i.e., power of hydrogen). This scale comprises values between 0 and 14 units. To measure this value, it is necessary to know the concentration of H+ ions, since it is calculated by the expression:
pH = -log[H+]
If a substance has a pH value within the interval [0,7) it exhibits acidic properties (being more acidic the smaller its pH value). Conversely, the pH range of a basic substance is within the range (7,14] (where the higher the pH, the more basic it is). The pH = 7 value is not included within any interval because it corresponds to a neutral pH (as may be the case of water).
We may sometimes need to know experimentally if a substance has acidic or basic nature. In view of what has been studied, we could go to the definition proposed by the Swedish chemist Arrhenius and dissociate the unknown substance in water in order to analyze what kind of ions emerge. Another possibility would be to use the properties of these substances and verify, for instance, whether or not it reacts with metals.
In practice, given the complexity of the proposals described above, it is customary to use substances known as indicators. Normally, these organic substances when added to a substance change the colour of the latter depending on pH.
There exist a variety of indicators but the most commonly used are bromothymol blue, phenolphthalein and universal indicator paper. When added to an acid solution bromothymol blue takes a characteristic yellow colour, while when added to a basic solution it turns blue. Phenolphthalein, when added to an acidic solution, does not acquire any colour (remains colourless); however, when added to a basic solution, phenolphthalein turns deep pink. The universal paper indicator turns deep red and deep blue when added to an acidic solution and basic solution, respectively,