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### Theory:

Due to the self-ionisation of water, all the aqueous solutions may contain hydrogen and hydroxyl ions. Substances dissolved in water may also form hydrogen ions or hydroxyl ions in addition to this ionisation. The amount of these ions in the solution determines whether it is acidic or basic.

The $$pH$$ scale is used to determine the concentration of hydrogen ions in a solution. The '$$p$$' in $$pH$$ stands for 'Potenz,' which means 'power' in German language. $$pH$$ refers to 'Power of Hydrogen' or 'Potential of Hydrogen' or strength of hydrogen. Sorensen, a Danish biologist, devised $$pH$$ notation in $$1909$$.

The $$pH$$ scale is a set of numbers that ranges from $$0$$ to $$14$$ and is used to determine whether a solution is acidic, basic, or neutral.

$$pH$$range of acids and bases:

 pH range Acids < $$7$$ Neutral solutions $$7$$ Base > $$7$$
The $$pH$$ is the negative logarithm of the hydrogen ion concentration.

i.e, $$pH= -log_{10}[H^+]$$
The $$pH$$ values of some common acids and bases are listed below:

 Common acids $$pH$$ Common bases $$pH$$ $$HCl$$ ($$4$$%) $$0$$ Blood plasma $$7.4$$ Stomach acid or gastric acid $$1$$ Egg white $$8$$ Lemon juice $$2$$ Sea water $$8$$ Vinegar $$3$$ Baking soda $$9$$ Oranges $$3.5$$ Antacids $$10$$ Sods, grapes $$4$$ Ammonia water $$11$$ Tomato juice $$4.2$$ Lime water $$12$$ Sour milk $$4.5$$ Drain cleaner $$13$$ Fresh milk $$5$$ Caustic soda $$4$$% ($$NaOH$$) $$14$$ Human saliva $$6 - 8$$ Milk of magnesia $$10$$ Pure water $$7$$ Coffee $$5.6$$