pH Scale

acid and a base, depending on the environment it is in. Figure 54 describes amphoteric molecules:

PH SCALE

Figure 54.

The pH is very important when it comes to recognizing whether or not a substance is an acid or base in solution chemistry. This is based on the concentration of hydrogen ions in a solution. The basic formula is that the pH equals the negative log of the hydrogen ion concentration. It is generally thought of as a scale from 1 to 14 but it is somewhat incorrect, as you’ll soon see. A pH of 7 is directly neutral, while a pH of 1-7 is acidic and a pH of 7-14 is considered basic. Figure 55 shows the pH scale:

Figure 55.

Because water is amphoteric, it can be considered acidic or basic. The fact of the matter is that two H2O molecules can make one H3O+ molecule and one OH- molecule. This is referred to as autoionization or self-ionization with equal concentrations of H3O+ and OH. This means that the molarity of each of these ions is 1 x 10-7 . Remember that the Kw of water is the multiple of each of these two ions so that it is 1 x 10-14 . This number should stick out because the maximum of the pH scale is 14. If an acid is added to water, the equilibrium shifts to a decrease in OH- ions. If a base is added to water, the equilibrium shifts to the right and there are more OH- ions.

Because of the Kw of water, the pKw is going to be 14. The pH becomes the negative logarithm of the molarity of hydrogen ions in water, while the pOH becomes the negative logarithm of the hydroxide ion concentration. The pH scale is considered logarithmic. This means that every increase or decrease of the pH by one is a change in concentration by tenfold of the hydrogen ion. A pH of 3 is 100 times more acidic than a pH of 5.