pH Science

More About pH ... A Little Chemistry

To understand more about pH, we need to know more about the chemistry of water. A molecule of water is composed of one oxygen atom and two hydrogen atoms and looks something like this.

H = Hydrogen: O = Oxygen:

Water Molecule ( H₂O ):

In pure water, most of the water molecules remain intact. However, a very small amount of them react with each other in the following manner.

H₂O + H₂O ===> H₃O⁺ + OH^–

Water + Water ===> Hydronium Ion⁺ ( an Acid ) + Hydroxyl Ion^– ( a Base )

The Hydronium ion ( H₃O⁺) is the chemical unit which accounts. for the acidic properties of a solution. The hydroxyl ion ( OH^– ) is the chemical which accounts for the basic or alkaline properties of a solution.

As you can see, when pure water reacts as described in Figure 2, it produces an equal amount of H₃O⁺ and OH^–. Thus, it does not have an excess of either ion. It is therefore called a neutral solution.

If a strong acid, such as hydrochloric acid ( HCl ) is added to water, it reacts with some of the water molecules as follows:

HCl + H₂O <=====> H₃O⁺ + Cl^–

Thus, the addition of HCl to water increases the H₃O⁺ or acid concentration of the resulting solution.

If a strong base, such as sodium hydroxide, is added to water, it ionizes as follows:

NaOH <=====> Na⁺ + OH^–

Thus, the addition of NaOH to water increases the OH^– or alkali concentration of the resulting solutions.

Another interesting aspect of water is that the concentration of H₃O⁺ and OH^– remain in balance with each other. An increase in the concentration of H₃O⁺ causes a proportional decrease in the concentration of OH^–.

Accordingly, a table can be constructed which shows the relationship of the pH's   H₃O⁺ concentration, and OH^– concentration.

Ion Activity ( Moles / liter )

         pH     H3O+ (Acid)               OH- (Base)
 
         0      1.0                       0.00000000000001
         1      0.1                       0.0000000000001
 
         |      |                         |
         |      |                         |
         |      |                         |
 
        13      0.0000000000001           0.1
        14      0.00000000000001          1.0

1. As the acid ( H₃O⁺) concentration decreases, the pH increases.

2. As the acid ( H₃O⁺) concentration decreases, the base ( OH^– ) concentration increases proportionately.

3. At pH 7 the acid ( H₃O⁺) and base ( OH^– ) concentrations are equal. This is called the neutral point.

4. The pH scale represents the number of places the decimal point is moved to the left of one in expressing the acid ( H₃O⁺) concentration.

5. Each pH unit represents a tenfold change in H₃O⁺ or OH^– concentration. For example, solution at pH 6 is 10 times more concentrated in H₃O⁺ ions than a solution at pH 7.

Thus, you can see from this chart that the pH scale is a far more concise way of quantitively expressing the acidity of a solution.

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