Introduction to pH, acids and bases

Although everything I’m about to write you can find on the internet in a more detailed form, and probably with some better explanations, I still thought it would be nice to have a place in Marie’s forum to talk about pH and help each other understand better what’s it all about. In the end of the post I’ve added a “Dictionary” list for any new terminology you’d encounter.

But before we talk about pH, let’s talk about WATER.

In any aqueous solution, a very small number of water molecules will dissociate to form hydrogen (H+) and hydroxide (OH-) ions:

The number of ions formed is small. At 25°C fewer than 2×10^-7% of the water molecules will dissociate. In terms of molar concentrations, water at 25°C contains 1×10-7 moles per liter of hydrogen ions and the same concentration of hydroxide ions.

In any aqueous solution, the concentration of hydrogen ions multiplied by the concentration of hydroxide ions is constant. Stated in equation form:

where the brackets signify molar concentrations and Kw is the dissociation constant for water.

The value of Kw depends on temperature. For example, at 25°C Kw = 1.00×10^-14 and at 35°C Kw=1.47×10^-14.

Now let’s talk about ACIDS and BASES.

Acids and bases are molecules, which when dissolved in water, simply alter the relative amounts of H+ and OH- in solution. Acids increase the hydrogen ion concentration, and, because the product [H+] [OH-] must remain constant, acids decrease the hydroxide ion concentration. Bases have the opposite effect. They increase hydroxide ion concentration and decrease hydrogen ion concentration.

For example, suppose an acid is added to water at 25°C and the acid raises the H+ concentration to 1.0×10^-4 M. Because the product [H+][OH-] must always equal 1.00×10^-14, [OH-] will be 1.0×10^-10 M.

Finally we get to pH definition.

pH is another way of expressing the hydrogen ion concentration. pH is defined as follows:

Which means that the pH is negative logarithmic scale of the H+ concentration. Therefore, if the hydrogen ion concentration is 1.0×10^-4 M, the pH is 4.00. The LOWER the pH, the more ACIDIC the solution is considered. The HIGHER the pH, the more BASIC it is considered.

The term NEUTRAL is often used in discussions about acids, bases, and pH. A neutral solution is one in which the hydrogen ion concentration exactly equals the hydroxide ion concentration. At 25°C, a neutral solution has pH 7.00. At 35°C, a neutral solution has pH 6.92. The common assertion that neutral solutions have pH 7 is not true. The statement is true only if the temperature is 25°C.

So to sum up:
1. Water constantly present as water molecules and some small amount of H+ and OH- ions.
2. At 25°C water contains 1×10^-7 M of H+ and OH- ions, which gives the known “neutral” pH of 7.00.
3. Acids and bases affect the pH by altering the relative amounts of H+ and OH- ions. Acids decrease the pH, making the solution more acidic. Bases increase the pH, making the solution more basic.

Next we’ll talk about specific acids and how to calculate the amount of the acid you’ll need to use in order to get a specific pH.

Dictionary:
– Aqueous solution – liquid solution, which contains water
– Ions – atoms or molecules having a positive or a negative charge
– Molar concentration – is a measure of the concentration of any chemical species, in terms of amount of substance in a given volume. A commonly used unit for molar concentration in chemistry is the MOLAR which is defined as the number of moles per liter (unit symbol: mol/L or M). A solution with a concentration of 1 mol/L is equivalent to 1 molar (1M).