In chemistry, pH (potential of hydrogen) is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions.

1. Physiochemical principles
Senior Lecturer
UIMLT, FAHS

2. Hydrogen ion concentration
Hydrogen ion concentration is the composition of hydrogen ions in a
solution.
The acidic, basic and neutral character of a solution can be found
out by the concentration of hydrogen ions [H+].
The large bracket sign, [ ], indicates molar concentration (mol/L).

3. pH
pH is defined as the negative logarithm (base 10) of hydrogen ion
concentration in moles/L.
The acidic or basic property of substances is measured in terms of pH. It
is a measurement of the hydrogen ion concentration.

4. The concept of pH was introduced by a Danish chemist Soren Peder
Lauritz Sorensen.

5. pH is a way of expressing the hydrogen ion concentration in water. It
is related to the acidic or alkaline nature of water.
Consideration of hydrogen ion concentration is important in almost all
uses of water.
In particular, pH balance is important in maintaining desirable
aquatic ecological conditions in natural waters.

6. Naturally, the pH of water lies between 6.5 and 8.
The pH scale
Pure water dissociates to yield 10-7 moles/L of H+ at 25 °C
H2O ⇔ H+ + OH-
Since water dissociates to produce one OH- ion for each H+ ion. It is
obvious that 10-7 OH ions are produced simultaneously.

7. The product of [H+] and [OH-] always remains constant even if the value
for one of the species changes.
[H+] x [OH-] = 10-14
The pH scale is usually represented as ranging from 0 to 14, with pH 7 at
25 °C representing neutrality.
Acid conditions increase as pH values decrease and alkaline conditions
increase as pH values increase.

9. Acidity and alkalinity
Acidity is a measure of a solution’s capacity to react with a strong
base (usually sodium hydroxide, NaOH) to a fixed (predetermined) pH
value.
Alkalinity is the measure of a solution’s capacity to react with a strong
acid (usually sulfuric acid H2SO4) to a predetermined pH.

10. Acidity or alkalinity of a solution is measured by concentration of
hydrogen ions (H+) versus hydroxyl ions (OH-) and is expressed as
pH level.

11. Indicators
Indicators are those substances that explain if a substance is an acid, a base
or neutral.
Changes colour when in contact with acidic, basic or neutral solutions.
Some of the most common indicators are:
Litmus paper
pH strips
Universal indicator
Phenophthalein

12. Litmus paper
Litmus paper comes in two colors, red and blue, and is useful for
finding out if the solution is an acid or a base although it doesn't tell
you the strength.
To use litmus paper you just dip the bottom of a strip into your
solution.
Red litmus paper will turn blue in a base but will stay red in an acid
or neutral solution.

13. Blue litmus paper will turn red in an acid but stays blue in a base or a
neutral solution.
So if you test a solution with one color paper and it changes color you
don't have to do another test. If it doesn't change color though you
have to test it with the other color paper to test if the solution is
possibly neutral.

15. pH strips
pH strips work similarly to litmus paper, but have
additional advantage in determining the pH of the solution.
To use a pH strip you dip the end with the three square into the
solution so that all three squares are covered. Then take out the strip
and compare it to the key on the box to find the most similar color
match. This gives a number value of the pH.

17. Universal indicator
Universal indicator is a liquid indicator, while it won't give an exact
number, can show the relative strength of the solution.
Universal Indicator can be made out of red cabbage.
To use Universal indicator you add a few drops to the solution and see
what color it turns.
Universal indicator will be red in a strong acid, purple in a strong
base, and green in a neutral solution.

18. The picture below shows how the color changes with different pH
values.

19. Phenophthalein
Phenophthalein is another liquid indicator that is similar to universal
indicator.
This is the indicator that is used in titrations as its color change is very
quick and obvious.
In a base, phenolphthalein turns pink and in a neutral solution it's
clear. To use this indicator you just add a few drops to your liquid
solution.

21. Buffers
A buffer is a chemical solution that is resistant to changes in its pH as
its pH is very stable.
Resist change of pH upon addition of small amount of acid or alkali.

22. A buffer is made by adding large volumes of a weak acid or base
to its conjugate acid or base as they can be in the same solution
without neutralizing each other. This ensures that if any acid or
base is added to the buffer it will be neutralized and
not significantly change the pH of the buffer.
Buffers keep things at a specific pH to allow them function correctly
and be used efficiently.