pH & Buffer solution

pH & Buffer solution

Acids

definition: 

these are substances that give hydrogen ions or proton in solution.

Acid is a proton donor e.g. HCl------------> H ⁺ + Cl^-

Types:

1. Strong acids: e.g. HCL gives a large number of protons (H⁺) and dissociates completely in solution. 
2. Weak acids :e.g. Acetic acid gives a low number of protons (H⁺) and partially dissociate in solution. 

Alkalies

definition: 

these are substances that give (OH^-) hydroxyl ions in solution

types:

1. Strong alkali: is that alkali which dissociates completely in water e.g. NaOH & KOH. 
2. Weak alkali : is that alkali which partially dissociate in water e.g Ammonium hydroxide (NH₄OH) .

Amphoteric substances

These are substances , which act as an acid (proton donor) or as a base (proton acceptor) e.g. H₂O and amino acids.

   H₂O            H₃O (base)          

   OH^- (acid)   H⁺ + H₂O         

pH

• pH of a solution: it is the negative logarithm of hydrogen ions concentration to the base 10 .  (  pH = - log [H⁺]  to the base 10 ) .

The smaller the pH ,the increase the acidity.

pH of blood

• 7.37- 7.43)) 0.03   Blood pH is normally 7.4 ±
• A decreases  in blood pH is called acidosis.
• An increases  in blood pH is called alkalosis.
• Slight change in blood pH will affect the functions of the body e.g. any enzyme needs special pH for its maximum action.
• Severe change in blood pH may lead to death
• The blood pH is kept within very narrow rang due to the presence of buffers in both blood and tissues 

Henderson – Hasselbalch equation

• This equation represents the relationship between pH and pK (acid dissociation constant) of a weak acid.
• a weak acid : HA ionizes as follows   HA= H ⁺ + A^-

• According the law of mass action :

• K = [A^-] X [H ⁺]

                                [HA]

• K [HA]  = [A^-] X [H ⁺]

By dividing both sides by [A^-]

    v   [H ⁺]  = K    [HA] 

                                            [A^-]

• By taking the  Log of both sides.

    v  Log [H ⁺]  = Log (K [HA])  = Log K + Log [HA]

                                  [A^-]                             [A^-]

• By multiplying both sides by -1

    v   - Log [H ⁺] = - Log K  - Log [HA]

                                              [A^-]                            

• Log [H ⁺] = pH
• Log K    =  PK
•  pH = pK  - Log    [HA]

                                         [A^-]

Buffers

• Buffer is a solution , which resists change in pH when an acid or alkali is added to it.
• Buffers are usually a mixture of a weak acid with salt of strong base, or a mixture of a weak base and its  salt of strong acid e.g. carbonic acid and sodium bicarbonate (H₂CO₃ / NaHCO₃)

Body buffer

Buffers of the body are either present in plasma and extracellular tissue or inside the RBCs. These buffer keep hydrogen ion concentration within narrow range suitable   0.03 for life   7.4  ±

Physiologic buffers:

•  Carbonic acid-bicarbonate buffer system H₂CO₃ -----> HCO₃^-  +  H⁺

    Phosphate buffer:  H₂PO₄ -------> PO^-₄ + H⁺

• proteins .