Final Practice Explanation of Buffer

Here is my link :  https://www.youtube.com/watch?v=6w9YbU_gk7w

What is a buffer solution? 

Definition 
The buffer solution is a substance which inhibits the change in pH when a small amount of acid or base is added thereto. 

Acidic buffer solution 

The acidic buffer solution is something that has a pH of less than 7. The acidic buffer solution is usually made of a weak acid and a salt - often a sodium salt.The usual example is a mixture of ethanoic acid and sodium ethanoate in solution. In this case, if the solution contains a molar concentration comparable between acid and salt, then the mixture will have a pH of 4.76. This is not a problem in terms of concentration, as long as both have the same concentration.You can change the pH of the buffer solution by changing the acid to salt ratio, or by choosing different acids and one of its salts. 

The alkaline buffer solution 

The alkaline buffer solution has a pH above 7. The alkaline buffer solution is usually made of weak base and its salt.Often used as an example is a mixture of ammonia solution and ammonium chloride solution. If both are in a comparable molar ratio, the solution will have a pH of 9.25. Again, that is not a problem as long as your chosen concentration is the same.How does the buffer solution work?The buffer solution contains something that will remove the hydrogen ions or hydroxide ions you might add - otherwise it will change the pH. Acid and alkaline buffer solutions reach these conditions in different ways.Acidic buffer solutionWe will take a mixture of ethanoic acid and sodium ethanoate as a typical example.Ethanoic acid is a weak acid, and the equilibrium position will shift to the left:

 

The addition of sodium ethanoate to this condition adds to the excess of ethanoate ions in large quantities. Based on the Le Chatelier Principle, the equilibrium position end is then shifted to the left.
Therefore the solution will contain something important:

     Many ethanoic acid is not ionized;
     Many ethanoic ions from sodium ethanoate:
     Enough hydrogen ions to make the solution acidic.

Something else (such as water and sodium ions) is not important in explanation.
 
Addition of acid to acidic buffer solution
The buffer solution must eliminate most of the new hydrogen ions, otherwise the pH will drop considerably.
The hydrogen ions join the ethanoate ion to produce ethanoic acid. Although the reaction is reversible, because ethanoic acid is a weak acid, most of the new hydrogen ions are removed in this way.
 
Since most of the new hydrogen ions are removed, the pH will not change too much - but because the equilibrium is involved, the pH will decrease slightly.
The addition of a base to an acidic buffer solution
The alkaline solution contains hydroxide ions and the buffer solution removes the hydroxide ions.

This time the situation is a little more complicated because there are two processes that can remove hydroxide ions.
The removal of hydroxide ions by reaction with ethanoic acid
Most of the acidic substances in which hydroxide ions collide with ethanoic acid molecules. Both will react to form ethanoic ions and water.

 

Since most of the hydroxide ions are removed, the pH does not change too large.
The removal of hydroxide ions by reaction with hydrogen ions
It should be remembered that some of the existing hydrogen ions come from ionization of aetanoic acid.
  
 
The hydroxide ion can join it to form water. As long as it happens, the equilibrium tip replaces it. This remains until most of the hydrogen ions are removed.
 
 Again, because you have the equilibrium involved, not all hydroxide ions are removed - because they are too much. The water formed becomes ionized back into a very small amount of water to provide some hydrogen ions and hydroxide ions.

The alkaline buffer solution

We will take a mixture of ammonia and ammonium chloride solutions as a typical example.
Ammonia is a weak base, and the equilibrium position moves to the left:
 

 The addition of ammonium chloride under these conditions adds excess ammonium ions in large quantities. Under the Le Chatelier Principle, it will cause the equilibrium position end to shift to the left.
Therefore the solution will contain several important things:

     Many unreacted ammonia;
     Many ammonia ions from ammonium chloride;
     Simply hydrogen ions to produce an alkaline solution.

Other things (such as water and chloride ions) are not important in explanation.
The addition of acids to an alkaline buffer solution
There are two processes that can remove the hydrogen ions you add.
Elimination of hydrogen ions by reaction with ammonia
Most of the basic substances in which hydrogen ions collide with them are ammonia molecules. Both will react to form ammonium ions.

Most, but not entirely, hydrogen ions will be removed. The ammonium ion is a slightly weak acid, and therefore the hydrohen ion will be released again.
Removal of hydrogen ions by reaction with hydroxide ions
It should be remembered that some of the existing hydroxide ions come from the reaction between ammonia and water.
 

The hydrogen ion can join the hydroxide ion to produce water. As it happens, the equilibrium tip replaces the hydroxide ion. This continues until most of the hydrogen ions are removed. 
 

 Again, because you have the equilibrium involved, not all of the hydrogen ions are removed - just the majority.
The addition of a base to an alkaline buffer solution
The hydroxide ion of the alkali is removed by a simple reaction with the ammonium ion.

Because the ammonia formed is a weak base, the ammonia will react with water - and therefore the reaction is slightly reversible. This means that, again, most (but not all) of the hydrogen ions are removed from the solution.

Dilution of buffer solution 

Why is the pH of the buffer solution unchanged if diluted? To understand this, it can be reviewed from the Henderson-Hasselbalch equation.Henderson-Hasselbalch equationThe pH value of the buffer solution is determined only by pKa and the molar concentration ratio of the conjugate acid base pair. The Ka or the pKa of the weak acid does not depend on the acid concentration, but depends on the temperature. Therefore, dilution of the buffer solution will not change the pKa value 
The dilution of the buffer solution only changes the molar concentration of the species in equilibrium, but does not alter the comparison of the speciesFigure 8.9 The dilution of the buffer solution changes only the molar concentration of the species in equilibrium, but does not alter the comparison of the species.The conjugate acid molar conjugate concentration will change if the volume of the solution changes because the concentration depends on the total volume of the solution.  

Dilution of the solution will change all the concentrations of the species present in the solution, but since the concentration changes are felt by all species then the molar concentration ratio of the acid-base conjugate pairs is unchanged. As a result, the pH of the solution has not changed.