Putting aside the titration calculations (a subject for another day!) this is what I always get asked about when it comes to titrations:

 

What is the difference between the equivalence point and the end point?

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First off all, excuse the crappy drawing. I got a D in GCSE Art as I’m sure you can tell.

The end point is the point at which the indicator changes colour. This is what you’re watching out for when you do the  titration. On the titration curve the point where the curve becomes vertical represents the volume added to reach the end point.

The equivalence point is the mid-point of the vertical section of the titration curve. It represents the point at which the acid and alkali are present in the proportions specified by the equation.

Let me explain that a bit more

For example, if HCl was reacted with NaOH as per this equation:

HCl + NaOH –> NaCl + H2O

The equation shows a 1 to 1 ratio of HCl to NaOH, so the equivalence point would be the point at which equal moles of HCl and NaOH mixed. So, if we started with O.O5 moles of HCl in the conical flask and added NaOH from the burette, the equivalence point would be when O.O5 moles of NaOH are added.

If H2SO4 was reacted with NaOH as per this equation:

H2SO4 + 2NaOH –> Na2SO4 + 2H2O

The equation shows a 1 to 2 ratio of H2SO4 to NaOH so the equivalence point would be the point at which there is twice as many moles of NaOH as there is of H2SO4. So, if we started with O.O5 moles of H2SO4 in the conical flask and added NaOH from the burette, the equivalence point would be when O.1 moles of NaOH are added.

 

What is the relationship between the equivalence point and the end point?

 

Take a look at the diagram. As you can see, the equivalence point and the end point have the pretty much the same volume. So, the volume of acid or base (depending on what is in the burette) that you have to add to get the colour change is also the volume you need to add to reach the equivalence point.

So, when you do a titration and write down the titre, strictly speaking you are writing down the volume for the end point but you are also assuming that this volume is the volume for the equivalence point. You can use this volume in your calculations.

 

Why is the equivalence point not always 7?

 

The pH of the equivalence point is determined by the products of the reaction.

If the products are neutral the pH of the equivalence point is neutral – i.e. 7.

In the following situations one of the products either acts as an acid or a base meaning that the pH is not neutral.

 

Weak acid – strong base titrations

If you titrate a weak acid (e.g. CH3COOH) with a strong base (e.g. NaOH) the salt produced (e.g. CH3COONa) is basic and the conjugate base from the salt (CH3COO-) reacts with water. Therefore the solution produced is weakly alkaline and the pH of the equivalence point will be greater than 7.

These equations illustrate what happens:

CH3COOH + NaOH –> CH3COONa + H2O

The salt dissociates to produce the conjugate base:

CH3COONa –> CH3COO- + Na+

The conjugate base reacts with water to produce hydroxide ions:

CH3COO- + H2O –> CH3COO- + OH-

This gives a slightly alkaline pH (> 7)

 

Strong acid – weak base titrations

If you titrate a strong acid (e.g. HCl) with a weak base (e.g. NH3) the salt formed (e.g. NH4Cl) is acidic. The salt will dissociate, releasing protons and producing a solution with a pH lower than 7.

These equations illustrate what happens:

HCl + NH3 –> NH4Cl

The NH4 dissociates to release H+:

NH4+ –> NH3 + H+

This gives a slightly acidic pH (< 7)

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