Question

1) Sketch four different titration curves including at least one for a polyprotic acid. On each curve indicate:

pH on one axis and concentration of _____________ on other axis.
Strength of acid and base
Equivalence point
Buffer region (if relevant)

Answer 1

the titrations can be of 4 types. each of the following with respective graphs and examples is shown below.

1 Strong acid - Strong base

titration between HCl and NaOH is a very good example. generally, acid from the burette is added to the alkali taken and the curve is represented accordingly. we can see a rapid pH change nearing to the equivalence point and at the approximate middle of the steep in the peak is determined as equivalence point.

Strong Acid Vs. Strong Base, PHO p147 - Equiralance poent volume of acid added

2. Strong acid - Weak base

HCl and NH3 are good examples. here also we run acid to the taken base.

here initially we will not have a decrease in pH but as the acid runs into the solution and near the equivalence point we can find a steep and hence the equivalence point.

Strong Acid Vs Weak Base- pt to tela Equivalance point volume of acid added.

3. Weak acid - Strong base

acetic acid and NaOH are examples of this category. as we take a strong base to which we add acid a fall in pH is visible and decreases at the equivalence point also. but after the equivalence point, we have sodium ethanoate and ethanoic acid together in the solution which serves as a buffer and any further decrease in the pH is avoided.

4. Weak acid - Weak base

acetic acid and NH3 can be taken as examples. here since both are weak we may not find an exact variation as in the above cases. and there will not any steep decrease in pH as well. so to determine an exact point is difficult.

- Weak Acid us. Weak Base : Equivalance Pant volume of aid added.

5. for polyprotic acid

let me explain with oxalic acid which is a diprotic weak acid against NaOH

oxalic acid has two protons, upon reaction with NaOH initially one of the protons will be removed completely. that will be the first reaction. we can observe a steep change in pH and an equivalence point for the completion of this reaction. only after completing the first reaction, the second proton will be removed and a similar change in pH will be observed at the equivalence point.