The formal potential of `Fe^(3+)//Fe^(2+)` in a sulphuric acid and phosphoric acid mixture `(E^@= + 0.61 V)` is much lower than the standard potential (`E^@` =+0.77 V). This is due to
(i) formation of the species `[FeHPO_4]^+`
(ii) lowering of potential upon complexation
(iii) formation of the species `[FeSO_4]^+`
(iv) high acidity of the medium.

Options

A. (i) and (ii) only
B. (i), (ii) and (iv) only
C. (iii) only
D. all of these.

Explanation

To solve the question regarding the formal potential of the `Fe^(3+)//Fe^(2+)` couple in a sulfuric acid and phosphoric acid mixture, we need to analyze the provided options and the reasons behind the observed lower formal potential compared to the standard potential.

### Step-by-Step Solution:

1. **Understanding Standard Potential**:
The standard potential (`E^@`) for the `Fe^(3+)//Fe^(2+)` couple is +0.77 V. This value is determined under standard conditions, where the concentration of ions is 1 M and the temperature is typically 25°C.

**Hint**: Recall that standard potentials are measured under specific conditions, and deviations can occur due to changes in concentration or medium.

2. **Analyzing the Given Formal Potential**:
The formal potential in the mixture of sulfuric acid and phosphoric acid is given as +0.61 V, which is significantly lower than the standard potential. This indicates that the conditions in the solution are affecting the electrochemical behavior of the iron ions.

**Hint**: Look for factors that can influence the electrochemical potential, such as concentration changes and complex formation.

3. **Considering the Role of Acidity**:
Both sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄) are strong acids. The presence of these acids increases the concentration of protons (H⁺) in the solution, which can influence the reduction potential of the `Fe^(3+)//Fe^(2+)` couple.

**Hint**: Strong acids can lead to increased proton concentration, which can affect the equilibrium of redox reactions.

4. **Complex Formation**:
The formation of complexes such as `[FeHPO₄]⁺` can occur when iron ions interact with the acidic medium. This complexation reduces the effective concentration of free `Fe^(3+)` ions in solution, which in turn lowers the formal potential.

**Hint**: Complexation can lead to a decrease in the concentration of the active species, affecting the overall potential.

5. **Evaluating the Options**:
- **(i)** Formation of the species `[FeHPO₄]⁺`: This is likely correct as it explains the decrease in concentration of `Fe^(3+)`.
- **(ii)** Lowering of potential upon complexation: This is also correct as complexation reduces the effective concentration of `Fe^(3+)`.
- **(iii)** Formation of the species `[FeSO₄]⁺`: This is incorrect as sulfuric acid does not significantly react with iron ions in this context.
- **(iv)** High acidity of the medium: This is correct as the increased acidity affects the reduction potential.

6. **Conclusion**:
The correct answer is that the lower formal potential is due to the formation of the species `[FeHPO₄]⁺`, lowering of potential upon complexation, and the high acidity of the medium. Therefore, the best answer is option **(i)**, **(ii)**, and **(iv)**.

### Final Answer:
The correct options are (i), (ii), and (iv).