Which of the following statements most correctly defines the
isoelectric point?

Options

A. The pH at which all molecular species are ionised and that carry the same charge.
B. The pH at which all molecular species are neutral and uncharge.
C. The pH at which half of the molecular species are ionised and the
other half unionised.
D. The pH at which negatively and positively charged molecular species are present in equal concentration.

Explanation

To determine which statement most correctly defines the isoelectric point (pI), we can analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understanding Isoelectric Point (pI)**:
- The isoelectric point is defined as the pH at which a molecule, typically an amino acid or protein, has no net charge. This means that the positive and negative charges are balanced.

2. **Analyzing the Statements**:
- **Statement 1**: "The pH at which all molecular species are ionized and carry the same charge."
- This statement is incorrect because at the isoelectric point, the molecule does not carry the same charge; rather, it has no net charge.

- **Statement 2**: "The pH value at which the cationic concentration will exist is equal to the anionic."
- This statement is correct. At the isoelectric point, the concentrations of cationic (positively charged) and anionic (negatively charged) forms are equal, resulting in a neutral dipolar ion.

- **Statement 3**: "The pH at which all molecular are neutral and uncharged."
- This statement is misleading. While the overall charge is neutral, the species are not uncharged; they exist as charged species that balance each other out.

- **Statement 4**: "The pH at which half of the molecular are ionized, other half."
- This statement is incorrect because it implies that half are ionized and half are not, which does not accurately describe the isoelectric point.

3. **Conclusion**:
- The most accurate definition of the isoelectric point is provided in **Statement 2**, which states that it is the pH value at which the cationic concentration is equal to the anionic concentration.

### Final Answer:
The correct statement that most accurately defines the isoelectric point is: **"The pH value at which the cationic concentration will exist is equal to the anionic."**