Problem 20 - Chemistry in 30 days

Question

The isoelectric point of colloidally dispersed material is the \(\mathrm{pH}\) value at which (a) the dispersed phase migrates in an electric field (b) the dispersed phase does not migrate in an electric field (c) the dispersed phase has \(\mathrm{pH}\) equal to 7 (d) the dispersed phase has \(p \mathrm{H}\) equal to zero

Step-by-Step Solution

Answer

The correct answer is (b): the dispersed phase does not migrate in an electric field.

Step 1: Understanding the Concept

The isoelectric point is a critical concept in colloidal dispersion, involving the stabilization of colloids in an electric field. This point is characterized by the absence of net charge on the particles, meaning they do not move under an electric field. Thus, the particles are at their least soluble state, leading us to explore the options given.

Step 2: Option Analysis: Part A

In option (a), it is suggested that at the isoelectric point, the dispersed phase migrates in an electric field. This is not characteristic of the isoelectric point as described, where particles should not migrate due to the absence of net charge at this point.

Step 3: Option Analysis: Part B

Option (b) suggests that the dispersed phase does not migrate in an electric field at the isoelectric point. This is consistent with the definition, as the isoelectric point is where the particles have no net charge and therefore are immobile under an electric field.

Step 4: Option Analysis: Part C

In option (c), the dispersed phase having a pH equal to 7 is considered as the isoelectric point. However, the isoelectric point is not related to a pH of 7 specifically, but rather to a pH at which the net charge is zero. This pH can vary depending on the material.

Step 5: Option Analysis: Part D

Option (d) implies that the isoelectric point occurs when the dispersed phase has a pH of zero. This is incorrect because, similar to option (c), the isoelectric point is not determined by a specific pH value but by a pH at which the net charge is neutral, and this value is not universally zero.

Key Concepts

Colloidal Dispersion

Colloidal dispersion involves a mixture where tiny particles of one substance are evenly distributed throughout another substance. These particles are very small, often nanometers in size, and remain suspended without settling out quickly.
The stability of colloids arises from the interaction between the particles themselves and their surrounding medium, maintaining an even state of distribution.

• This suspension is possible because of forces such as gravitational force being too weak to dominate the natural tendency of particles to move chaotically (Brownian motion).
• Additionally, electrostatic repulsion usually prevents the particles from clumping together.

This stability is vital in various applications, such as in medicine, food science, and industrial processes where specific particle behavior is essential.

Electric Field

An electric field is a force field surrounding a charged particle. It represents how the particle influences other charged particles within the field.
In the context of colloidal dispersions, the electric field plays a role in determining the movement and stability of particles.

• When particles have a net charge, they experience force under an electric field, causing them to move.
• This motion can help separate materials based on charge, which is useful in various technologies.

During the isoelectric point, however, particles lose their net charge, and hence, they remain motionless in an electric field, serving as a critical tool for analyzing colloidal properties.

pH Value

The pH value is a measure of how acidic or basic a solution is.
The scale ranges from 0 to 14, where lower numbers indicate acidity, higher numbers indicate alkalinity, and 7 represents a neutral state.

• In terms of colloidal systems, the pH is crucial because it affects the charge on the surface of particles within the dispersion.
• Adjusting the pH can alter the stability of a colloidal dispersion by influencing the ionization of surface groups, particularly around the isoelectric point where the net charge is zero.

This ensures that processes needing stable dispersions, such as drug formulation, can be accurately controlled by maintaining specific pH conditions.

Net Charge

Net charge refers to the overall charge of particles, which is essential in their interaction and behavior in colloidal dispersions.
Particles in a dispersion have charged groups on their surface; how these groups are arranged and the environment they're in, especially pH and ionic strength, dictates the net charge.

• This charge determines how particles interact with each other and their stability within the dispersion.
• If particles have a net charge, they repel each other, leading to a stable dispersion, but if their net charge is zero, they will likely aggregate.

At the isoelectric point, the net charge becomes neutral, particles attract due to van der Waals forces, leading to agglomeration and affecting the dispersion's properties.