In analytical chemistry, a solution that resists significant pH change upon addition of small amounts of acid or base is called a what?

Introduction / Context:
Being able to name and recognize a solution that maintains an approximately constant pH is fundamental in biochemistry, pharmaceutics, and analytical chemistry. Such solutions are essential for enzyme assays, titrations, and many industrial processes where pH stability is critical.

Given Data / Assumptions:

• The solution exhibits a reasonably constant pH when small amounts of acid or base are added.
• We are not discussing colloidal stability, ideality, or phase equilibria; the focus is pH control.

Concept / Approach:
A buffer solution is typically prepared from a weak acid and its conjugate base (or a weak base and its conjugate acid). The Henderson–Hasselbalch relationship describes how the ratio of conjugate base to acid controls pH. The capacity to neutralize small additions of H+ or OH- is the buffering action.

Step-by-Step Solution:

Verification / Alternative check:
Common examples include acetate buffers (acetic acid/acetate), phosphate buffers (various H2PO4-/HPO4^2- pairs), and Tris buffers in biochemistry. Each resists pH change within a characteristic pH range near the pKa of the weak acid/base pair.

Why Other Options Are Wrong:

• Ideal/non-ideal solutions concern activity behavior, not pH stabilization.
• Colloidal solution relates to particle dispersion, not acid–base buffering.
• Azeotropic solution refers to constant-boiling mixtures, unrelated to pH control.

Common Pitfalls:
Assuming strong acid plus strong base mixture forms a buffer—true buffers require a weak component and its conjugate pair in appropriate ratios.

Final Answer:
Buffer solution