**Question:** Most commonly used retarder in gypsum:

A. Calcium chloride
B. Calcium nitrate
C. Polyphosphoric acid
D. Sodium chloride

**Core Concept:**

The question is related to the choice of retarders in gypsum, which are substances added to gypsum mixtures to control setting time. Retarders are essential in dental and orthodontic applications, allowing the gypsum to set slowly, enabling accurate impression-taking and model casting procedures.

**Why the Correct Answer is Right:**

The correct answer is **C. Polyphosphoric acid**. Polyphosphoric acid is commonly used as a retarder in gypsum because it effectively slows down the setting time of gypsum pastes. This allows more time for accurate impression-taking and model casting. Polyphosphoric acid acts as a complexing agent, forming a complex with calcium ions in the gypsum mixture, which reduces the rate of hydration and ultimately slows down the setting process.

**Why Each Wrong Option is Incorrect:**

A. **Calcium chloride (D):** Calcium chloride is an accelerator, not a retarder. It increases the rate of hydration, leading to a faster setting time, which is counterproductive in gypsum applications.

B. **Calcium nitrate (B):** Similar to calcium chloride, calcium nitrate is also an accelerator that enhances the hydration process and shortens the setting time, making it an incorrect choice for retarding gypsum setting.

D. **Sodium chloride (D):** Although sodium chloride is a common table salt, it is not used as a retarder in gypsum applications. Sodium chloride does not specifically target the gypsum hydration process, making it an unsuitable choice for retarding gypsum setting.

**Clinical Pearl:**

Understanding the role of retarders and accelerators in gypsum mixtures is essential for dental professionals. Retarders like polyphosphoric acid allow for precise impression-taking and casting procedures, whereas accelerators like calcium compounds should be avoided to ensure accurate and reliable end results in dental applications involving gypsum.