Soluble Blueing Agent Identification

A blueing agent is used to impart a blue color. The key requirement here is solubility, meaning the agent must dissolve in a solvent, typically water.

Analysis of Blueing Agent Options

Let's examine the solubility of each option:

• Ultramarine: This is a deep blue pigment, generally considered insoluble in water and common solvents.
• Indigo: While a well-known blue dye, natural indigo has low water solubility. Soluble forms exist (like Indigo Carmine), but indigo itself isn't typically described as readily soluble.
• Methylene blue: This is a cationic dye that is highly soluble in water, appearing as a deep blue solution. It is widely used as a stain and indicator precisely because of its solubility.
• Prussian blue: This is another common blue pigment, known for being insoluble in water.

Based on the solubility requirement, Methylene blue is the correct choice.