Problem 4 - Chemistry in 30 days

Question

Which of the following is a mordant? (a) Tannic acid (b) Metallic hydroxides (c) Salts of \(\mathrm{Al}, \mathrm{Cr} . \mathrm{Fe}, \mathrm{Sn}\) (d) All of these

Step-by-Step Solution

Answer

The mordant is (d) All of these.

Step 1: Understanding the Term 'Mordant'

A mordant is a substance used in dyeing that fixes a dye to the material by combining with the dye to form an insoluble compound. In textile dyeing, mordants are crucial because they create a strong bond between the dye and the fibers, ensuring colorfastness.

Step 2: Analyzing Option (a): Tannic Acid

Tannic acid is a type of polyphenol that can act as a mordant in dyeing processes, primarily for vegetable dyes on cotton and wool. It aids in binding the dye to the fabric fibers.

Step 3: Analyzing Option (b): Metallic Hydroxides

Metallic hydroxides, such as those of aluminum and iron, serve as mordants by interacting with dyes to form a bridge between the dye and fabric, making the dye more permanent and lightfast.

Step 4: Analyzing Option (c): Salts of Al, Cr, Fe, Sn

These metallic salts are traditional mordants used extensively in the dyeing industry. They work by forming complex molecules with dyes, which adhere strongly to textile fibers, making the dye insoluble.

Step 5: Evaluating Option (d): All of These

Each listed substance or class of substances (tannic acid, metallic hydroxides, and salts of certain metals) can function as a mordant in specific dyeing processes. This suggests that all options have the capability to act as mordants.

Key Concepts

Tannic Acid in Dyeing

Tannic acid is a naturally occurring compound found in plants, and it belongs to a class of substances called polyphenols. In the world of dyeing, tannic acid is recognized for its ability to act as a mordant, especially when working with vegetable dyes on materials like cotton and wool. Mordants are essential in the dyeing process because they help the dye adhere better to the fabric fibers, providing better color retention and stability.

• Tannic acid binds with the dye molecules, creating a strong connection with the fibers.
• This connection ensures that the color remains vibrant and does not wash out easily over time.

When used as a mordant, tannic acid forms a stable complex with the dye, making it less soluble in water, and thus, more "set" in the fabric. It is especially effective with lighter, pastel tones and can help in achieving a smoother coloration on natural fibers.

Role of Metallic Hydroxides

Metallic hydroxides, such as those derived from aluminum or iron, function as potent mordants in the dyeing industry. They work by creating a bridge between the dye molecules and the textile fibers, which makes the dye adhere more firmly. This process is crucial because it enhances both the intensity and the durability of the color once applied to fabrics.

• Aluminum hydroxide is commonly used because of its effectiveness in linking dyes to the fibers without compromising the texture of the material.
• Iron hydroxide is employed particularly in achieving darker hues, as it can deepen the dye colors.

The use of metallic hydroxides not only contributes to the colorfastness of the dye but also often enhances its resistance to fading under light exposure.

Significance of Metal Salts in Dyeing

Metal salts, such as those containing aluminum (Al), chromium (Cr), iron (Fe), and tin (Sn), are traditional and highly effective mordants used in various dyeing techniques. Their primary role is to form complex molecules with the dyes. These newly formed complexes adhere tightly to textile fibers, making the dye less soluble.

• Aluminum salts are favored for bright and brilliant colors, often used in combination with other mordants for best results.
• Chromium salts are known for providing excellent washfastness, meaning the dyed fabric withstands multiple wash cycles without significant color loss.
• Iron salts create deeper, more subdued tones, which are ideal for achieving earthy colors.
• Tin salts amplify the brilliance of the dye, making them suitable for vivid color applications.

Using metal salts in dyeing helps stabilize the dye on the fabric, ensures long-lasting vibrancy, and provides resistance against fading and washing.