Which of the following can be used in place of tannin for the process of tanning?

Chromium salts

The correct answer is option 3 .Chromium salts.

In the process of tanning, chromium salts are commonly used in place of tannin.

Tanning is the process of treating animal hides to produce leather. Traditional tanning uses tannins, which are naturally occurring polyphenolic compounds found in plant extracts. Tannins form strong bonds with the proteins in the hide, stabilizing the collagen fibers and making the material durable and less prone to decomposition.

Chromium Salts: Chromium tanning, specifically using chromium(III) salts, has become the most prevalent method in the leather industry. Chromium salts provide several advantages:

Speed: Chromium tanning is much faster than vegetable tanning with tannins.

Quality: It produces leather that is more durable, more resistant to water, and more supple compared to vegetable-tanned leather.

Color: Chromium-tanned leather tends to have a blueish color, often referred to as "wet blue," which can be dyed to a variety of colors.

The other salts mentioned (zinc, copper, and cobalt) are not typically used for tanning leather due to their inefficacy in stabilizing the collagen fibers in hides.

Therefore, the correct choice is Chromium salts.