Schoder's electro neutralization theory:
When the collagen is treated with nitrous acid. Nitrogen gas is evolved due to destruction of basic amino group of collagen.
This de-aminated collagen fixes less amount of vegetable tannin. The reduction of fixed tannin proves that amino group of collagen takes part in vegetable tannin.
The percentage of fixed tannin increases as the pH value of tan-liquor is lowered. At low pH the collagen is positively changed & the no. of positive charge increases as the pH goes down.
Schoeder concluded that in vegetable tanning the positively charged collagen neutralises the negative charged tannin molecule.
