What is the primary chemical function of ammonium hydroxide in permanent hair coloring?

Ammonium hydroxide plays a crucial role in permanent hair coloring primarily by adjusting the pH of the hair and the dye mixture. In the context of hair coloring, a higher pH is essential because it opens up the hair's cuticle layer, allowing the dye molecules to penetrate the hair shaft effectively. When the pH is adjusted appropriately, it enhances the color's absorption and helps achieve a longer-lasting result.

The alkalinity of ammonium hydroxide also activates the oxidation process necessary for the color development, facilitating the chemical reaction that allows the dye to adhere to the hair. Therefore, its ability to modify the pH is fundamental to the effectiveness of the permanent hair coloring process.