What is the primary ingredient in acid-based neutralizers for permanent waves?

The primary ingredient in acid-based neutralizers for permanent waves is sodium bisulfite. When a permanent wave is processed, the hair’s disulfide bonds are broken down using a reducing agent such as ammonium thioglycolate. After this process, it's essential to neutralize and reform those bonds to set the new curl structure.

Sodium bisulfite works effectively because it has a pH that is lower than alkaline products and helps to stabilize the newly formed curl by reforming the disulfide bonds. It also helps to bring the hair back to its natural pH level after the wave has been processed. This is crucial in ensuring that the hair maintains its health and integrity following a chemical treatment.

While calcium hydroxide and sodium hydroxide are also involved in the chemical processes related to hair treatments, they primarily serve different roles, such as raising the pH level rather than neutralizing. Therefore, for acid-based neutralizers specifically, sodium bisulfite is the accurate choice.