Final answer:
The primary low pH reducing agent in acid waves is ammonium thioglycolate, used to break disulfide bonds in the hair for reshaping.
Step-by-step explanation:
The primary low pH reducing agent in acid waves is ammonium thioglycolate. During the permanent wave process, ammonium thioglycolate is used to reduce disulfide bonds in the hair, allowing it to be reshaped. This chemical reduction is crucial to the process, distinguishing it from other potential agents like ammonium persulfate, hydrogen peroxide, and sodium hydroxide, which do not serve the same function.