pH for acidifier used in fixer should be:

The fixing solution contains an acetic acid buffer system (pH 4 to 4.5) to keep the fixer pH constant. The acidic pH is required to promote good diffusion of thiosulfate into the emulsion and of silver thiosulfate complex out of the emulsion. The acid-fixing solution also inactivates any residual developing agents in the film emulsion, blocking continued development of any unexposed crystals while the film is in the fixing tank. White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition