Are you seeing yellowing in wash water or later? If you are seeing this yellowing thing during wash... read on...
If you are using direct sulfide toner using manufacturer's recommended dilutions, curious things happen. It will tone in the toner bath. When you *think* you are done and put it in wash water, it speeds up and moves toward more toning effect. It may look like yellowing but it's actually toning more and more from black into light brown.
To stop this, you don't do acid/alkaline thing. You make a 10% solution of sodium sulfite (aka strong HCA) and dip the print after toning is done to your liking. That'll stop it right way. This is the method I use when I want a precise control.
Roger (who posted above) have a different method. He uses much weaker solution to tone then washes it after it's done. He has his system calibrated so that this toning faster in water effect does not happen.
Again, PH has nothing to do with it and it is unlike developer where acid will stop the action.