The pH of the hardener should be alkaline and varies slightly due to the fact that unstabilzed formalin can oxidize to form formic acid as a contaminant. This causes some variation in pH. I used to adjust the pH to the pH of the developer I was using.

The sulfate is in addition, not in place of carbonate as the solution should be buffered with an alkali.

The hardener I refer to may be a different formula number, it is just the preferred one we used in-house.