Instead of Sodium Metabisulfite and Hexametaphosphate one can simply add Citric Acid to the milky clearing bath. Not only does it form a nice buffer with Sulfite between pH 6 and 8, the Citric Acid also works as chelating agent for all these ions which tend to form white scum with Sulfite or Carbonate, and it dissolves easily in water.
Instructions for making such a mix are very simple:
- Prepare working solution of hypo clearing agent with Sodium Sulfite, Sodium Carbonate (or whatever your recipe says) and tap water.
- The mix will be milky depending on your water hardness. Now add Citric Acid (anhydrous, monohydrate, it doesn't matter) until the working solution clears. Don't add too much Citric Acid or your bath will smell from SO2. If it does, add Sulfite or Carbonate to raise pH back to where it doesn't smell.