| 423 / | HD | CHEMISTRY OF INORGANIC COMPOUNDS |
| 511 | DF | SULFUR OR COMPOUND THEREOF {4} |
| 512.1 | DF | .~ Oxygen containing {11} |
| 532 | DF | .~.~ Sulfur trioxide {1} |
| 533 | DF | .~.~.~ Utilizing catalyst in reaction {4} |
| 535 | | .~.~.~.~ Catalyst contains oxygen, vanadium, and another metal |