| 423 / | HD | CHEMISTRY OF INORGANIC COMPOUNDS |
|
| 511 | DF | SULFUR OR COMPOUND THEREOF {4} |
| 512.1 | DF | .~ Oxygen containing {11} |
| 521 | DF | .~.~ Ternary compound containing hydrogen {1} |
| 522 |  | .~.~.~ Sulfuric acid {5} |
| 523 | DF | .~.~.~.~> Nitrogenous impurity or utilizing nitrogenous catalyst or reactant {1} |
| 525 | DF | .~.~.~.~> Starting material includes organic or carbonaceous impurity {3} |
| 529 | DF | .~.~.~.~> Subjecting reactants to pressure, vacuum, or steam |
| 530 | DF | .~.~.~.~> Utilizing metal sulfate |
| 531 | DF | .~.~.~.~> Purifying acid or reactant |