caustic soda vs sodium

caustic soda

noun
  • sodium hydroxide (NaOH), a strong alkaline substance used in the manufacture of paint, soap and detergents, or as a drain cleaner. 

sodium

noun
  • Employing sodium. 

  • The chemical element (symbol Na) with an atomic number of 11 and atomic weight of 22.990. It is a soft, waxy, silvery, reactive alkali metal that is never found unbound in nature. 

How often have the words caustic soda and sodium occurred in a corpus of books? (source: Google Ngram Viewer )