Tris-HCl is a buffer that can be used to control the pH of many solutions, including buffers used in ELISAs, cell and tissue lysis buffers, and buffers for fluorogenic assays.
Tris-HCl can be prepared using Tris base (molecular weight: 121.14 g/mol), or Tris-HCl (Tris base which is already combined with HCl in a 1:1 molar ratio, so the molecular weight is 157.6 g/mol).
It is important to note which form you use to make the solution, because the initial pH of the solution you make depends on which form you use.