View source for PCR 2X Master Mix

The master mix can be purchased premixed commercially or mixed from the individual components. 

For a single 25 µl reaction
:2.5 µl 10X buffer (usually this is supplied with the polymerase, this assumes it does not already have 
:0.5 µl 10mM dNTPs
:1 µl 50mM MgCl<sub>2</sub> (adjusting the Mg concentration affects the specificity of the polymerase and can be optimized)
:0.5 µl (5 U / µl) thermostable (Taq) polymerase
:8 µl nuclease-free H<sub>2</sub>O (adjust if necessary if MgCl<sub>2</sub> or Taq volume is adjusted for a 12.5 µl total volume). 

In general a mix would be made for many reactions, say for 100, and the volumes multiplied accordingly. The mix can be stored at -20 C.