# How to make a buffered 10% formalin solution

How can you prepare a solution of 50 mL buffered 10% formalin using the following reagents?

100% formalin (100 mL)
Phosphate buffer (100 mL)

I know I can mix 5 mL of formalin and 45 mL of distilled water together to make 50 mL of 10% formalin, but if I add phosphate buffer later the volume of formalin would exceed 50 mL.

Can I mix like 1 mL of formalin and 9 mL of distilled water together and then make up the remaining volume with 40 mL of phosphate buffer? Is the concentration of formalin remaining the same as long as the proportion of distilled water to formalin kept constant no matter how much phosphate solution I add?