pH

 TIP: 

If 25% sulfuric acid is desired instead of 50%, the mixture needs to be diluted: 50% water and 50% sulfuric acid.

E.g. 10 liters of 50% acid are poured into 10 liters of water = 20 liters of 25% sulfuric acid.

WARNING: Never pour the water into the acid. Risk of very high heat generation / explosion. Instead, pour the acid into the water slowly.