I'm guessing you're talking about a pump up sprayer? Next time you empty it out, put a quart of household ammonia in it and fill it with water. It will take all the chemical residue out of the sprayer. I use a quart of ammonia per 25 gallons of tank size on a tractor sprayer. You'll be surprised what comes out of a "clean" tank if you put ammonia in it.

If my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then will I hear from heaven and will forgive their sin and will heal their land. 2 Chronicles 7:14