Hello, Bill,

let c1 be the concentration of effective substance in the 1rst solution

let v1 the volume of the 1rst solution

let c2 be the concentration of effective substance in the 2nd solution

let v2 the volume of the 2nd solution

These 4 values must satisfy the equation:

c1 * v1 = c2 * v2

(c1 * v1 is the total amount of effective substance which doesn't change here)

You already know 3 of these values, thus you can calculate the 4th one:

500ppm * 10 ml = 4 ppm * v2

v2 = 1250 ml that means you have to add 1240 ml.

EB