Answer

 The disodium salt of EDTA will not dissolve until the pH of the solution is adjusted to pH8.0, by the addition of Sodium hydroxide (NaOH).

 Recipe:
 • Add 18.6 g of disodium EDTA (Na2EDTA) to 80 mL of distilled / deionised water
 • Adjust the pH to 8.0 with NaOH (~5 ml of NaOH)
 • Bring the volume to 100ml with distilled / deionised water
 • Stir vigorously on a magnetic stirrer
 • Sterilize by autoclaving
 • Store at room temperature