How do you prepare a potassium hydrogen phthalate buffer solution?

Potassium hydrogen phthalate (KHP) buffer solution is a highly stable standard solution, primarily used for calibrating pH meters and electrodes. Method for preparing a 0.05 M standard KHP solution (pH 4.00): Accurately weigh out 10.21 g of KHP and place it in a clean 500 mL beaker. Add 400 mL of deionised water to the beaker, allow to dissolve, and leave to stand for 10 minutes. Transfer the solution to a 1000 mL volumetric flask, make up to the 1000 mL mark with water, secure the stopper tightly, and invert the flask several times to ensure the solution is homogeneous.