1. Calculate mass of Na2HPO4 which cointains 1,000 mg PO43-.
2. Using laboratory scale weigh mass of Na2HPO4 calculated in 1. dissolve it in water and dilute to 100 ml.
3. Dissolve some (for example 0,1 g) Na2HPO4 in water and dilute to requested concentration.
4. Compare obtained solution to some standard - by spectrometry or titration.