How could I prepare a 1 ppm solution of ammonium chloride in water?
You could dissolve 4 mg of ammonium chloride in 1 L of water.
1 ppm means 1 g of solute in 10⁶ g of solution.
Therefore, it is common to equate 1 mL of solution with 1 g of solution.
Assume that you want to make 1 L of solution. Then
You could dissolve 4 mg of NH₄Cl in 1 L of water in a volumetric flask.
It is difficult to measure 4 mg of NH₄Cl accurately.
It would be easier to dissolve 400 mg of NH₄ in 1 L. This would give you a concentration of 400 ppm.
Then you could dilute 10 mL of this solution to 1 L in another volumetric flask to get a final solution with a concentration of 4 ppm.