Always check to be sure that your solution is at room temperature before diluting it to the mark.

Meniscus close to mark (at 20 °C)	Meniscus above mark (at 25 °C)

On the left is ammonium nitrate dissolved in water, mixed and diluted to the mark. But ammonium nitrate cools when it dissolves, so the solution is below room temperature. At the right is the same solution after it has warmed to room temperature. The solution cannot be used, because its exact concentration is no longer known.