The standard state for liquids and solids is simply the state of the pure substance subjected to a total pressure of 10 Pa (or 1 bar). For most elements, the reference point of Δ f H = 0 is defined for the most stable allotrope of the element, such as graphite in the case of carbon, and the β-phase (white tin) in the case of tin. An exception is white phosphorus, the most common allotrope of phosphorus, which is defined as the standard state despite the fact that it is only metastable. This is because the thermodynamically stable black allotrope is difficult to prepare pure.