The standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions.
[7][8] No real gas has perfectly ideal behavior, but this definition of the standard state allows corrections for non-ideality to be made consistently for all the different gases.
For example, the use of a standard state concentration of 10−7 mol/L for the hydrogen ion in a real, aqueous solution is common in the field of biochemistry.
For adsorption that occurs on specific sites (Langmuir adsorption isotherm) the most common standard state is a relative coverage of θ° = 0.5, as this choice results in a cancellation of the configurational entropy term and is also consistent with neglecting to include the standard state (which is a common error).
[14] The advantage of using θ° = 0.5 is that the configurational term cancels and the entropy extracted from thermodynamic analyses is thus reflective of intra-molecular changes between the bulk phase (such as gas or liquid) and the adsorbed state.
[14] At the time of development in the nineteenth century, the superscript Plimsoll symbol (⦵) was adopted to indicate the non-zero nature of the standard state.
[19][20] Compared to the plimsoll symbol used in 1800s text, the U+29B5 glyph is too large and its horizontal line does not sufficiently extend beyond the boundaries of the circle.
[21] It is a regular-sized Unicode symbol meant to be used in superscripted form when denoting standard state, replacing U+29B5 for this purpose.
[22][23] Ian M. Mills, who was involved in producing a revision of Quantities, Units and Symbols in Physical Chemistry, suggested that a superscript zero (