The plimsoll symbol as used in shipping

It should be noted that the standard state is an arbitrarily chosen non-zero value, not a natural zero point.

For a given material or substance, the standard state is the reference state for the material's thermodynamic state properties such as enthalpy, entropy, Gibbs free energy, and for many other material standards. The standard enthalpy of formation for an element in its standard state is 0.

When the standard state is referred to a solute in a solution, or to a chemical reaction, it also includes the condition that the concentrations of all solutes are at unity (or another designated quantity) for whatever measure of concentration is specified. If that is molarity that would be 1 mol·dm^-3 and for molality 1 mol·kg^-1 assuming the solution infinite-dilution behaviour. If mole fraction is used, the pure liquid or solid is the standard state (x=1). As it is possible (and in principle legitimate) to take a different unit for each of the species in the reaction, the nature of the standard state needs to be specified when reporting or tabulating. Although a definition involving 1 mol/L (molarity at unity) of A in combination with pure B (mole fraction at unity) is clearly a condition that can never be met, such a non-existent standard state nevertheless leads to a consistent system of tabulated values, provided it is used consistently by all. Of course these values are different from those where a different standard state is adopted.

Symbol

In the time of their development (the 19th century) the Plimsoll - symbol was adopted as a superscript ^o to indicate the non-zero nature of the chosen reference state. For typographic reasons this symbol is often abridged to a rather misleading zero superscript ⁰ in later texts.

At elevated temperature and pressure

In chemistry of solutions at elevated temperatures and pressures, the term "standard state" often denotes the hypothetical standard concentration, typically:

• 1 mol/kg for solutes assuming an ideal behaviour (i.e., an infinite dilution),
• a unity molar fraction (for pure substances),
• a partial pressure of 100 kPa for gaseous reagents.

It does not imply any particular temperature or total system pressure because, although contrary to IUPAC recommendation, it is more convenient when describing solutions over a wide temperature and pressure ranges.^[2]

See also

• Standard conditions for temperature and pressure

Notes

1. ↑
2. ↑ V. Majer, J. Sedelbauer and Wood, "Calculations of standard thermodynamic properties of aqueous electrolytes and nonelectrolytes." Chapter 4 in: "Aqueous Systems at Elevated Temperatures and Pressures. Physical Chemistry of Water, Steam and Hydrothermal Solutions", D.A.Palmer, R. Fernandez-Prini, and A.Harvey (editors), Elsevier, 2004.

External links

• Standard-state enthalpies

nl:Standaardomstandigheden pl:Warunki standardowe pt:Condições normais ur:?????? ?????