The Wikipedia article Standard conditions for temperature and pressure notes that IUPAC and NIST are two of the most common standards for temperature and pressure in use.
Even if I could have gotten my hands on the IUPAC standards, I wouldn't have wanted to use them as the standard temperature according to IUPAC is 0 °C. For my present work, which is related to fluid dynamics, NIST standards seem to be useful.
However I couldn't find any solid reference where I could get the related data. The reference listed on wikipedia are weak. NIST's own website is very difficult to navigate. Is there a place where I can find NIST published values for standard temperature and pressure? I am not asking for just the values for temperature and pressure, which if I were to trust most sources, are 293.15 K and 101.325 kPa respectively. But how about the value of density of air at that T&P? What about dynamic viscosity?
The problems created by this inaccessibility of data are so rampant that the aforementioned Wikipedia article includes the following quote (2015 March 25 Wed 2243 hrs):
However, many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often leading to confusion and errors. Good practice is to always incorporate the reference conditions of temperature and pressure.
The changes in these quantities (and perhaps in other related quantities), for small changes in both temperature and pressure, are a few percentage points at worst. That's assuming a typical lab setting. However, a great amount of noise could have been eliminated had NIST simply published this data (and made it accessible).
Best Answer
This is a good question. Wikipedia says the NIST standard is $20^\circ {\rm C}$ and $1\,{\rm atm}$, but the WebBook guide does not define standard conditions. The guide does say "Unless stated otherwise, all data are for reactions occurring at $298\,{\rm K}$," which is $25^\circ{\rm C}$.
Modern Thermodynamics with Statistical Mechanics by Carl S. Helrich says the NIST standard in $25^\circ{\rm C}$ and $1\,{\rm bar}$.
One other thing. The NIST Chemistry WebBook references their enthalpy data to $25^\circ{\rm C}$. That is, if you use the Shomate coefficients to calculate the the enthalpy as a function of temperature, the equation gives $H(T) - H(298.15\,{\rm K})$.