Sciencemadness Discussion Board

Accurate Constants

vehmently - 10-4-2018 at 16:56

Where can one find an online source of most-recent, accurate and precise constants for pure elemental substances? In particular, I require the best density at STP, linear coefficient of expansion for Cu, Al, Ag, Pd, Au, ... - all the common industrial/pgm metals.

I've found Atomic Mass Units reliably here:

Ideally, I'd like to get the linear expansions of each bulk isotope (Ag107 versus Ag109) instead of just a terrestrial sample but I know that is a stretch to get.

I've also noticed Avogadro's Number varies from place to place.

As many of you probably know, the standards is going to be updated by the end of the year. A couple articles have come out in the past ten years also talking about how some physics constants seems to be slightly changing and were trying to ascertain if via contamination, etc.

So... where does one find the good stuff ?


vehmently - 10-4-2018 at 17:00

As a side note, I've found I'm relying more and more on my Alfa Aesar ordering book of research chemicals from 2008-09 and the constants therein. The Aluminum-27 expansion coefficient, for example, is all over the place when comparing various sources. Anyone have any particular favorites they'd like to share?

Sulaiman - 23-4-2019 at 04:20

I do not know the best source of constants,
but Wikipedia is often my first choice, especially the references at the bottom of pages.

When dealing with engineering/real world materials (alloys, plastics, wood, glasses, ceramics etc.) the precise formulation, manufacturing processes and even past history can affect parameters such as thermal expansion.

Mesauring thermal expansion is not easy for an amateur,
not least is the expense of any measuring device (thermometer, ruler, caliper etc.) accurate to better than 100 ppm.
and the thermal expansion coeficient of the measuring device itself (e.g. steel>10ppm/oC)

e.g. I am presently making a clock based on a pendulum,
even when I obtain a material of low thermal expansion coeficient for the rod,
(e.g. pultruded carbon fibre in epoxy resin),
I can't accurately measure micron changes over 1 metre.
(except maybe by making it into a pendulum ;)

So, if its for engineering you need to consult with the manufacturer,
if its for practical hobby science you will need excellent measuring devices,
otherwise ... who cares ?

Why do you want to know constants to greater accuracy than you can measure results?

P.S. regarding ".. precise constants for pure elemental substances .."
the implication is that you would measure the constants for a single pure perfect crystal of the element
... AFAIK, 'perfect and pure' does not exist in this solar system.

Also, the ratios of different isotopes of 'pure' elements obtained from geographically different sources can differ.
So even 'atomic mass' is no longer a constant for many elements.

[Edited on 23-4-2019 by Sulaiman]

Heptylene - 24-4-2019 at 01:46

Have you tried NIST? National institute of standards and technology in the US.

[Edited on 24-4-2019 by Heptylene]