Re: ml and cm^3

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

Regarding Michael E.'s comment:

> Robert Cohen asks, "Isn't 1 ml equivalent to 1 cm^3?  Isn't g/ml
> equivalent to g/cm^3?"
>
> I think it depends upon whom you ask.  Now that we have a meter (and
> from that, centimeter) defined by light, it turns out that 1 cm^3 of
> water, at water's maximum density (at 3.98 Celsius) is not exactly one;
> rather, it is 0.999972 g/cm^3.

The 1983 redefinition of the meter in terms of c being exactly
299792458 m/s by definition has nothing to do with the noted
discrepancy.  The discrepancy involving the density of water at 3.98 C
occurs in the 5th sig fig.  The 1983 redefinition of the meter involved
the 9th sig. fig.

> Therefore, water never gets more dense
> than this, i.e. it never reaches exactly one.  For chemists it is most
> convenient to calibrate volumes using mass of water.  Therefore chemists
> still want one milliliter of water at 3.98 Celsius to come out exactly
> one gram.
>
> My Merck Index says... density of water at 3.98 Celsius = 1.000000 g/ml
> = 0.999972 g/cc.

Then your Merck Index is quite out of date.

> However, I believe NIST does define one liter (and
> from that, ml) as one cubic decimeter.

Yes.  But it has not always been that way.  In the past a liter was
taken as the volume of a kilogram of pure water (with average
terrestrial isotopic concentrations) under 1 atmosphere of pressure
at the temperature of its greatest density (i.e. 3.98 C).  This
definition of the liter was adopted in 1901.  Since then the
definition of the liter was changed 39 years ago, back in 1964, to
be 1 dm^3.

> I don't know what the latest
> word is, but in chemistry circles the people and texts I am familiar
> with make a slight distinction between ml and cm^3.

Probably because either they are old or because chemists may have an
affinity for a volume unit based on the density of water and don't
want to change with the redefinition of the liter.

> Does anyone have updated information on that?

For more info see my post in the PHYS-L archives:
http://lists.nau.edu/cgi-bin/wa?A2=ind9803&L=phys-l&F=&S=&P=41871
and
http://lists.nau.edu/cgi-bin/wa?A2=ind9803&L=phys-l&F=&S=&P=42373

My post above was prepared with info found in the appendix 1 of NIST
Special Publication 330.  For more details see:
http://physics.nist.gov/Document/sp330.pdf

David Bowman