In Lesson 4
we examined the water content of solutions containing more
than one salt. In Lesson 2
it was shown that
the deliquescence relative humidity is the relative humidity
at which the transition between solid salt and aqueous solution
occurs. Ammonium nitrate was found to have a deliquescence relative
humidity of about 61%. Here we will determine the
the deliquescence point of first sodium nitrate and then of
mixtures of the two salts.
Before starting, ensure that this browser window occupies only the
left half of your screen. You should leave enough
space for another browser window where you can
enter data into E-AIM
and read the results. If your screen is too small for
two windows, print out this tutorial and use to run the E-AIM
model. In these lessons we assume that you will
have two browser windows open.
link to open a second browser window containing the data
input page for "variable relative humidity, or total water" calculations
using Model III (http://www.aim.env.uea.ac.uk/aim/model3/mod3rhw.php). Arrange
the windows on your screen so that both are visible and the left window
contains this text.
This consists of the two sets of calculations described in the links below,
which should be done in the order listed.
You have completed Lesson 5, and learned that:
- The deliquescence relative humidity of sodium nitrate at
298.15 K is 73.75%.
- Mixtures of more than one salt have a mutual deliquescence
point – where water is first taken up &nash; that is lower than either
of the single salts.
- The value of the relative humidity at the mutual
deliquescence point is unique for each pair of salts (and
temperature) and does not vary with the relative amounts
of the salts present.
- The composition of the aqueous phase at the mutual
deliquescence point is similarly invariant and is called the
- Unless a mixture of solid salts has a composition corresponding
exactly to the eutectic composition, some solid salt(s) will
exist at relative humidities above the mutual deliquescence point in
equilibrium with the saturated aqueous solution.
Now proceed to Lesson 6, which
explores aerosol/gas partitioning in mixed salt systems.