Introduction

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.

Preparation

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.

Select this 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.

The Lesson

This consists of the two sets of calculations described in the links below, which should be done in the order listed.

a.  Deliquescence of sodium nitrate

b.  Deliquescence of ammonium/sodium/nitrate mixtures

Conclusion

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 eutectic composition.

• 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.