Lesson 6a: solid NaNO3, and gaseous NH3 and HNO3 at 60% relative humidity


In Lesson 3 the existence of particulate phase ammonium nitrate was shown to depend on the amounts of ammonia and nitric acid present and the relative humidity. Here we study a similar case but with added sodium nitrate.

Entering the Data

The following example is an equilibration between gas, liquid and solid phases of a system containing 0.1 micromole each of nitric acid, ammonium nitrate, and sodium nitrate:

1st Calculation
  1. Enter the values and options under the following headings:

    Ambient Conditions
    Relative humidity = 0.60 (60%).

    Ionic Composition in Moles
    Hydrogen = 1E-7, Ammonium = 1E-7, Sodium = 1E-7,
    Nitrate = 3E-7.

    Trace Gases
    There are no entries under this heading.
  2. Solid Phases
    There are no entries under this heading.

  3. Click on the "Run" button at the end of the page to do the calculation.
Note:  the above should be entered on the "comprehensive" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3b.php).

Interpreting the Results

We learned in Lesson 5 that a system containing both NH4NO3 and NaNO3 has a mutual deliquescence relative humidity of about 50%, compared to values for the two individual salts of 73.75% (NaNO3) and 61% (NH4NO3). In this example we are above the mutual deliquescence relative humidity, but solid NaNO3 is the only condensed phase that exists. This is primarily because the HNO3 and NH3 are allowed to partition into the gas phase.

The equilibrium between solid NH4NO3(s) and the two gases is described by:

NH4NO3(s) = NH3(g) + HNO3(g)

Kp = pNH3 × pHNO3 = 4.356E-17 atm2

The output in the other browser window shows that the actual partial pressures – calculated from the number of moles of each gas present in the system volume of 1 m3 – are 2.4E-9 atm (NH3) and 4.8E-9 atm (HNO3), yielding a product of 1.15E-17 atm2. Because this is less than the value of Kp above, no solid NH4NO3(s) can form. In contrast, the constituent ions of NaNO3(s) are not volatile and the ambient relative humidity is just below the deliquescence relative humidity of NaNO3(s). Consequently, this salt exists as a solid at equilibrium.

If no partitioning of NH3 and HNO3 was allowed into the gas phase, then a aqueous phase containing both salts would exist. (You can try this by repeating the calculation above but with trace gas partitioning switched off.) In this case the equilibrium partial pressure of HNO3 above the liquid (4.067E-5 atm) corresponds to 1.663E-4 moles of the acid in 1 m3 of gas phase – orders of magnitude greater than is present in the system. Consequently the HNO3 would be expected to evaporate along with the NH3 whose equilibrium partial pressure above the liquid would increase as it became less acidic. The end result would therefore be expected to be a particle containing only NaNO3, which is what we find.

Proceed to Lesson 6b to learn how the system behaves at a high enough relative humidity for deliquescence to take place, or return to the main page for this lesson.