NH3 Addition to NH4Cl Solution

Problem

Given is a 0.001 molar NH4Cl solution. What is the pH after addition of 0.2 mmol NH3 per liter?

Answer

We solve this task in two steps: First, we calculate the pH of the NH4Cl solution, then we add ammonia (NH3).

Step 1. We start with pure water (button New) and switch to molar units (activate checkbox mmol/L). Now enter the following values into the two fields, Amm (i.e. total ammonia to which all NH4+ species and NH3 belong) and Cl:

Amm = 1 mM (equivalent to 1 mM NH4Cl)
Cl = 1 mM (equivalent to 1 mM NH4Cl)

Run the calculation with Start. The program tells you that the solution is not charge-balanced. Now adjust charge-balance by pH (default value). The result will be immediately displayed:

pH = 6.13

The 0.001 molar NH4Cl solution has pH 6.13.

Step 2. After return to the input window you can add NH3. For this purpose increase the value of Amm from 1.0 to 1.2 mmol (which is the sum of NH4+ plus NH3; both species belong to Amm). Then click on Start and adjust the solution with pH.

The final solution after addition of ammonia has

pH = 8.55

Note. In equilibrium thermodynamics the sequence order of (reversible) processes can be changed without altering the final result. For example, you can start with a 0.0002 molar NH3 solution and then add 1 mM NH4Cl. The pH will evolve in the following way:

7.00 → 9.71 → 8.55

Not surprising, the final solution is the same; the final pH is the same.

[last modified: 2013-08-05]