pH of Phosphate Buffer with addition of NaOH and HCl

Problem

Given is a 0.03 molar phosphate buffer that consists of 4 mM KH2PO4 and 26 mM K2HPO4.

Task 1.  What is the pH of this buffer solution at 25°C?

Task 2.  How does the pH of the buffer solution change after

  • addition of 1 mM NaOH?
  • addition of 1 mM HCl?
  • equilibrium with the atmospheric CO2?

Task 3.  How does the pH of the buffer solution change after addition of 1 mM FeCl2 and 1 mM FeCl3? What happens when the obtained solution is oxidized by O2?

phosphate_buffer_input_1

Task 1

We start with pure water (button New) and open the reaction module (button Reac). Then we enter two reactants: 4 mM KH2PO4 and 26 mM K2HPO4 as shown in the right screenshot.

By click on the Start button we immediatelly get the result

  pH = 7.70

In words: The 0.03 molar phosphate buffer has a pH of 7.70 at 25°C.

phosphate_buffer_input_2

Task 2

We repeat the calculation, but now we add into the input panel shown above 1 mM NaOH as third reactant. The same will be done for 1 mM HCl (also as third reactant).

Finally, we set the buffer solution equilibrium with the atmospheric CO2. For this purpose click on button Setup and activate the checkbox “Open CO2 System” – as shown in the right screenshot.

The obtained results for the buffer (in comparison with pure water) are:

buffer + 1 mM NaOH: 7.70 ⇒ 7.84   ( H2O + 1 mM NaOH: 7.00 ⇒ 10.98 )
buffer + 1 mM HCl: 7.70 ⇒ 7.59   ( H2O + 1 mM HCl: 7.00 ⇒ 3.01 )
buffer + atmospheric CO2: 7.70 ⇒ 7.66   ( H2O + atmosph. CO2: 7.00 ⇒ 5.61 )

In fact, this example demonstrates the buffer’s resistance against pH changes through acids or bases.

phosphate_buffer_plus_FeCl2_FeCl3

Task 3

The aim of the third task is to demonstrate the influence of redox reactions and mineral precipitation.

For this purpose we add to the phosphate buffer two reactants wich differ in their oxidation state: 1 mM FeCl2 (i.e., Fe(II) in the oxidation state 2) and 1 mM FeCl3 (i.e., Fe(III) in the oxidation state 3).

The result is dispayed in the right screenshot. Two output values for pH are given:

before precipitation: pH = 7.42
after precipitation: pH = 7.56

The mineral that precipitates is strengite (FePO4:2H2O) – an Fe(III) mineral.

From the total amount of 2 mM Fe that we added to the buffer solution, 1 mM precipitates as strengite (which corresponds to the amount of FeCl3). The other 1 mM Fe remains in the solution in form of Fe(II).

phosphate_buffer_inp_FeCl2_FeCl3

Oxidation. The results above are valid for ambient redox conditions, i.e. for pe = 4.1 Let us now change the redox conditions by oxidation with O2 (aeration).

The corresponding input panel is shown on the right. We have four reactants (where the first two reactants define the phosphate buffer) plus the additional condition of an Open Redox System at pe = 10.2 This means that O2 is added until pe = 10 is obtained.  (In particular, 0.25 mM O2 is added to the system.3)

phosphate_buffer_plus_FeCl2_FeCl3_oxy

The obtained results are shown on the last screenshot on the right. The only difference to the above calculation is the fact that all Fe(II) is oxidized to Fe(III). In total, there are now 2 mM Fe(III) which completely precipitates as strengite. What remains in the solution is a vanishing amount of 4.5·10-7 mM Fe.

[Note: The input panel for reactions is designed for four reactants – see next-to-last screenshot. However, the activation of the “Open CO2 System” and “Open Redox System” provides two additional reactants, namely CO2 and O2. So, in total six reactants can be added to any input solution.]

Remarks & Footnotes

  1. pe = 4 is the default parameter for the redox potential in PhreeqC.

  2. The results are independent of the exact pe value. You can choose any value pe > 8 (which characterizes an oxidative aquatic environment).

  3. The value of 0.25 mM O2 is presented in the subsequent output tables.

[last modified: 2015-05-04]