Buffers

The aim of this experiment is to determine the pH of buffered and unbuffered solutions. You are provided with different solutions from which you will prepare two buffers (150 mM and 15 mMPhosphate-buffer, respectively). Subsequently, you then measure the pH of 20 mL of teh different solutions with and without the addition of 1 mL 1 M HCl.

For this experiment you need the following solutions:

Distilled water

150 mM NaH₂PO₄ (50 mL)

150 mM K₂ HPO₄ (50 mL)

1 M HCl (5 mL)

Prepare:

1. 20 mL of distilled water.

2. 20 mL of 150 mM Phosphate-buffer by mixing 10 mL of the 150 mM NaH₂PO₄ with 10 mL of the 150 mM K₂ HPO₄ solution (labelled "150 mM Buffer").

3. 20 mL of 15 mM Phosphate-buffer by mixing 2 mL of the 150 mM Phosphate-bufferwith 18 mL distilled water (labelled "15 mM Buffer").

Instructions:

Measure the pH of the different solutions. Make sure that you 'clean' the pH-electrode after every use (by clicking on the "Clean"-button", otherwise the pH-Meter will show a blank screen. Record your observations and explain the results (the pka of the relevant acid = 7.2).