Experiment Title
Indication of pH and Buffer Solutions
Statement of Purpose
Train the preparation of the acetate buffer solution (pH = 4.8) and other solutions. Practice the pH measurements. Test the buffer capacity of the prepared buffer solution.
Hypothesis to test:
- property of buffer solution to keep pH, when the strong acid or base are added;
- red cabbage juice acts as a natural indicator, which helps to identify the pH of the solution.
Description of the Experiment
Acetate Buffer Preparation: mix 10 ml of 0.2 M acetic acid with 15 ml of 0.2 M sodium acetate, add 75 ml of distilled water was added. Store in an orange cap flask.
Preparation of Biological pH indicator: shred red cabbage, put into boiling water for ten minutes. Decant and allow to cool.
Solutions for buffer testing: 10 ml of 0.1 M HCl stock solution into the test tube 1. 1 ml of the stock solution into tube 2, add 9 ml of distilled water. Dilute solution from tube 2 10 times – tube 3. Dilute solution 3 10 times – tube 4. These solutions used to test the buffer.
The same operations repeated with NaOH stock solutions. Used to test the buffer.
Testing: 2 ml of red cabbage juice (anthocyanin) added to each test tube.
Acetate Buffer Activity: two test tubes with 10 ml of acetate buffer and two tubes with distilled water. Add 2 ml of anthocyanin to each tube. Add HCl by 100 µl to the test tubes with acetate buffer and water. Observe the color change, if appears. Procedure repeated with 0.1 M NaOH solution, acetate buffer and water.
Results
Calculations for Preparation of Acetate Buffer.
Needed: 100 ml of acetate buffer with 50 mM of acetate ions, which is the sum of acetic acid and sodium acetate. Thus, 20 mM of tic acid and 30 mM and sodium acetate are needed. Stock solutions used: 0.2 M (both).
The volumes of the stock solutions necessary for preparation of 100 ml of 50 mM acetate buffer are obtained from the basic formulae for concentrations:
Cacetic acid=υacetic acidV; (1)
Csodium acetate=υsodium acetateV, (2)
V = 100 ml (necessary volume of the acetate buffer);
Cacetic acid=20 mM;
Csodium acetate=30 mM,
υacetic acid and υsodium acetate – the quantities of moles of acetic acid and sodium acetate that are obtained by dilution of the stock solutions.
υcetic acid=CstockVstock1; (3)
υsodium acetate=CstockVstock2. (4)
Vstock1 and Vstock2 are the volumes of the stock solutions used to prepare acetate buffer.
υacetic acid=Cacetic acidV; (5)
υsodium acetate=Csodium acetateV; (6)
CstockVstock1=Cacetic acidV; (7)
CstockVstock2=Csodium acetateV; (8)
Vstock1=Cacetic acidVCstock; (9)
Vstock2=Csodium acetateVCstock; (10)
Vstock1=20∙10-3M∙100 ml0.2 M=10 ml; (9)
Vstock2=30∙10-3M∙100 ml0.2 M=15 ml; (10)
The volume of water to add:
V = Vstock1 + Vstock2 + Vwater;
Vwater = V - Vstock1 + Vstock2;
Vwater = 100 ml – 10 ml – 15 ml = 75 ml.
10 ml of acetic acid + 15 ml of sodium acetate stock + 75 ml of water = 100 ml of 50 mM of acetate buffer.
pH Tests
