Chemical buffers prevent sudden changes in a solution's acidity. Buffer solutions keep a certain pH level nearly constant during the course of biological reactions and are crucial for many physiological processes. All laboratory experiments in biochemistry or biophysics require buffer solutions. Citric acid is an organic acid that can efficiently maintain pH range from 3 to 6.2. In this example, you would make 200 ml of a citric acid buffer solution with the concentration 0.1 molar and pH 5.5.
Calculate the mass of citric acid needed to make the buffer solution using the following formula: mass(citric acid)=192.12 x molarity (solution) x volume (solution)/1,000 ml. (The molar mass of citric acid is 192.12 g/mole.) In our example, mass(citric acid)=192.12 g/mole x 0.1 M x 200 ml/1,000 ml= 3.842 g.
Weigh citric acid on the scale and place the chemical in the beaker. Pour the distilled water into the beaker. The volume of the water added should be about 10 ml less than the final volume. In our example, you would add 190 ml.
Place a stir bar in the beaker with the solution. Put the beaker on the magnetic stirrer and turn it on to start stirring the solution.
Place the glass electrode of the pH meter in the solution and wait 20 to 30 seconds. Read the pH displayed by the instrument. It should be below 3.
Fill the plastic pipette with the sodium hydroxide solution.
Add three to five drops from the plastic pipette to the solution in the beaker. Wait 20 to 30 seconds.
Read the value displayed by pH meter and compare it with the desired pH.
Repeat Steps 6 and 7 until the pH of the solution reaches the designated value. In our example, it is 5.5.
Switch off the magnetic stirrer and take off the pH glass electrode.
Pour the solution into the graduated cylinder and add the distilled water to the final volume. In our example, add water to the mark "200 ml."