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Partner: Alisa 1 March 2012
Preparation and Properties of Buffer Solutions
Purpose: The purpose of this experiment is to compare the pH effect on buffered and non-buffered solutions as well as making a buffer of a certain pH. This can be done by observing the change in pH of the buffered solution and non-buffered solutions. The buffer of a certain pH can be made by mixing the conjugate base and acid solution. The application of this lab is that it could be use to make a model of the human body which has a certain pH and conduct experiments on it. For example, this model of the human body could be used to figure out the buffer capacity of the human body, the medicine effect on it, etc.
Hypothesis: The hypothesis is that the buffered will have a much higher resistance to pH change than does a non-buffered solution and that the buffered of a certain pH can be made by mixing the appropriate amount of conjugate acid and base. This is because the buffered solutions have the
conjugate acid-base pair that nullifies the effect of acid and base added to the solution. The pKa can
be determined from using a half-neutralization technique. Once the pKa has been found, the volume
of acid and base needed can be calculated with the Henderson-Hasselbach equation.
Materials:
Materials Quantities Distilled water 220 mL Sodium chloride solution (NaCl) 40 mL Acetic acid solution (CH 3 COOH) 20 mL Sodium acetate solution (CH 3 COONa) 20 mL Ammonia solution (NH 3 ) 20 mL Ammonium chloride solution (NH 4 Cl) 20 mL 0.1 M Sodium hydroxide solution (NaOH) 5 mL 0.2 M Sodium hydroxide solution (NaOH) 30 mL Potassium hydrogen phthalate (KHP) 1.9 g 1.0 % phenolphthalein 1 mL 0.1 M Hydrochloric acid solution (HCl) 5 mL 0.0001-g precision balance 1 balance pH meter 2 meters 10-mL graduated cylinder 5 cylinders 25-mL graduated cylinder 6 cylinders 50-mL graduated cylinder 2 cylinders 250-mL graduated cylinder 1 cylinder Pipette 10 pipettes Size-12 stopper 4 stoppers Burette 1 burette 250-mL Erlenmeyer flasks 2 flasks Wash glass 1 wash glass Ring stand 1 ring stand Burette clamp 1 clamp 100-mL beaker 11 beakers
600 - mL beaker 2 beakers Stirring rod 1 stirring rod Small funnel 1 funnel
Procedure:
Part 1. pH Changes in Non-buffered Solutions
1.) Prepare two beakers of 20 mL of distilled water 2.) Prepare two beakers of 20 mL of NaCl solution 3.) Record the pH of the solution each beaker 4.) Add one drop of 0.1 M HCl into a beaker with distilled water 5.) Record the pH of the solution 6.) Add one more drop of 0.1 M HCl into the same beaker with distilled water 7.) Record the pH of the solution 8.) Put an additional one drop of 0.1 M HCl into the same beaker with distilled water 9.) Record the pH of the solution 10.) Repeat step 4 to step 9 for a beaker of NaCl solution 11.) Add one drop of 0.1 M NaOH into the other beaker distilled water 12.) Record the pH of the solution 13.) Add one more drop of 0.1 M NaOH into that beaker with distilled water 14.) Record the pH of the solution 15.) Put an additional one drop of 0.1 M NaOH into the same beaker with distilled water 16.) Record the pH of the solution 17.) Repeat step 11 to step 16 for the other beaker of NaCl solution
Part 2. pH Changes in Buffered Solutions
1.) Prepare two beakers of 10 mL of 0.1 M CH 3 COOH 2.) Add 10 mL of 0.1 M CH 3 COONa into each of the beaker 3.) Record the pH of the solution in each beaker 4.) Add a drop of 0.1 M HCl into a beaker of the buffered solution 5.) Record the pH of the solution 6.) Add another drop of 0.1 M HCl into the same beaker 7.) Record the pH of the solution 8.) Add one more drop of HCl into the same beaker 9.) Record the pH of the solution 10.) Add a drop of 0.1 M NaOH into the other CH 3 COOH beaker 11.) Record the pH of the solution 12.) Add one more drop of NaOH into the beaker 13.) Record the pH of the solution 14.) Add another drop of NaOH into the same beaker 15.) Record the pH of the solution 16.) Prepare two beakers of 10 mL of 0.1 M NH 3
Part 1. pH Changes in Non-buffered Solutions Step number Non-buffered Solution^ Initial pH^
pH after Addition of HCl 1 st^ drop 2 nd^ drop 3 rd^ drop
- Distilled water 6.04 3.725 3.39 3.
- 0.10 M NaCl solution 6.29 3.82 3.435 3. Step number Non-buffered Solution^ Initial pH^
pH after Addition of NaOH 1 st^ drop 2 nd^ drop 3 rd^ drop
- Distilled water 5.935 10.005 10.40 10.
- 0.10 M NaCl solution 6.10 9.57 10.085 10.
Part 2. pH Changes in Buffered Solutions Step number Buffered Solution^ Initial pH^
pH after Addition of HCl 1 st^ drop 2 nd^ drop 3 rd^ drop
- Acetate buffer 4.51 4.49 4.48 4.
- Ammonia buffer 9.015 9.00 8.98 8. Step number Buffered Solution^ Initial pH^
pH after Addition of NaOH 1 st^ drop 2 nd^ drop 3 rd^ drop
- Acetate buffer 4.46 4.455 4.45 4.
- Ammonia buffer 9.00 8.98 8.97 8.
Part 3. Preparation of a Buffer of a Given pH Weak acid Potassium Hydrogen Phthalate Mass of weak acid 1.8801 g Volume of NaOH titrant 21.6 mL pH of buffer solution 5.
pKa of weak acid 5.
Assigned pH 5. Volume of weak acid solution 5.59 mL Volume of conjugate base solution 44.41 mL Measured pH 5. Initial pH (^1) st (^) drop pH after Addition of HCl 2 nd (^) drop 3 rd (^) drop 5.905 5.89 5.875 5. Initial pH (^1) st (^) drop pH after Addition of NaOH 2 nd (^) drop 3 rd (^) drop 5.905 5.91 5.915 5.
Calculation process of the Volume Needed of the certain pH
1.)
2.)
3.)
4.) 5.) 6.) 7.) 8.) 9.) 10.) 11.)
Calculation
1.) 2.) 3.) 4.) 5.) 6.) 7.) 8.) 9.) 10.) 11.) 12.) 13.)
Analysis: The hypothesis could be verified. The non-buffered solution has a much greater change in pH than did the buffered solution. Also, the buffer prepared was very close to the assigned pH. In step 1, the strong acid greatly decreases the pH of distilled water, making it acidic. The addition of NaCl did not buffer the pH changes because both the Na+^ and Cl-^ ions are conjugate of a strong base or acid. The conjugate of strong base or acid is negligibly weak. The Cl-^ ions, therefore, do not react with the strong acid and do not buffer the solution. Strong base causes the pH of distilled water to increase greatly, in contrast to adding strong acid to distilled water. NaCl did not buffer the solution because the Na+^ that dissociates from the salt were a conjugate of a strong base thus has a
