Partner: Alisa 1 March Preparation and Properties of Buffer Solutions

Transcription

1 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 pk a can be determined from using a half-neutralization technique. Once the pk a has been found, the volume of acid and base needed can be calculated with the Henderson-Hasselbach equation. Materials: Materials Quantities Distilled water 2 Sodium chloride solution (NaCl) 40 ml Acetic acid solution (CH 3COOH) Sodium acetate solution (CH 3COONa) Ammonia solution (NH 3) Ammonium chloride solution (NH 4Cl) 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 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

2 600-mL beaker Stirring rod Small funnel 2 beakers 1 stirring rod 1 funnel Procedure: Part 1. ph Changes in Non-buffered Solutions 1.) Prepare two beakers of of distilled water 2.) Prepare two beakers of 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 3COOH 2.) Add 10 ml of 0.1 M CH 3COONa 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 3COOH 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

3 17.) Add 10 ml of 0.1 M NH 4Cl into each of the beaker 18.) Record the initial ph of the solution 19.) Repeat step 4 to step 9 for a beaker of ammonia buffered solution 20.) Repeat step 10 to 15 for the other beaker of the buffered solution Part 3. Preparation of a Buffer of a Given ph 1.) Weigh out about 1.9 g of KHP 2.) Add the mass of KHP into a 250-mL graduated cylinder 3.) Add distilled water to the graduated cylinder until the volume reaches 150 ml 4.) Add the KHP solution into a 250-mL Erlenmeyer flask 5.) Label the flask. 6.) Pour 75 ml of the KHP solution into another flask 7.) Titrate it with 0.2 M NaOH 8.) Add the same volume of distilled water as the volume of NaOH used to titrate to the flask that was not titrated 9.) Pour 10 ml of the titrated KHP solution into a beaker 10.) Add 10 ml of the non-titrated KHP solution into the same beaker 11.) Record the ph of the solution 12.) Calculate the volume of the weak acid and conjugate base needed to make 50 ml of the solution of a given ph 13.) Make the buffer of the desired ph using the volumes calculated 14.) Record the ph of the solution 15.) Pour of the buffered solution into two beakers 16.) Add a drop of 0.1 M HCl solution into a beaker 17.) Record the ph of the solution in the beaker 18.) Add another drop of 0.1 M HCl into the same beaker 19.) Record the ph of the solution 20.) Add one more drop of the HCl into the beaker 21.) Record the ph of the solution 22.) Add a drop of 0.1 M NaOH solution into the other beaker of buffer 23.) Record the ph 24.) Add one more drop of NaOH into the same beaker 25.) Record the ph 26.) Add another drop of NaOH solution into the beaker 27.) Record the ph of the solution in the beaker Results: In the non-buffered solution, ph changes greatly when the first drop was added. However, the second and third drops have less much less effect on the ph of the non-buffered solution. The acid and base effect on ph decrease as more drops are added. In the buffered solution, the ph changes were small. The changes in ph were so small that no consistent pattern could be found.

4 Part 1. ph Changes in Non-buffered Solutions ph after Addition of HCl Non-buffered Solution Initial ph 1. Distilled water M NaCl solution ph after Addition of NaOH Non-buffered Solution Initial ph 3. Distilled water M NaCl solution Part 2. ph Changes in Buffered Solutions ph after Addition of HCl Buffered Solution Initial ph 5. Acetate buffer Ammonia buffer ph after Addition of NaOH Buffered Solution Initial ph 7. Acetate buffer Ammonia buffer Part 3. Preparation of a Buffer of a Given ph Weak acid Potassium Hydrogen Phthalate Mass of weak acid g Volume of NaOH titrant 21.6 ml ph of buffer solution 5.00 pk a of weak acid 5.00 Assigned ph 5.90 Volume of weak acid solution 5.59 ml Volume of conjugate base solution ml Measured ph Initial ph ph after Addition of HCl Initial ph ph after Addition of NaOH

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

6 negligible tendency as an acid. Therefore, the Na + ions do not react with the strong base added and do not buffer the ph. When the HCl was added to the acetic buffer, the CH 3COO - reacted with the HCl, buffering it. The CH 3COO - is the conjugate of a weak acid, CH 3COOH, thus is a weak base and is able to neutralize HCl. This causes the ph to have only a slight change in ph. When the NaOH was added to the solution, the CH 3COOH ions in the buffer react with NaOH, forming the CH 3COO -, Na +, and water. CH 3COOH has the ability to suppress the change in ph because it is a weak acid and is able to react with NaOH. The ph decrease after adding base could be because the CH 3COO - formed from the reaction between NaOH and CH 3COOH reacts with water to form CH 3COOH. When HCl was added to the ammonia buffer, ammonia ions acted as weak base and react with HCl, partly neutralizing it. In the case of adding NaOH to buffer, the NH 4 + ions is a weak acid and reacts with NaOH to reduce its effect on the ph. The ph also decreases when strong base was added. A possible explanation for this is that the NH 3 formed then reacts with water to reform NH 4 +. The prepared buffer was close to the assigned ph. The ph of the prepared buffer was 5.905, more than the assigned buffer. The ability of the prepared buffer to resist change was relatively good. The ph change was about 0.01 when a drop of strong acid or base was added. It was not genuinely good because a very good buffer would probably has less change in from one drop of 0.1 M strong acid and base. Conclusion: The hypothesis was could be confirmed true in this experiment. There could be some possible errors during the course of the experiment. An error could be that when the ph meter was rinsed repeatedly, the water that was used to rinse the ph meter became contaminated with other solution. This could have caused the latter solution to be affected by the ph of the contaminated water. Another error could be that the results were inconsistent due to using two ph meters. The two ph meter could be calibrated with a slight difference. Thus, this could affect the ph in making it difference for each step. There are ways to prevent these errors in the future. The first error could be prevent by constantly changing the distilled water use for rinsing. The only reason that the ph of would be contaminate enough to affect the ph of the buffer solution is because that the contamination builds up. Therefore, if the rinsing distilled water is constantly changed, the contamination would not build up. The other error could be prevent by using only one ph meter throughout the whole duration of the lab. This may cause the experiment to take longer, but it should make the data more consistent.

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