Buffers. This experiment lets us to know more about the buffer solutions and how the pH changes by adding acid and base to the buffers.

1. Get 7 beakers.
2. Label each of them.
3. Make a buffer solution contains Na2CO3 and NaHCO3.
4. Put the buffer solution equally into two beakers.
5. Make another buffer solution contains Na2HPO4 and NaH2PO4.
6. Put the buffer solution equally into another two beakers.
7. Get NaHCO3, NaHPO4 and water separately into the rest three beakers.
8. Use the pH paper to measure the approximate pH value.
9. Use the pH meter to measure the exact pH value.
10. Add one drop of HCl into all the beakers except those repeated two.
11. Measure the pH value again by pH meter and pH paper.
12. Add two drops of NaOH into the five beakers just measured.
13. Measure the pH one more time by pH meter and pH paper.
14. Put the pH meter into the rest two beakers.
15. Add HCl to the two solutions drop by drop until the pH reaches 2
16. Record the number of drops needed.

Apparatus:

Beakers (50ml) 8

Test-tube rack 1

Dropper 2

Graduated cylinder (10ml) 2

Glass stir bar 1

pH meter 1

Chemicals:

Distilled water

Universal pH paper (pH 1-14)

1M hydrochloric acid, HCl

1Msodium hydroxide, NaOH

0.1M sodium carbonate, Na2CO3

0.1M sodium hydrogen carbonate, NaHCO3

0.1M sodium hydrogen phosphate, Na2HPO4

0.1M sodium dihydrogen phosphate, NaH2PO4

Safety:

Follow the rules in the laboratory and wear the safety goggles. Watch out those strong acid and base that are extremely dangerous.

Procedure:

1. Get 7 beakers.
2. Label each of them from 1 to 7.
3. Get 10ml Na2CO3 solution and 10ml NaHCO3 solution.
4. Make a buffer solution contains Na2CO3 and NaHCO3 by mixing them together.
5. Put the buffer solution equally into beaker 1 and 2.
6. Get 10ml Na2HPO4 solution and 10ml NaH2PO4 solution.
7. Make another buffer solution contains Na2HPO4 and NaH2PO4 by mixing them together.
8. Put the buffer solution equally into beaker 2 and 4.
9. Get 10ml of NaHCO3, NaHPO4 and water separately into beaker 5, 6 and 7.
10. Use the pH paper to measure the approximate pH value of beaker 1, 3, 5, 6 and 7.
11. Use the pH meter to measure the exact pH value of beaker 1, 3, 5, 6 and 7.
12. Add one drop of HCl into beaker 1, 3, 5, 6 and 7.
13. Measure the pH value again by pH meter and pH paper.
14. Add two drops of NaOH into the five beakers just measured.
15. Measure the pH one more time by pH meter and pH paper.
16. Put the pH meter into beaker 2 and 4.
17. Add HCl to the two solutions drop by drop until the pH reaches 2
18. Record the number of drops needed.

Raw data:

Intial pH of beaker 1, 3, 5, 6 and 7

pH of beaker 1, 3, 5, 6 and 7 after adding HCl

pH of beaker 1, 3, 5, 6 and 7 after adding NaOH

pH of beaker 2 and 4

To decrease the pH of solutions in beaker 2 to 2 needs 22 drops of HCl

To decrease the pH of solutions in beaker 4 to 2 needs 13 drops of HCl

Photo of the results:

Initial pH of beaker 1 and 2                     Initial pH of beaker 3 and 4

     

Initial pH of beaker 5                          Initial pH of beaker 6

       

Initial pH of beaker 7

pH of beaker 1 after adding HCl                  pH of beaker 3 after adding HCl

       

pH of beaker 5 after adding HCl                  pH of beaker 6 after adding HCl

       

pH of beaker 7 after adding HCl

pH of beaker 1 after adding NaOH                  pH of beaker 3 after adding NaOH

       

pH of beaker 5 after adding NaOH                  pH of beaker 6 after adding NaOH

       

pH of beaker 7 after adding NaOH

The final pH of beaker 4 after adding HCl           The final pH of beaker 2 after adding HCl

       

Data analysis:

1. Through comparing we can see that when adding same volume of HCl and NaOH to different solutions, the pH changes are different. Solution 1 and 3, mixture of two solutions only change a little while solution 5 and 6 and beaker 7, water, change greatly.
2. The pH change of solution 1 is less than that of solution 3.
3. Water changes its pH the most.
4. Solution 2 needs more HCl to decrease the pH of solution 2 and 4 to 2 than solution 4.

Conclusion:

1. Buffer solutions, the mixture solutions, can resist a great pH change when adding small amount of acid and base into that.
2. Solution 1, the mixture of Na2CO3 and NaHCO3 is a better buffer solution than solution 2, the mixture of Na2HPO4 and NaH2PO4.
3. Water cannot resist a pH change at all.

Evaluation and improvement:

1. The pH value of water is much greater than the literature value. The beaker must contain some basic solutions that affect the pH of water. Next time the beakers must be fully cleaned and dried so there will be no impurities.
2. When adding HCl to beaker 2 and 4, the final pH is far less than 2. This is because the acid was added so fast and the whole solution was not stirred very well. Next time the solution should be added slowly, drop by drop. After adding one drop, the whole solution should be stirred for a long time.
3. The pH meter has been put into different solutions. It may affect the result. Next time after each trial, take the detector of the pH meter out and clean it. Then put it into another solution.

Reference:

[1] Buffer solution

John Green and Sadru Damji, Chemistry 3rd edition (Victoria: IBID press, 2007), P221