Buffered Solutions

Summary:   The buffering capacity of two solutions are compared with small additions of acid and base.

Hazards: 

Chemicals and Solutions: 

Buffered solution at pH 7

Unbuffered HCl solution at pH 7

Universal indicator

Universal indicator key

0.1 M HCl

0.1 M NaOH

Materials: 

4-standard hydrometer cylinders

Procedure: 

Fill 2 cylinders half full of unbuffered solution.

Fill the remaining 2 cylinders half full of buffer solution.

Arrange solutions as two pairs of unbuffered and buffered solution.

Use small additions of HCl to show buffering capacity of buffered solution vs. unbuffered water.  Do the same using NaOH.

Hint: 

Discussion: 

Disposal:

After neutralization solutions can be poured down the drain. Demo usually generates 300 mL neutralized 2.5 x 10-5 M HCl and 300 mL 0.5M sodium acetate/acetic acid buffer.

References:

Method 2:

DEMO #45

BUFFERED SOLUTIONS

Hazards: 

Chemicals and Solutions: 

Buffered solution at pH 4

Unbuffered HCl solution at pH 4

Methyl purple indicator, dropper bottle

Thymol blue indicator

tap water (NOT deionized water)

0.1 M HCl

0.1 M NaOH

Materials: 

6-standard hydrometer cylinders

Procedure: 

Fill 3 cylinders half full of unbuffered solution.

Fill the remaining 3 cylinders half full of buffer solution.

Arrange solutions as three pairs of unbuffered and buffered solution.

Dilution effect using water and methyl purple:

To first set (one buffered and one non-buffered) add methyl purple (6-8 drops).

Dilute each with tap water.

Hint: 

Hint:  This only works if the solution is at or above the pH of 4.0.  More concentrated solutions require too great a dilution.  Bagley’s DI water is acidic, so it won’t work with the DI water either.

Discussion: 

Buffer:            no change

Non-buffer:     turns green

Acid effect using hydrochloric acid and thymol blue indicator:

To second set add thymol blue indicator.  Add: 0.1 M HCl to each.

Buffer:            no change

Non-buffer:     turns pink

Base effect using thymol blue and sodium hydroxide:

To third set add thymol blue indicator. Add: 0.1 M NaOH to each.

Buffer:            no change

Non-buffer:     turns blue

After neutralization solutions can be poured down the drain. Demo usually generates 300 mL neutralized 2.5 x 10-5 M HCl and 300 mL 0.5M sodium acetate/acetic acid buffer.

To schedule a demonstration, please send an email to the demonstration lab.

 

Contact:

Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606

ericcamp@uw.edu

 

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