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Buffer Systems Experiment: Measuring pH Changes in Deionized Water and Acetate Buffers, Lab Reports of Chemistry

A laboratory procedure for measuring the ph changes in deionized water and acetate buffers after adding hydrochloric acid (hcl) and sodium hydroxide (naoh). The experiment involves preparing the buffers, measuring their initial ph, and then adding acid or base dropwise while recording the ph at each interval until a significant change is observed. The document also includes instructions for preparing sorensen's phosphate buffer and determining its buffer capacity.

Typology: Lab Reports

Pre 2010

Uploaded on 08/16/2009

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Buffer Systems
PROCEDURE
System #1 – Deionized water – NO buffer.
1. Place 10.0 mL of deionized water into each of two sample cups.
2. Measure the pH. (Should be the same in both cups – if not, they were likely not properly cleaned.)
3. Into one cup, add one drop (assume 1 drop ~ 0.05 mL) of 0.10 M HCl. Measure the pH.
4. Add an additional 9 drops of the HCl. Measure the pH.
5. Into one cup, add one drop of 0.10 M NaOH. Measure the pH.
6. Add an additional 9 drops of the NaOH. Measure the pH.
Solution Calculated pH Observed pH
DI water
DI water + 1 drop 0.1 M HCl
DI water + 10 drops 0.1 M HCl
DI water + 1 drop 0.1 M NaOH
DI water + 10 drops 0.1 M NaOH
System #2 – pH of Acetate Buffer solutions
7. Obtain ~ 40.0 mL each of acetic acid (CH3CO2H = HOAc) and sodium acetate (NaCH3CO2 = NaOAc) and put
into separate burettes.
8. Deliver 10.0 mL of each solution to separate sample cups. Measure the pH of both solutions
9. Prepare Buffer A by mixing these two solutions. Measure the pH of the buffer solution. Save this solution.
10. Prepare Buffers B and C as indicated in the table below. Measure their pH’s. Save the solutions.
pH of acetic acid: pH of acetate:
Buffer Volume (mL)
0.10 M HOAc
Volume (mL) 0.10
M NaOAc
NaOAc/HOAc log(NaOAc/HOAc) Calculated pH Observed
pH
A 10.0 10.0
B 5.0 15.0
C 15.0 5.0
pf3

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Buffer Systems

PROCEDURE

System #1 – Deionized water – NO buffer.

  1. Place 10.0 mL of deionized water into each of two sample cups.
  2. Measure the pH. (Should be the same in both cups – if not, they were likely not properly cleaned.)
  3. Into one cup, add one drop (assume 1 drop ~ 0.05 mL) of 0.10 M HCl. Measure the pH.
  4. Add an additional 9 drops of the HCl. Measure the pH.
  5. Into one cup, add one drop of 0.10 M NaOH. Measure the pH.
  6. Add an additional 9 drops of the NaOH. Measure the pH.

Solution Calculated pH Observed pH DI water DI water + 1 drop 0.1 M HCl DI water + 10 drops 0.1 M HCl DI water + 1 drop 0.1 M NaOH DI water + 10 drops 0.1 M NaOH

System #2 – pH of Acetate Buffer solutions

  1. Obtain ~ 40.0 mL each of acetic acid (CH (^) 3CO (^) 2H = HOAc) and sodium acetate (NaCH3CO 2 = NaOAc) and put into separate burettes.
  2. Deliver 10.0 mL of each solution to separate sample cups. Measure the pH of both solutions
  3. Prepare Buffer A by mixing these two solutions. Measure the pH of the buffer solution. Save this solution.
  4. Prepare Buffers B and C as indicated in the table below. Measure their pH’s. Save the solutions.

pH of acetic acid: pH of acetate:

Buffer Volume (mL)0.10 M HOAc Volume (mL) 0.10 M NaOAc NaOAc^ / HOAc log ( NaOAc^ / HOAc ) Calculated pH ObservedpH

A 10.0 10.

B 5.0 15.

C 15.0 5.

  1. Divide each of the 3 buffers into two equal parts (10.0 mL each).
  2. For one sample of each buffer: a. Add one drop 0.10 M HCl and measure the pH. b. Add 9 additional drops and measure the pH. c. Add enough to change the pH by one unit from the start. Record the volume. d. Add enough to change the pH by two units (or more) from the start. Record the volume. e. Add enough to change the pH by three units (or more) from the start. Record the volume.

Buffer ObservedpH

Predicted Resistance: More to acid, base, or the same?

pH after 1 drop 0. M HCl

pH after 10 drops 0. M HCl

TOTAL volume HCl to change pH by one unit

TOTAL volume HCl to change pH by two units

TOTAL volume HCl to change pH by three units A B C

  1. For one sample of each buffer: a. Add one drop 0.10 M NaOH and measure the pH. b. Add 9 additional drops and measure the pH. c. Add enough to change the pH by one unit from the start. Record the total volume added at that point. d. Add enough to change the pH by two units (or more) from the start. Record the total volume added at that point. e. Add enough to change the pH by four units (or more) from the start. Record the total volume added at that point.

Buffer ObservedpH

Predicted Resistance: More to acid, base, or the same?

pH after 1 drop 0. M NaOH

pH after 10 drops 0. M NaOH

TOTAL volume NaOH to change pH by one unit

TOTAL volume NaOH to change pH by two units

TOTAL volume to change pH by four units A B C

Diluting a buffer:

  1. Prepare an additional sample of “Buffer A” by mixing 5.0 mL of sodium acetate and 5.0 mL of acetic acid. These volumes can be approximate. (You may use your 10-mL graduated cylinder to measure.)
  2. Measure the pH of the buffer.
  3. Dilute the buffer by adding 90.0 mL of DI water. The total volume is now 100. mL (therefore the concentrations of the buffer ions have decreased by a factor of ten.
  4. Measure the pH of the resulting solution.

pH of buffer before dilution: pH of buffer after dilution: