Experiment 3

Experiment 3

Preparation of Buffer Solutions

Experimental Determination of pH

Buffer Action

Agustin, Aileen Concepcion M. Doloroso, Gianfranco Emmanuel M.

MD-1

Dr. Gracia Urubio Urmeneta

Date of Experiment:

August 10, 17, 2015

Date of Submission:

ABSTRACT

The purpose of this laboratory activity was to understand

INTRODUCTION

The experiment was conducted

METHODOLOGY Experiment 3

Prepare a 0.10 M buffer solution using 0.20 M salt solutions whose pH and volume will be assigned to you. The pH range will be between 2.0 to 8.2. Show your calculations for the volumes of the acid (base) and the salt to be used. Measure the calculated volumes using burettes. Mix and check the pH using a pH meter. Adjust the pH if necessary.

Prepare the following solutions and keep them for experiment 4: 1. 200 ml of 0.10 M NaOAc/HOAc buffer, pH 3.8

2. 200 ml of 0.10 M NaOAc/HOAc buffer, pH 4.7 3. 200 ml of 0.10 M Na2HPO4/NaH2PO4 buffer, pH 6.8

4. 200 ml of 0.01 M Na2HPO4/NaH2PO4 buffer, pH 6.8

RESULTS AND DISCUSSION

Questions

1. Review the following: a) concepts of acids and bases, b) methods of expressing concentrations of solutions, c) Ka, pKa, pH, Kb, pOH, Kw, pKw.

2. Derive the Henderson-Hasselbach equations for a solution of a weak acid, HA, and its salt, NaA. Do the same for a solution of a weak base, MOH, and its salt, MA.

3. What is the biological importance of pH?

5. Explain the possible discrepancy between the calculated pH and the experimental pH of the prepared buffer.

6. A buffer solution is prepared by mixing 200 ml of 0.20 M salt solution and 400 ml of 0.2 M acid solution. What is the concentration of the resulting buffer solution?

7. The pKa of HOAc is 4.7. Calculate the concentration of NaOAc that must be present in 0.1

M solution of acetic acid so that the solution will have a pH of a) 5.7 and b) 3.7.

8. How would you prepare 400ml of a 0.10 M buffer solution with a pH of 6.6 from 0.2 M solutions of acid and its salt?

9. Calculate the pH of a solution prepared by mixing 80.0 ml of 0.20 M NaH2PO4 and 120.0

ml of 0.20 M NaH2PO4 solution.

METHODOLOGY Experiment 4

A. Preparation of Colorimetric Buffer Standard Solutions

Prepare the colorimetric buffer standard solution assigned to you by pipetting 4.0 ml of the buffer into a vial and adding 2 drops (small vial dropper) of the indicator for this pH. (Refer to the preceding table). Label the vial with the pH and the initials of the indicator and submit to the instructor.

B. Colorimetric Determination of pH

Determine colorimetrically the pH of 0.01 N HCl, 0.01 N HOAc and 0.2% gelatin solution. Determine the approximate pH range of the samples by using either a universal indicator (example: Bogen indicator) or a set of trial indicators. If Bogen indicator is being used, touch its dropper to a cavity of your spot plate and add a drop of the solution. The colors given below will indicate the approximate pH.

Color pH Red 2.0 Orange 4.0 Yellow 6.0 Green 8.0 Blue 10.0

Sample

MR-MB

Purple (pH ≤ 4.5) Brown (pH = 4.6-5.9) Green (pH ≥ 6.0)

MO H2Ph Red (pH ≤ 3.1) Orange (pH = 3.2-4.3) Yellow (pH ≥ 4.4) Colorless (pH ≤ 8.2) Pink (pH = 8.3-9.9) Red (pH ≥ 10.0)

A set of three trial indicators:

Trial Indicators pH range Color Change

Methyl Red-Methylene Blue (MR-MB) 4.5-6.0 Purple-Green

Methyl Orange (MO) 3.1-4.4 Red-Yellow

Phenolpthalein (H2Ph) 8.3-10.0 Colorless-Red

The pH range of the sample is first determined by the use of methyl red-methylene blue (MR-MB). Place 2 drops of the sample in a cavity of a spot plate and add a drop of MR-MB. If the color obtained is purple or pink, the pH of the sample is 4.5 or below. Place another 2 drops of the sample in a cavity of a spot plate and add a drop of methyl orange (MO). If the color obtained with MO is red, then the approximate pH range of the sample is 3.1 or below; if orange, the pH range is between 3.1 to 4.4. If the color obtained with MR-MB is green, the pH of the sample is 6.0 or above. Place another 5 drops of the sample in the spot plate and add a drop of phenolphthalein (H2Ph)). If the color with H2Ph is colorless, the pH range is between 6.0 to 8.3,

if it is red or red, the pH is 8.3 or above.

Knowing the approximate pH range, make the final pH determination by pipetting 4.0 ml of the solution into a vial and adding 2 drops (small vial dropper) of the indicator used in the preparation of the colorimetric buffer standards covering this pH range. Compare with the buffer

standards containing this same indicator. Use a clean piece of bond paper as background. The pH of the solution is the pH of the buffer standard that matches closely in color intensity.

For solutions that are slightly colored or turbid like the gelatin solution, place a vial of distilled water behind the gelatin with the indicator and a vial with gelatin behind the buffer standard during the color comparison.

Calculate the hydrogen ion concentration of the sample tested.

C. Electrometric Determination of pH

Determine the pH of the following samples using the pH meter: soft drinks, vinegar, filtered fresh pineapple juice, calamansi juice, urine, filtrate from soil suspension, sea water, wine.

RESULTS AND DISCUSSION

Questions

1. Compare the true or actual acidities of 0.01 N HCl and 0.01 N HOAc and account for the differences, if any. How would their total or titratable acidities compare?

2. Can the colorimetric method be used to determine the pH of water? Milk? Blood? Why? 3. What physical qualities of vinegar, fruit juices, soft drinks, and wine are affected by pH? METHODOLOGY

Experiment 5

A. Prepare the following samples:

1. Distilled H2O (freshly boiled in an Erlenmeyer flask and cooled under tap H2O with a

cover) 2. 0.10 M NaOAc 3. 0.10 M HOAc 4. 0.1 M acetate buffer (pH 4.74) 5. 0.1 M acetate buffer (pH 3.8) 6. 0.1 M phosphate buffer (pH 6.8) 7. 0.01 M phosphate buffer (pH 6.8) 8. Fruit homogenate or juice

B. Pipet 20.o ml each of samples 1, 2, 3, 4, 8, and 9 into separate beakers. Determine the pH electrometrically. Add 0.1 ml of 0.01 M NaOH to solutions 1-4. For solutions 8 and 9, add 0.01 ml of 0.01 M NaOH. Mix well. Determine the pH up to the second decimal place. C. Repeat B adding 0.1 ml of 0.01 M HCl instead of NaOH to solutions 1-4 and 0.01 ml of 0.01

M HCl to solutions 8 and 9. D. Buffer Capacity

*Pipet 20.0 ml each of samples 4-7 and determine the pH electrometrically. To each of the solutions, add 2.5 ml of standard 0.1 N NaOH. Mix well and determine the pH

electrometrically. Repeat from * but use 0.1 N HCl instead of NaOH. Calculate the buffer capacity of the four solutions.

RESULTS AND DISCUSSION

Questions

1. Which of the samples exhibit buffer action? Give the chemical equations involved. 2. Represent with ionic equations the buffering action of a HCO3-/H2CO3 system.

3. Why should the distilled water be boiled and cooled before determining its pH? 4. What factors affect buffer capacity?

5. At what pH does a buffer solution exhibit maximum buffer efficiency? 6. What are universal buffer solutions?

7. What is the pH of a buffer solution containing equal volumes of 0.10 M HOAc and 0.10 M NaOAc? Calculate the pH when a) 0.01 mole of HCl has been added to 1 liter of the buffer; b) 0.01 mole of NaOH has been added to 1 liter of the buffer.

CONCLUSION

REFERENCES