Objectives:
The objective of this lab was to learn how to measure the pH using the correct tools and procedures. Preparing a buffer from acetic acid and sodium acetate. Testing the ability of buffered and unbuffered solutions to resist pH changes when strong acids and bases are added
4.0 Materials and Methods:
Materials:
Beakers, distilled water, sodium chloride, solid sodium acetate, acetic acid, stirring rod, hydrochloric acid, pipette, pH meter, magnetic stirrer, spin bar, sodium hydroxide
Methods:
Objectives:
The objective of this lab was to learn how to measure the pH using the correct tools and procedures. Preparing a buffer from acetic acid and sodium acetate. Testing the ability of buffered and unbuffered solutions to resist pH changes when strong acids and bases are added
4.0 Materials and Methods:
Materials:
Beakers, distilled water, sodium chloride, solid sodium acetate, acetic acid, stirring rod, hydrochloric acid, pipette, pH meter, magnetic stirrer, spin bar, sodium hydroxide
Methods:
The preparation of the Buffer Solution
Ten 100 ml beakers are labelled as 1 to 10. 50 ml distilled water is added into beaker 1 and 6 and 50 ml 0.1 M sodium chloride into beaker 2 and 7. 1 g solid sodium acetate (CH3COONa) is weighed and transfer into beaker 3 and 8. 5 g solid sodium acetate (CH3COONa) is weighed and transfer into beaker 4 and 9. 10 g solid sodium acetate (CH3COONa) is weighed and transfer into beaker 5 and 10. 50 ml 0.1 M acetic acid is added to beaker 3, 4, 5, 8, 9 and 10. The solution is stirred until all the solid dissolve.
The Determination of Buffering Action Toward Acid
The pH meter is calibrated. The pH value of distilled water is determined in beaker 1. The value is recorded. 1 ml 6.0 M hydrochloric acid is added into beaker 1. The solution is mixed and the new pH value of the solution is determined. The pH value is recorded. Step 3 is repeated until there is only a slight change in the pH value. Step 2 to Step 4 is repeated for each beaker 2, 3, 4 and 5.
The Determination of Buffering Action Toward Base
All the steps in Section B are repeated by replacing 6.0 M hydrochloric acid with 6.0 M sodium hydroxide as well as beaker 1-5 with beaker 6-10.
5.0 Results & Discussion:
Results
The preparation of the Buffer Solution
- The Determination of Buffering Action toward Acid
- The Determination of Buffering Action toward Base
Discussion:
- From your results in this experiment, which solution of those you tested had the greatest buffer capacity:
- Toward strong base? Discuss.
Buffer capacity is the ability of a solution to resist changes in pH by either absorbing or desorbing H+ and OH- ions when small amount of acid or base is added. Buffer capacity is also a quantitative measure of resistance to pH change upon the addition of H+ or OH- ions. From the results of the experiment, solution in test tube 5 had the greatest buffer capacity toward strong acid, which is 10g of solid sodium acetate (CH3COONa) added with 50 ml 0.1 M acetic acid. This solution has the smallest change in the pH value when 1 ml 6.0 M hydrochloric acid is added into it. The pH value is changed from 8 to 7. With the addition of hydrochloric acid (HCl), the increase of H+ ions are counteracted with the excess of acetate ions to form CH3COOH. Thus, the added H+ ions are neutralized and the pH of the buffer solution is changed. However, due to the increased mass of sodium acetate (CH3COONa) in the solution in test tube 5, the equilibrium shifts only slightly to the right to increase H+ ions compare to solution in test tube 1 to 4. This explains the marginal increase of pH of the buffer solution on addition of hydrochloric acid (HCl).
CH3COO- + H+ CH3COOH
On the other hand, solution in test tube 10 had the greatest buffer capacity toward strong base, which is also 10g of solid sodium acetate (CH3COONa) added with 50 ml 0.1 M acetic acid. This is because the smallest change in pH value is detected in this solution when 6.0 M of sodium hydroxide (NaOH) is added into it. During the experiment, the pH value of the solution is test tube 10 remain unchanged which is 7. When NaOH is added into test tube 10, the additional OH– ions of NaOH combine with H+ ions of the buffer solution to form H2O. When the mass of sodium acetate (CH3COONa) has increased in the solution of test tube 10, and NaOH is added into it, OH– ions
associate with excess of NH +
ions to form unassociated NH4OH. As a result, the equilibrium shifts to
the right to produce more H+ ions till the excess OH– ions are neutralized. Hence the pH value of the buffer solution in test tube 10 is maintained approximately constant.
NH4+ + OH-NH4OH
- Why was distilled water used to rinse off the pH probe?
A pH meter is an electronic device that used to measure the pH value of the acidity and alkalinity of substances. The accuracy of pH meters will decrease with every use and need to be calibrated on a regular basis. During regular calibrating, the probe of pH meter needs to be cleaned before and after every use to avoid contamination of the substances when a solution is being measured. Distilled water will work as a general cleaner to rinse off the pH probe. Distilled water is used to clean the probe because it does not contain any dissolved salts and minerals in it and has lowest levels of contaminants. It is composed of nothing more than water (H2O).
- Define buffer solution.
Buffer solution is an aqueous solution containing weak acid and its conjugate base or a weak base and its conjugate acid. Buffers can be prepared in multiple ways as in this experiment by creating a solution of an acid and mixed with its conjugate base. Buffer solution is able to resists the change in pH when a small amount of either an acid or a base is added. It is used to avoid any change in the pH of a solution, regardless of solute. Buffer solutions are used to keep the pH value at a nearly constant value in a wide range of chemical applications. Buffer solutions has ability to avoid pH change because of the presence of an equilibrium between the acid (HX) and its conjugate base (X–). The balanced equation for this reaction is:
HX ⇌ H++X−
In accordance with Le Chatelier’s principle, if strong acid (more H+) is added to an equilibrium
mixture of the weak acid and its conjugate base (salt), the equilibrium is shifted to the left. This causes the hydrogen ion (H+) concentration to increase by less than the quantity expected for the amount of strong acid added. Correspondingly, when a strong base is added to the mixture, the hydrogen ion concentration (H+) decreases by less than the quantity expected for the amount of base added. This is because the reaction shifts to the right to accommodate for the loss of hydrogen ion (H+) in the reaction with the base.
6.0 Conclusion:
In a nutshell, we are able to understand the nature of buffer. In other words, we could define nature of buffer in which solution that resists a change in pH when acids or bases are added. Besides, we are able to prepare a buffer from acetic acid and sodium acetate. Sodium acetate is a weakly basic salt and the conjugate of acetic acid. A mixture of sodium acetate and acetic acid makes good buffer for a weakly acidic solution. If hydrochloric acid is added to acetate or acetic solution, the hydronium ions produced by the nearly complete dissociation of the hydrochloric acid react with the acetate ions to form molecular (non-dissociated) acetic acid. According to Le Chatelier’s Principle, the equilibrium is forced to the left, reducing the concentration of hydronium and acetate ions, and increasing the concentration of the molecular acetic acid in the solution. If sodium hydroxide is added to the buffer solution, the NaOH ionizes completely in solution, yielding hydroxide ions and sodium ions. According to Le Chatelier’s Principle, the increase in hydroxide ions forces the acetic acid equilibrium to the right, decreasing the concentration of hydroxide ions and increasing the concentration of acetate ions.
7.0 References:
Reference:
Hula Nicki, A. (1987). Reactions of acids and bases in analytical chemistry. Translated by Masson, Mary R. Horwood. ISBN 0-85312-330-6.
Scorpio, R. (2000). Fundamentals of Acids, Bases, Buffers & Their Application to Biochemical Systems. ISBN 0-7872-7374-0.
Mendham, J.; Denny, R. C.; Barnes, J. D.; Thomas, M. (2000). "Appendix 5". Vogel's textbook of quantitative chemical analysis (5th ed.). Harlow: Pearson Education. ISBN 0-582-22628-7.
Appendixes:
The Preparation of the Buffer Solution in 10 beakers
Initial pH Value from beaker 1 to 10
Final pH Value from beaker 1 to 10
The calculation of molarity of chemical needed before
preparation stock solution for chemical in solid or liquid