Goals– learn how to use pH sensor and monitor changes in the pH of a buffer; observe the effects of buffering on the pH titration curve
Purpose– the purpose of this experiment is to titrate the lemonade drink and the 0.010 M citric acid with a base solution of sodium hydroxide, NaOH and compare the pH changes in the buffered lemonade drink and unbuffered solution of citric acid.
Introduction
Buffers are solutions that resist a change in pH when small amounts of acid H3O+ or base OH– are added. Buffers consist of a weak acid and its conjugate base (or a weak base and its conjugate acid). Both components must be present in the solution at approximately equal concentration and in substantial amounts. A buffer resists pH changes because it contains both an acid to neutralize the OH– added and a base to neutralize the H3O+ions added. An example of a buffer solution is weak acetic acid, HC2H3O2, and its salt sodium actetate, NaC2H3O2.
The dissociation of acetic acid in an aqueous solution is shown in equation (1). HC2H3O2(aq) + H2O(l) ←⎯ç→ H3O+(aq) + C2H3O2–(aq) (1)
The dissociation of sodium acetate in an aqueous solution is shown in equation (2). C2H3O2Na(aq) → C2H3O2–(aq) + Na+(aq) (2)
The buffer components are conjugate acid/base pair, HC2H3O2 and C2H3O2–
. When a small amount of acid solution is added to the buffer, most of the H3O+ ions are consumed reacting with the acetate ions, to form acetic acid, equation (3).
The equilibrium shifts to the left in the equation (1), decreasing added concentration of H+, keeping the pH constant.
H3O+(aq) + C2H3O2–(aq) ←⎯ç→ HC2H3O2 (aq) + H2O(l) (3)
When a solution of a base is added to the buffer, most of the OH– ions are removed when neutralized by acidic component of the buffer, more acetate is formed, equation (4). H3O+ and OH– will form water, and the equilibrium in equation (1) shifts to the right, restoring the H3O+ concentration and keeping the pH constant.
OH– (aq) + HC2H3O2(aq) ←⎯ç→ H2O(l) + C2H3O2– (aq) (4)
The buffering capacity represents the effectiveness of a buffer to resist the large changes in pH when a base or an acid is added. The amount of the conjugate acid–base pair will define the buffer capacity. The greater the amounts of the conjugate acid–base pair, the more resistant buffers will be to pH changes. When the species capable of neutralizing acid and base added are exhausted, the pH will change drastically.
Buffers are very important in biological and chemical systems. In the human body, buffers maintain the pH of many bodily fluids. For example, the pH of blood is 7.35–7.40 and it is maintained by the carbonic acid–bicarbonate ions (H2CO3 /HCO3– ). pH changes larger than 0.4 can be fatal.
In the case when the excess of H3O+ ions enter the blood stream, the pH is maintained by the following reaction: HCO3–(aq) + H3O+(aq) → H2CO3(aq) + H2O(l) (5) The excess of OH– ions is removed by the reaction :
H2CO3(aq) + OH–(aq) → HCO3–(aq) + H2O(l) (6)
Buffers are used in food industry as part of various consumer products to help control the effects of varying pH. The lemonade drink used in this experiment uses a citric acid– sodium citrate buffer to control the pH according to its label.
In this experiment, you will titrate the two solutions (buffered and unbuffered solution) of citric acid with 0.10 M NaOH , monitoring pH using pH Sensor and plot titration curves as pH versus volume of the titrant NaOH.
Processing the data
1. To calculate the number of moles of NaOH used in both titrations, you will need the
volume at the equivalence point and the molarity of NaOH solution.
Data and Results
Complete the data table below.
Include the raw LoggerPro data and the graph.
Data Lemonade Citric Acid
Concentration of NaOH (M) 0.10 0.10
Initial volume of NaOH, mL 100 100
Final volume of NaOH, mL 28.4 48.9
NaOH volume added before the largest pH increase (mL) 57 16
NaOH volume added after the largest pH increase (mL) 62 26
Results
Volume of NaOH added at the equivalence point (mL) 59.5 21
Moles NaOH (mol)
Discussion
1. Determine which solution, buffered or unbuffered reacted with more moles of NaOH when the equivalence point was reached?
2. Observe the graph of each titration. How does the titration curve of the buffered lemonade compare to the curve of the unbuffered citric acid solution?
3. What is the formula of citric acid and the sodium salt of citric acid?
