How can I make buffered solutions with specific solute concentrations and pH? PreLab 3.6

BIOL 1406

PreLab 3.6

How can I make buffered solutions with specific solute concentrations and pH?

A buffered solution generally contains several different solutes. At a minimum, it must contain both the acid and base components of the buffer. In addition, buffered solutions often contain other solutes such as nutrients, electrolytes, and/or various biomolecules. To prepare a buffer with specific concentrations of several different solutes, do a separate calculation to determine the amount of each solute needed (using the procedures you learned in Exercise #2), mix all of the solutes together, and then add enough solvent to bring the solution to its final volume.

In addition to controlling solute concentrations, making a buffer also involves controlling pH. The pH of a buffer is determined by the relative concentrations of the acid and base components. For example, in lab you will be preparing an acetate buffer. In this buffer, the acid component is acetic acid and the base component is the acetate ion. As long as the concentrations of these 2 components are equal, the pH of the acetate buffer will be 4.6. Therefore, if you prepare one acetate buffer where the concentration of each component is 50 mM and a second acetate buffer where the concentration of each component is 900 mM, both buffers will have a pH of 4.6. On the other hand, even though both buffers will have the same pH, the second buffer will have a much higher buffering capacity, meaning it will have a much greater ability to resist changes in pH as additional acid or base is added to the buffer.

It is also important to realize that as the relative concentration of the acid component of a buffer increases, the pH of the buffer will decrease. For example, an acetate buffer containing 88 mM acetic acid and 16 mM sodium acetate (a source of acetate ions) will have a pH of 3.8 (compared to a pH of 4.6 when both components have the same concentration.) On the other hand, as the relative concentration of the acid component of a buffer decreases, the pH of the buffer will increase. For example, an acetate buffer containing 21 mM acetic acid and 79 mM sodium acetate will have a pH of 5.2.

There are several ways to control the pH of a buffer. One method is to start with one buffer component (either acid or base) and then gradually add the second component until the desired pH is reached (the pH can be measured with a pH meter.) A second method is to use an equation to calculate the concentrations of acid and base components needed to produce a buffer with a specific pH. A third method is to consult a reference to determine the concentrations of acid and base components needed to produce a buffer with a specific pH. In this lab you will make two buffered solutions using the third method.

YOUR TURN
In lab, the following stock solutions will be available: 0.1 M sucrose 0.2 M acetic acid 0.2 M sodium acetate 0.2 M anhydrous sodium carbonate 0.2 M sodium bicarbonate
A. Calculate the amount of 0.1 M sucrose solution and the amount of dH₂O needed to prepare 100 mL of unbuffered 20 mM sucrose. You will need this information for lab, so write your answers in the spaces below and in your lab notebook.
Amount of 0.1 M sucrose solution needed	Hint	Check your answer.
Amount of dH₂O needed	Hint	Check your answer.
B. The acetate buffer consists of acetic acid (CH₃COOH) and acetate ions (CH₃COO^-). A chemistry reference indicates that a solution containing 40 mM acetic acid and 60 mM sodium acetate (a source of acetate ions) will produce an acetate buffer with a pH of 4.8. Calculate the amount of each stock solution and the amount of dH₂O needed to prepare 100 mL of an acetate buffer that contains 20 mM sucrose, 40 mM acetic acid, and 60 mM sodium acetate. You will need this information for lab, so write your answers in the spaces below and in your lab notebook.
Amount of 0.1 M sucrose solution needed	Hint	Check your answer.
Amount of 0.2 M acetic acid solution needed	Hint	Check your answer.
Amount of 0.2 M sodium acetate solution needed	Hint	Check your answer.
Amount of dH₂O needed	Hint	Check your answer.
C. The bicarbonate buffer consists of carbonate ions (CO₃^-2) and bicarbonate ions (HCO₃^-). A chemistry reference indicates that a solution containing 22 mM anhydrous sodium carbonate (a source of carbonate ions) and 28 mM sodium bicarbonate (a source of bicarbonate ions) will produce a bicarbonate buffer with a pH of 9.8. Calculate the amount of each stock solution and the amount of dH₂O needed to prepare 100 mL of a bicarbonate buffer that contains 20 mM sucrose, 22 mM anhydrous sodium carbonate, and 28 mM sodium bicarbonate. You will need this information for lab, so write your answers in the spaces below and in your lab notebook.
Amount of 0.1 M sucrose solution needed	Hint	Check your answer.
Amount of 0.2 M anhydrous sodium carbonate solution needed	Hint	Check your answer.
Amount of 0.2 M sodium bicarbonate solution needed	Hint	Check your answer.
Amount of dH₂O needed	Hint	Check your answer.

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