Question

1.

A. Calculate, stepwise, the weights of NaH₂PO₄ and K₂HPO₄ you need to prepare 1200 mL of 0.5 M phosphate buffer, pH 7.4.

B. What does pK_a mean in terms of concentrations of conjugate acid and conjugate base?

C. Why does glycine have buffering ability

Answer 1

A buffer is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer’s pH changes very little when a small amount of strong acid or base is added to it. It is used to prevent any change in the pH of a solution, regardless of solute. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. For example, blood in the human body is a buffer solution.

Buffer solutions are resistant to pH change because of the presence of an equilibrium between the acid (HA) and its conjugate base (A^–). The balanced equation for this reaction is:

HA⇌H++A−HA⇌H++A−

When some strong acid (more H⁺) is added to an equilibrium mixture of the weak acid and its conjugate base, the equilibrium is shifted to the left, in accordance with Le Chatelier’s principle. This causes the hydrogen ion (H⁺) concentration to increase by less than the amount expected for the quantity of strong acid added. Similarly, if a strong base is added to the mixture, the hydrogen ion concentration decreases by less than the amount expected for the quantity of base added. This is because the reaction shifts to the right to accommodate for the loss of H⁺ in the reaction with the base.

Buffer solutions are necessary in a wide range of applications. In biology, they are necessary for keeping the correct pH for proteins to work; if the pH moves outside of a narrow range, the proteins stop working and can fall apart. A buffer of carbonic acid (H₂CO₃) and bicarbonate (HCO₃⁻) is needed in blood plasma to maintain a pH between 7.35 and 7.45. Industrially, buffer solutions are used in fermentation processes and in setting the correct conditions for dyes used in coloring fabrics.

The perturbed pKa of Glycine is caused by repulsion between the departing proton and the nearby positively charged amino group on the α-carbon atom. The titration curve ofGlycine has two regions of buffering power. At pKa 2.34, glycine is a good buffer near this pH. The other buffering zone is centered on pH 9.60.

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