0.5 Acid and bases to buffers

Acids and bases to buffers

Objective

• To reinforce the importance of titration as an analytical tool.
• To graphically verify the number of donated protons per molecule of phosphoric acid.
• To prepare a phosphate buffer and realize the importance of buffers in our everyday life.

Grading

• Pre-Lab (10%)
• Lab Report Form (80%)
• TA Points (10%)

Background information

Phosphoric acid (H ₃ PO ₄ ) is a chemical that is commonly found in everyday products such as soft drinks and cleaning agents. It is called a polyprotic acid because it can donate more than one proton (H ⁺ ion) per phosphoric acid molecule. The released protons combine with water to form hydronium ions (H ₃ O ⁺ ).

Phosphoric acid releases its protons in a step-wise manner:

H ₃ PO ₄ + H ₂ O $\leftrightarrow$ H ₃ O ⁺ _­ + H ₂ PO _{${}_4^{-}$} K _a1 = 7.5 ´10 ^-3 (1)

H ₂ PO ₄ ^- + H ₂ O $\leftrightarrow$ H ₃ O ⁺ _­ + HPO _{${}_4^{2-}$} K _a2 = 6.2´10 ^-8 (2)

HPO _{${}_4^{2-}$} + H ₂ O $\leftrightarrow$ H ₃ O ⁺ _­ + PO _{${}_4^{3-}$} K _a3 = 4.2´10 ^-13 (3)

For example, reaction (2) will not occur until reaction (1) is complete.

The K _a values listed after each reaction are called acid ionization constants. They indicate the relative ease with which each reaction occurs. A small K _a value shows that a reaction does not occur easily. The K _a value for phosphoric acid’s second donated proton is much smaller than for the first donated proton, while the third K _a is five orders of magnitude smaller than the second.

To determine the amount of acid in an unknown sample, you will need to add a known amount of base until the acid and base are neutralized. This technique is known as titration , and it is widely used in chemistry and other natural sciences.

During a titration, the pH of the solution is constantly monitored while the known acid or base (called the titrant) is slowly added to the unknown solution. The pH of the unknown solution will stay fairly constant until the moles of titrant added equals the moles of unknown acid or base. When the moles of acid and base are the same, further additions of titrant will cause a dramatic change in pH until the pH eventually stabilizes. A graph of pH versus added titrant is called a titration curve , and the point at which the pH changes drastically is called the equivalence point .

The titration curve for a polyprotic acid will have more than one equivalence point. As the added base completely removes each proton from the acid, the pH will jump significantly. Figure 1 shows the titration curve for ascorbic acid, a polyprotic acid also known as Vitamin C:

Figure 1. Titration curve for ascorbic acid.

2 ^nd equiv.point

1 ^st equiv.point

By graphing the pH versus volume of base added during an acid-base titration, you can easily see the successive ionization steps taking place. To find the concentration of a polyprotic acid, the volume of base required to reach the first equivalence point is needed. The half-equivalence points on this graph can also be used to obtain the K _a value of each successive ionization.

In the third part of the lab, you will be making a buffer solution. Buffers are important in everyday life because they regulate the pH in our blood, keeping the pH between 7.35 and 7.45; if pH values for our blood go outside this range, death can result. A buffer is composed of a weak acid and its conjugate base (or a weak base and its conjugate acid). When a strong acid or base is added to a buffer, one of the species will react to maintain the pH within a small range.