Acid- Base Chemistry

Titration

With the help of computer-interfaced pH probes, you will investigate the qualitative and quantitative aspects of acid-base reactions. Such reactions are in a class known as neutralization reactions. Determining the molarities and/or volumes involved in a neutralization reaction involves the technique called titration (a titer refers to a known or fixed volume).

Terms

For our purpose the operative definition of an acid is that of Lowry-Bronsted:

An acid is a proton (H+ ) donor and a base is a proton acceptor. In donating its proton, an acid produces a base, called its conjugate base. The converse is true of a base.

Therefore every acid-base neutralization reaction involves acid-base pairs. In fact, the act of dissolving an acid in water is an acid-base reaction as shown to the left.

Neutralization reactions take the general form shown below: an acid plus a base yield a salt and water. The heart of this type of reaction is the combination of proton and proton receiver to form water.

There are several factors which describe acids as well as bases - strong, weak, mono-, di-, triprotic/basic. If an acid is effectively 100% dissociated in water, it is considered strong; less than 100%, weak. A monoprotic acid dissociates one mol of H+ per mol of acid, while a diprotic acid produces two mols of H+, triprotic three, etc. We can investigate these factors using titration since this process produces graphs (titration curves) which have distinguishing characteristics.

Dissociation of Weak Acids

The relative strength of weak acids can be found by comparing their pKas. The Ka is the special equilibrium constant for acids, called the acid dissociation constant.

The pKa can be found experimentally from the data accumulated during a titration. pKa is equal to the pH halfway to neutralization. The mathematical justification for this determination follows.

When [A- ] = [HA], log10 = 0 and the pH = pKa.

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EXPERIMENTAL PROCEDURE

Part I: The Process of Titration (Qualitative Analysis)

• Work with a partner for this experiment.
• Obtain one 25.0 mL buret.
• Pour about 30 mL of 0.050 M NaOH from the reagent container into a clean beaker; rinse each buret with this solution and fill them according to your instructor's directions.
• Use a 125 mL beaker for your reaction vessel.
• Pipet 3.0 mL of 0.10 M HCl into the beaker and add roughly 20 mL of deionized water.
• Set up your beaker with a small stir bar on a stirring motor and turn on the motor.
• Clamp the buret containing base above the reaction beaker.
• Clamp your pH probe so that the tip of the plastic probe covering is resting gently on the bottom of the beaker and is not in the way of the stir bar.
• Select the pH file under Experiment Files on the Datalogger software on your computer.
• Simultaneously, click the mouse on START and open up the buret stopcock completely.
• Click STOP when either the titration curve has "topped off" or when 10 mL of NaOH has been delivered.
• Refill your buret with NaOH and repeat the titration using 3.0 mL of maleic acid.
• Find the pKa values for maleic acid.
• Follow the instructions for washing and storing the pH probe.

Part II: Titration (Quantitative Analysis)

• Each person will determine the molarity of an unknown acid solution.
• Prepare your base buret as in Part 1.
• Obtain an unknown solution of acid; be sure to record the unknown number.
• Proceed as in Part 1 but use the pH Titration file from the Datalogger files. This data collection program is set up to monitor pH and volume. Data collection is a triggered event. This means you have to participate in the titration.
• With the probe in your unknown-water solution, click the mouse when the pH has stabilized. Record 0 mL as your volume.
• Add 0.10 mL of NaOH from your buret to the reaction beaker.
• When the pH has stabilized, click the mouse and record your volume.
• As the graph begins its slope change, go to smaller increments.
• When the titration is complete, save your data and then transfer it to the Graphical Analysis program.
• Determine the volume required for neutralization, classify the type of acid you titrated, find the pKa value(s), and calculate the molarity of your unknown acid solution.

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