Weak acid titration and buffer solutions

Purpose:

Our goal is to identify at least one of four unknown weak acids by titration with a strong base (NaOH). We will do this using a pH meter and the resulting titration curve (as shown in figure 1), from which we can determine the molecular weight and the acid dissociation constant (K_a) of the unknown. We will also prepare a buffer solution, and determine its effectiveness.

Materials:

• Four solid weak acids (unknowns)

and their conjugate salts

• Orion pH meters
• Standardized buffer solutions (pH 4

and 7)

• 0.05M NaOH (standardized)
• 0.05M HCl
• 50 mL burets
• 50 mL volumetric flask
• 10 mL volumetric pipet
• beakers
• stir plate and magnetic stir bar

Please make sure you are familiar with the concepts of titration curves and buffers by reading sections 19.1 and 19.2 in Silberberg.

Procedure:

Preparing your weak acid sample

• Determine approximately how much weak acid to use. The amount should be selected to allow for the efficient collection of data and to provide the most accurate results. You should determine the sample size based on the fact that the titrant (NaOH) has a concentration of about 0.05 M, and you want to have a target titrant volume of about 30mL. (If the volume is too small, it’s difficult to get an accurate curve, if it’s greater than 50mL, you’ll have to refill the buret, again decreasing accuracy.) Although you don’t know the exact molecular weight of the weak acid, you can approximate the mass you’ll need from the number of moles of base in your titrant, and assuming an average molecular weight of the acid of about 120g/mol.
• This brings about the question of how to handle the sample. It may limit your accuracy to directly weigh out a small mass required for a single titration, and keep in mind you may want to perform more than one run to ensure accuracy. One way to solve this is to make up a stock solution with an estimated concentration that contains the desired number of moles in a convenient volume. For example, a 10mL sample size will yield 0.0015 moles of weak acid if the concentration is 0.15M. If you make 50mL of this 0.15M stock solution, you can weight out five times the amount needed for a single run, and have enough solution with a constant concentration to perform multiple titrations. (This may seem wasteful if you don’t use all the stock solution, but along with serial dilution, it is a common method to ensure an accurate sample concentration when the actual amount of moles needed is very small.) Because accuracy is very important here, make sure to use an analytical balance to weigh out the weak acid, and a volumetric flask to prepare the stock solution.

Calibrating the pH electrode

• You will monitor the titration using a pH electrode, which must be calibrated prior to use. You will do this by calibrating to standardized buffer solutions of pH 4 and 7 because the bulk of your measurements will be on the acidic side of neutrality.
• Rinse the electrode well with DI water (a squeeze bottle works well, with a beaker to catch the rinse water) and blot it dry with a Kimwipe. You need to do this every time you move the electrode from one solution to another, to avoid contamination and dilution errors.
• Place the electrode in the pH 7 buffer solution, making sure the glass bulb is fully immersed. Gently stir the solution, then adjust the intercept control until the meter displays 7.00.
• Remove the electrode from the solution, rinse it with DI water, and place it in the pH 4 buffer solution. Adjust the slope control until the meter displays 4.00.
• Repeat these two steps until no further adjustment of the controls is necessary (this can take as many as five times). Once you’re satisfied with the calibration, place the electrode in a beaker of DI water for temporary storage.

Preparing to titrate

• You will produce the titration curve by incrementally adding NaOH to the weak acid sample, recording the volume of NaOH added and the corresponding pH. You will want to plot the data as you collect it, to detect any problems as they arise. Create a graph in your notebook, plotting pH vs. volume of NaOH, as in figure 1.
• Clean your buret, and rinse well with DI water. Add a small amount of the titrant (NaOH), swirl it around the buret, making sure it is thoroughly rinsed with the solution. This is to prevent any dilution errors that can arise from water droplets clinging to the glass.
• Fill the buret with the standardized NaOH (don’t forget to record the concentration!) and securely clamp the buret to a ring stand. Let a few milliliters flow out the tip, getting rid of any air bubbles that may remain there, which can skew your volume readings. Record the initial volume in the buret.
• Set up a clean beaker (at least 250mL) on a stir plate. Transfer 10.00mL of your weak acid sample to the beaker using a volumetric pipet, add a clean stir bar, and start stirring at a steady moderate pace. (If the solution does not completely cover the stir bar, you can add some DI water. This will change the concentration of the weak acid, but not the amount of moles present, which is what’s important in this case.)
• Lower the pH electrode into the solution, making sure it doesn’t come into contact with the stir bar. Make sure the bulb is completely immersed. If not, add some DI water until it is.
• Align the buret over the beaker so that the titrant can be added without striking the sides of the beaker or the electrode.

Performing the titration

• The titration is performed incrementally. A small amount of titrant is added, equilibrium is established, and the pH is measured. This is repeated as many times as necessary until the solution is distinctly basic. Look at titration curve in figure 1 for an idea of what to expect. At the beginning of the titration, the pH should increase considerably, as OH^- is introduced into the acid solution. Near the half equivalence point enough conjugate base has been produced to create a buffer solution, so that a relatively large volume of NaOH added causes only a slight change in pH. As you approach the equivalence point, the acid is present in smaller and smaller quantities, therefore the addition of a small amount of OH^- causes a dramatic change in pH. In order to produce an accurate curve near the equivalence point, it is important to add the NaOH in very small increments, or a drop at a time. If you’re not careful and miss the equivalence point, you won’t be able to calculate the moles of weak acid present in the sample. It’s not a bad idea to perform a quick trial run to get a feel for the apparatus, and at what approximate volume will you have to be careful.
• Record the reading of the pH meter before any NaOH is added. Plot this value on your graph, where volume NaOH = 0.
• Add a few drops of NaOH, then allow about 30 seconds for the system to equilibrate. Record the resultant pH and volume of NaOH added, (not the actual volume in the buret) and plot this point on your graph.
• Continue with this stepwise addition, allowing a short time to equilibrate before recording pH, until you’re sure you’ve passed the equivalence point. The volume of each increment shouldn’t be more than 2mL, nor should it produce a pH change larger than 0.2.
• You may want to repeat the titration to ensure accurate results to be used in your determination of the identity of the weak acid. You probably will want to check the calibration of the pH meter between runs (see above).
• If you have time, identify another unknown weak acid using the same procedure.

Making a buffer solution

• You want to make a buffer solution with a pH of 5. As you know, a buffer can be made of a weak acid and its conjugate base in solution. You have available the conjugate salts of the weak acids you titrated. You can prepare the buffer in one of two ways. You can determine the identity of your weak acid, then add an amount of its conjugate base according to the Henderson-Hasselbalch equation:

pH = pK_a + log([A^-]_eq/[HA]_eq)

Or you could calculate the amount of strong acid (0.05M HCl is provided) needed to add to one of the selected salts in order to produce the appropriate ratio of [A^-]:[HA}.

• Once you’ve made up the buffer solution, determine its effectiveness by comparing the pH change after the addition of strong acid and/or strong base with that produced by adding the same amount of DI water.

When you’re done

• Test the acidity of any waste or left over solutions. This can be done either by using litmus paper or the pH meter. Neutralize any and all solutions before disposing down the drain.
• Rinse the pH electrode well with DI water, and place in the designated storage solution.

Calculations

· Determining the molecular weight of your unknown acid:

At the equivalence point, you know the moles of base added equals the moles of acid you originally started with. You can calculate the moles of base that you added from the volume you added and the molarity of the standardized NaOH solution. (Be sure to use the exact molarity, to 4 sig figs.) Once you know this, you know how many moles of acid you started with. From this and the weight of the acid that you started with, you can find the molecular weight.

· Determining K_a:

At the half equivalence point, half of the weak acid has been converted into its conjugate base, so [HA] = [A^-]. According to the Henderson-Hasselbalch equation, at this point the pH of the solution is equal to the pK_a. You know the volume of base at this point, (half the volume at the equivalence point) so you can use your curve to find the corresponding pH (the more data points you have on the curve near this point, the more accurate your determination will be.) Once you know the pH at the half equivalence point, you know the pK_a and hence the K_a.

Report considerations

· Describe in detail your titration curve: what is happening in each region, why it is that particular shape, label important points, etc.

· Describe how you used that curve to determine the identity of the weak acid. Comment on any discrepancies between your calculated values and the actual values, and how specific errors could have caused them.

· Describe how you prepared your buffer solution, whether it was effective, and reasons why or why not.