Determination of the Dissociation Constant of a Polyprotic
SCHA-266 20073
Experiment
Apparatus:
pH meter with electrode
sodium hydroxide solution, about 0.1 M (standardized)
unknown solution of a weak acid (100mL of approx. solution)
25-mL pipet
buret, buret clamp, and ring stand
150-mL beaker
250-mL beakers (3)
standard buffer solutions, pH 4, 7 and 10
General Theory
This lab will be very similar to the first. However, you will be given a polyprotic acid in this case. In principle you might expect to see a clear endpoint for each proton that you titrate from your acid. This is not always the case however and you might see just one break for a diprotic acid. The pKa values and the concentrations can still be determined from these titrations. The acids that you might possibly get in this lab are fumaric acid, maleic acid, salicylic acid, thioglycolic acid, phosphoric acid, citric acid or phthalic acid.
You will still determine the endpoint(s) that you observe but you might have to make an assumption that this might be the second endpoint. From this point the first endpoint would be 0.5 Veobserved and the pKa’s would be at the 0.25 Veobserved and 0.75 Veobserved . If you observe endpoints that are not multiples of each other than this is an indication that you might have such a situation.
Operation of the pH Meter
In order to measure the pH during the course of the titration, we shall use an electronic instrument called a pH meter. This device consists of a meter and two electrodes.
The main variations among different pH meters involve the positions of the control knobs and the types of electrodes and electrode-mounting devices. The measurement of pH requires two electrodes: a sensing electrode that is sensitive to [H3O+] concentrations, and a reference electrode. This is because the pH meter is really just a voltmeter that measures the electrical potential of a solution.
The reference electrode is just an electrode that develops a known potential that is essentially independent of the contents of the solution into which it is placed. The glass electrode is sensitive to the H3O+ concentration of the solution into which it is placed; its potential is a function of [H3O+]. It operates by mobility of H3O+ ions in the glass membrane. This can be more precisely described, but for our purposes here it is sufficient for you to understand that two electrodes are required. These two electrodes are sometimes combined into an electrode called a combination electrode, which appears to be a single electrode. The combination electrode, however, does contain both a reference and a sensing electrode.
Preliminary Operations with the pH meter
1. Obtain a standard buffer solutions of known pH.
2. Plug in the pH meter to line current. Our meter will not require a warm up time. It should be left plugged in until you are completely finished with it.
3. The meters used in the lab can read in units of mV or pH. Select the pH mode.
4. Prepare the electrodes. Make certain that the solution in the reference electrode extends well
above the internal electrode. If it does not, ask your instructor to fill it with saturated KCl solution. Slide down the rubber collar on the reference electrode. Rinse the outsides well with distilled water. (Our electrodes now are gel filled and require no maintenance)
5. Standardize the pH meter. Set the pH meter control to correct room temperature. Carefully immerse the electrodes into the first buffer solution contained in, you should select the two buffers that will come close to the values you will measure (4 and 7 are a good starting pair) a small beaker. Remember that the glass electrode is very fragile; it breaks easily!
Don’t touch the bottom of the beaker with the electrode surface!!
Set the function to “read” or “pH”. The meter will adjust itself to the value of the buffer you have placed it in. When the meter indicates ready then hit yes, rinse the electrode with distilled water and move it to the next buffer. When this value is obtained enter yes. Record the slope that is briefly displayed. The pH meter is now ready to measure pH.
RECORD ALL DATA DIRECTLY INTO your notebook!
Procedure
A. Determination of pKa’s of an Unknown polyprotic Acid.
With the aid of a pipet bulb, pipet a 25.00 mL aliquot of your unknown acid solution into a 250-mL beaker and carefully immerse the previously rinsed electrode into this solution. Measure the pH of this solution. Record the pH in your notebook. Begin your titration by adding 0.5 mL of your standardized base from a buret and record the volume of titrant and pH. Repeat with successive additions of 0.5 mL of base until you approach the end point; then add 0.1-mL increments of base and record the pH and milliliters of NaOH added. For the first run plot your titration in your lab book as it progresses. When you have added about 20 mL you may discontinue data collection. From these data plot a titration curve of pH versus mL titrant added. Repeat the titration with two more 25-mL aliquots of your unknown acid and plot the titration curves. From these curves calculate the ionization constants. Time may be saved if the first titration is run with larger-volume increments of the titrant to locate an approximate equivalence point; then the second and third titrations may be run with the small increments indicated above.
B. Concentration of Unknown Acid
Using the volume at the equivalence point(s), calculate the concentration of the unknown acid, and record this in your notebook.
Prelab Questions
1. You carry out a titration of a pure weak triprotic acid and only see two major pH jumps. One at 10.0 mL and the second one at 15.0 mL. At what volume is the other equivalence point?
2. You carry out the titration of a weak diprotic solid acid. You take 178.0 mg of the solid acid and dissolve it in about 50 mL of distilled water. You titrate with 0.1005 M NaOH and arrive at an endpoint at 24.20 ml. You know that at the final pH both protons have been titrated. What is the formula weight of the acid? At what volumes would you determine the pKa’s of this diprotic acid?
Report
1. Provide properly labeled Data Tables for the data you have collected for the experiment.
2. Provide Excel plots of the titrations, properly labeled as Figures.
3. Provide first and second derivative plots of the three titrations. See Table 11-3 (page 210) in your text for how to construct these plots. These will be additional figures for your report.
4. Provide a Results Tables with the endpoints and pKa values determined.
5. Give a brief Results and Discussion section. Propose an acid that might be your unknown and explain the difference from the book pKa value.
6. The answers to the above questions should be in your report.
7. A copy of the notebook pages should be included. They should contain one plot. (as above)
8. Prepare a proper abstract for the lab. It of course will be the first element of the report after the title page.
SCHA-266 Page 1 Polyprotic Acid