VOLUMETRIC ACID DETERMINATION
The objective of this experiment is to apply the methods of volumetric analysis to the determination of (a) the percentage of potassium hydrogen phthalate, KHC8H4O4, in a sample, (b) the molecular weight and ionization constant (Ka) of an unknown weak acid, and (c) the titration curve of the same weak acid when it is titrated with a strong base, NaOH. In achieving this objective, experience will also be gained in preparing and standardizing a base solution and in using a pH meter.
PRINCIPLES
The theory behind the methods used in this experiment is described in your text. The principles upon which the operation of the pH meter are based cannot be fully understood until after the nature of electron-transfer reactions has been studied later in the course, but a few brief notes concerning the pH meter are included in Appendix D. Review the details on the care and attention that you should observe in the use of the buret on pp. 22-24 of the introductory section of the lab manual and on titrations on pp. 24-25.
EXPERIMENTAL PROCEDURE
1. Preparation of Carbonate-free NaOH Solution. Boil one liter of reverse osmosis water for about 5 minutes to remove dissolved CO2 and then cool the covered flask without shaking (so as to avoid re-incorporation of CO2 from the air). Pour the water into the container which will ultimately store the NaOH, leaving an air space of approximately 50 ml. Take the bottle to the laboratory assistant who will add 4 ml of 50% NaOH to the water. Stopper the container immediately and thoroughly mix the contents by vigorous shaking and repeated inversion of the container. The air space is large enough to make an effective mixing bubble, but not so large as to introduce an undesirable amount of CO2. This solution is approximately O.08 M NaOH and it should be carefully protected against CO2 in the air at all times except when removing some of the solution from the container. A polyethylene or Pyrex bottle is the most desirable container, though a soft glass bottle with polyethylene stopper will be satisfactory for a period of a month. Label the bottle.
2. Standardization of NaOH Solution.
(a) Preliminary operations. If your NaOH solution has stood for a long period of time (such as from one lab period to the next) some water from the solution may have vaporized and then recondensed on the walls of the air space above the solution. If this has happened, shake the contents thoroughly for re-mixing before using the solution. Thoroughly rinse your buret (including stopcock and tip) with three successive small portions of the 0.08 M NaOH solution, then fill the buret to somewhat above the "0" level and withdraw solution from the tip in the proper manner (see p. 23). Avoid undue exposure to the air and keep the storage bottle well-stoppered.
(b) Weighing of standard sample.Weigh to the nearest 0.1 mg three approximately 0.5-g samples of primary standard potassium hydrogen phthalate (abbreviated hereafter as KHP, but note that KHP is NOT the molecular formula) that has been previously dried at 120 °C. The KHP is kept in small weighing bottles stored in desiccators located near the analytical balances; it should be returned to the desiccator when not in use. Weigh the samples on weighing paper and then, after sliding the samples into clean (but not necessarily dry) 250-ml conical flasks, reweigh the paper to take account of any crystals which remain on it. The solid should be added to the weighing paper OUTSIDE THE BALANCE CASE. The flasks should be numbered on the frosted spot for identification. Once weighed, the samples may be used immediately or stored in your desk (with watch glass over mouth of flask) until another lab period.
(c) Titration of standard sample. For titration, dissolve a sample in about 50 ml of reverse osmosis water, heating gently as necessary to aid in the dissolution , add 2-3 drops of O.l% phenolphthalein indicator, then add NaOH solution from the buret until a faint permanent pink endpoint has been achieved. Initial and final buret readings should be recorded to the nearest 0.01 ml, and the appropriate buret corrections applied. Being acidic, the titration solution will not absorb any appreciable amount of CO2 until near the endpoint. At the endpoint the color will doubtless fade after several minutes due to absorption of CO2 from the air. To avoid a reflected pink color, do not wear pink colored clothing to the lab.
The initial amounts of NaOH solution may be added rapidly with swirling of the flask contents, but when the color tends to persist in solution at the point where the NaOH mixes with the flask contents, the addition should be slowed and the subsequent quantities added dropwise. Just prior to the endpoint the flask walls should be rinsed down with a stream of distilled water from your wash bottle. Partial drops may be added as discussed on p. 24.
Once one titration has been completed, the others may be done more rapidly by first making a rough calculation of the approximate volumes to be expected for the other weighed samples. Almost all of the calculated volume may be added at once, followed by the final careful procedure used with the first sample. A quick check for satisfactory precision may be made before going on to the next section by simply comparing the ratio of base volume to weight of KHP for each sample. In the case of poor precision (e.g., a range of values of 2 p.p.t. or more) additional samples should be run.
3. Determination of the Percentage of KHP in a Sample. Obtain a previously dried sample of the unknown from the desiccator on the side shelf and place it in a small desiccator for your own use. Record the number of your unknown. Just as you did with the primary standard, weigh out three samples to the nearest 0.1 mg. Calculate the approximate weight of sample assuming that your sample contains about 50% KHP. Perform the titrations in the same manner as carried out for the primary standard.
4. Molecular Weight and Ionization Constant. Titrate a sample of an unknown weak acid, using a pH meter to follow the course of its titration, and from the titration curve determine Ka and the molecular weight of the acid. For the purpose of the experiment, all the unknown acids are assumed to be monoprotic.
(a) Sample preparation. Obtain an unknown weak acid and weigh a 0.4 g sample using the same method as for previous samples, but this time transfer the sample to a clean (but not necessarily dry) 250-ml beaker. Dissolve the sample in about 75 ml of reverse osmosis water. Record the sample number of your unknown.
(b) Standardization of the pH meter. Normally the pH meter is standardized for you with a standard buffer solution. Hence, do not touch the cal 2 and cal 1 controls.
(c) pH Measurements. Make sure the power switch is in the power on () position. Since you will be measuring pH using the automatic temperature compensation (ATC) probe make sure that the measurement mode switch is in the ATC position. When the ATC mode is used the manual temperature adjustment control is not used. To measure the pH of a solution raise the combination glass electrode and calomel electrode probe and the ATC probe from the soaking solution, rinse them thoroughly with reverse osmosis water, and wipe them carefully with a Kimwipe. Lower the probes into the sample solution and note the reading when relatively stable. When you are finished rinse the electrodes and place them in the soaking solution.
(d) Titration with the pH meter. So as to avoid any splashing, carefully place a well-rinsed magnetic stirring bar in the beaker containing your unknown acid solution. Position the beaker on the magnetic stirrer and lower the electrodes into the solution
being careful so that the electrodes are not hit by the stirring bar. The electrodes should be positioned to one side of the beaker so as to provide easy access for your buret tip in the titration operation.
Clamp your buret in position with the tip close enough to the surface of the solution so that there is no danger of loss by splashing as the base solution is run into the beaker. The buret should already have been properly filled with your standard base solution, the tip readied for titration, and the meniscus level near, but below, the "O" mark. Turn the magnetic stirrer ON and adjust the speed so as to give good mixing, but not so fast as to risk splashing.
Record the initial buret reading and the initial pH of the solution. Add standard NaOH solution in such quantity (but in no case more than 5 ml) as to give a change of about 0.2 or 0.3 pH units. Record the new buret reading and the corresponding pH of the solution, allowing sufficient mixing time after the addition so that you know you are observing the correct pH. Continue this stepwise addition of standard NaOH and the recording of the volumes and related pH values until you are well past the endpoint; very small additions must be made near the endpoint.
On the completion of the titration, turn the magnetic stirrer OFF, remove your buret and lift the electrodes to a raised position. Rinse the electrodes with reverse osmosis water (catching the rinsings in your titration solution), put the magnetic stirring bar in the designated place, discard your titration solution, and replace the electrodes in the beaker of soaking water. Rinse your buret with several portions of reverse osmosis water and finally fill it with reverse osmosis water and stopper it for storage.
CALCULATIONS
1. Calculate the molarity of your NaOH solution from the titration of weighed samples of KHP. Calculate the standard deviation of the molarity values and the relative 95% confidence interval of the mean in parts per thousand (p.p.t.).
2. Calculate the percentage of KHP in your unknown from the titration of weighed samples of unknown. Calculate the standard deviation of the calculated percentages, and the relative 95% confidence interval of the mean in p.p.t.
3. Graph the pH of the solution of your unknown acid (as ordinate) as a function of the volume of standard NaOH solution added (as abscissa). Draw this graph on millimeter graph paper using a convenient and sufficiently large scale so that the majority of the graph paper is used. Draw a smooth curve that best represents all of the points; do not draw a series of straight lines connecting each point in succession. Label both axes carefully, including the units. Draw a vertical line corresponding to the volume of base used at the equivalence point; draw another vertical line corresponding to half the volume of base used at the equivalence point. Draw horizontal lines corresponding to the pH at the equivalence point and at half-titration.
4. Calculate the molecular weight of your unknown acid and the value of its ionization constant, Ka. Use the weight of your sample and the necessary data from the graph prepared in 3 above.
5. Turn in the report sheet summarizing your results. Be sure to show your sample calculations and to answer the question on the report.
Name______Lab Section
Date Report Submitted______Sample No.______
VOLUMETRIC ACID DETERMINATION
1.Standardization of NaOH Solution
Sample No Wt. of KHP Moles KHP Vol. of NaOH Molarity
1. ______
2. ______
3. ______
4. ______
Average ______
Standard deviation ______
Relative 95% confidence interval of the mean, p.p.t ______
2.Percentage of KHP in Unknown
Sample No. Sample Wt. Vol. NaOH Moles KHP %KHP
1. ______
2. ______
3. ______
4. ______
Average ______
Standard deviation ______
Relative 95% confidence interval of the mean, p.p.t ______
Name______Sample No.______
VOLUMETRIC ACID DETERMINATION
(report sheet for Chemistry 51)
[You are only performing part 3 of the experiment.]
3.Characterization of a Weak Acid
Weight of sample______
pH at the endpoint______
pH at half-titration______
Volume of NaOH at endpoint ______
Ka of acid______
Molecular weight of acid______
4.Show on the back of the report sheet, sample calculations for the calculations of the molecularweight and Ka in 3. Attach a properly labeled graph of your titration curve.
5.Solve the following problem.
Using the sheet on indicators with each setup and information in your text about indicators, select an indicator which would be suitable for the titration of your unknown weak acid. Show on your graph the pH range in which the indicator you selected would change color and indicate on the graph what the colors are.
Indicator______
1