A two-week lab. Purpose: To standardize a sodium hydroxide solution for use in subsequent quantitative determinations.

Transcription

1 CHM 212 Experiment 1: Standardization 1 of a 0.1M sodium hydroxide solution. A two-week lab Purpose: To standardize a sodium hydroxide solution for use in subsequent quantitative determinations. The accuracy of your subsequent experiments depends on the accuracy of this standardization. Each of you will have a unique NaOH solution concentration and the accuracy of your standardization will be checked against known values in subsequent experiments. Background: In order to accurately determine the composition of a sample by titration, it is first necessary to accurately know the concentration of the titrant. There are two basic methods for determining the titrant composition. The direct method involves dilution of an accurately known amount of a primary standard to a known final volume, using a properly calibrated volumetric flask. A primary standard is characterized by its high purity, environmental (temperature, oxygen, etc.) stability, and freedom from waters of hydration so that its composition is not affected by atmospheric humidity. It should ideally be commercially available for low cost, have reasonable solubility in the titration medium and have a high molecular weight so that relative weighing errors are minimal. Very few compounds can meet these criteria, so so-called secondary standards must often be used, following careful chemical analysis of their composition. The second basic method is standardization of the titrant to yield a standard solution a solution of accurately known concentration. The titrant of approximately known concentration is titrated against either (1) a known amount of primary or, less ideally, secondary standard, or (2) a known amount of a different standard solution. Solid sodium hydroxide cannot be used for the direct preparation of a standard solution because it is too hygroscopic, too reactive with CO 2 in the air, and is not available in sufficiently pure form. For the accuracy needed in these experiments, it is therefore not possible to simply weigh out a known amount of NaOH, dissolve it and use the nominal calculated solution concentration. Instead, it is necessary to standardize a sodium hydroxide solution. There are several primary standards useful for the standardization of NaOH. Constant-boiling hydrochloric acid is one, but its preparation is tedious, and many of the other primary standards share similar deficiencies of convenience. Potassium hydrogen phthalate, in contrast, has favorable properties and is therefore frequently used for standardizing NaOH solutions. To standardize a sodium hydroxide solution, a sample containing an accurately known amount of potassium hydrogen phthalate is titrated with the NaOH concentration. With the reaction gone to 1 Make sure to see your textbook glossary for the meaning of new terms.

2 completion, the stoichiometry below can be used to accurately calculate the concentration of the sodium hydroxide solution, that is to standardize it. CO 2 K CO 2 H + NaOH CO 2 K CO 2 Na + H 2 O Eq. 1 aoh The change in color of the indicator phenolphthalein from colorless to pink indicates the endpoint of the titration: the indicator is chosen so that the endpoint matches as closely as possible to the equivalence point of the reaction. The difference between the endpoint and equivalence point is the titration error. To attempt to compensate for the titration error, one can create a solution with species concentrations similar to those of the KHP equivalence point, and measure the volume of NaOH required to titrate to endpoint. That volume is the titration error, and can be subtracted from the KHP standardizing titration to yield a more accurate value for [NaOH]. Ideally, one would prepare a solution of the product salt, CO 2 K CO 2 Na + H 2 O, at the concentration of the KHP equivalence point and titrate it to the endpoint. This salt is difficult to procure, but one may substitute a base with a similar pk a2 (sodium succinate) and titrate it to the endpoint in order to compensate for most of the titration error. The sodium succinate solution is used for this purpose. It is more usual to simply subtract off the volume of NaOH needed to titrate a pure water solution to phenolphthalein endpoint (if this is done at all), so you will also perform this measurement and compare results. You will determine which method gives you the more accurate standardized concentration by testing each value in the analysis of the results of subsequent experiments. Reagents wt % NaOH 2. Phenolphthalein indicator 3. Boiled distilled water 4. Water-free potassium hydrogen phthalate 5. Sodium succinate 6. Hydrochloric acid, ~1M Procedure (adapted from your textbook, pages 235, 42, 49, 54.): Plan ahead so that you can work on something else when you are waiting for water to boil or for a piece of equipment to become free. A. Calibration of buret 1. Fill the buret with distilled water and ensure proper flow. Air bubbles can be removed by fully opening the stopcock, tapping if necessary (see pages 35-36). If you wash the buret, be sure to wash the outside first, do not scratch the interior, and thoroughly rinse it with distilled water. Then dry the outside.

3 2. Fill the buret above the 0.00mL mark, and let it drain until the meniscus is at or slightly below 0.00mL. Touch the drop at the tip against a beaker to remove the drop from the buret tip. 3. Place a dry Kimwipe or paper towel underneath the closed buret and wait several minutes to ensure the buret does not leak. If it does, adjust the stopcock tension and inform your TA. 4. Weigh an empty 125mL Erlenmeyer flask. Then fill it with 10mL of water from the buret, drained at <20mL/min into the flask. Reweigh the flask with water to determine how much water you delivered. Using the lab temperature, the mass of water delivered and the known temperature-dependent density of water, determine the actual volume of water delivered. Weights to the nearest milligram are preferable, but to the nearest hundredth of a gram will do. 5. Repeat these steps for draining the buret from 10 to 20mL, 20 to 30mL until you have delivered 50mL. Repeat twice and average the results before plotting them. Determine and state whether or not your buret meets the tolerance limits of its class. 6. See the example on page 50 of your text and the caveat that you should start all titrations from near the 0.00mL mark. B. 250mL Volumetric flask calibration. This is in preparation for Experiment 2 you may do this calibration at any point during Experiment 1 or 2. See your textbook, Section Weigh your clean, dry, stoppered flask: do not leave fingerprint residue on the glass. 2. Fill the flask almost to the mark using a funnel extending below the calibration mark. 3. Adjust the level to the mark using water from a dropper or pipet. 4. Remove any droplets of water above the mark using a paper-wrapped glass rod or similar method. 5. Weigh the stoppered flask again. 6. Drain the flask and weigh it again, stoppered. Estimate the thickness of the remaining water layer coating the inside glass surface, assuming the flask is a sphere. 7. Repeat steps 1 5 two more times and average the calibration results, calculating also the standard deviation. C. Standardization of sodium hydroxide solution 1. Dry the primary-standard-grade potassium hydrogen phthalate (KHP) for at least 1 hour at 110 C and then store it in a dessicator to cool. KHP is not hygroscopic, so weighing by difference is not necessary. 2. Boil 1L of water (start with a little bit more than 1L to account for evaporation losses you will need at least 1L for subsequent experiments) for 5 minutes to expel carbon dioxide. This is time-consuming, so start early. Pour into a polyethylene storage bottle and keep it tightly capped whenever possible. Before closing the bottle, squeeze it to expel as much air as possible.

4 3. Boil another 1L of water and store it in another polyethylene storage bottle. This will be your standard stock of CO 2 -free distilled water; you may need to replenish it occasionally as the course goes on. 4. Using a plastic graduated cylinder, add enough of the supplied 50 wt% NaOH to your 1L water to create a 0.1M solution. Be cautious when working with such a concentrated base and if you spill it on yourself, immediately rinse with copious amounts of water. This solution should be room temperature before you attempt to standardize it you may have to wait until next week. 5. Accurately weigh (analytical, 4-place balance) four ~0.51g portions of the cooled, dry KHP. 6. Drain the first ~1mL from the buret before starting the titration: it is most susceptible to contamination from atmospheric CO Dissolve each of the KHP portions in ~25mL of distilled water in a 125 ml Erlenmeyer flask and add 3 drops of phenolphthalein indicator. Your endpoint will be the first appearance of the pink color that persists for 15 s. Perform one rough titration (given [NaOH]=~0.1M and calculating [KHP], what is the expected endpoint volume?). Keep the top of the buret covered with a small beaker to minimize exposure of the NaOH to CO Standardize the NaOH using 3 replicate titrations of KHP. 9. Perform titrations of 3 50mL distilled water blanks each containing three drops of phenolphthalein indicator. Repeat 3 more times with 50mL of a comparison solution composed of 0.05M sodium succinate. Calculate the means of each. You will have two blanks in this experiment, so you will have two different values for the standardized [NaOH] depending on which blank you have subtracted. 10. Once you have titrated all of your samples, immediately drain the buret and then wash with ~1M HCl and then rinse three times with distilled water. 11. Report your standardized sodium hydroxide concentration for the distilled water and sodium succinate blanks. Notes on recommended procedures: Weighing and analytical balances. Read Section 2-3 of your textbook. Always start and finish your measurement by ensuring the balance is clean and free of debris. Always check that the balance is level and not near a sources of heat, vibration or air currents. Don t even lean on the countertop when you are weighing something. Close the doors of the balance when you are measuring. Do not touch anything you are weighing with your bare hands: use tongs or a piece of paper folded over multiple times: fingerprints can affect your masses. The heat from your hand may affect the measurements.

5 Your sample should be at ambient temperatures to prevent errors due to convective air currents. Understand weighing by difference and why it is important. KHP is not especially hygroscopic, although it does contain waters of hydration. Read about buoyancy; we will not make the correction, however. Sample Transfer. When transferring a sample from one container to another for dissolution or dilution, rinse the first container out with your solvent (so long as the solvent is compatible with the first container). If you are pouring from one container to another, always wipe off the outside of the source bottle to ensure the next person isn t holding a wet and possibly caustic-coated bottle. Titration. During titrations, you should wash liquid from the walls of the flask into the bulk solution by tilting and rotating the flask carefully. Close to the endpoint of your titration (you will have done an initial rough titration to determine this, and once you have done a first careful titration, you can calculate more closely the expected endpoint of the remaining aliquots), deliver less than 1 drop of titrant at a time: suspend a fraction of a drop from the buret tip and touch the inside wall of the flask to the drop. Then wash the fractional drop into the solution and swirl. If you overrun an endpoint, simply add 1.00mL of the solution being titrated by means of a 1mL pipet and again titrate to the transition color. For this experiment, you would have to add more solid KHP. Dessicator Use. Special precautions for hot objects in dessicators and for opening dessicators: see text, page 41. Buret Use. See pages 35-36, especially for Errors from air bubbles and tricks to expel them How to deliver sample from a buret, especially washing Rinsing your buret with a small amount of wash solution Hazards of alkaline solutions for glass. After you have had an alkaline solution in a buret, you should rinse it with ~1M HCl (if chemically compatible) followed by several water rinses. See checklist at the top of page 36 know it! After closing the stopcock, wait a moment before making the reading: the flow of liquid down the walls of the buret must first catch up to the bulk fluid flow.

6 To minimize parallax, take advantage of the fact that there are graduations that extend all the way around the cylinder. Always interpolate your readings. Read the class of the buret to know its intended accuracy. Drying. See page 41, especially drying for constant mass. Vacuum ovens can aid in removing water from samples that is hard to remove Heat-sensitive substances should not be dried at elevated temperatures Solution blanks/blank titrations. See pages 23, 56 (margin note), 87. Transferring/weighing out chemicals. See bullet list page 23 in Box 1-1.