Experiments based on citric acid content in fruit juice by titration
Answers
Procedure
Note: do this lab with a partner.
Standardization of a Solution of Sodium Hydroxide
Thoroughly clean and rinse with distilled water your supplies: a burette, a 25-mL graduated cylinder, a 500 mL boiling flask, and three 250 or 300-mL Erlenmeyer flasks.
Put your NaOH pellets (mass calculated in the pre-lab) in the 500 mL boiling flask. Cover them with a small amount of water and swirl until they dissolve. The heat of solution produced by the NaOH helps to speed the dissolving process. Once the pellets are dissolved, fill the flask to the base of the neck. Using Parafilm, mix the solution thoroughly by inverting several times (the flask not you!). It is best to keep the flask covered with Parafilm when not in use as CO2 from the air can slowly neutralize the NaOH.
Using an analytical balance, weigh out the oxalic acid dihydrate in a weigh boat (mass calculated in the pre-lab). Place it in a clean, clearly labeled Erlenmeyer flask. Note: It is not necessary to weigh out exactly the amount calculated (although you should be close), however it is imperative that the mass of each sample of oxalic acid dihydrate be known precisely for each flask. Label each clearly.
Dissolve the oxalic acid dihydrate in 25 mL of distilled water and add three drops of phenolphthalein indicator solution.
Repeat steps 3 and 4 for 2 more samples of oxalic acid dihydrate so you have a total of 3 flasks containing oxalic acid solutions.
Rinse your buret once with water and then twice with 5-mL portions of the solution of sodium hydroxide which you have prepared, draining the solution off through the burette tip into a beaker for waste reagents. Fill the buret nearly to the top of the graduated portion with the solution of sodium hydroxide you have prepared, making sure that the buret tip is completely filled with the solution. Touch the inner wall of a beaker for waste reagents to the buret tip to remove any hanging drop of solution.
Make a preliminary titration using one of your solutions of oxalic acid to learn approximately how the neutralization proceeds. Place a sheet of white paper under the Erlenmeyer flask so that the color of the solution is more easily observed. Make sure to either adjust the meniscus to the 0.00 mL line or record the exact volume to the nearest 0.05 mL. Swirl the sample in the flask throughout the titration. Add sodium hydroxide rather rapidly from the buret until the color of the solution where the sodium hydroxide is entering the solution begins to linger, then add the base dropwise until, finally, one drop of the alkaline solution of base changes the colorless solution to a permanent pink, not red. A drop should not be left hanging on the buret tip. You probably will overrun the endpoint in this first titration, but it will provide a useful rough measure of the volume of the sodium hydroxide solution needed to neutralize the acid and provide you with valuable experience. Read and record the level of the meniscus in the buret (to the nearest 0.05 mL), and compute the volume of basic solution used in the titration.
Now titrate the remaining two samples of standard acid, being certain each time to refill the burette nearly to the top graduation with your sodium hydroxide solution and to record the burette reading. In these runs, add the sodium hydroxide from the buret very rapidly, again with swirling, until you are ~ 2 mL short of the volume that you estimate will be needed on the basis of your first titration. Then carefully add base drop by drop so that you can determine the equivalence point accurately. Record the data for your titrations in a table and perform the calculations necessary to yield the exact molarity of the NaOH solution. Show your results to your instructor. The concentrations of your titrations should agree within 5 % of each other.
The solution of sodium hydroxide that you have just standardized will be used in Part II, so do not waste it.