CHEM 240
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Experiment 2

Lab 4: Acid-base titration

Version Date: 01/10/2014

Lab Objectives

  • To understand the importance of instrument calibration and to calibrate a constant-volume drop dispenser
  • To understand the principle of acid-base titration and the use of an indicator to find an endpoint
  • To understand the principles and methods behind standardization of a solution
  • To practice error analysis and propagation of uncertainty in a laboratory setting

Reading from Exploring Chemical Analysis. Harris, D.C. 5th  ed. (4th ed.)
      Ch 2-9, p. 48 (p. 54):  Calibration of volumetric glassware, correction factors
      Ch 4-1, pp. 77-80 (pp. 83-86):  Mean and standard deviation
      Ch 9-6, pp. 200-203 (pp. 206-209):  How indicators work
      Ch 10-1, 10-2, 10-3, 10-4, and 10-5, pp. 207-221 (pp. 213-228):  Review the information on titrations in this chapter, especially section 5 on practical notes, primary standards

Prelab

  1. Write the structural formula of KHP and the balanced chemical equation for the reaction of KHP with hydroxide ion (see Harris p. 221 (p. 227)).
  2. Calculate the amounts of materials you will need to use to make up necessary solutions for this lab.  Calculations required for this lab should be worked out in advance and can be reported in a Sample Calculations section of your notebook.  (Formula and formula weight information for potassium hydrogen phthalate (KHP) can be found in Harris - see p. 221 (p. 227).) 
  3. Briefly describe how an indicator works and explain the choice of phenolphthalein for the titration of KHP with sodium hydroxide.

Introduction
            One common method for quantitative analysis of unknown samples is titration.  Indeed, titration is one of the most common analytical tools in a chemist’s toolbox.  The most common of these is an acid-base titration.  Titration can be a powerful tool for determination of unknown concentrations, pKa values, and other properties of acidic or basic compounds, of which there are countless biological examples.
            To do a titration properly, with suitable attention to precision and accuracy, we must first calibrate and standardize our equipment and solutions.  This helps to reduce the amount of systematic error in our experiment.  In today’s experiment, you will calibrate a constant volume drop dispenser, which can be used to deliver acid or base in a titration experiment, and standardize a sample of sodium hydroxide so you know its concentration and can use it in next week’s lab.


Experimental Procedure

A. Calibration of Constant Volume Drop Dispenser

  1. Record the ID number on your MicroLab setup, pH probe, and drop dispenser.  Make sure you use this same setup next week in lab.
  2. You will need to accurately determine the volume of one drop that the drop dispenser delivers so that later you can calculate the volume of a particular number of drops.  Follow the instructions in Appendix 2E of the lab manual (or here) to set up the MicroLab™ instrument with the drop counter sensor.
  3. Set up your drop dispenser and drop counter as described by your instructor.  Add deionized water to the drop dispenser and set the stopcock horizontally, so it is closed.  Place the drop dispenser in its holder on the ring stand.  To adjust the flow of water, open the stopcock all the way (vertical position) and adjust the black knob until the water is coming out at the speed you desire.  Note that the stopcock will only turn in one direction.  If you turn it the wrong way, it will snap off.  You will want a flow rate of about 1 – 2 seconds per drop.  Align the drop dispenser and the drop counter so that each drop makes the red light flash.  When the red light flashes, the drop counter will increase by 1 in the Digital Display.  Test this out with several drops until you are satisfied that everything is set up properly.  When your drop dispenser is set at a flow rate you are happy with, you can stop the flow by turning the stopcock.  When you re-open the stopcock, water will flow at your desired rate.  Do NOT allow any liquid to drop on any part of the drop counter.  If this does occur, dry it off immediately. 
  4. To determine the volume of 1 drop, you will want to very accurately measure the mass and temperature of approximately 150 drops.  Repeat this process at least 4 times.  Use a clean, dry, preweighed 50 mL Erlenmeyer flask equipped with a rubber stopper (to prevent evaporation).  After you have dispensed the desired number of drops, recap your flask with the same stopper and measure its mass.  Do a quick calculation to determine the approximate volume of one drop to estimate the number of drops you will need later in Part B.  You will perform a more careful calculation (including error) later in your analysis.

B. Standardization of NaOH

  1. A stock solution of approximately 0.1 M NaOH will be made available for use as a titrant.  Collect this 0.1 M NaOH solution in a polyethylene bottle, which should be kept tightly capped whenever not in use to minimize CO2 entry from the air.  You will only have one bottle worth of NaOH to complete this lab and next week’s lab, so do not waste it.
  2. You will weigh out three samples of solid potassium hydrogen phthalate (KHP) to the nearest 0.1 mg and dissolve each in ~25 mL of deionized water in a 125 mL Erlenmeyer flask.  Each sample should contain enough solid to react with approximately 25 mL of 0.1 M NaOH.  Based on the stoichiometry of the reaction between KHP and NaOH determined in your prelab, the mass of KHP to be weighed out should be calculated in your prelab.  Add four drops of phenolphthalein indicator to each flask.
  3. Once the exact mass of your KHP samples is known, estimate the volume of 0.1 M NaOH that you expect will be required to reach the endpoint of your titration, and the number of drops of NaOH that would require using the drop counter you just calibrated, so you have an approximate idea of when to expect a change to occur.
  4. To prepare the drop dispenser, drain the drop dispenser and replace the water with your NaOH solution.  Add about 5 mL of NaOH, rinse it around the drop dispenser, and drain it from the bottom.  Repeat this process several times.  Then fill the drop dispenser with NaOH and drain a small amount from the bottom to ensure no air is trapped.  Align the drop dispenser with the drop counter, and set the flow rate so each drop is counted.
  5. Titrate each of your three KHP samples carefully with the 0.1 M NaOH solution from your polyethylene bottle.  Add the NaOH slowly and dropwise, making sure the drop counter is counting each drop.  Stir your solutions on a stir plate during your titrations.  During each titration, periodically tilt and rotate the flask to wash liquid from the walls of the flask into the solution.  A pipet may also be used to take up solution from the flask and rinse its sides.  The endpoint is the first appearance of pink color that persists at least 15 seconds.  As you approach the endpoint you will want to slow your flow rate significantly or you will miss the endpoint.  Your flow rate should be at a slow enough speed such that the color completely dissipates before the next drop is added.  Be very careful not to overshoot the endpoint.  The color change should occur with the addition of your final drop.  Note that over time, this color will slowly fade as CO2 from the air dissolves in the solution.
  6. Record the number of drops of NaOH delivered in each titration.
  7. Label the polyethylene bottle with the remaining NaOH solution with the calculated average molarity and store it in your drawer (or elsewhere on advice of your instructor) until next week.  Be sure the bottle is tightly capped.

In your EXCEL Spreadsheet, make sure you do the following:

  1. Input all of your raw data in organized tables to be analyzed.
  2. Calculate the average volume of 1 drop from the drop dispenser, including its absolute error, and percent relative error (correctly propagated).
  3. Calculate the average concentration (in M) of your NaOH solution, including its absolute error, and percent relative error (correctly propagated).
  4. As always, make sure your formulas reference other cells, organize your tables well, label columns, include units, and display final values (and error) to the proper number of significant figures.

Discussion questions:
Answer the following questions fully, using complete sentences:

  1. How did you calibrate the drop dispenser.  What was the purpose of the calibration?
  2. What was the purpose of standardizing your NaOH, and why/how did you use KHP for this purpose.
  3. Describe the acid-base chemistry that occurred in your titration, including a chemical reaction.
  4. What does it mean chemically to “reach an endpoint” for a titration?  How do you know you have reached it?  Compare the “endpoint” to the “equivalence point” and relate these concepts to your experiment.
  5. What is the value of your concentration of NaOH (including its associated error)?  Comment on the accuracy of your measurement.  What could you have done to improve this accuracy? 
  6. In step B2, you added approximately 25 mL of water to your KHP.  Was the exact volume that you added here important to note?  Why or why not?
  7. For the solution of NaOH that you are storing for next week, why does exposure to open air need to be avoided?  Include a chemical reaction showing what could happen if it is exposed to air, and explain why that is bad.

Instructor's notes for this lab

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