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Titrations

(revised 4 April 2023)

Introduction

In this experiment you will perform two titrations:
During each titration, you will measure the pH with a pH meter after the addition of certain quantities of base.

Vinegar literally means sour wine in French, because it is the unwanted product of the fermentation of glucose (sugar) by yeast (bacteria). The desired product of the fermentation or the oxidation of glucose is alcohol (ethanol), the alcohol that we buy in the form of wine, beer, and spirits.  Commercial white vinegar is a solution consisting mainly of acetic acid and water, and thus the source of the acidity in vinegar is acetic acid.  Acetic acid, also called ethanoic acid, is an organic acid that contains the carboxylic acid functional group.  It is this functional group that determines the acidity of aqueous solutions of acetic acid.  Commercial vinegars are regulated by the Food and Drug Administration (FDA) and must contain a certain quantity of acetic acid to be sold in stores as vinegar. Please refer to the following FDA link for more information: (http://www.fda.gov/ICECI/ComplianceManuals/CompliancePolicyGuidanceManual/ucm074471.htm). 

The objectives of this experiment are to graph pH versus the volume of base added (pH titration curve), and to determine the equivalence point and the half equivalence point of each titration. From the titration of HCl with NaOH, you will calculate the molarity of the HCl. From the titration of commercial white vinegar with NaOH, you will calculate the quantity in % by m/v of acetic acid in commercial vinegar and compare it to that reported on the vinegar’s label to determine if it meets the minimum acetic acid content required by the FDA.  Furthermore, you will determine pKa of acetic acid. 

Upon titrating a weak acid (HA) with a base, there is a point in the titration at which the number of moles of base added is equal to half the number of moles of acid initially present.  This is the point at which 50% of the acid has been titrated to produce A- and 50% remains as HA.  At this point [HA] = [A-], the ratio [HA] / [A-] = 1, and the log [HA] / [A-] = 0.  Hence, at this point in a titration, that is, at the half equivalence point, pH = pKa.  Therefore, by determining the equivalence point graphically (ask your instructor), the half equivalence point can be found and it is possible to determine the pKa of the weak acid.

Please refer to your textbook for the theory pertaining to titrations.

Prelab

Safety

Wear your safety glasses. Sodium hydroxide solutions are corrosive!

Procedure

Titration 1 – strong acid with strong base

Using a pipet, add 20 mL of 0.0300 M HCl to a clean, dry 150-mL beaker.  Add 50 mL of distilled water using a graduated cylinder and 4 drops of the indicator phenolphthalein to the beaker, as well as at magnetic stir bar (gently!!) and place the beaker on a stir plate. Ensure the pH meter is in STAND-BY position. The meter will have been calibrated by the technician. Please do not change it!  Gently remove the electrodes from the storage solution, rinse them with some distilled water, dab them with a Kim wipe, and place them into the beaker containing the acidic solution. The pH meter is a delicate and expensive piece of laboratory equipment.  Handle it with great care!!  Begin gentle stirring by turning the STIR button to a low setting.  Very important – adjust the position of the beaker on the stir plate so that the magnetic stir bar does not touch the electrodes.  The electrodes are very fragile. Note the initial pH of the solution.  Fill the burette with 0.0300 M NaOH, after having purged the burette tip of air.  In case of spills, perform this step away from your beaker containing the acidic solution. Adjust the volume to 0.00 mL. Add successive volumes of NaOH to the beaker as indicated in Table 1 of the report sheet. Note the pH after each addition. Please note that equal volumes of base are not always added.  Note as well, the pH and the volume when the indicator changes color.  To finish, place the pH meter in STANDBY position, remove the electrodes from the solution, rinse them with water and place them in the storage solution.  Place the solution in the waste beaker and continue with the next titration.

Titration 2 – weak acid with strong base

Using an Eppendorf pipet, pipet 0.500 mL of commercial white vinegar into a clean dry 150-mL beaker.  Add 80 mL of distilled water using a graduated cylinder. Add 4 drops of the indicator, phenolphthalein, to the beaker, as well as a magnetic stir bar (gently!!) and place the beaker on a stir plate.  Titrate as above. Add successive volumes of NaOH as indicated in Table 3 of the report sheet. Note the pH after each addition. Please note that equal volumes of base are not always added.  Note as well, the pH and the volume when the indicator changes color. 

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