Introduction:

One of the most important techniques for chemical analysis is titration to an equivalence-point. Suppose that an investigator wishes to know the exact quantity of acid present in a certain mixture. He can find this value by determining the quantity of a base that must be added to the mixture just to neutralize the acid. The quantity of base needed can be measured by preparing a solution of a known concentration of the base and measuring the volume of it needed for the neutralization.

The volume of the base solution needed is found by adding the solution from a buret until an indicator in the mixture signified that the end point of the titration had been reached, or that exactly enough base had been added to neutralize the acid present. The volume of base solution used would then be read from the buret. The indicator commonly used in acid-base titrations is a dye that changes color sharply at the end-point. The end-point can also be identified in ways other than through a color change, such as by monitoring with a pH probe interfaced to a computer.

A titrating solution must be of known concentration and must contain only a single chemically active reagent. Such solutions are known as standard solutions.

A widely used procedure for preparing and standardizing solutions for titrations is to prepare a solution of approximately the desired concentration and then to titrate it against an accurately massed quantity of a compound of known purity. The compound of known purity used in this manner is called a primary standard. Nearly all standard sodium hydroxide solutions can be standardized against a known mass of potassium hydrogen phthalate (KHP), KHC₈H₄O₄.

KHP is a monobassic acid; it contains one mole of neutralizable hydrogen per mole of substance used. This compound can be highly purified (99.97% pure), it is not easily oxidized, it can be dried to a constant mass, and it has a relatively high molar mass, permitting accurate massing to be obtained on samples as small as 0.002 mole. All of these are desirable characteristics of a primary standard. One not-so desirable property of KHP, however, it that it is a weak acid; therefore, the end point of its titration is not so sharp as might be desired. It is necessary, therefore, to carry out the titrations in the absence of carbon dioxide; boiled water must be used to dissolve the acid and the base to be standarized. Phenolphthalein, colorless in acid solutions, but pink at a pH of 8 or higher, is a suitable indicator in titrations of KHP with NaOH solutions.

Procedure A: Standardization of NaOH

Carbon dioxide-free water is prepared by pouring about 200 mL of boiling distilled water into a flat-bottomed flask and cooling the flask under the water tap, taking care not to shake the water unnecessarily which would permit carbon dioxide to dissolve in it again. Add (using a graduate) to the cooled water, 20 mL of carbonate-free 6M NaOH solution, stopper the flask to prevent absorption of carbon dioxide, and mix it thoroughly by shaking. Keep this flask stoppered as much as possible hereafter.

The diluted sodium hydroxide solution must now be standardized; i.e., its exact concentration must be determined. This is accomplished by titrating the solution against a known mass of KHP.

Obtain a dried sample of pure KHP and prepare three samples for titration against the NaOH solution. Carefully mass the vial containing the KHP, and then transfer about 1.2 g of KHP to a 250 mL beaker, and remass the vial and its remaining contents. Dissolve the solid in about 40 mL of recently boiled distilled water. Prepare the second and third samples the same way. Be sure you mark the beakers to keep them straight. Add 2 drops of phenolphthalein to each beaker

Click here for directions on how to set up the pH probe-computer interface equipment. Check to see if the pH probe is attached to Port 1 or Port 2.

Clean, rinse, and fill a buret with the diluted NaOH solution. Titrate the acid using 1 mL increments at a time of 10 seconds per reading. Set the x-axis for at least 180 seconds. Be sure to swirl the beaker to ensure complete mixing of the NaOH with the acid solution. Add NaOH until the pH increases sharply (the indicator whould turn pink) and then add 1-2 mL more NaOH. Save the data table as a text file on your own diskette or your file server folder for importing into KaleidaGraph or Graphical Analysis later.

Repeat with the second and third samples.

Procedure B: Titration of Acetic Acid

Titrate three 10.00 mL samples of the vinegar (measured out with a pipet) with the newly standardized NaOH solution. Leave out the phenolphthalein indicator, as the final endpoint with be determined from the computer generated titration graph.

Analysis:

You will first need to calculate the number of moles of KHP being titrated. This will give you the moles of hydroxide ions used, which along with the volume of NaOH added allows you to calculate the molarity of the NaOH solution. Calculate the molarity of NaOH in each of the three trials and average the results. Also calculate the average deviation.

Use the newly calculated molarity of the NaOH solution and calculate the moles of acetic acid titrated and the % of acid in the vinegar.

Discussion:

1. Explain the effect, if any, of each of the following sources of error upon the molarity of the base as determined in the experiment; i.e., would the experimental value for molarity be too high or too low, and why?

a. If the beaker in which the titration was performed contained several mL of distilled water from the rinsing at the time the KHP was massed into it.

b. If the tip of the buret was not filled with solution before the initial reading was taken.

c. If a bubble appeared in the tip during the titration.

d. If liquid splashed from the titration beaker before the end point had been reached.

e. If the buret was not rinsed with the NaOH solution following the rinsing with distilled water.

2. Suppose that, instead of using NaOH, a base such as Ba(OH)₂ had been used. What changes in the calculations would then have to be made to determine the molar concentrations of the base? Answer this question in words, and illustrate you answer by calculating M_B from the following data:

Mol KHP used	0.040 mol
Ba(OH)2	Initial buret level = 0.020 mL Final buret level = 36.70 mL