TITRATION OF AN ACID; USING A ph METER. The ph meter is an instrument that measures the ph of a solution and affords a

TITRATION OF AN ACID; USING A pH METER Introduction

The pH meter is an instrument that measures the pH of a solution and affords a direct method of obtaining a titration curve. A titration curve is a graph of measured pH values versus the volume (milliliters) of titrant added. The figure below is an example of a titration curve, illustrating the numerous data points and the best smooth curve drawn through the points.

Figure 1. Titration Curve of acid HA

The equivalence point is the point at which an equal amount of acid has been added to the amount of base present or vice versa. The equivalence point occurs on the titration curve in the region where there is a relatively large change in pH with a relatively small change in volume. The steeper the curve in the region of the equivalence point the more precisely it may be established. Once a titration curve is constructed and the equivalence point established the experimenter could then choose an indicator that would give a suitable endpoint (point at which the indicator changes colour).

Selection of the equivalence point

In this experiment you will graph the measured pH against the volume of standard NaOH solution added. The best smooth curve should be drawn through these points. The equivalence point can be established using the steepest tangent to the smooth curve where the pH changes rapidly. The equivalence point is the mid point between the two lines intersecting the volume axis.

The method is summarized below:

3 4 5 6 7 8 9 1 0 1 1 23.5 2 4 24.5 2 5 25.5 2 6

Equivalence Point Determination

pH

Volume NaOH (mL)

Figure 2: Equivalence Point Determination for acid HA

The equivalence point selected using this method is a more accurate method than using an indicator in the titration.

A second method maybe used to determine the equivalence point. To use this method a graph is constructed of !pH

!V vs

V

0 1 0 2 0 3 0 4 0 5 0 2 4 24.5 2 5 25.5 2 6 26.5 2 7 Equivalence Point Determination

dpH/dV

V average

Figure 3: Equivalence Point Determination for acid HA using the First Derivative Method

The volume at the point where the graph reaches the maxima is the equivalence point of the titration.

A disadvantage of the titration curve method is the time and effort required to make the measurements and to construct the graph. This disadvantage can be overcome by using a recording pH meter, which provides a chart record of the pH of a solution as a function of time.

The pH of a solution is related to the H+ _{ion concentration by the equation:}

(1) pH = –log[H+_]

The ionization constant, Ka, for a generic acid HA is:

HA  H+ _{+ A}–

(2) Ka = [H

+_][A–_]

[HA]

and the pKaof an acid is simply:

(3) pKa = –log Ka

(2) Ka = [H

+_][A–_]

[HA]

and now take –log of both sides: –log Ka = –log[H+] –log_[HA] [A–]

from equation (3): pKa= -log[H+] –log_[HA] [A–]

and therefore from (1): pKa = pH –log_[HA] [A–]

But since pKais constant and pH varies: pH = pKa + log_[HA] [A–] Therefore when [A–_{] = [HA]}

pH = pKa

This is the midway point to the equivalence point. Therefore pKa values can also be directly read from your titration curves.

Procedure

In preparation for this procedure, read Appendix B for directions on the proper use of a balance and how to weigh by difference, and Appendix C for directions on the proper use of a buret in a titration. Before lab have a draft of the procedures for the preparation and the standardization of ~300mL of ~1M NaOH solution ready for your instructor to look over.

Obtain from your instructor an unknown acid sample. You will also be given instructions on the pH meter before you start.

Hint: A similar procedure was used in the Chemistry 1000 laboratory.

SAFETY NOTE: Sodium hydroxide is a strong base and can cause burns if

Weigh accurately 2.00 - 2.25 g (+ 0.0001 g) of your sample into a 250 mL beaker. Dissolve the sample in 50 mL of water. Prepare your buret for titration (clean, rinse, etc.) and fill the buret with standardized ~1 M NaOH solution. Obtain a pH probe from your instructor and set up your apparatus as shown. Check with your instructor before starting to be sure everything is set up correctly.

Proceed to add standardized ~1 M base at the rate of 1 mL per addition taking the buret reading and also the pH reading after each addition. Once the pH starts to change more rapidly the size of the additions should be reduced to 0.5 mL or less. As the pH changes become larger (this may occur at a pH of about 4 to 5) reduce the size of the NaOH additions until single drops are being added. Continue until you are satisfied the equivalence point has been passed or the pH is approximately 11 to 11.5. When you are finished, remove your apparatus, rinse the pH electrode and return the pH probe to your instructor.

Note that your unknown acid will have more than one equivalence point if it is polyprotic.

Report

1. Construct a titration curve graph of pH values versus mL of 1M NaOH solution added.

2. Construct a graph of !pH

!V vs

V

3. Calculate and report the Ka value(s) and the molar mass of your unknown sample.

Titration of an Acid; Using a pH Meter

DATA SHEET: Name: _______________________

Partner Credit: Y (Circle if yes) Partner's Name: ________________ Unknown # or letter: _______________ Lab Section: _______________

Experimental Procedure (draft only): Please prepare before the start of class.