6268 Expt5_pH Meter Titration

8/12/2019 6268 Expt5_pH Meter Titration

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62 Experiment #5. Titration of an Acid; Using a pH Meter

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).


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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 betweenthe 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

!VvsVaverage . The graph below illustrates this

method.


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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

If we take equation (2)


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(2) Ka=[H+][A]

[HA]

and now take log of both sides:

log Ka= log[H+

] log

[A]

[HA]

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

[HA]

and therefore from (1): pKa = pH log[A]

[HA]

But since pKais constant and pH varies: pH = pKa+ log[A]

[HA]

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 balanceand how to weigh by difference, and Appendix C for directions on the

proper use of a buretin 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 similarprocedure was used in the Chemistry 1000 laboratory.

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

it is left on the skin for too long. If your hands feel slippery or soapy at any

point during the lab, wash them well to remove the sodium hydroxide.


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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 vsVaverage . Using this graph determine theequivalence point for the titration. Illustrate on the titration curve the pKa(s)

of your unknown acid.

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

sample.

4. Give a balanced, generic reaction equation for your unknown acid.


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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: _______________

Instructor's signature: _______________________


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

Instructor's signature: _______________________

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6268 Expt5_pH Meter Titration