experiment 4 - potentiometric titration

MALAYAN COLLEGES LAGUNA

EXPERIMENT NO. 4

POTENTIOMETRIC TITRATION

Experiment 4: Potentiometric Titration Page | 1

CH

E130L

Analytical C

hemistry

Laborato

ry

I. OBJECTIVES

Upon completion of the experiment, the student should be able to:

calibrate an electrometric pH meter;

use potentiometric measurements to determine the pH of an unknown;

determine the factors affecting pH of solutions; and

determine the ionization constant of a weak acid by potentiometric titration.

II. A. LABORATORY EQUIPMENT / INSTRUMENTS

Equipment/ Accessories Quantity

250-mL beaker 2 pH meter 1 Magnetic stirrer 2

Iron stand 1 Pipet 1

Rubber aspirator 1 Stirring rod 1 Iron stand 1

Iron clamp 1

B. CHEMICALS AND REAGANTS

Chemical/ Reagent

Standard NaOH solution Distilled water in wash bottle Standard buffer solutions (ph 4.0 and pH 7.0)

Unknown acid

III. DISCUSSION OF FUNDAMENTALS

Introduction

In the previous experiment, we are already concerned of the different concepts that underlies

about acid-base titrations. Acid-base titrations, by concept, is also called a neutralization reaction,

since it involves an acid and a base (that is regardless of strength, either one is weak or one is strong

or either weak or both strong and ad infinitum), and regularly have the products of salt and water.


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The completion of the reaction is reached when the number of moles of the acid is equal to the

moles of the base, or technically the equivalence point is reached. In this point, if the stoichiometric

ratios of the acid and base are equal, we can solve for the concentration of the unknown by using

the dilution equation, that is, M1V1=M2V2. For diprotic and triprotic acids and bases that are titrated

with a strong acid/base, however, have many equivalence points, depending on how protic it is(i.e. a

diprotic acid/base has two equivalence points, whereas a triprotic acid/base has three) since the

acid/base has to converted to its less acidic/basic form, or its intermediates. A good example would

be a diprotic acid titrated using a strong base (in this case, sodium hydroxide), or H2A. The reaction

would proceed, forming HA-, and ultimately forming the base, A2-. But in this titration, you cannot

determine its actual pH at a certain aliquot of the titrant, since what we are using in this type of

titration is an indicator, an organic compound added to the solution to see the equivalence point by

a change of color. In doing so, a pH meter is used to monitor the measurement of pH of the titration

process at hand. The pH meter measures the [H+] concentration of the solution, so if the titration

goes from base to acid, the pH must be computed as pH=14-pOH, where pOH is the initial reading of

pH. By using a pH meter in an acid-base titration to monitor the pH variations of the titration

process, we call this process a potentiometric titration, wherein it finds the equivalence point not

through an indicator, but through a graph that signifies the relationship between the recorded

amount of titrant used, and the measured pH of the system.

Discussion

Potentiometric titration as has been introduced

in the previous experiment also needs the slow

addition of a titrant to an analyte to determine the

endpoint. Potentiometric titration makes use of a

pH meter to monitor the pH of the resulting

solution upon addition of the titrant. A plot is then

made of the resulting pH against volume of added

titrant. In cases where the change in pH at the

equivalence point are too small to obtain a sharp

endpoint, the slope of the titration curve can be

plotted against the volume of added titrant and a sharp peak corresponding to the equivalence

point can be obtained The tip of the sharp peak, known as the inflection point, corresponds to a

value where the slope goes through a maximum (increasing to decreasing values).

Figure 1. Sample of titration curve


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Aside from determining amounts of analytes,

titration data, especially the titration curve can help in

the determination of identities of unknown acids and

bases. Various portions of the curve show species that

are present during the course of the titration. The

equivalence point also known as the neutralization

point shows all analyte has been used up, and after

which, the titrant is in excess. For the titration of an

unknown weak acid, the pH at the midpoint (half-way

point), also termed as the point of 50% neutralization

point, gives the ionization constant of the acid.

Applications

The potentiometric titration method has many applications. These applications, however, seek

proper tests to satisfy the conditions for quantitative analysis, i.e. starting a test of a sample with

known concentration, etc. Potentiometry is mainly applied to acid-base, precipitation and redox

reactions. On acid-base reactions, its practical application would be on the field of knowing the pH

levels of the different manufactured food and many others, as well as common household items. For

precipitation reactions, the determination of how chlorinated the water sample is done by finding

the chloride ions in different water samples, and comparing them. The chloride ions are generally

insoluble to water, so by a precipitation reaction, it will do the trick. Lastly, for redox reactions,

potentiometry is used to determine the ferrous (Fe2+) ions in certain medicinal tablets, as iron goes

on two states, one having a 2+ charge(ferrous) and one with a 3+ charge(ferric). By the redox

reaction and its counterpart potentiometric titration, we can successfully determine if the iron in

the tablet is ferrous or is ferric.

Figure 2. 1st plot derivative of sample titration curve


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IV. METHODOLOGY

The pH meter was calibrated

according to the instructions given.

The pHs of the standard buffers used

for calibration were recorded.

Figure 3. Calibration of the pH meter

After calibrating, the pH of the unknown

acid was recorded.

The acid was titrated potentiometrically

with 0.50-mL aliquots until there was no

change in its pH.

When the results stray far from the

expected, the probe was calibrated using

the two buffers.

After every pH measurement, the probe

was washed with distilled water.

Volume and pH were recorded with every

addition.

A graph showing the relationship between

the pH and titrant volume was made.

Figure 4. Potentiometric titration of unknown acid


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V. DESCRIPTION OF THE APPARATUS / SET – UP

A pH meter is an instrument that measures the hydrogen ion concentration of the solution it will

be used into. The experiment we’ve done uses a portable version, one that can run without plugging

in an electric socket since it runs on batteries. Beside the probe is a metal that detects the

temperature, which can be seen on the display screen. The probe, metal and the surrounding parts

are washed with distilled water as to remove the remaining traces of the last measured sample. The

pH meter has two modes, calibrate and measure. Before measuring any sample, it is a good practice

to always calibrate the probe with the green and red buffer solutions of 4.01 and 7.00 pH

measurements. This is to ensure the accuracy and precision of the results of the measuring to be

done.

VI. DATA SHEET

TITRANT VOLUME pH TITRANT VOLUME pH

6 1.94 22 7.52

6.5 1.99 22.5 7.64

7 2.26 23 7.71

7.5 2.34 23.5 7.86

8 2.4 24 7.99

8.5 2.52 24.5 8.13

Figure 5. Potentiometric titration set-up Figure 6. pH meter device


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9 2.64 25 8.3

9.5 2.64 25.5 8.6

10 2.55 26 8.82

10.5 2.82 26.5 9.18

11 2.71 27 9.33

11.5 2.85 27.5 9.72

12 2.8 28 9.9

12.5 3.04 28.5 9.97

13 3.28 29 10.07

13.5 3.45 29.5 10.11

14 4.14 30 10.34

14.5 4.48 30.5 10.57

15 4.95 31 10.74

15.5 5.45 31.5 10.82

16 5.78 32 10.85

16.5 6.24 32.5 11.05

17 6.45 33 11.12

17.5 6.89 33.5 11.21

18 6.95 34 11.45

18.5 6.98 34.5 11.53

19 7.02 35 11.6

19.5 7.05 35.5 11.6

20 7.12 36 11.61

20.5 7.2 36.5 11.62

21 7.37 37 11.65

21.5 7.51 37.5 11.65

VII. SAMPLE COMPUTATIONS

( )

| |

(

)

Probable identity of acid: Citric acid (7.4 x 10-3) or Phosphoric acid (7.5 x 10-3)


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VIII. RESULTS AND DISCUSSIONS

Titration is the controlled

neutralization of an acid and a

base. If we consider the titration

of a weak acid by a strong base,

we can analyze the situation

completely and determine all the

concentrations of the aqueous

species at any volume addition of

the titrant. From the figure, one

can notice that the titration of a

weak acid with a strong base is

different from that of a strong base to

a relatively strong acid titration, primarily in the beginning of the titration, or before the equivalence

point is reached. Moreover, the equivalence point itself is shifted upward in pH, making the

equivalence point arrive in the basic pH.

The experiment started with the calibration of the

pH meter. A pH meter is an instrument that measures the

hydrogen ion concentration of the solution it partakes unto,

and if this should be done, one must need to calibrate it first

to ensure accuracy for the proceeding myriad of

measurements to follow. There are various types of pH

meters, and the one in this experiment is the portable

version, one that can run without plugging it in since it runs

on batteries. By having two standardized buffer solutions of

different concentrations (one preferably neutral and the

other acidic/basic), the pH meter is calibrated accordingly by

the following steps:

1. Turn on meter and set MODE to CAL 2. Wash probe with distilled water. 3. Put the probe on one solution. When display

registers ‘READY’, press enter. Read until the desired concentration is registered.

4. Rinse the probe with distilled water and slightly shake to dry.

5. Repeat 3 for the other solution. When finished, put MODE to MEAS (Measure).

Figure 7. Titration curve of weak acid titrated by a strong base

Figure 8. Example of a portable pH meter.


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The titration process is started with the usage of the standardized NaOH solution from

Experiment 3(Acid-base titrations). The NaOH solution is used to fill the burette to the 0.00 mL

mark. By having a known volume of the unknown acid, the initial pH is measured by the pH meter.

This initial pH will be used for the computation of the ionization constant of the unknown acid. After

this, the potentiometric titration process will now commence. Potentiometric titration is mainly

used in this because it does not need an indicator to determine the equivalence point, but by

titrating in aliquots, not considering whether it will break through the equivalence point or not. By

using 1.50, 0.50 and 0.20 (from which our group only did the second aliquot) mL aliquots at different

containers, the acid is to be titrated with it, and measuring the pH for every aliquot that is added to

it. In doing this, one can observe that the change in pH is small, but there would be a sharp change

in pH upon the middle of the titration. This sharp change is characterized by the solution’s tendency

to reach the equivalence point of the titration process. The titration is stopped when the pH of the

system is constant. This pH is referred to as the pH of the titrant, from which it will be already

constant since all of the unknown acid has been converted to its salt and water form by reacting

with NaOH. To understand what the values basically mean, the values are plotted as the pH on the

y-axis, and the volume of the titrant (cumulative) on the x-axis. This graph is now what we call as the

titration curve. By observation, the graph looks like an intestine shaped like an ‘S’. This type of

graph is what we call a sigmoidal curve, a logistic curve (or one that is related to exponential

functions). The end point will be found in this graph, as the location of the sudden change in pH will

be clear. Since this experiment will be under the preference that it assumes the acid is a

hypothetical monoprotic acid HA , and if was made to react to a strong base, then the resulting

graph would only have one equivalence point.


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Based on the graph will be the determination of the ionization constant Ka of the unknown acid.

This will come in two ways. First would be the calculation of the ionization constant using titration

data. By getting the volume of the titrant at the equivalence point, on the half -equivalence point,

and the pH value of 50% neutralization, we can compute for the pH of the system by using the

equation: and

. After this, the other method would be the

calculation of the ionization constant based on initial pH. By having the assumed concentration of

the unknown acid to be equal to the concentration of the titrant and its initial pH, the pH will be

computed by the following equation:

. By comparing both values to some known

acid’s ionization constants, the identity of the unknown acid will be clarified.

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30 35 40

pH

Figure 9. pH vs. Volume of titrant Volume of titrant (mL)


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IX. SUMMARY AND CONCLUSIONS

This experiment concluded the concepts involving potentiometric titration. By specifically using

a calibrated pH meter using buffer solutions of standardized concentrations, the pH of the solution

is constantly measured as to adding an exact amount of the titrant that is measured accordingly to

its equivalent aliquot. This is done until the pH of the system is constant and more or less basic

(since we are handling an analyte that is classified as an unknown acid, which is being titrated by a

strong base). After plotting the values of pH against volume of the titrant, we form a graph that will

determine the equivalence point of the reaction. By using the values of the volume @50%

neutralization, the equivalence point and the initial value of the pH, we can compute for the

ionization constant of the unknown acid, as well as identifying the molecular formula of the acid and

identifying the acid itself. Errors should be avoided especially on the consistency of putting an exact

amount of the titrant, as this will in small amounts, will collectively deter the position as to where

the equivalence point should be, therefore getting an incorrect ionization constant in the following

changes, and it further deteriorates the consistency of the experiment itself.

X. POST LAB QUESTIONS

Which of the two methods of calculation can give the more accurate value for the ionization

constant? Explain your reason. Cite possible errors incurred.

Answer: The more accurate method of knowing the ionization constant is the one using the

initial pH, by the equation:

. This is more accurate than the other one, since the

50% neutralization volume in our graph is not clearly determined at the first sight of it, therefore

it will leave us to do interpolation, which might not be needed to do for there is a given formula

for getting the ionization constant, and there wouldn’t be any need to do that. Moreover, the

initial pH is among the most accurate of all the pH measurements, since it is the first to be

measured by the freshly calibrated pH meter, and in effect, will have the most accurate

computed ionization constant, Ka.


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XI. REFERENCES

Christian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc.

Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative analysis. New

Jersey: Pearson Prentice Hall.

Skoog, Douglas et. al. 2004. Fundamentals of Analytical Chemistry (8th ed.). Singapore:

Thomson Learning.

experiment 4 - potentiometric titration

Documents

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