INDEX

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Criteria PageIndex 1

Objectives 2

Summary 2

Introduction 3-7

Materials and Apparatus 8

Procedures 9-10

Results 11-14

Discussion 15-19

Conclusion 19

Recommendation 20

Tutorial 21-22

References 23

OBJECTIVE

To determine the pKa of methyl red by using the absorbance spectra as a function of pH.

SUMMARY

Part A: Preparation Standard Solution of Methyl Red

Methyl red (MR) was pipette into a volumetric flask. Ethanol (95%) was added and was diluted

with distilled water.

Part B: Preparation Acid and Basic Solution of Methyl Red

i) Acid Solution of MR (pH ≈ 1)

MR standard solution was pipette into volumetric flask and HCl (0.1M) was added. Distilled

water was used to dilute the solution. The pH value of the solution was checked and the colour of

the solution was observed. The absorption spectrum for the solution was recorded.

ii) Basic Solution of MR (pH ≈ 13)

MR standard solution was pipette into volumetric flask and NaOH (1.0M) was added. Distilled

water was used to dilute the solution. The pH value of the solution was checked and the colour of

the solution was observed. The absorption spectrum for the solution was recorded.

Part C: Absorption Spectrum of MR at various pH

Absorption spectrum at the eight different pH values was recorded. The volume of the mix

solution was given in the table 1.0. The quantity of the substance as in table 1.0 was pipette in

volumetric flask and diluted with distilled water. The flask was shook well to homogenize the

solution. The absorption spectrum for each solution was recorded on a different paper. The

remaining solution was kept to determine the true pH value by using the pH meter.

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INTRODUCTION

Methyl red (4-dimethylaminobenzene-2-carboxylic acid) is a commonly used indicator for acid-

base titrations. The visible absorption spectra of the acidic and basic forms of this compound will

be measure. Then, a series of buffered solutions of methyl red at known pH will prepare. By

following the change in absorbance as a function of pH the acid dissociation constant, or pKa

will determine. This technique is not restricted to indicators, and can be used with any substance

whose absorption spectrum changes with pH. The acid form of the indicator, which designate as

[HMR], is zwitter ionic, Figure 1. The basic form is designated as [MR-].

Figure 1.0: Acid and base forms of methyl red.

The equilibrium of interest is

HMR+H 2 O→ M R−¿+H 3 O+¿¿ ¿ -1-

The equilibrium constant is the acid dissociation constant:

pKa Methyl Red

Ka=¿¿ -2-

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The prime indicates that used concentrations rather than activities. Activities are necessary in

true thermodynamic equilibrium constants. Using concentrations, instead, gives the effective or

conditional equilibrium constant.

By definition pH = – log [H+] and pKa = – log Ka. Taking the (– log) of both sides of equation 2

gives:

pKa=pH−log¿¿¿ -3-

In this experiment, this equilibrium constant, pKa', will be determine by varying the pH and

measuring the ratio ¿¿. The acetic acid-acetate buffers will be use to control the pH values, since

the Ka value for acetic acid is in the same range as the Ka value for methyl red. The pH of these

buffers force methyl red to distribute itself somewhat evenly between the two coloured forms.

In chemistry, spectrophotometry is the quantitative study of electromagnetic spectra. It is more

specific than the general term electromagnetic spectroscopy in that spectrophotometry deals with

visible light, near-ultraviolet, and near-infrared. Also, the term does not cover time-resolved

spectroscopic techniques.[2]

Spectrophotometry involves the use of a spectrophotometer. A spectrophotometer is a

photometer (a device for measuring light intensity) that can measure intensity as a function of the

color, or more specifically, the wavelength of light. There are many kinds of spectrophotometers.

Among the most important distinctions used to classify them are the wavelengths they work

with, the measurement techniques they use, how they acquire a spectrum, and the sources of

intensity variation they are designed to measure. Other important features of spectrophotometers

include the spectral bandwidth and linear range.

The absorption spectrum can be symbolized as lambda (λ). Lambda (λ) max can be obtained at

the highest peak of the absorption spectrum graph.

The values of the [HMR] and [MR-] can be obtained by using the equation given:

HMR=λ pH 1− λ solution X -4-

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M R−¿=λ solution X− λ pH 13¿ -5-

The absorption of light is governed by the Beer-Lambert Law:

A = e ℓ [X] -6-

where,

A = absorbance

e = molar absorption coefficient

ℓ = path length of the cell in centimetres

[X] = concentration of the absorbing species in moles per litre

The absorbance of mixtures is the sum of the separate absorbencies. In mixtures of the acid and

base forms of methyl red the total absorbance is

A=A MR−¿+A HMR¿ -7-

The absorption spectra of [HMR] and [MR-] are given schematically in Figure 2. For two

components in solution, the absorbance must be measured at two different wavelengths. The best

wavelengths to choose for the analysis are where one form absorbs strongly and the absorbance

of the other form is negligible. Examination of Figure 2 reveals that there are no wavelengths

where one form, acid or base, absorbs exclusively. For this case, two equations need to be set up

in two unknowns, one equation for each wavelength. Call the two wavelengths l1 and l2. The

absorbance at l1 is A1 and at l2 is A2.The two measurements then provide two simultaneous

equations with two unknowns:

A1=a1 ,HMR

[HMR ]+a1 ,MR−¿ ¿¿ -8-

A2=a2 ,HMR

[HMR ]+a2 ,MR−¿ ¿¿ -9-

The molar absorbance coefficients are illustrated in Figure 2. The molar absorbance coefficients

are determined from standard solutions that contain one component alone. Equation 6 and 7

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provide two equations in two unknowns. For an unknown solution, the absorbance at the two

wavelengths, A1 and A2, are determined and then equation 6 and 7 are solved for the unknown

concentrations [MR-] and [HMR] at each given pH.

pKa Methyl Red

Figure 2.0: Absorbance of a solution is the sum of the absorbencies of the constituents.

Measurements at two wavelengths are necessary to determine the

composition of a two- constituent solution if the absorbance bands overlap.

The first subscript indexes the wavelength and the second subscript indexes

the constituent.

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In spectroscopy, an isosbestic point is a specific wavelength at which two chemical species have

the same molar absorptivity (ε) or -more generally- are linearly related. The word derives from

two Greek words: isos: equal and sbestos: extinguishable. [1]

Figure 3.0

Then, the theoretical graph that should be obtained between absorption spectrum against pH

values of the mixtures is like Figure 4.0. Draw a straight line corresponding to the absorption at

pH 1 (Line 1) and pH 13 (Line II). Hence, the graph obtained as in Figure 4.0.

Figure 4.0: Absorption graph (at specific wavelength) against pH

MATERIALS

Methyl Red

Ethanol (95%)

Hydrochloric Acid, HCl (0.1M)

Sodium Hydroxide, NaOH (1.0M)

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Sodium Acetate (0.2M)

Acetic Acid (0.2M)

APPARATUS

Ultraviolet Visible Spectrophotometer

Silica cell 10mm

Volumetric Flask (12 x 50ml)

Pipette (1mL, 5mL and 10mL)

Beaker (50mL and 100mL)

Dropper

pH meter

PROCEDURE

Part A: Preparation Standard Solution of Methyl Red

1. 5.0mL of methyl red (MR) stock solution was pipette into a 100mL volumetric flask.

2. 50mL of ethanol (95%) was added and diluted with distilled water to obtain 100mL

solution.

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Part B: Preparation Acid and Basic Solution of Methyl Red

i) Acid Solution of MR (pH ≈ 1)

1. 6.0mL of MR standard solution was pipette into 50mL volumetric flask.

2. 10.0mL of HCl (0.1M) was added.

3. The solution was diluted to 50mL with distilled water.

4. The pH of the solution was checked using a pH meter. The colour of the solution was

stated.

5. The absorption spectrum for the solution was recorded in the wavelength range of 350-

600 nm.

ii) Basic Solution of MR (pH ≈ 13)

1. 5.0mL of MR standard solution was pipette into a 50mL volumetric flask.

2. 10.0mL of NaOH (1.0M) was added.

3. The solution was diluted to 50mL volumetric flask.

4. The pH value was checked and the colour of the solution was stated.

5. The absorption spectrum of the solution was recorded as i(5) above.

Part C: Absorption Spectrum of MR at various pH

1. The absorption spectrum was recorded at the eight different pH values; besides the two

pHs in the Part B above.

2. As a guideline, to obtain the specific pH value, the volume of the mix solution was given

in Table 1.0.

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3. To prepare a solution with a specific pH, the quantity of the substance was pipette as in

Table 1.0 in 50mL volumetric flask. The solution was diluted with distilled water to the

50mL calibration mark.

4. The flask was shook well to homogenize the solution.

5. The absorption spectrum for each solution was recorded on a different paper. Recording

should be carried out immediately after preparing the solution.

6. The remaining solution was kept to determining the pH value.

7. The true pH value for all the solutions was determined by using the pH meter.

SolutionVolume of

MR/mL

Volume of Acetic

Acid (0.2M)

Volume of Sodium

Acetate (0.2M)pH Abs (λ max)

A 6.0 12.0 8.0

B 6.0 10.0 10.0

C 6.0 8.0 12.0

D 6.0 6.0 14.0

E 6.0 4.0 16.0

F 6.0 3.0 17.0

G 6.0 2.0 18.0

H 6.0 1.0 19.0

RESULT

Part B:

Methyl Red Solution pH Values Colour Changes

Acid solution 1.8 Light red to dark red

Basic solution 13.45 Light red to yellow

Table 1.0

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Part C:

SolutionVolume of

MR/mL

Volume of Acetic

Acid (0.2M)/mL

Volume of Sodium

Acetate (0.2M)/mLpH Abs (λ max)

A 6.0 12.0 8.0 4.36 2.469

B 6.0 10.0 10.0 4.52 1.587

C 6.0 8.0 12.0 4.70 1.402

D 6.0 6.0 14.0 4.93 1.232

E 6.0 4.0 16.0 5.19 0.979

F 6.0 3.0 17.0 5.32 0.748

G 6.0 2.0 18.0 5.58 0.738

H 6.0 1.0 19.0 5.97 0.908

Table 2.0

Overall results:

SOLUTION pH Absorption (λ max)pH ≈ 1 1.8 2.469

A 4.36 1.587B 4.52 1.402C 4.70 1.232D 4.93 0.979

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E 5.19 0.748F 5.32 0.738G 5.58 0.908H 5.97 0.984

pH ≈ 13 13.45 0.748

Table 3.0

0 2 4 6 8 10 12 14 160

0.5

1

1.5

2

2.5

3

Absorption of spectrum against pH values

AbsorptionLinear (Absorption)

pH values

Abso

rptio

n

Graph 1.0

Wavelength (nm)Absorption, λ

pH≈1 A B C D E F G H pH≈13

600.0 0.0550.07

00.063 0.057 0.048

0.037

0.032 0.030 0.028 0.007

575.0 0.5360.64

50.575 0.503 0.402

0.304

0.246 0.200 0.120 0.009

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550.0 1.8631.47

21.298 1.138 0.899

0.676

0.549 0.433 0.247 0.013

525.0 2.4321.58

71.402 1.232 0.977

0.743

0.609 0.492 0.296 0.036

500.0 2.1491.30

71.171 1.044 0.860

0.700

0.605 0.553 0.423 0.144

475.0 1.2720.85

00.810 0.780 0.729

0.707

0.681 0.749 0.725 0.361

450.0 0.5390.50

80.541 0.584 0.632

0.714

0.738 0.898 0.952 0.522

425.0 0.0180.33

70.397 0.465 0.551

0.667

0.712 0.896 0.980 0.549

Table 4.0

425 445 465 485 505 525 545 565 585 6050

0.5

1

1.5

2

2.5

3

Absorption against wavelength (nm)

pH≈1ABCDEFGHpH≈13

Wavelength (nm)

Abso

rptio

n

Isosbestic point(470, 0.75)

Graph 2.0

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Use these equations for calculating the pKa, [MR-] and [HMR] values.

pKa=pH−log¿¿¿ -3-M R−¿=λ solutionX− λ pH 13¿ -4-

HMR=λ pH 1− λ solution X -5-

Solution pHAbsorption, λ

max MR- HMR [MR-] / [HMR]

Log [MR-] / [HMR]

pKa

A 4.36 1.587 1.037 0.882 1.176 0.071 4.289B 4.52 1.402 0.852 1.067 0.799 -0.097 4.617C 4.70 1.232 0.682 1.237 0.551 -0.259 4.959D 4.93 0.979 0.429 1.490 0.288 -0.541 5.471E 5.19 0.748 0.198 1.721 0.115 -0.940 6.130F 5.32 0.738 0.188 1.731 0.109 -0.963 6.283G 5.58 0.908 0.358 1.561 0.229 -0.640 6.220H 5.97 0.984 0.434 1.485 0.292 -0.535 6.505

Table 5.0

3 3.5 4 4.5 5 5.5 6 6.5

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

f(x) = − 0.465411979361046 x + 1.87222050033471R² = 0.468371793784018

Log [MR-] / [HMR] against pH values

Log [MR-] / [HMR]Linear (Log [MR-] / [HMR])

pH values

Log

[M

R-] /

[HM

R]

Graph 3.0

DISCUSSION

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Part B: Preparation Acid and Basic Solution of Methyl Red (MR)

The acid solution of Methyl Red (MR) was prepared at the pH value of 1.8 and the colour

change for this solution is light red to dark red while the basic solution of methyl red obtained

was 13.45 of pH value. The colour of the basic solution turned from light red to yellow. The

absorption spectrums of the solution for both solutions were recorded in the length of 350 –

600nm. The absorption spectrum of acidic solution will produce high absorption of spectrum

compare to the solution that has more basic.

Part C: Absorption Spectrum of Methyl Red at various pH

Absorption spectrum is the characteristic pattern of dark lines or bands that occurs

when electromagnetic radiation is passed through an absorbing medium into a spectroscope. An

equivalent pattern occurs as coloured lines or bands in the emission spectrum of that medium.[3]

For solution A, 12.0mL of Acetic Acid (0.2M) was mix with 8.0mL of Sodium Acetate

(0.2M) and 6.0mL of Methyl Red solution. The pH obtained by using pH meter was 4.36 and the

maximum absorption of spectrum was 1.587. Hence the higher the volume or concentration of

acid used, the maximum absorption of spectrum will be higher. The pH value of the solution will

be lesser and shows high acidity of the solution.

Overall Reactions

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Graph 1.0 shows the relationship between the absorption of spectrums against pH values.

Line I represents for the absorption of spectrum at pH 1 and Line II represents for the absorption

of spectrum at pH 13. The theoretical graph that should be obtained is shown below.

Figure 4.0: Absorption graph (at specific wavelength) against pH

The graph that that has been plotted was not similar with the theoretical graph because of several

problems happen while conducting this experiment such as the cuvette was not cleaned properly

and there were finger printing on the cuvette that caused the UV and invisible light cannot

penetrate through the solution.

Graph 2.0 shows the relationship between the absorption of spectrum against wavelength

(nm). The isosbestic point that has been obtained when plotting all the graphs was (470, 0.75)

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where 470 and 0.75 represented the wavelength and the absorption of spectrum respectively.

From the graph obtained, the isosbestic point of the overall lines of absorption can be determined

by marked the intersection point of the similar level of absorption of the each pH values. The

levels of the absorption spectrum of pH 1 and pH 13 were not same to the others because of

some problems occur when handling the experiment.

During a chemical reaction, a point in the absorption spectrum (that is, a wavelength)

where at least two chemical species (for example, reactant and product) have identical molar

absorption coefficients, which remain constant as the reaction proceeds. A stable isosbestic point

is evidence that a reaction is proceeding without forming an intermediate or multiple products.

Graph 3.0 shows the relationship between log ¿¿¿ and pH values. The graph obtained

was inversely proportional. The intersection point at X-axis is the point where the indicator

concentration in the acidic medium is the same as the concentration in the basic medium. At the

pH where the intersection occurs, the pKa of the MR indicator will be determined.

From each results obtained, that graph 3.0 shown log ¿¿¿ always inversely proportional

against pH value. The indicator concentration in the acidic medium must be equally as the

concentration in the basic medium. So, the intersection point of the graph at x-axis must be taken

to get the value of pKa of the methyl red indicator. The pKa values must be same with the pH

values obtained by using theoretical method. The value of pKa can be determined by using the

equation:

pKa = pH + log [HMR]/[MR-]

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log [HMR]/[MR-]= pKa – pH

When log [HMR]/[MR-]= 0

0 = pKa – pH

pKa = pH

There are few points why the lines of the graphs deviate from the best fit line.

i. The mixtures had been contaminated during the experiment.

ii. The volumes of the mixtures were incorrect due to parallax error during measuring the

mixtures. (The eyes level of observer was not at the same level of the meniscus.)

iii. The solution may vaporize due to the property of strong oxidizing agent. Vaporization

may cause the volume of the solution being reduced without any notice.

iv. Different surrounding temperature. Should be conducted in a closed room with a stable

room temperature.

There were few weaknesses in this experiment. Firstly, there were bubbles in the pipette

appeared when the solutions were pipette. Then, the volume will decrease and the concentration

changed thus affecting the pH values. Furthermore, the apparatus used for the experiment were

not well cleaned. There was lot of impurities or known as ‘foreign substances’ stacked with the

apparatus that has been used. The contamination of the solution caused error to the result.

Parallax error caused different volume when measuring the solution thus, the pH values obtained

will be wrong. Moreover, the cuvette from the spectrophotometer used was not clean properly

and there was fingerprint on it. So, the UV light that went through the silica cell will be affected.

There are few precautions and safety steps that need to be taken when conducting this

experiment.

1. Avoid skin contact with acid and basic solution (use gloves when handling the acid).

2. Use stopper to prevent the spilling of the solutions from the volumetric flasks.

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3. Wear googles throughout the experiment.

CONCLUSION

The objective of this experiment of this experiment is to determine the pKa of monobase

indicator that is methyl red by measuring the absorbance spectra as a function of pH by using

ultraviolet visible spectrophotometer. The values of pKa can be determined by using the

formula given:

pKa=pH−log¿¿¿ -3-

At this point, the overall lines of absorption can be determined by marked the intersection

point of the similar level of absorption of the each pH values. This point is called the isosbestic

point where the coordinate obtained is (470, 0.75).

From each results obtained, that graph 3.0 shown log ¿¿¿ always inversely proportional

against pH value. The indicator concentration in the acidic medium must be equally as the

concentration in the basic medium. So, the intersection point of the graph at x-axis must be taken

to get the value of pKa of the methyl red indicator and the pH value of the solution can be

determined by using the formula given at equation 3.

RECOMMENDATION

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To run this experiment, there are few recommendations that need to be followed to ensure the

result of the experiment follow theory given.

1. Use stopper to close and prevent the impurities from entering the volumetric

flask.

2. Make sure the eye level and the meniscus of the solution and calibration mark are

perpendicular to prevent the parallax error.

3. Before use the apparatus, make sure that the apparatus is cleaned perfectly by

using distilled water to remove impurities.

4. Clean the pH meter using distilled water before use to measure the pH value.

QUESTIONS

1. Why is the graph log [MR-]/[HMR] against pH is a straight line?

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From each results obtained, it shows that graph log [MR-]/[HMR] against pH values

always in a straight line. The indicator concentration in the acidic medium need to be

same as the concentration in the basic medium hence the graph will have the intersection

point at x axis. Then, from the intersection at the graph, we can get the value of pKa of the

Methyl Red indicator.

2. From the data, is the pKa value similar with pH? If so, state the condition why it is

shows the same value.

The pKa values almost same with the pH values obtained using pH meter. In theoretical

method, the pKa value is similar with pH value.

The value of pKa can be determined by using the equation:

pKa = pH + log [HMR]/[MR-]

log [HMR]/[MR-]= pKa – pH

When log [HMR]/[MR-]= 0

0 = pKa – pH

pKa = pH

3. How do you calculate the pH of the buffer solution prepared? (One example is

sufficient). Is the pH value from the calculation are the same as the value determined

using pH meter?

Take solution A as example.

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pH = pKa + log [MR-]/[HMR]

pH = 4.50 + log [1.037]/[0.882]

pH = 4.50 + 0.0703

pH = 4.57

where

pKa = the intersection of the graph 3.0 at x-axis that 4.50

So, the difference is 0.07 which is not so far compared with the value from pH meter.

When comparing the pH value by calculated with the value that obtained from the pH

meter, the values are different because by using formula the value will be fixed. It is

different with pH meter which is not stable at all and depends on how the user use the pH

meter.

4. Discuss the weakness of this experiment?

Less accuracy measurement of the mixtures. Its known as parallax error which is the

position of eyes are not same level with the meniscus.

The dilutions of the mixtures are not exact. The meniscus level more than the calibration

mark of the volumetric flask.

The cuvette is not well cleaned. It will cause the cuvette less transparent and interfere the

absorption of the spectrum.

REFERENCES

1. Isosbestic Point,

http://en.wikipedia.org/wiki/Isosbestic_point, 17 September 2011/14:32

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2. Spectrophotometer,

http://en.wikipedia.org/wiki/Spectrophotometer, 18 September 2011/17:46

3. Absorption Spectrum,

http://www.thefreedictionary.com/absorption+spectrum, 18 September 2011/20:15

4. Isosbestic Point,

http://www.answers.com/topic/isosbestic-point#ixzz1YIFI1Nnm, 18 September 2011/

20:59

5. Physical Chemistry Laboratory manual

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