INDEX
Criteria
Page
Index
1
Objectives
2
Summary
2
Introduction Materials and Apparatus
3-7 8
Procedures
9-10
Results
11-14
Discussion
15-19
Conclusion
19
Recommendation
20
Tutorial References
21-22 23
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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 acidbase 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
-1-
The equilibrium constant is the acid dissociation constant:
pKa Methyl Red
[ ][ ] []
-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:
[ ] []
-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.
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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:
-4-
-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
-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 A 1 and at l2 is A 2.The two measurements then provide two simultaneous equations with two unknowns:
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[] [ ]
-8-
[] [ ]
-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 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
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MATERIALS
Methyl Red
Ethanol (95%)
Hydrochloric Acid, HCl (0.1M)
Sodium Hydroxide, NaOH (1.0M)
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
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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.
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 350600 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.
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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.
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.
Volume of
Volume of Acetic
Volume of Sodium
MR/mL
Acid (0.2M)
Acetate (0.2M)
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
Solution
pH
Abs (λ max)
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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
Part C: Volume of
Volume of Acetic
Volume of Sodium
MR/mL
Acid (0.2M)/mL
Acetate (0.2M)/mL
A
6.0
12.0
B
6.0
C
pH
Abs (λ max)
8.0
4.36
2.469
10.0
10.0
4.52
1.587
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
Solution
Table 2.0
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Overall results:
SOLUTION pH ≈ 1
pH
Absorption (λ max)
1.8
2.469
A B C
4.36 4.52 4.70
1.587 1.402 1.232
D E
4.93 5.19
0.979 0.748
F G H
5.32 5.58 5.97
0.738 0.908 0.984
pH ≈ 13
13.45
0.748
Table 3.0
Absorption of spectrum against pH values 3
2.5
2 n o i t p r 1.5 o s b A
Absorption Linear (Absorption)
1
0.5
0 0
5
10
15
pH values
Graph 1.0
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Wavelength (nm)
Absor ption, λ pH≈1
A
B
C
D
E
F
G
H
pH≈13
600.0
0.055
0.070
0.063
0.057
0.048
0.037
0.032
0.030
0.028
0.007
575.0 550.0
0.536 1.863
0.645 1.472
0.575 1.298
0.503 1.138
0.402 0.899
0.304 0.676
0.246 0.549
0.200 0.433
0.120 0.247
0.009 0.013
525.0 500.0 475.0 450.0
2.432 2.149 1.272 0.539
1.587 1.307 0.850 0.508
1.402 1.171 0.810 0.541
1.232 1.044 0.780 0.584
0.977 0.860 0.729 0.632
0.743 0.700 0.707 0.714
0.609 0.605 0.681 0.738
0.492 0.553 0.749 0.898
0.296 0.423 0.725 0.952
0.036 0.144 0.361 0.522
425.0
0.018
0.337
0.397
0.465
0.551
0.667
0.712
0.896
0.980
0.549
Table 4.0
Absorption against wavelength (nm) 3
2.5 pH≈1
A
Isosbestic point (470, 0.75)
2 n o i t p r 1.5 o s b A
B C D E
1
F G
0.5
H pH≈13
0 425
445
465
485
505
525
545
565
585
605
Wavelength (nm)
Graph 2.0
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-
Use these equations for calculating the pKa, [MR ] and [HMR] values.
[ ]
-3-
[]
Solution
pH
Absorption, λ max
A B C D E F G H
4.36
1.587
4.52
1.402
4.70 4.93
1.232 0.979
5.19 5.32
0.748 0.738
5.58 5.97
0.908 0.984
-4-
-5-
-
MR
HMR
[MR ] / [HMR]
1.037 0.852 0.682 0.429 0.198 0.188 0.358 0.434
0.882 1.067 1.237 1.490 1.721 1.731 1.561 1.485
1.176 0.799 0.551 0.288 0.115 0.109 0.229 0.292
-
-
Log [MR ] / [HMR]
pKa
0.071 -0.097 -0.259 -0.541 -0.940 -0.963 -0.640 -0.535
4.289 4.617 4.959 5.471 6.130 6.283 6.220 6.505
Table 5.0
Log [MR-] / [HMR] against pH values 0.2 0 3
3.5
4
4.5
5
5.5
6
6.5
-0.2
] R M H [ -0.4 / ] R M-0.6 [ g o L
Log [MR-] / [HMR] Linear (Log [MR-] / [HMR])
-0.8 -1 -1.2
y = -0.4654x + 1.8722 R² = 0.4684
pH values
Graph 3.0
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DISCUSSION
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.
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Overall Reactions
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.
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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) 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
[ ] []
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
[ ] []
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 pK a of the methyl red indicator. The pKa values must be same
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with the pH values obtained by using theoretical method. The value of pK a 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
There are few points why the lines of the graphs deviate from the best fit line. i. ii.
The mixtures had been contaminated during the experiment. 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.
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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.
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:
[ ]
-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
[ ] []
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 pK a of the methyl red indicator and the pH value of the solution can be determined by using the formula given at equation 3.
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RECOMMENDATION
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.
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QUESTIONS
1.
-
Why is the graph log [MR ]/[HMR] against pH is a straight line? -
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 pK a 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 pK a 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
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How do you calculate the pH of the buffer solution prepared? (One example is
3.
sufficient). Is the pH value from the calculation are the same as the value determined using pH meter? Take solution A as example. -
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.
Discuss the weakness of this experiment?
4.
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.
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REFERENCES
1.
Isosbestic Point, http://en.wikipedia.org/wiki/Isosbestic_point, 17 September 2011/14:32
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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