Friday, 31 May 2013

Practical 4


Title

Practical 4:  Determination of Diffusion Coefficient

Objective

To determine the diffusion coefficient of crystal violet and bromothymol blue.

Introduction

Diffusion is the process of material transport by atomic motion where the movement of solutes from a high concentration area to a lower concentration area occur spontaneously. The Fick’s Law states that the diffusion flux of material ( amount dm in time dt ) across a given plane ( area A ) is proportional to the concentration gradient dc/dx. The first Fick’s Law equation is

dm = -DA(dc/dx)dt -------------------- (Equation 1)

where D is the diffusion coefficient or diffusivity for the solute, in unit m2s-1. Diffusion coefficient is generally prescribed for a given pair of species. For a multi-component system, it is prescribed for each pair of species in the system. The higher the diffusivity (of one substance with respect to another), the faster they diffuse into each other. For dc/dx which is the concentration gradient is often called the driving force in diffusion and the minus sign in the equation means that diffusion is down the concentration gradient. This first law is applied to systems in which the conditions are not steady but for the second law is applied to system in which the conditions are non-steady. In most real situations the concentration profile and the concentration gradient are changing with time in the second law.

If a solution containing neutral particles with the concentration Mo, is placed within a cylindrical tube next to a water column, the diffusion can be stated as

M = M0 eksp (-x²/ 4 Dt)  --------------------- (Equation 2)

Where M is the concentration at distance x from the intersection between and solution that is measured at time t. by changing the Equation 2 to its logarithmic form which is

ln M = ln M0 – x2 / 4Dt  or
2.303 x 4D (log10M0 – log10 M) t = x2 ----------------------- (Equation 3)

Thus, a graph of x2 against t can produce a straight line that passed through the origin with the slope 2.303 x 4D (log10M0 – log10 M). As a result, D can be calculated. If the particle in the solution are assumed to be spherical, their size and molecular weight can be calculated by the Stokes – Einstein equation which is

D = kT / 6пŋa 

Heat causes atoms to vibrate and the Vibration amplitude increases with Temperature. Melting occurs when vibrations are sufficient to rupture bonds. the Vibrational frequency is approximately 1013 H Average atomic / electronic energy due to thermal excitation is of order kT
where k is the Boltzmann constant constant 1.38 x 1023 Jk-1
T is the temperature in Kelvin
П is the viscosity of the solvent in Nm-2s
a is the radius of particle in M.

The volume of spherical particle is 4/3 пa3, thus its weight M is equivalent to 4/3 пa3Np (p = density). It is known that molecular weight M = mN (N is the Avogadro’s number 6.023 x 1023 mol-1). Therefore,

M = 4/3 пa3Np   ------------------------ (Equation 4)

If the diffusion is for charged particles, the Equation 3 should be modified which we need to include potential gradient effect that exist between the solution and solvent. However, by adding a little sodium chlorine into the solvent, the problem where formation of this potential gradient can be solved.

            Agar gel contains a semi-solid network of molecular that can be penetrated by water molecules. The water molecules will form a continuous phase around the agar gel. Then, the solute molecules can be diffused freely in the water, if there are no chemical interactions and adsorption effects occur. Thus, the agar gels will provide a suitable supportive system that can be used in the experiment for diffusion of certain molecules in a aqueous medium.
Procedures


1 1) 7g of agar powder is weighed and mixed with 425ml of Ringer solution in the 500mL beaker.
2 2) The mixture in the beaker is stirred and boiled on a hot plate until a transparent yellowish solution was obtained.
3.      About 20ml of the agar solution is pour into 6 test tubes. The test tubes are then put into the fridge to let them cool.
4.      An agar test tube which contained 5ml of 1:500,000 crystal violet is being prepared where it is used as a standard system to measure the distance of the colour as a result of the diffusion of crystal violet.
5.      After the agar solutions in the test tubes solidifying, 5ml of each 1:200, 1:400, 1:600 crystal violet solution is pour into each test tube.
6.       The test tubes are closed immediately to prevent the evaporation.
7.      Three test tubes are put in room temperature (28 ºC) while another three are put in water bath of 37 oC.
8.      The distance between the agar surface and the end of crystal violet where that area has the same colour as in the indicator was measured accurately.
9.      Average of the readings are obtained, this value is x in meter.
10.  The x values are recorded after 2 hours and at appropriate intervals for 2 weeks.
11.  Steps 3 to 10 are repeated for bromothymol blue solutions.
12.  Graph of x² values (in m²) versus time (in hours) are plotted.
13.  The diffusion coefficient , D was determined from the graph gradient for both 28 ºC and 37 ºC and the molecular mass of crystal violet and bromothymol blue are also determined by using N and V equation.

Result :

Crystal Violet system

System
Time (seconds)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:200
0
0
0
3.5x10-5
1.12 x
10-6
28
1.0617 x 10-6
86400
1.5
2.25
172800
2.0
4.00
259200
2.6
6.76
345600
3.2
10.24
432000
3.8
14.44
518400
4.2
17.64
604800
4.5
20.25














System
Time (seconds)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:400
0
0
0
3.24x10-5
1.136 x 10-6
28
1.0617 x 10-6
86400
1.0
1.00
172800
1.8
3.24
259200
2.5
6.25
345600
3.0
9.00
432000
3.7
13.69
518400
4.0
16.00
604800
4.2
17.64














System
Time (seconds)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:600
0
0
0
2.5x10-5
9.291 x 10-7
28
1.0617 x 10-6
86400
0.8
0.64
172800
1.2
1.44
259200
2.0
4.00
345600
2.7
7.29
432000
3.0
9.00
518400
3.5
12.25
604800
3.8
14.44














System
Time (seconds)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:200
0
0
0
3.41x10-5
1.089 x 10-6
37
1.073 x 10-6
86400
1.8
3.24
172800
2.2
4.84
259200
2.5
6.25
345600
3.0
9.00
432000
3.4
11.56
518400
4.1
16.81
604800
4.7
22.09














System
Time (seconds)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:400
0
0
0
3.043x10-5
1.067 x 10-6
37
1.073 x 10-6
86400
1.5
2.25
172800
2.1
4.41
259200
2.6
6.76
345600
3.2
10.24
432000
3.5
12.25
518400
3.7
13.69
604800
4.4
19.36














System
Time (seconds)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:600
0
0
0
2.86x10-5
1.063 x 10-6
37
1.073 x 10-6
86400
0.8
0.64
172800
1.5
2.25
259200
2.4
5.76
345600
2.8
7.84
432000
3.3
10.89
518400
3.7
13.69
604800
4.0
16.00














Bromothymol Blue system

System
Time (s)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:200
0
0
0
3.704x10-5
1.1833x10-6
28
1.10873 x 10-6
86400
1.4
1.96
172800
2.2
4.84
259200
2.5
6.25
345600
3.1
9.61
432000
3.8
14.44
518400
4.2
17.64
604800
4.9
24.01














System
Time (s)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:400
0
0
0
3.472x10-5
1.2170x10-6
28
1.10873 x 10-6
86400
1.2
1.44
172800
1.6
2.56
259200
2.4
5.76
345600
3.0
9.00
432000
3.4
11.56
518400
3.7
13.69
604800
4.3
18.49














System
Time (ss)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:600
0
0
0
2.49x10-5
9.2589x10-7
28
1.10873 x 10-6
86400
1.0
1.00
172800
1.4
1.96
259200
2.0
4.00
345600
2.7
7.29
432000
3.1
9.61
518400
3.5
12.25
604800
3.8
14.44














System
Time (s)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:200
0
0
0
2.716x10-5
8.6768x10-7
37
8.8445 x 10-7
86400
1.2
1.44
172800
1.7
2.89
259200
2.0
4.00
345600
2.6
6.76
432000
3.4
11.56
518400
4.0
16.00
604800
4.5
20.25














System
Time (s)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:400
0
0
0
2.862x10-5
1.0032x10-6
37
8.8445 x 10-7
86400
1.0
1.00
172800
1.5
2.25
259200
2.1
4.41
345600
2.5
6.25
432000
3.3
10.89
518400
3.7
13.69
604800
4.0
16.00














System
Time (s)
x, M
x2, M2
Slope of graph
D, M2S-1
Temperature, oC
Average Diffusion Coefficient, D (M2S-1)
1:600
0
0
0
2.106x10-5
7.8248x10-7
37
8.8445 x 10-7
86400
0.8
0.64
172800
1.3
1.69
259200
2.0
4.00
345600
2.5
6.25
432000
2.8
7.84
518400
3.2
10.24
604800
3.4
11.56
















Graph of x2 against time (Crystal violet at 280C)

Click to enlarged


Graph of x2 against time (Crystal violet at 370C)


Click to enlarged


Graph of x2 against time (Bromothymol Blue at 280C)


Click to enlarged


Graph of x2 against time (Bromothymol Blue at 370C)


Click to enlarged


Calculation:

The slope = 2.303 x 4D (log10 M0 – log10 M)
Crystal violet system with dilution 1:200 (280C)

M = 1:500000                       M0 = 1:200
    =1/500000                              = 1/200    
    = 2.0 x 10-6                             = 5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 3.5 x 10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 3.5 x 10-5
D = 1.12 x 10-6 cm2/ s

Crystal violet system with dilution 1:400 (280C)

M = 1:500000                       M0 = 1:400
    =1/500000                              = 1/400    
    = 2.0 x 10-6                             = 2.5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 3.24x 10-5
2.303 x 4D (log10 2.5 x 10-3 – log102 x 10-6) = 3.24 x 10-5
D = 1.136 x 10-6 cm2/ s

Crystal violet system with dilution 1:600 (280C)

M = 1:500000                       M0 = 1:600
    =1/500000                              = 1/600    
    = 2.0 x 10-6                             = 1.67 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 2.5 x 10-5
2.303 x 4D (log10 1.67 x 10-3 – log102 x 10-6) = 2.5 x 10-5
D = 9.29 x 10-7 cm2/ s

Average of Diffusion Coefficient, m²/hour for Crystal Violet system at 28ºC
= (1.12 x 10-6 cm2/ s + 1.136 x 10-6 cm2/ s +9.29 x 10-7 cm2/ s) / 3
= 1.0617 x 10-6 cm2/ s

Crystal violet with dilution 1:200 (370C)

M = 1:500000                       M0 = 1:200
    =1/500000                              = 1/200    
    = 2.0 x 10-6                             = 5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 3.41 x 10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 3.41 x 10-5
D = 1.089 x 10-6 cm2/ s
  
Crystal violet with dilution 1:400 (370C)

M = 1:500000                       M0 = 1:400
    =1/500000                              = 1/400    
    = 2.0 x 10-6                             = 2.5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 3.043 x 10-5
2.303 x 4D (log10 2.5 x 10-3 – log102 x 10-6) = 3.043 x 10-5
D = 1.067 x 10-6 cm2/ s

Crystal violet with dilution 1:600 (370C)

M = 1:500000                       M0 = 1:600
    =1/500000                              = 1/600    
    = 2.0 x 10-6                             = 1.67 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 2.86 x 10-5
2.303 x 4D (log10 1.67 x 10-3 – log102 x 10-6) = 2.86 x 10-5
D = 1.063 x 10-6 cm2/ s

Average of Diffusion Coefficient, m²/hour for Crystal Violet system at 37ºC
= (1.089 x 10-6 cm2/ s + 1.067 x 10-6 cm2/ s + 1.063 x 10-6 cm2/ s) / 3
= 1.073 x 10-6 cm2/ s

Bromothymol blue system with dilution 1:200 (280C)

M = 1:500000                       M0 = 1:200
    =1/500000                              = 1/200    
    = 2.0 x 10-6                             = 5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 3.704x10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 3.704x10-5
D = 1.1833 x 10-6 cm2/sec

Bromothymol blue system with dilution 1:400 (280C)

M = 1:500000                       M0 = 1:400
    =1/500000                              = 1/400    
    = 2.0 x 10-6                             = 2.5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 3.472x10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 3.472x10-5
D = 1.2170 x 10-6 cm2/sec

Bromothymol blue system with dilution 1:600 (280C)

M = 1:500000                       M0 = 1:600
    =1/500000                              = 1/600    
    = 2.0 x 10-6                             = 1.67 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 2.492x10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 2.492x10-5
D = 9.2589 x 10-7 cm2/sec

Average of Diffusion Coefficient, m²/hour for Bromothymol Blue system at 28ºC
= (1.1833 x 10-6 cm2/sec + 1.2170 x 10-6 cm2/sec + 9.2589 x 10-7 cm2/sec) / 3
= 1.10873 x 10-6 cm2/sec

Bromothymol blue system with dilution 1:200 (370C)

M = 1:500000                       M0 = 1:200
    =1/500000                              = 1/200    
    = 2.0 x 10-6                             = 5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 2.716 x 10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 2.716 x 10-5
D = 8.6768 x 10-7 cm2/sec

Bromothymol blue system with dilution 1:400 (370C)

M = 1:500000                       M0 = 1:400
    =1/500000                              = 1/400    
    = 2.0 x 10-6                             = 2.5 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 2.862 x 10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 2.862 x 10-5
D = 1.0032 x10-6 cm2/sec

Bromothymol blue system with dilution 1:600 (370C)

M = 1:500000                       M0 = 1:600
    =1/500000                              = 1/600    
    = 2.0 x 10-6                             = 1.67 x 10-3
2.303 x 4D (log10 M0 – log10 M) = 2.106 x 10-5
2.303 x 4D (log10 5 x 10-3 – log102 x 10-6) = 2.106 x 10-5
D = 7.8248 x 10-7 cm2/sec

Average of Diffusion Coefficient, m²/hour for Bromothymol Blue system at 37ºC
= (8.6768 x 10-7 cm2/sec + 1.0032 x10-6 cm2/sec + 8.8445 x 10-7 cm2/sec) / 3
= 8.8445 x 10-7 cm2/sec


Discussion:

            Fick's first law relates the diffusive flux to the concentration under the assumption of steady state. It postulates that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative). In one (spatial) dimension, the law is

 This experiment is carried out to determine the diffusion coefficient (D) of the crystal violet and bromothymol blue. A graph x² against time t is plotted, a straight line is obtained with the gradient of 2.303 x 4D (log 10 Mo- log 10 M) . From here D can be calculated. This is because we already controlled some of the variables such as the size of the particles and also the temperature. Factors such as viscosity and concentration of agar gel may affect the rate of diffusion too. From this experiment we can know that both 28ºC and 37 ºC system have high rate of diffusion at concentration 1:200 and low rate of diffusion at 1:600.

            During this experiment, we are preparing a standard system with the dilution 1:500 000 that is known as M. the (log 10 Mo- log 10  M) will increased when Mo is increased. This will cause the rate of diffusion increase because the concentration gradient become larger and therefore the driving forces for the occurance of diffusion would be larger also. 

            One of the other factor that will affect the rate of diffusion is temperature. As we can see from the result we get from the experiment, it shows that test tube that is located in the water bath 37°C has a high rate of diffusion than test tube located at room temperature 28°C. This result is due to the kinetic energy theory. The kinetic energy of the molecule will increase when the temperature is increased. It will provide the molecule the energy to free from the intermolecular forces and make the easily escape and enter the agar.

            Other than that the agar gel also can influence the rate of diffusion. When the concentration of gel substance is increase, the size of the hole will decrease and the diffusion rate will decrease too as the hole size same with the size of the diffuse molecule. Moreover, the viscosity of the solution in the hole also can influence the diffusion rate. When the crystallinity of the gel medium is increased, the diffusion rate will decrease. The larger the volume fraction of crystalline material, the slower the movement of diffusion molecules. This can be happened because crystalline regions of the gel medium represent an impenetrable barrier to the movement of solute particles where it have to circumnavigate through it.

            From the result that we obtained, at room temperature 28°C, the diffusion coefficient for crystal violet is 1.0617x10-6 cm2/sec and bromothymol blue is 1.10873 x 10-6 cm2/sec while for the experiment that carried out in the water bath with temperature 37°C, the D for crystal violet and bromothymol blue are 1.073x10-6 cm2/sec and 8.8445 x10-7cm2/sec respectively. The D for crystal violet supposes to be higher than that of bromothymol blue. This may due to some errors that occurs during the experiment. The measurement taken be different people may be a little bit different and this will lead to the inconsistency of the readings. Besides, the colour of the dyes are not very obvious and this cause the measuring process become difficult and in accurate.

Question:

1.      From the experiment value for D28, estimate the value of D37 using the following equation
D28                        T28
------         =         --------
D37                        T37
Where ƞ1 and ƞ2 is the viscosity of water at temperature 280C and 370C.

D37 = D28 x (T37/ T28)
       = 1.0617x10-6 x ((37+273)/(28+273)
       = 1.093x10-6 cm2/ sec.

2.      Is the calculated value of D37 the same as the value from the experiment? Give some explanation if it is different. Is there any difference between calculated molecular weight with the real molecular weight?

            No. D37 from this experiment for crystal violet is 1.073x10-6 cm2/sec and for bromothymol blue is 8.8445 x10-7cm2/sec. This may due to some errors that occur during the experiment. There might be an inaccuracy while measure the length of dye   diffuse because our eye is not parallel to the meniscus and the colour is hard to be seen. Other than that viscosity of the gel may be not uniform which may affect the length of dye diffuse.

3.      Between crystal violet and bromothymol blue, which diffuse quicker? Explain if ther are any differences in the diffusion coefficient values?

The crystal violet will diffuse faster as its molecular size is smaller than bromothymol blue. From the diffusion coefficient calculated, the D for crystal violet is bigger than D of bromothymol blue. The higher the diffusion coefficient, the faster the diffusion rate. Thus, the statement that crystal violet diffuse faster is proven correct.
.
Conclusion:

From this experiment, we can determine the factor which effect the diffusion rate which are temperature and concentration of diffusing molecules. Therefore, the rate of diffusion is increase in high temperature and faster in the concentration of diffusing molecules 1:200> 1:400> 1:600. The rate of diffusion of crystal violet also higher than bromotymol blue. Diffusion coefficient, D for Crystal Violet system at 28ºC is 9.85x10-7 cm2/sec while at 37ºC is 1.088x10-6 cm2/sec. The diffusion coefficient, D for Bromothymol Blue system at 28ºC is 1.10873 x 10-6 cm2/sec while at 37ºC is 8.8445 x10-7cm2/sec

References:

1)  http://www.eng.utah.edu/~lzang/images/lecture-5.pdf
2)  http://web.utk.edu/~prack/mse201/Chapter%205%20Diffusion%20.pdf































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