Expt AS
1
EXPERIMENTS
Adsorption from Solution (AS)
Objective
The purpose of this experiment is to determine the surface area of carbon powder by
studying the adsorption of the soluble dye methylene blue on the powder.
Special Instructions
Only use spatula and glassware that is in the AS drawer with the methylene blue. Do not use
any general glassware. Do not use spatula with methylene blue unless it is tagged "for
methylene blue." You do not need to try to clean blue color off glassware since it is used with
Methylene Blue each week.
Determine exact mass of carbon powder in each flask.
Your adsorption experiment will be done at room temperature.
Remember accurate concentrations require use of pipets and volumetric flasks using formula
M1V1=M2V2.
Prepare three plots:
(1) Beer law calibration curve of Absorbance versus concentration of methylene blue.
(2) N, mg of methylene blue adsorbed per gram of powder, versus Cf , final equilibrium
concentration. This curve is not linear and must go through the x,y origin. Draw the best hand
drawn smooth curve from the origin—point (0,0)— through these points for this graph.
(3) Cf/N versus Cf (these points should be fit with a straight line using linear regression software
to find the slope and intercept). This plot should NOT include the (0, 0) data point.
Report the specific surface area in square meters per gram of carbon powder. Note that this
should be in the range of 10 to 50 square meters per gram (m2/g) and you are likely to have
calculation error if your value is not in this range. We don't have specific literature value for this
powder.
Expt AS
2
Introduction
In this experiment we are concerned with the adsorption of the dye molecule methylene
blue on carbon powder. Dye molecules can become concentrated on the surface of a solid and
thus be removed from solution. This reduction in solution concentration can be observed
visually and can be quantified by the use of a uv/visible spectrophotometer. The decrease in
concentration of dye molecules is related to the number of dye molecules adsorbed on the
surface of the carbon. Since each dye molecule occupies a known surface area, the total surface
area of the carbon powder can ultimately be determined.
The substance used in this work is 3,7-Bis(dimethylamino)-phenothiazin-5-ium chloride, or
more simply methylene blue. The formula is C16H18ClN3S and the molecular weight is 373.9
g/mol. It was first prepared in 1876 and is now usually produced from dimethylaniline and
thiosulfuric acid. It consists of dark blue crystals and is used as a stain in bacteriology, as a
redox indicator, and it can be used as an antidote for cyanide poisoning. The structure is shown
below.
N
ClN
S
+
N
Adsorption
Adsorption is the sticking of molecules from the gas or liquid phase onto the surface of a
solid. A molecule that undergoes adsorption is referred to as the adsorbate, and the solid is the
adsorbent. Another process, absorption refers to a phenomenon where something is taken up
throughout - such as a sponge absorbs water. Adsorption refers to a phenomenon where
something is taken up only at the surface - such as carbon powder adsorbs a dye. The general
term for absorption and adsorption is sorption.
In this experiment, we are concerned only with adsorption. There are two types of
adsorption: (1) chemical adsorption (chemisorption), and (2) physical adsorption (physisorption).
Chemical adsorption involves the formation of chemical bonds such as in the surface oxidation
of a metal. A new penny gradually turns to a darker brown color and changes from being shiny
to being a dull color because of the formation of a coating of copper oxide on the surface of the
penny. Physical adsorption involves nonspecific attraction due to weaker van der Waals or
dipole forces and is similar to the condensation of a gas to a liquid.
Expt AS
3
Chemical adsorption involves higher heats of adsorption (80 to 400 kJ/mol of molecules)
and is enhanced by raising the temperature. There is an energy barrier for chemisorption based
on the fact that a chemical bond in the gas-phase or liquid-phase molecule must be broken first
before the molecule or its fragments can bond to the surface of the solid.
Physical adsorption involves low heats of adsorption (less than 40 kJ/mol) and is enhanced
by lowering the temperature. Molar heats of adsorption are similar in magnitude to molar heats
of vaporization. Water purification filters use carbon powder to physically adsorb organic
molecules from the water. Military gas masks use carbon powder to physically adsorb poisonous
gases from the air passing through the filter.
Theory
The number of molecules adsorbed per gram of solid, N (mol/g), depends on the specific
surface area of the solid, S(m2/g), the final liquid phase concentration Cf (mol/L) or equilibrium
gas phase pressure p (atm or kPa), and the specific molecules undergoing adsorption. A plot of
N versus Cf or N versus p, where the temperature is held constant, is referred to as an adsorption
isotherm. There are a variety of equations used to relate the moles adsorbed to the concentration
of adsorbate molecules. These equations are based on different models and three models (virial,
BET, and Langmuir) are discussed below.
In the Henry's law region of low coverage, the moles adsorbed are directly proportional to
the concentration or pressure. One may write
N = (B2s / RT) p
(1)
where R is the gas constant, T is the temperature, and B2s is the second gas-solid virial
coefficient. The second gas-solid virial coefficient is analogous to the gas virial coefficient,
which is a measure of the attraction between pairs of gas molecules. However, B2s is a measure
of the attraction between an isolated molecule and the surface of a solid. B2s can be related to an
integral that includes the exponential of the potential describing the exact form of the gas solid
interaction.
Whereas Eq. (1) is most useful for theoretical studies of the nature of gas-solid interactions,
the BET equation (developed by Brunauer, Emmett, and Teller in 1938) is widely used to
determine specific surface areas of solids, S (m2/g). The BET equation may be expressed as
1/[N(po/p -1] = 1/[Nm G] + [ (G-1) p]/[Nm G po ]
(2)
Expt AS
4
where N is the moles adsorbed at a relative pressure of p/po, po is the saturation pressure where
adsorbate condensation occurs, Nm is the moles adsorbed at monolayer coverage, and G is a
constant related to gas-solid interaction strength. The BET equation assumes that multilayer
adsorption can occur at sites covering the surface. A balance between adsorption and desorption
for each adsorbed layer is used to derive Eq. (2).
The model developed by Langmuir in 1916 differs from the BET model in that it does not
allow for adsorption beyond monolayer coverage(ooooooooo). This model is most appropriate
for gas phase chemisorption and adsorption from solution. The Langmuir equation may be
written as
N/Nm = k Cf / [1 + k Cf ]
(3)
where N/Nm is the fraction of surface covered by adsorbate molecules, Cf is the final molarity
concentration of adsorbate (mol/L), and k is a constant at a fixed temperature. A plot of N/Nm
versus Cf yields a plot that is linear at low Cf (1>>kCf: N/Nm = kCf) then curves and reaches a
flat region with a maximum value of 1 where N = Nm at higher concentration of Cf (1<<kCf :
N/Nm = kCf/kCf = 1). When the curve is in this flat portion then N = Nm since N/Nm = 1
A rearrangement of Eq. (3) yields a linear form for the Langmuir isotherm equation of
Cf/N =
(1/Nm) Cf + (1/kNm)
(4)
where (1/Nm) is the slope, and (1/kNm) is the intercept, when Cf/N is plotted versus the
concentration Cf. The inverse of the slope is Nm,and this represents the moles adsorbed at
monolayer coverage. Nm can be used in Eq. (5) to determine the specific surface area of a solid.
The moles adsorbed at monolayer coverage, Nm (mol), is based on a complete coverage of
the solid surface with a layer of adsorbate molecules. This value can be used to determine the
specific surface area, S (m2/g) from
S = Nm L am
(5)
where L is Avogadro's number (6.022x1023 molecules/mol), and am (m2/molecule) is the cross
sectional area for a single molecule. The BET equation is most often based on low pressure
nitrogen adsorption at 77K and the am value for nitrogen is 16.2 x10-20 m2/molecule ( the same
as 16.2 angstrom2 per molecule).
Note, we are using Cf to distinguish from Ci which is the initial concentration before the
dye solution is exposed to carbon powder. As dye molecules adsorb on the carbon surface then
Expt AS
5
the concentration in solution decreases until a dynamic equilibrium is reached and there is a
balance between dye molecules adsorbed on the surface of the carbon and present in the solution.
At this final equilibrium value the concentration is Cf . Cf should obviously be less than Ci .
Experimental
Use seven clean 125ml Erlenmeyer flasks that have rubber stoppers. Label each flask with
the amount of carbon it contains as you place the following approximate amounts (weigh to the
nearest milligram) of decolorizing carbon (a type of activated carbon or charcoal) in each flask:
#1 - 0.040g, #2 - 0.080g, #3 - 0.120g, #4 - 0.160g, #5 - 0.200 g, #6- 0.240g, and #7 (control) no powder. Flask #7 represents the initial concentration Ci since there is no carbon and hence no
change in dye concentration.
Caution: Methylene blue stains everything, and a tiny amount goes a long, long way. Be
cautious not to spill any of the methylene blue crystals or solutions. Use only the glassware and
spatula in the AS drawer! You may want to wear gloves to avoid blue hands.
Carefully measure out 0.0200g of methylene blue into a weigh boat. Weigh to the nearest
0.0001g realizing that the mass may be slightly over or under 0.02000g. Transfer the methylene
blue crystals to a beaker and wash the weigh boat with distilled water repeatedly to make sure all
the methylene blue is transferred into the beaker. Pour this solution into a 1L volumetric flask,
swirl the flask to mix, and rinse the beaker with water several times to get all the solution into the
volumetric flask. Bring the liquid to volume so that you have prepared a solution of
approximately 20.0 mg of dye per liter of solution.
Use 50ml or 100mL pipets to transfer exactly 100 mL of solution into each of the 125ml
Erlenmeyer flasks in order from #1 to #7. Carry out this procedure quickly so the carbon
powders are in contact with the dye solution at about the same starting time.
After the solutions have been added, the powder should be kept distributed in the liquid
solution as much as possible so the dye molecules can come in contact with the carbon surface.
However, the carbon powder tends to settle out of solution so some method is needed to keep the
powder in the liquid. Make sure each flask has a rubber stopper and then place all the flasks in
the provided carrier. Ask your instructor to show you the location and use of the shaker to
agitate flasks. If a shaker is not available you may have to swirl each of the flasks to mix the
powder and keep the powder suspend in solution. Make sure flasks are fixed where they will not
tip over. Whichever method is used continue for a period of one hour in order to allow the
adsorption process to reach equilibrium. If time was not a consideration we could just leave
overnight to insure that equilibrium was reached.
Expt AS
6
During this equilibration time, use the colorimeter cell to prepare solutions needed for
Beer's Law calibration curve. See the detailed instructions in the next section to complete this
part of the experiment. You can do the Beer's law calibration measurements (A through E) while
carbon containing flasks are shaking and then you complete the absorbance measurements as
indicated below.
Allow each flask to sit undisturbed for ten minutes after the one-hour shaking equilibration
time span so that the carbon powder settles to the bottom of the flask. You should be able to
decant the liquid without disturbing the settled powder. If this is not the case, you need to ask
your instructor how to filter the mixtures.
Use the colorimeter cell to measure the absorbance of each of the solutions in flasks #1
through #6.
Analysis
Recall the Beer law relates Absorbance to extinction coefficient, ε, path length, l, and
concentration of light absorbing molecule, C.
Abs = ε l C
(6)
Use Excel to determine the Beer Law plot for data from flasks A through E. Require the best
line to go through (0,0) point (intercept set at zero). Note that since the path length is 1.00cm
then the slope of the line is the molar extinction coefficient, also known as molar absorptivity
constant.
Using the Beer Law, you will be able to calculate the equilibrium concentration Cf (mg/L)
of the dye solution in flask #1 based on the absorbance reading for this solution and Eq. (6)
where C = Abs/( ε l ). Repeat for flasks #2 through #6.
Use Excel to calculate each of the following five quantities for flasks #1 through #6 (see
the instructions in the next section):
m
mass of powder in each flask, g
Abs
measured absorbance at for each dye solution
Cf
final concentration of dye (mg/L)
Ci-Cf
difference between initial and final concentrations (mg/L)
N
mass of methylene blue dye adsorbed per gram of carbon (mg dye/g carbon)
N = (0.100L)(Ci-Cf)/m
Cf/N
ratio (g/L) which is needed in Langmuir equation
Expt AS
7
Note that in all this work we are using mg of dye in our calculations of concentration and only at
the end will we consider the molecular mass of dye and convert to moles. This approach allows
the initial calculations to be independent of the particular dye used.
After you complete the calculations listed above, make a plot of N versus Cf and compare
to the expected form based on Eq. (4). This plot should go through the origin point (0,0), since
no dye in solution means no dye adsorbed. Draw the best curve through the points. Note this is
NOT a linear plot. This plot should follow the Langmuir equation.
Next plot Cf/N versus Cf, as shown in Eq. (5). Do NOT use the (0,0) point–only use the
measured data values. This is a linear plot. Determine the slope, intercept, and correlation
coefficient, and report these values on the plot. Determine the Nm value from the slope and
convert from (mg of dye/g of carbon) to (mol of dye/g of carbon).
The molecular weight of methylene blue is 373.9 g/mol. The molecular cross sectional
area, am, of the methylene blue dye molecule has been reported to be 120x10-20m2. Use the
value of Nm and am in conjunction with Eq. (5) to determine the specific surface area of the
carbon powder in (m2/g).
References
1.
S. Lowell, Introduction to Powder Surface Area (Wiley-Interscience, New York, 1979).
2.
J. H. Potgieter, J. Chem. Educ. 68, 349 (1991).
3.
R. Brina and A. De Battisti, J. Chem. Educ. 64, 175 (1987).
4.
D. P. Shoemaker et. al, Experiments in Physical Chemistry 6th edition (McGraw-Hill,
New York, 1996).
Adsorption from Solution (AS) and Colorimeter
You will be using the Colorimeter shown in Figure 1. In this device, red light from the LED light
source will pass through the solution and strike a photocell. The dye solution used in this
Expt AS
8
experiment has a deep blue color. A higher concentration of the colored solution absorbs more
light (and transmits less) than a solution of lower concentration. The Colorimeter monitors the
light received by the photocell as either an absorbance or a percent transmittance value.
Figure 1
Figure 2
Calibration Curve
You are to use the original solution and prepare dilute solutions of known concentrations in order
to generate a five-point calibration-curve plot. To measure the light-absorbing power of a
solution, a sample of the solution is transferred to a cuvette that is then placed into the
Colorimeter. The amount of light that passes through the solution and strikes the photocell is
used to compute the absorbance of each solution. When a graph of absorbance vs. concentration
is plotted for the standard solutions, a direct relationship should result, as shown in Figure 2. The
direct relationship between absorbance and concentration for a solution is known as Beer’s law.
In this experiment we are going to a much higher absorbance than normally used for calibration
curve just to make your data collection easier and avoid additional dilutions. As a result you
may observe some curvature of Beer law at highest absorbance value.
The concentration of an unknown dye solution is then determined by measuring its absorbance
with the Colorimeter. By locating the absorbance of the unknown on the vertical axis of the
graph, the corresponding concentration can be found on the horizontal axis (follow the arrows in
Figure 2). The concentration of the unknown can also be found using the slope of the Beer’s law
curve.
Obtain four 10 mL volumetric Flasks, labeled A through D. Into Flask A, pipet 2 mL of the dye
solution from Flask #7. Into Flask B, 4 mL. Flask C, 6 mL. Flask D, 8 mL. Fill each of the
four flasks to the mark with distilled water and mix well. Solution E is the starting solution
Expt AS
9
which is the same as the solution in Flask #7 that was used as a control. That is the control
should show no change or little change (within experimental uncertainty), since the control flask
has no carbon powder to remove dye molecules. Calculate the concentration of dye solution
(mg/L) in each of the four flasks using the known concentration of the solution in Flask #7 as
your starting point.
You are now ready to calibrate the Colorimeter. Prepare a blank by filling a cuvette up to lined
mark on cuvette with distilled water. To correctly use a Colorimeter cuvette, remember:
•
All cuvettes should be wiped clean and dry on the outside with a tissue.
•
Handle cuvettes only by the top edge of the ribbed sides.
•
All solutions should be free of bubbles.
•
Always position the cuvette with its reference mark facing toward the white reference mark
at the right of the cuvette slot on the Colorimeter.
Calibrate the Colorimeter.
a. Plug Colorimeter into LabQuest device. On Vernier LabQuest device select
“SENSORS” from main menu then select COLORIMETER from SELECT SENSOR
menu.
b. Select CALIBRATE and then select CALIBRATE NOW from the CALIBRATION
menu.
c. Turn the wavelength knob on the Colorimeter to the 0 %T position. When the voltage
reading stablizes press ENTER. Enter 0 as the percentage transmittance and then press
ENTER. Note, this sets the value when no light reaches the detector.
d. Place a cuvette containing only water into the Colorimeter and close lid. Turn the
wavelength knob to the red LED position (635nm). Note the methylene blue solution
appears blue because it absorbs red light. After the voltage reading stabilizes, press
ENTER. Then type “100” as the percent transmittance and press ENTER. Note, this
selects the maximum %T value when there is no chromophore to absorb any light so
100% of the light reaches the detector.
e. Select OK to return to the Setup screen
Expt AS
10
You are now ready to collect absorbance data for the five standard solutions A – E (the solution
in Flask #7 is solution E). Fill a clean curvette with the solution in Flask A. Wipe the outside
with a tissue and place it in the Colorimeter. After closing the lid, wait for the absorbance value
displayed on the monitor to stabilize. Record the absorbance in the lab notebook.
After samples of solutions (#1 – #7) are removed from the carbon containing flasks, then
absorbance data should be collected for these samples as you have done above.
We will use colorimeter only to gather data and then use a spreadsheet program like EXCEL for
the actual analysis. Be sure to record Absorbance values as collected.
Processing the Data
You will need a known calibration table with columns for flask (letter A, B,C, D, and E),
concentration (mg/L), and measured absorbance. A second table for adsorption data should have
columns for flask (#), powder mass (g), Absorbance, Ci(mg/L), Cf (mg/L) Ci – Cf (mg/L), N
(mg dye/g carbon), and Cf / N (g carbon/ L). Include all data tables and graphs in your report.
Remember that we are using mg dye/L for concentration and initially you will find Nm in units of
(mg dye/ g Carbon). It is necessary to change methylene blue amounts from mg to g to mol,
but these amounts are always expressed per g of carbon in Nm.