Chemical Engineering 401
ION EXCHANGE LABORATORY AND ANALYSIS
PURPOSE
This laboratory is designed to help the students understand the principles and physics of ion
exchange chromatography and to allow the students to observe the practical relationships
between theory and experiment. The laboratory consists of two experimental parts, linked by a
section of theoretical, computer modeling. The modeling is used to predict the results from the
second (column) experiments, given data from the first (batch). This protocol is intended to
complement the laboratory overview found in Chemical Engineering Education.
I. MATERIALS AND EQUIPMENT
Materials
Cupric Sulfate pentahydrate (Sigma)
Dowex 50W-X8 cationic resin
20-50 mesh (Bio-Rad)
200-400 mesh (J.T. Baker)
Hydrochloric Acid (EM Science)
For the experimental portion of the laboratory some light-absorbing ion is needed to monitor
the adsorption of the ion within the ion exchange resin. Cupric sulfate is a good substance for
these experiments as the cupric ion absorbs strongly in the blue region of the visible spectrum,
and the sulfate counterion is non-absorbing. The substance itself is fairly common and the
aqueous solutions have been well characterized in literature (densities found in the CRC
Handbook of Chemistry and Physics Vol. 47). One drawback for cupric sulfate is its ability to
hydrate. For this reason we use the substance in its pentahydrate form. The lid should always be
kept tightly closed to prevent changes in the degree of hydration. For the cationic resin we use
the Dowex 50W-X8 resin in the acid form at sizes of 20-50 and 200-400 mesh. The two sizes
have different advantages for certain experiments. This resin is composed of beads of crosslinked sulfonated polystyrene, where the sulfonated groups are protonated. These groups are
strongly acidic, and the protons are removed easily allowing for a high adsorption capacity. One
may notice the leeching of a yellowish substance (probably unreacted monomer) from the beads
into the aqueous solution. The wavelength of absorption of this substance is well that of the
cuprate ion and should not interfere with measurements. To remove the cuprate ion after
experiments (regeneration of beads) a 1 or 2 M HCl solution should be used to re-protonate the
resin. Washing or eluting should be done until the blue color is absent from solution and beads.
Note that the companies listed in parentheses indicate the source of materials for our
experiments, but any source should be sufficient.
Equipment
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Spectrophotometer (Perkin Elmer UV-Visible))
Persomal Computer
A/D Board
Data Acquisition software
Peristaltic Pump - calibrated (Watson-Marlow)
Chromatography Column - 1 cm diameter (Omnifit Columns from Alltech)
Plastic Cuvets (Fisher)
Flow-through Cuvets (Fisher)
Glass Vials - 20ml (Fisher)
Balance (Mettler PC 440 - down to mgs)
Tubing
Beakers
Stopwatch
To analyze the cuprate ion concentration, a spectrophotometer is used to monitor the
absorption of high wavelength light in solution. We made measurements at 660 nm. The plastic
cuvets are used for batch experiment measurements while the flow-through cuvet is used for
column experiments. The Macintosh computer and A/D board are used to capture data in the
column experiments. This requires a spectrophotometer with an analog output connection.
Alternatively, data may be sent to a printer and scanned into the computer. The balance needs to
measure down to milligrams for the batch experiments. The column should allow for the bed
length to be adjusted. The tubing should be of small diameter (inside diameter about 1/16") to
prevent excessive mixing.
II. BATCH EXPERIMENTS
A. Calibration
1) Prepare 8 vials filled with 8 g of deionized water.
2) To 7 of the vials add enough Cupric Sulfate Pentahydrate (CSP) to create solutions of
1.3, 2.3, 5.3, 9.0, 12.0, 15.9, and 21.6% w/w based on the amount of added solute.
The eighth sample will be 0% w/w.
3) After the Cupric Sulfate has dissolved obtain absorbance values for the solutions on
the spectrophotometer.
4) Plot absorbance versus concentration (g/ml of cuprate ion) and find the linear best fit
so you will have an equation to translate absorbance values into concentration
values.
The absorbance quantity is linearly related to the concentration of the ion, so if your data is
not linear look for sources of error. The units of concentration need to be mass/volume and not
mass fraction, so a table of density values and molecular weight values are necessary here. The
absorbance values appear to be temperature dependent, especially at higher concentrations, so if
cuvets are left in the spectraphotometer the readings will drift upwards. Obviously the values
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here should be used as a guide but in no way are set in stone. Most of the experiments are done
at relatively low concentrations, so the calibration should be weighted more to these values.
B. Equilibrium Adsorption - Isotherm
1) Prepare 9 vials filled with 15 g of deionized water.
2) To 8 of the vials add enough CSP to create solutions of 1.0, 1.8, 3.6, 5.9, 7.3, 10.0,
13.3, 17.8% w/w. The ninth sample will be 0% w/w. Keep track of exact
quantities of CSP added.
3) After the solute has dissolved add 1.0g of cationic resin (20-50 mesh) to each sample
and shake well.
4) Allow these to equilibrate with time and occasional shaking (about 6 hours).
5) Obtain absorbance values for the equilibrated solutions.
6) Plot the concentration of cuprate ion in the resin (g of cuprate ion adsorbed per g of
resin) versus the concentration of cuprate ion in solution (g/ml). This is the
adsorption isotherm.
7) Repeat for the 200-400 mesh resin.
We have found the cuprate ion to be very strongly binding. The results are Langmuir
isotherms with very steep linear regions and small saturation concentrations (1/K). Our result for
the 200-400 mesh resin is shown in Figure 1. Judging by our kinetics experiments (see next
section) adsorption is very rapid at moderate concentrations (4% w/w) and especially so for the
smaller particles. At 4% the mixture should be well equilibrated in 30 minutes, but it is unclear
how long the smaller concentrations will take. Two hours is probably adequate, but we let ours
sit overnight. The smaller particles also do take a long time to settle (at least a couple of hours),
so we suggest letting samples sit overnight. Again our concentrations are only a guide. You may
wish to add a few more samples with concentrations below 5% w/w, for this is where region of
saturation transition for the isotherm.
C. Adsorption Kinetics
1) In a 250 ml jar add 150 g of deionized water and a stir bar.
2) Add enough CSP to make a 4.2% w/w solution.
3)
4)
5)
6)
Dissolve the solute.
Take out a small amount of solution for absorbance reading (~1.5ml).
Add 18 g of the 20-50 mesh particles, start stopwatch, and begin to stir the mixture.
To take a sample remove jar from stir plate, let particles settle for a few seconds, and
put small amount of solution into cuvet. Put the jar back on stir plate to continue.
7) Take samples at 0.5, 1.5, 2.5, 3.5, 4.25, 5, 6, 7.5, 12.5, 15.5, 20, 30, 45, and 60
minutes.
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8) Get absorbance values for these samples and plot cuprate concentration (g/ml) versus
time.
Our results are shown in Figure 2. This experiment may be repeated for the smaller particles,
but we found that adsorption was basically instantaneous at 4% w/w (within 30 sec) and that
there is a big problem with sample taking due to slow settling times of the particles. The
presence of particles in a sample will really confound absorbance measurements. We also found
that the removal of samples (loss of material in the batch) does little to affect results.
III. COLUMN EXPERIMENTS
1) In a 400 or 500 ml jar add 300 g of deionized water and enough CSP to make an
8.66%w/w solution (0.0233 g Cu++/ml).
2) Arrange the experiment such that fluid flows from a reservoir (jar), through the
peristaltic pump, through the column (top to bottom), through the
spectrophotometer, and out to drain into another jar.
3) The column should be filled with 3cm of the 200-400 mesh resin. Adding the resin
with water helps to prevent air bubbles.
4) The lines and column should be filled with deionized water from a reservoir. Care
must be taken to keep bubbles out of the column.
5) Connect the computer to the spectrophotometer to acquire data.
6)
7)
8)
9)
Switch to the cupric sulfate solution as the feed reservoir.
Speed the pump up a bit to bring the solution to the top of the bed.
Set pump to about 0.044 ml/min.
Plot normalized cuprate ion concentration (1.0 is the final value) versus eluted
volume.
10) Repeat runs for a flow rate of 0.375 ml/min and for a CSP concentration of
0.326%w/w (0.00083 g Cu++/ml) with a flow rate of 0.375 ml/min.
Our results are shown in Figure 3. We found that the larger particles had problems with
channeling and non-uniform flow. We chose concentrations and flow rates to illustrate the
effects (or lack of) of large changes in flow rates and changes in concentrations to cover the
saturated region and linear region of the isotherm. Obviously other values and effects may be
chosen. The experiments could also be designed in such a way to teach statistically designed
experimental methods, matrices, and factorial design. Complete breakthrough takes at least a
few hours, but the actual breakthrough transition may be quite sharp, so we suggest taking data
about every 10 seconds.
As an alternative to monitoring the cupric ion concentration in the effluent, we have also
done a visualization experiment. With the beads packed in a clear glass column it is possible to
measure the velocity of the blue front as it moves through the bed. The students measured the
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frontal position with a ruler and plotted it versus time. The resulting line is straight and can be
compared to the capacity determined in the batch experiments. The students also noted the
breadth of the front when the experiment is conducted at different flow rates.
Developed by Mark R. Anklam and Robert K. Prud'homme
Department of Chemical Engineering
Princeton University
January 1996
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