LIQUID-LIQUID EXTRACTION OF IODINE
CHE 342 TEAM 11 Alfred Chung Ben Krekeler Kegan Lovelace Ali Moughania
December 09, 2008
INTRODUCTION This experiment demonstrates the mass transfer of iodine from an aqueous solution to an organic layer through the use of an oxidizing agent. Complexed iodine, soluble in water as triiodide, forms a brown solution in the denser aqueous layer while free iodine forms a faint pink color in the organic solution. Upon addition of an oxidizing agent, such as bleach, to the bottom of the vial, the triiodide complex is oxidized to free iodine. The free iodine, which is only sparingly soluble in water, then undergoes diffusive mass transfer into the organic layer. In the end, the organic solvent becomes dark pink as iodine leaves the aqueous layer and the aqueous layer becomes clear. This report is written for the use of high school chemistry instructors to demonstrate the roles of mass transfer and diffusion in liquid extraction cases. It also illustrates the importance of convective mass transfer for decreasing time for complete iodine extraction. The report is divided into two sections: the first section is a general outline of the methodology and set-up of the experimental outline for high school students and the second section is a detailed analysis of the theories behind the experiment. Both sections should be read by the instructor, and he or she should devise experimental questions based on guidelines found in this report. SECTION 1: STUDENT PART MATERIALS 4 x 6mL vials Iodine Tincture (2% Iodine and 2% Iodide) Water Mineral Spirits Oxiclean (Sodium Percarbonate) Bleach (6.5% Sodium Hypochlorite) Small Spatula
80 degree Celsius Hot-water bath (a thermos or other insulated device can be used) Thermometer Stopwatch/clock Plastic Pipettes (3 or more)
PROCEDURE 1) Add 3 mL of room temperature water into one 6mL vial. 2) Add 0.5 mL of iodine solution into the water and mix thoroughly by pipetting up and down. The solution should turn yellowish-brown. 3) Carefully add 2 mL of mineral spirits on the top. Try to avoid mixing the mineral spirits with the aqueous layer. This organic layer should be clear to light pink if done correctly. 4) Take another 6 mL vial and repeat step 1-3 but with 80 degrees Celsius water. Set this vial in the hot water bath to maintain the temperature of this second vial at 80 degrees Celsius. 5) Add 5 drops of bleach (about 0.25 mL) to the bottom of the first vial. A clear aqueous layer should develop. 6) Immediately record the time on a clock. The clear layer will eventually propagate upward to the rest of the aqueous layer, while the organic layer will become darker. These conditions will take about an hour. 7) Repeat step 5 with the second vial at 80 degrees Celsius. Record the time and let sit in the 80 degree Celsius bath. (about 15 minutes for completion) 8) While these two vials are diffusing, take an empty vial(3rd) and measure out enough Oxiclean to cover the bottom of the vial. 9) Using the last vial, repeat steps 1-3 with room temperature water. 10) Now add the pre-measured Oxiclean into the 4th vial. Try to add the Oxiclean directly into the aqueous layer (to prevent surface tension of water from keeping Oxiclean afloat) and try to add it so it maximizes the bottom coverage of the vial. 11) Immediately start a stop-watch and record the time it takes for the bottom to clear. (about 2 minutes). 12) When the vials 1 and 2 are complete, record the time and calculate the elapsed time.
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MEASUREMENTS Height of Aqueous Layer (cm)
Temperature of Aqueous Layer (°C)
Substance Added to Aqueous Layer
~3.0 ~3.0 ~3.0
~20 ~80 ~20
Bleach Bleach Oxiclean
Experimental Time For Iodine Diffusion Through Aqueous Layer (minutes) About 15 min About 1 hr About 2 min
The main measurements that need to be taken are the height of the aqueous layer as well as the initial temperature. In addition, the experimental time for diffusion of iodine through the aqueous layer was recorded after the addition of either bleach or Oxiclean to the aqueous layer. QUESTIONS FOR STUDENTS We have devised some calculations and conceptual questions for basic to advanced students in chemistry. Basic Questions: 1) Was the process of diffusion in low temperature or high temperature faster? Why? a. High temperature is faster because molecules have greater kinetic energy and the diffusivity is higher. 2) Is diffusion a relatively fast process? Do you hypothesize that in chemical engineering applications diffusion alone is used for mass transfer? a. No, diffusion is commonly the rate-limiting step for chemical engineering applications. Chemical engineering applications employ convective mass transfer processes with velocity to expedite mass transfer. Advanced Questions/Calculations 1) Use reaction stoichiometry to predict concentrations of added reagents and determine the limiting reagent for the reaction. a. The limiting reagent is the triiodide complex and the oxidizing agent is in excess. 2) Given an equation to fit the behavior (Model I-3c described later), use the recorded time and algebraic skills to calculate the diffusivity. Compare this to another model, Wilke-Chang (Model I-2) model 3) Estimate the velocity of the oxygen bubbles released when Oxiclean is added based on observation and calculate the velocity based upon a given fit equation (Model II-2). SECTION 2: INSTRUCTOR PART EXPECTED OUTCOME The first step in performing the experiment is to make a two phase solution of water/triiodide and organic solvent/iodine. The solution should have a pinkish tint in the organic layer and a yellow/orange color in the aqueous layer. When the bleach reaches the bottom, the yellow/orange of the bottom aqueous layer will begin to turn clear. This is because the bleach has reacted with the triiodide at the bottom and the iodine is diffusing into the organic layer. Since the iodine is above the solubility limit as it is formed, the iodine diffuses into the organic layer. As the experiment continues the yellow iodide line will move closer to the organic layer until the aqueous layer becomes clear. As temperature increases, the experimental time for completion decreases. Addition of Oxiclean introduces convective mass transfer, yielding the shortest time for completion. ANALYSIS Iodine is only sparingly soluble in water. The solubility of iodine in water at 20 degrees Celsius is 0.03 g/100g water and at 80 degrees is 0.06g/100g water. When the iodine solution is added to water, it stays in solution after forming the complex I3-. This reaction is shown below: a) I2(aq) + I-(aq) I3-(aq)
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This triiodide complex is responsible for the yellowish-brown color in the water. It is also the complex that serves as the indicator for starch, staining starch pitch black. Iodine is responsible for turning the mineral spirit purplish-pink. The idea behind the experiment is to oxidize the triiodide complex to form I2, which is sparingly soluble in water. The actual species undergoing oxidation is iodide, which exists in equilibrium with the triiodide complex. However, for simplicity, we will assume that the triiodide complex is undergoing oxidation. Bleach (sodium hypochlorite) is used as the oxidizing agent, and the balanced net chemical reaction is shown below: b) 2I3- (aq) + ClO- (aq) + 2H+ (aq)3I2(aq) + Cl-(aq) + 2H2O(l) For the Oxiclean reaction, Oxiclean in water forms hydrogen peroxide, which is used as the oxidizing agent instead of bleach, and releases oxygen bubbles as it dissolves. The experimental observation shows that when Oxiclean is added, the diffusion of iodine is much more rapid than when Bleach is added. This may be explained by the convective mass transfer of water molecules which were observed as a byproduct of the oxidation-reduction reaction. c) 2I3- (aq) + 2H+(aq) + H2O2 (aq)3I2 (aq) + 2H2O(l) The chemistry behind the experiment is relatively straight-forward, but modeling the diffusion is very difficult. The reaction, multiple chemical entities, and partition equilibrium of iodine complicates the use of simple mass-transfer models. This experiment was designed to offer students a more qualitative understanding of mass transfer. Equations and measurements can be performed for this experiment, although the results entail numerous assumptions. EXPERIMENTAL MODELING/EQUATIONS Model I: Diffusion of Iodine Key Assumptions: We have made several important assumptions when constructing our model. These assumptions with our explanations are listed below. We assume diffusion of iodine is occurring in 1-D pseudo-steady state from the bottom of the vial (clear aqueous layer with bleach) to the boundary between the organic layer and aqueous layer. The diffusion path length decreases with time as the clear bleach/aqueous layer rises as the triiodide complex is converted to iodine and diffuses into the organic layer. An illustration of this is shown below.
Organic and I2
Aqueous I3-
Path length
Aqueous and Bleach
We assume that the aqueous/bleach layer is a separate layer from the aqueous I 3- layer. We also assume that the concentration of bleach in the aqueous/bleach layer remains constant to simplify calculations since the bleach is in excess throughout the progress of iodine diffusion (in reality, the bleach gets diluted as the aqueous/ bleach layer expands and as bleach is reacted with triiodide). Another important assumption is that the aqueous/bleach layer has uniform concentration throughout. However, in order to focus on the diffusion of iodine, we will make the assumption that the aqueous/bleach layer is well-mixed and our model will be an underestimate of the time it takes for this reaction to finish (we neglect diffusion of bleach through the aqueous bleach layer). We also assume that bleach reactions only at the interface between the aqueous/bleach layer and brown aqueous I3- layer. The reaction is instantaneous and not occurring throughout the diffusion path length. Before addition of bleach, we assume that the aqueous layer is well-mixed and saturated with iodine before forming the triiodide complex. This allows us to develop a starting point in terms of concentration of iodine
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in solution and the rest of the iodine is stored in the I 3- complex. When the reaction occurs at the boundary of the brown aqueous I3- layer (also saturated with I2), an excess of I2 is created, eliciting a concentration gradient between this boundary and the other boundary (with a concentration of I2 saturation) bordering the organic layer. In other words, the lower portion of the path length has I 2 concentration of (solubility limit+I2 generated by interface reaction) and the top portion of the path length has I2 concentration of solubility limit. This is the driving force of I2 diffusion. As I2 travels upward, excess iodine diffuses into the organic layer, turning it darker purple. When all the triiodide is oxidized, the reaction is complete and the bottom layer becomes entirely clear. Our model takes into account iodine diffusing with other components. Based on the equation b, NB(I3-)= -2 NA/3. We can ignore the contribution of Cl- and H2O formed since Cl- and H2O cannot diffuse into the organic layer (much like the simple case A in non-diffusing B). We can assume that the hypochlorite and H+ are not in our control volume since they are part of the lower boundary. Also assume cAV = 55M (molarity of water), since we are working with dilute solutions. We assume that adding the bleach dropwise does not cause convection and all diffusion takes place by concentration gradient. We also assume constant concentration throughout the diffusion path length (roughly equal to concentration of water). When all the triiodide is oxidized, the reaction is complete and the bottom layer becomes entirely clear.
Key Equations: I-1) NA=-DAB(dcA/dz)+cA/cav(NA+NB)= -DAB(dcA/dz)+cA/cav(NA-(2/3)NA) Steady-state, 1-D, Multi-component diffusion.. I-1a) NA=3DAB*cAV*ln((1-(1/3)x2)/(1-(1/3)x1))/(z2-z1) Integrated flux equation. Let xdr =ln((1-(1/3)x2)/(1-(1/3)x1)): This is the driving force for diffusion. I-2) DAB=1.173*10-16(φMB)0.5*(T/(μBVA0.6)) Wilke-Chang Correlation for diffusivity in liquids I-3) Mass Balance In-Out=Accumulation I-3a) -NA*Area=ρA(dz*Area)/(MA dt) Mass balance for our system I-3b) –3DAB*cAV*xdr*tf = ρA(-Z12)/(2MA) where Z1 is the initial path length Integrated mass balance I-3c) tf = ρA*Z12 /(6*xdr* MA*DAB*cAV) Calculations and Analysis: The calculations for this experiment are shown in the appendix. The sample experiment was done for two temperatures: room temperature for control and a 80 degree hot water bath. The results for the sample experiments were that diffusion in the bath took 13 minutes for completion (feasible for classroom setting), while the room temperature took 72 minutes (not feasible to complete whole reaction in classroom setting). This indicated that temperature increased the rate of diffusion substantially. Our models were used to calculate the diffusivity of iodine. For room temperature, the diffusivity from the WilkeChang model predicts 2.38*10-8 m2/s but our experimental value was 7.67*10-8 m2/s based on our time and models solving for diffusivity. For 80 degrees Celsius, the diffusivity of Wilke-Chang predicts 9.72*10-8 m2/s but our experimental value 4.74*10-7 m2/s. We noticed that in all cases, our experimental values of D AB are greater than those predicted by Wilke-Chang. This is most likely due to the induced convective mass transfer as we were adding the bleach.
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Model II: Oxiclean Addition Key Assumptions: See above for most of the assumptions used. Assume that Oxiclean covers the entire bottom of the vial. We model the oxygen bubbles flowing through the vial as forming an imaginary “pipe” with an average velocity of the air stream. Assume flux is constant. Assume velocity is great enough to ignore multi-component species. The model used is iodine in non-diffusing aqueous solution. Key Equations: II-1) Na=kc(cA1-cA2) Convective mass transfer equation for A non-diffusing B II-2)NSh=kc’ D/DAB=0.023(Dvρ/μ)0.83*( μ/ρDAB)0.33 Mass transfer: turbulent flow in pipe (valid for Reynold’s number above 2100) II-3)NRE=ρvD/ μ Reynold’s Number Calculations: The purpose of this section is to illustrate that most chemical engineering applications are run under conditions of turbulence or assisted with convective mass transfer. Diffusional mass transfer alone is too slow to be much use in most applications. We designed this portion of the experiment for students to calculate the time it takes for the solution to clear after addition of Oxiclean. Students can calculate the time it takes for the reaction to reach completion and then use equation II-2 to solve for velocity. In order to illustrate how this works, we have used our own test data at 20 degrees Celsius. The recorded time for the solution to clear up was 2 minutes. If we assume constant flux and assume that all the iodine added is removed, NA=Total Iodine in original aqueous solution (mol)/(Area*time)=kc(cA1-cA2) From this equation, we can solve for a value of kc since we know all the values of every other variable based on our assumptions stated earlier. We find that kc = 7.23*10-5 m/s. Next we assume that kc=kc’ because the iodine is dilute relative to water (55M). Now we can model this as mass transfer for turbulent flow inside pipes. By using equation 2 from above, we can solve for v. These calculations are shown in the appendix. Based on our data, the velocity of the stream is 0.147 m/s, which is intuitively reasonable. Next, we validated that turbulent flow can be used. We calculated Reynold’s number and obtained 2633 > 2100 which means that our assumption for turbulent flow is valid. CONCLUSION This experiment effectively utilizes solubility and concentration gradients to separate iodine from an aqueous solution. The simple set up demonstrates fundamental chemical engineering mass transfer principles. Factors such as temperature and convection influence the rate of mass transfer. The experiment shows that as temperature increases, the rate of mass transfer increases. Adding a convective component (generated by Oxiclean) also increases the rate when compared with simple diffusion.
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