Student Name: TA Name Experiment 3: Acid/Base Extraction the following questions
ID: 546994 • Letter: S
Question
Student Name: TA Name Experiment 3: Acid/Base Extraction the following questions. Limited partial credit will be given. 1. A student performs the following to biphenyl and salicylic acid. steps to isolate biphenyl from a solid 1:1 mixture of a. Dissolve 2.0 grams of the mixture in 20 mL of ether b. Extract the ether layer 2-3 times with an aqueous solution of 5% sodium c. Add 2-3 spatula tips of sodium sulfate to the ether layer d. Extract the ether layer with a saturated aqueous solution e. Isolate the organic layer and evaporate the ether. sold they collect at the end weighs 1.23 g and has a wide melting point range. based on the The If the student expected to recover 1 gram of pure biphenyl, explain procedural information given above, why this student did not see results. Hint: The order of steps in an extraction can be important. their expected The mixture of benzoic acid, biphenyl, and benzocaine used in this experiment is soluble in acetone, a low boiling point organic solvent. Why would this be a poor choice of organic solvent for this experiment? Hint: think about what the requirements are for the solvents in an effective liquid-liquid extraction 2. 3. Explain why saturated sodium chloride solution is used in liquid-liquid extractions. Can the mixture of benzoic acid, biphenyl, and benzocaine still be separated from one another if 5% hydrochloric acid is added at the beginning of the extraction instead of 5% sodium bicarbonate? Explain your answer 4.Explanation / Answer
Solution:-
The reason behind not obtaining accurate results:-
Weigh mixture into a 125 ml Erlenmeyer flask, and dissolve in 20 ml of ether (this may require gentle warming on a hot plate). Pour the solution into a separatory funnel (use a funnel, make sure that the stopcock is closed), and rinse the flask with an additional 10 ml of ether. Add 15 ml of 5% of sodium bi carbonate, stopper the sep funnel, and mix the phases, venting the funnel frequently. Allow the layers to separate, and draw off the lower layer into the 125 ml Erlenmeyer. Determine which of these is the organic phase (see “Other Aspects of Extraction”). Set the aqueous phase aside in a labeled Erlenmeyer flask, and extract the organic phase one more time. Combine the aqueous layer with that from the first extraction. Repeat the process, extracting the organic phase twice. The aqueous layers from the base extraction are combined, but kept separate from those from the acid extraction. The organic phase which remains after the extractions is dried.
Now, Definetely you will get accurate result.
2.
The solvent is the key to a successful separation by liquid-liquid extraction. The several criteria are:
1. Distribution Coefficient
This is the ratio (at equilibrium) of the concentration of solute in the extract and raffinate phases. It gives a measure of the affinity of the solute for the two phases.
A distribution coefficient other than unity implies that the solute must have different affinity in the two phases. If only one solute is involved (such as in the recovery of an impurity from an effluent stream), only the distribution coefficient need be considered, and it is desirable for this to be as large as possible.
2. Selectivity (Separation Factor)
If there are more than one solutes (say two solutes A and B), then consideration should be given to the selectivity of the solvent for solute A as against B. The selectivity between the 2 solutes A and B is defined as the ratio of the distribution coefficient of A to the distribution coefficient of B. For all useful extraction operation the selectivity must exceed unity. If the selectivity is unity, no separation is possible.
3. Insolubility of Solvent
The solvent should have low solubility in the feed solution, otherwise the separation is not "clean". For example, if there is significant solubility of solvent in the raffinate stream, an additional separation step is required to recover the solvent.
4. Recoverability
It is always necessary to recover the solvent for re-use, and this must ordinarily be done by other means, eg. distillation. If distillation is to be used, the solvent should form no azeotrope with the extracted solute and mixtures should show high relative volatility. The solvent should also be thermally-stable under the distillation temperature.
5. Density
A large difference in density between extract and raffinate phases permits high capacities in equipment. This is especially important for extraction devices utilizing gravity for phase separation.
6. Interfacial Tension
The larger the interfacial tension, the more readily coalescence of emulsions will occur but the more difficult the dispersion of one liquid in the other will be. The more readily coalesces the emulsions the easier phase separation will be. Low interfacial tension aids dispersion and thus improves contacting mass-transfer efficiency. Coalescence is usually of greater importance, and interfacial tension should therefore be high.
7. Chemical Reactivity
The solvent should be stable chemically and inert toward the other components of the system and toward the common materials of construction.
8. Viscosity, Vapour Pressure, Freezing Point
These should be low for ease in handling and storage, for example, a high viscosity leads to difficulties with pumping , dispersion and mass-transfer rate.
9. Availability and Cost
An excellent solvent may not be commercially available. Or it may represent a large initial cost for charging the system, and a heavy continuing expense for replacing inevitable operating losses.
3. The saturated sodium chloride is used in liquid-liquid extaction because,The bulk of the water can often be removed by shaking or "washing" the organic layer with saturated aqueous sodium chloride (otherwise known as brine). This is because the concentrated salt solution wants to become more dilute and because salts have a stronger attraction to water than to organic solvents.
4.Yes ,benzoic acid ,biphenyl,benzocaine because The liquids involved have to be immiscible in order to form two layers upon contact and both sodium bicarbonate and Hcl both are compatible for this .So we can extract.