Talk:Liquid-liquid extraction
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Liquid-liquid extraction is based on the transfer of a solute substance from one liquid phase into another liquid phase according to the solubility. Extraction becomes a very useful tool if you choose a suitable extraction solvent. You can use extraction to separate a substance selectively from a mixture, or to remove unwanted impurities from a solution. In our experiment one phase is a water layer and the other an organic solvent which is immiscible with water. Liquid-Liquid extraction involves the distribution, or partitioning, of a solute between two immiscible liquid phases. Extraction is a separation technique based on the distribution of the organic compounds between immiscible solvents. Most often, water is used as one of the solvents. Organic solvents, from which the organic compound is soluble, are used to extract it from the aqueous mixture. All impurities, particularly inorganic impurities are left in the aqueous layer. The separatory funnel is used in any liquid-liquid extraction. The mixture to be extracted is placed in the separatory funnel and the organic solvent is added. The mixture is shaken until the organic compound is distributed into two miscible solvents. If the density is less than that of water, the layer will be the upper layer, whereas if it is greater than that of water it will be the lower layer. Depending on whether the organic layer is the upper or the lower layer.
The aqueous layer is transferred into a 250mL Erlenmeyer flask and titrated it with 0.02M NaOH solution up to the phenolphthalein endpoint. The distribution of the organic compound between the immiscible solvents is referred to as the distribution coefficient, KD. KD gives the ratio of the concentration of the solute in each solvent. The amount of solute extracted in each solvent can be determined using KD of the system. Applying multiple extractions can increase the efficiency of the extraction. In this technique, the aqueous mixture is repeatedly extracted with the organic solvent using small volumes of solvent. The amount of the solute extracted is computed in each step and the total amount of extracted solute is determined.
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[edit] Distribution coefficient
This may seem trivial but I believe the "distribution ratio" discussed in the article is usually referred to as the "distribution coefficient," with a symbol of K not D. It is written this way in my lab manual, text book and other sources on the internet. The term "distribution coefficient" also seems to be more specific and may help eliminate confusion. Nilboarder 02:52, 4 October 2006 (UTC)
- In the solvent extraction of metals, it is normal to sum all the concentrations of the different forms of the metal in each phase to get a total metal content. This allows the calculation of a "distribution ratio" for the metal. Each of the different species will have a "distribution coefficient" which will be different. By combining the contributions made by each of the different species (with its own "distribution coefficient") you can then get the "distribution ratio" (D) for that metal. Principles and Practices of Solvent Extraction by Jan Rydberg, Claude Musikas and Gregory R. Choppin will explain more of the way in which a D value is obtained (also this book tends to use D values).Cadmium
[edit] Inorganic Bias
This is again a very trivial matter. This page reflects strongly the historic origins of liquid:liquid extraction but the majority (in dollar value) of extractions are performed as part of work-up of organic processes. This does not seem to be reflected in the bias of the page. Patricksears
[edit] Micro vs Macro scale LLE
In my opinion, this is not a very clear description on what LLE is and its applications. For me as an analytical chemist, LLE does not mean the unit operation but a sample workup method. The process is the same but calcuations and concerns are different. Should this be pointed out in the article?Mia Legato 19:42, 7 July 2007 (UTC)
[edit] Removed chunk
===One big batch of solvent or several smaller batches?=== When a solute is being extracted from an aqueous phase using an organic solvent, a better recovery will be obtained by using two equal volumes of solvent than the recovery that would be obtained using all the solvent in one large volume. This can be shown by experiment and can be explained by the following example.
If the aqueous solution is shaken until a dynamic equilibrium is reached with one liter of nitrobenzene, then five grams of solute would be transferred. The aqueous raffinate will contain five grams of solute. 50% of the solute has been recovered.
- Imagine that you have a liter of water containing 10 grams of a solute which has a distribution ratio of one when the solvent is nitrobenzene.
If the two layers are now separated and the nitrobenzene extract is kept, then the aqueous layer (raffinate) can be treated further.
- Imagine that the same original aqueous mixture was shaken with 500 ml of nitrobenzene. When the system has been brought to equilibrium, the concentration of the solute in each phase is the same. As the organic to aqueous ratio is now 1:2 the aqueous phase will now contain 6.666 g of solute while the organic layer will have 3.333 g of solute.
If the nitrobenzene was to be used in four batches of 250 ml then the outcome is theoretically better still. The first 250 ml will extract 2 grams. The second 250 ml will extract 1.6 grams. The third 250 ml will extract 1.28 grams. The fourth 250 ml will extract 1.024 grams. In total 5.904 grams is extracted; this is a recovery of 59%.
- If the raffinate from this first extraction is extracted with 500 ml of new nitrobenzene, then 33.33% of the remaining solute will be extracted. This will be 2.222 g of solute in the new nitrobenzene extract. In total, 3.333 + 2.222 grams of solute for a total of 5.555 grams will have been extracted. This is a recovery of 55.56%. An additional 5.56% of product has been obtained.
- The process of using one large batch of solvent to extract solute is called a single extraction. The process of using several small batches of solvent to extract solute is called multiple extractions.
I've removed this section per WP:NOT a textbook. Perhaps it can be condensed and re-added. I might do it later, but if someone is game... --Rifleman 82 03:49, 13 September 2007 (UTC) I've removed
