One of the most important laboratory tasks is the separation of mixtures. We invariably are faced with mixtures when we perform a chemical synthesis, when we try to isolate a material from plant tissue, when we try to analyze a sample or when we want to purify a reasonably clean material. Entire industries are based on the ability to separate what we want from whatever else is present (ore processing, recycling and sugar refining. ) This laboratory is designed to illustrate a number of the most important laboratory scale processes. Some of these methods will be familiar, but others may be new to you.
This is one of the simplest processes and you probably don't even think of it as a separation. If you have a solution (like sea water), you can let all of the solvent (water) evaporate and you are left with the nonvolatile components (mostly NaCl here.) We can speed up the process with a fan (good air circulation), by heating or by applying a vacuum. In this approach we lose the solvent and recover the solute.
is a related process, but here the usual goal is to remove the solvent or bound water without the use of elevated temperature. This is often important in the processing of foods like instant coffee or orange juice concentrates. Many of the components which provide desired taste are actually volatile compounds that would be lost on boiling; other samples contain components that would decompose at the boiling point of water.
This is a refinement of the evaporation technique used to purify lower grade material. We start with a reasonably pure solid and dissolve it in a minimal amount of a suitable solvent (like hot water.) If some of the impurities are insoluble, we can separate them by
filtering the solution. Then we change the conditions so the solution can no longer keep all of the compound in solution. Some useful techniques:
modify the solvent to reduce the solubility (add alcohol)
Since most impurities are present in low concentrations and are far from saturation, they remain in solution. We can filter off and recover the solid (purified material.) This results in a product of much higher purity than the starting material. (The method is more difficult if the impurities constitute more than 10-20% of the material.)
- Sea water is really a mixture of ionic compounds so we won't get pure NaCl if we evaporate to dyness. If, however, we let about 90% of the water evaporate we will crystallize out most of the NaCl while other components (like KCl) will remain in solution.
Distillation:
Here the focus is on the volatile components of a solution. We heat the material to boiling and we condense the resulting vapors. In the simplest case, we prepare distilled water from sea water (desalination) or from drinking water. Petroleum (crude oil) is a mixture of many hydrocarbons and it is separated by distillation. If the solution contains several volatile materials the one with the lowest normal boiling point will emerge first, followed by those with higher boiling points. Alcoholic beverages are made by yeast fermentation, producing a solution with 5-10% ethanol. Distilling the fermentation mixture produces a product with much higher ethanol content.
Sublimation:
Many solids have a significant vapor pressure below their melting points. Often these materials will not melt at normal atmospheric conditions, yet we can purify them by heating and condensing the vapors. This process is known as sublimation. Some materials which are purified in this manner: Iodine, camphor, naphthalene, p-dichlorobenzene (moth balls), Zinc and Cadmium metals.
Freezing as a Separation Process:
If you partially freeze a sample of sea water or a bottle of fermented apple cider you will form pure ice. All of the solutes (NaCl or ethanol and sugars) will remain in solution. This has been used for desalination plants in Saudi Arabia and elsewhere, converting sea water into pure water (although it has not proven to be economical.) It is also the way New England Colonists made strong alcoholic beverages without paying the Crown's taxes on distilled drinks. (Applejack can be made by freezing fermented cider and discarding the block of ice that formed.)
Extraction:
Sometimes we can remove one component from a mixture by extracting it with a solvent. This is how we brew coffee and tea-- we extract the interesting components with hot water. This produces a solution that contains almost all of the caffeine in the bean or leaf, but most of the other components remain in the tea bag or in the grounds. Of course we extract other components which give these drinks their characteristic flavor and aroma. It is worth noting that extraction can be used to purify a very small component in the original mixture.
Clearly the choice of solvent and temperature determine which components are extracted and which remain in the solid residues. If we extract coffee beans with Methylene Chloride (decaffeination) we will extract the caffeine, but leave most of the interesting coffee flavor components intact.
Solvent Partitioning:
Extraction can also be used to selectively remove material from a solution, either to purify it or to concentrate it. The requirement is a second liquid which is not soluble in the solvent of the original mixture. If you put water and hexane in a bottle and shake, the two liquids will separate into layers, with the lower density hexane on the top. If the water contained a yellow and a blue dye, we might find that almost all of the yellow dye remained in the water and almost all of the blue dye ends up in the hexane layer.
In the laboratory this process is usually done with a separatory funnel. This is a glass bulb which allows us to shake the two solvents and allow them to settle. A stopcock on the bottom allows us to drain off the lower layer, separating it from the upper layer.
One important use is to concentrate samples. A liter of industrial waste water might contain 2 mg of chloroform. If our instrument can detect 20 mg we'd need a 20 ml sample. (The instrument can't handle that large a sample.) If we extracted this liter of water with 1 ml of hexane we'd find all the chloroform in the hexane layer. The concentration is now 2000 mg/L (or 2 mg/1.0 ml) and we would only need a 0.02 ml sample.
Adsorption:
Some solids have a strong affinity for specific materials. Activated charcoal is one of the most widely used materials. It has little tendency to adsorb nitrogen, oxygen or water but it will adsorb many organic compounds. A charcoal filter on a water line will remove most of the chlorine and many of the materials that produce bad taste, even at very low levels. Sugar is usually treated with activated charcoal to remove the brown coloration (caramel) that forms when sugar solutions are boiled down.
We usually use charcoal and other adsorbents to remove
unwanted materials. However, it is also possible to use charcoal to collect and isolate something we want to measure. Air quality can be measured by pumping 1-100 liters of air through a small tube packed with charcoal or a similar adsorbent. Most of the organic compounds (pollutants) will be retained by the adsorbent. We can extract these species with a suitable solvent and analyze the solution with an instrument like a gas chromatorgraph. In practice, we end up introducing all of the impurities from 10 liters of air in a 5 microliter syringe. In this way we can detect very small concentrations of air pollutants.
Precipitation Methods:
Frequently the best way to separate a mixture may be to convert the species of interest into another chemical compound. (This assumes, of course, that we will be satisfied with the modified material or that we can regenerate the desired species.) Sea water contains very small traces of Mg2+. If you add a small amount of OH-, almost all of the Mg will precipitate as Mg(OH)2. It will settle to the bottom of the tank; the water is decanted (poured off) and the solids are collected. The material is virtually pure Mg(OH)2 because there are no other significant impurities in sea water which precipitate as the hydroxide. Much of the Mg metal produced today is made by this method.
We can also modify the solvent to reduce the solubility. In today's experiments we will acidify milk, causing some of the proteins to become insoluble. We can then separate these proteins from the original sample.
Leaching:
Here the process is to extract a species of interest, changing it into another chemical species in the process. Many minerals are extracted by leaching the ores with strong acids or strong bases to dissolve the material of interest. Of course, our interest here is the metal content and not the specific compound found in the ore. (This is similar to the extraction process, but the material is converted to another chemical form.)
Chromatography:
This is a major laboratory method and most chemistry laboratories are equipped with several types of chromatographic instruments or equipment. This discussion will illustrate the process with one form-- paper chromatography.
We start with two phases solid (the paper) and a liquid (a suitable solvent.) We will make one phase (the liquid) flow through the other phase. (The paper serves at the stationary phase.) This sounds quite formal, but we just let liquid rise up the paper.
The sample starts as a small dot on the paper. As the liquid passes past the dot, some of the inks dissolve and travel with the liquid. If the ink interacts strongly with the paper it will travel slowly, lagging well behind the solvent front. If the ink is less tightly bound by the paper (or interacts strongly with the solvent) it will travel much faster. The end result is that the components of the ink spreads out. Obviously, this example relies on our ability to see dyes directly, but the separation process is much more general.
Paper Chromatography TLC or Thin Layer Chromatography
Column Chromatography Gas Chromatography
Experimental Work
We want to cover many of the methods described above, but we have a limited time. You should operate with a
team of four persons and divide the projects between members of the team. Each of you should view each of the projects, understand the process and interpret the results.
You are to use your notebook to briefly describe your work. List your observations. If calculations or conclusions are requested, include them on the pages of your notebook.
Notice that each member of your team is expected to complete his or her notebook at the time of the experiment. It is not permitted to wait until the end of the period and then copy sections from your partners' notebooks.
CAUTION: We will use alcohol and hexane. Since these are flammable
no flames are
to be lit during the period. The distillation, sublimation, and solvent extraction exercises should be done in a hood. We will use electrical hot plates when we need heat. The hexane, the camphor, the methylene chloride and the dichromate solutions are to be collected for disposal; the other liquids can be discarded safely in the drains.
Part 1-- Distillation
Set up the glassware for a small still. The liquid is boiled in a round bottom flask heated with an electric heating mantle. (Our distillate is flammable so we don't want to heat with an open fire.) A distilling head lets us pass the vapors past a thermometer and into a condenser. In the condenser, a stream of cold water cools and condenses the vapors which are collected. (Notice that water enters via the lower connection on a condensor.) Have one of the instructors help you with the setup and have it inspected before you begin the distillation.
Place 50 ml of a 15% alcohol solution in the flask. Add a few drops of a color indicator (just for appearance) and a boiling chip. (The boiling chip will prevent superheating.) As you distill, collect 5 samples of 5 ml each. Record the boiling temperature (range) for each product.
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Analysis of the distillate:
Accurately pipet into a text tube 10.00 ml of the dichromate reagent
- Add 25 microliters of the distillate using the special pipet
- place the tube in hot water (90-100 degrees) for five minutes
- record the spectrum for each solution at two wavelengths (values will be posted)
- include a reagent blank (25 microliters of water.)
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Consult the posted notes for determining the % alcohol in the sample.
- Plot % alcohol in each distillate fraction.
The reagent contains 5 g of Potassium Dichromate in 1 liter of 3 M sulfuric acid. Treat this material with caution. Protect your hands with gloves. The reaction oxidizes ethanol to acetic acid. We will observe a color change associated with loss of the orange dichromate color and the gain of the blue-green Cr3+ ion.
- 3 Cr2O72- + 4 C2H5OH + H+ ----> 6Cr+3 +4 CH3 CO2H + 11 H2O
Part 2-- Sublimation:
Select a 100 ml and a 150 ml beaker.
