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Chemistry LibreTexts

General Lab Techniques

Welcome to the online depository for basic chemistry techniques.

  • Acid-Base Extraction
    An acid-base extraction is a type of liquid-liquid extraction. It typically involves different solubility levels in water and an organic solvent. The organic solvent may be any carbon-based liqiuid that does not dissolve very well in water; common ones are ether, ethyl acetate, or dichloromethane. Acid-base extraction is typically used to separate organic compounds from each other based on their acid-base properties.
  • Calibration of a Buret
    To carry out this procedure you will require, in addition to a volumetric buret, two clean, dry 125 mL Erlenmeyer flasks and one #5 rubber stopper.
  • Condensing Volatile Gases
    Ever had to run a reaction with a volatile gas? It's not a very common thing to have to do, but every once in a while, it needs to be done.
  • Cooling baths
    Cooling baths are used extensively in organic chemistry for a variety of reasons. The low temperature of these baths is determined both by the appropriate use of solvent as well as a cryogenic agent such as liquid nitrogen, dry ice  or ice. Temperatures between -20 and -80° can be obtained using varied mixtures of ethylene glycol and ethanol over dry ice.
  • Distillation
    Distillation of compounds is a method of separation which exploits the differences in boiling point of a crude mixture.  Several methods exist.
  • Distillation II
    Distillation is a method of purifying organic compounds. It takes advantage of the fact that two different compounds probably have two different boiling points. Suppose two different liquids are present in a homogeneous mixture (they are completely miscible, or they mix completely together, like water and alcohol). If they have two different boiling points, one of the compounds will evaporate before the other one does.
  • Drying Solvents
    hese days many laboratories will use a commercially available solvent purification system, others will distil solvents using more traditional techniques. Tetrahydrofuran, dichloromethane, dimethylformamide, chloroform, acetonitrile, methanol, diethyl ether and toluene are all commonly used solvents, and in many cases they are required in anhydrous form. In some cases there are multiple ways to dry a given solvent.
  • Fractional crystallization
    Fractional crystallization is a method of refining substances based on differences in solubility. It fractionates via differences in crystallization (forming of crystals). If a mixture of two or more substances in solution are allowed to crystallize, for example by allowing the temperature of the solution to decrease, the precipitate will contain more of the least soluble substance. The proportion of components in the precipitate will depend on their solubility products.
  • Heating a Crucible to Constant Weight
    Your first exercise teaches you some skills on the proper use of the laboratory burner (in this case called a Tirill Burner), the adjustment of the flame and the proper placement of a crucible which is to be heated to constant weight.
  • Liquid-Liquid Extraction
    Liquid-Liquid extraction is a method by which a compound is pulled from solvent A to solvent B where solvents A and B are not miscible. The most common method of liquid-liquid extraction is performed using a separatory funnel.
  • Packing Columns
    Column chromatography in chemistry is a chromatography method used to purify individual chemical compounds from mixtures of compounds. It is often used for preparative applications on scales from micrograms up to kilograms.
  • Precipitation from a Homogeneous Solution
    If a precipitating agent is produced over a long period of time in a homogeneous solution the level of supersaturation remains low and compact crystal precipitates usually result instead of coagulated colloids. The resulting suspension of precipitate is compact, crystalline and easily filtered, whereas a precipitate formed by the addition of a precipitating agent is not easily filtered owing to a high level of relative supersaturation at the point where the reagent is added.
  • Preparing your Filter Paper
    Folding a piece of filter paper for insertion into a conical filter consists of a simple set of steps shown here in the six photographs below.
  • Proper Use of a Buret
    The volumetric analysis exercises will make use of a 50 mL buret.
  • Proper Use of a Desiccator
    A desiccator is an airtight container which maintains an atmosphere of low humidity through the use of a suitable drying agent which occupies the bottom part of the desiccator. It is used both for the cooling of heated objects and for the storage of dry objects that must not be exposed to the moisture normally present in the atmosphere.
  • Proper Use of Balances
    For a chemical reaction to be successful, reactants must be added with accurate, specific masses, and products must be accurately weighed at the end of the reaction. Therefore, balances are of immense importance in a chemistry lab.
  • Quenching Reactions
    Quenching a reaction refers to the deactivate any unreacted reagents.
  • Recrystallization (Advantages)
    This technique is no longer as widely used as it was before the advent of flash chromatography, but it's still quite useful! A particular advantage is that compounds can be recrystallized in amounts that are somewhere between streaky and impossible to column.
  • Reflux
    Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply heat to reactions over a long period of time.
  • Rotary Evaporation
    Rotary evaporation is the process of reducing the volume of a solvent by distributing it as a thin film across the interior of a vessel at elevated temperature and reduced pressure. This promotes the rapid removal of excess solvent from less volatile samples. Most rotary evaporators have four major components: heat bath, rotor, condenser, and solvent trap. An aspirator or vacuum pump needs to be attached, as well as a bump trap and round bottom flask containing the concentrated sample.
  • Thin Layer Chromatography
    Thin layer chromatography (TLC) is a chromatographic technique used to separate the components of a mixture using a thin stationary phase supported by an inert backing. It may be performed on the analytical scale as a means of monitoring the progress of a reaction, or on the preparative scale to purify small amounts of a compound. TLC is an analytical tool widely used because of its simplicity, relative low cost, high sensitivity, and speed of separation.
  • Titration
    Titration is the slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization, which is often indicated by a color change. The solution called the titrant must satisfy the necessary requirements to be a primary or secondary standard. In a broad sense, titration is a technique to determine the concentration of an unknown solution.
  • Use of a Volumetric Pipet
    Volumetric glassware is capable of measurements of volume that are good to four significant digits and is consequently expensive. You should be careful in handling this type of equipment so that breakage losses are minimized. Be particularly careful with the tips of pipets and burets.
  • Vacuum Equipment
    Vacuum equipment is used to generate, maintain, and manipulate pressures below that of the ambient atmosphere. Many common lab procedures require vacuum conditions, such as inert gas purging, cannulation, and solvent evaporation. Vacuum equipment often requires special care to maintain.
  • Vacuum Filtration
    Suction filtration is a chemistry laboratory technique which allows for a greater rate of filtration. Whereas in normal filtration gravity provides the force which draws the liquid through the filter paper, in suction filtration a pressure gradient performs this function. This has the advantage of offering a variable rate depending on the strength of the pump being used to extract air from the Büchner flask.