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Semimicro Analytical Techniques

  • Page ID
    97253
  • Performing qualitative analysis properly requires a knowledge of certain basic laboratory techniques. In order to speed up procedures, all techniques will be on a semimicro scale. This scale involves volumes of 1­2 mL of solutions and adding reagents dropwise with eye droppers. Containers will generally be standard 75 mm test tubes which hold about 3 mL. Techniques for working with volumes of this magnitude will be outlined below.

    Water

    Whenever it is necessary to use water in a procedure, use distilled water. Ordinary tap water is not completely pure and may introduce substances for which you are trying to test or it may introduce incompatible contamination.

    Dispensing Reagent Solutions

    When obtaining reagents from the reagent bottles, always dispense the reagent with the dropper contained in the reagent bottle, whether dispensing the reagent directly into your sample, or obtaining a quantity of reagent in another container. Do not touch the dropper to the solution to which you are adding the reagent or to your sample container. Do not set the dropper on the reagent bench or lab bench. Return the stopper promptly to the reagent bottle from which it originated. Do not place anything into a reagent bottle other than the dropper which is contained in it. If you need a volume greater than 2 mL, use a graduated cylinder. For lesser volumes, you may want to calibrate one of your eye droppers by counting how many drops of water it takes to deliver 1 mL into a graduated cylinder.

    Stirring Rods

    When reagents are added to a solution, it is essential that the solution be stirred thoroughly. Stirring rods can be prepared by cutting short lengths of thin glass rod and fire­polishing the ends. The stirring rods get wet with each usage, and if not properly cleaned, will contaminate the next solution. A simple way to keep stirring rods clean is to place them in a beaker of clean distilled water and swirl them about after each use. The contamination will be highly diluted and can remain in the water. It is advisable to change the water periodically to minimize contamination however.

    Adjusting pH

    At times you will want to make a solution acidic or basic. Add the proper reagent dropwise, stirring well with a stirring rod after each addition, and test the pH at appropriate intervals by touching the tip of the stirring rod to litmus or other pH indicating paper. Continue this procedure until the paper turns the proper color. If litmus paper is not sufficiently sensitive, obtain some pH indicator paper, which is available for various ranges of the pH scale.

    Precipitation

    In order to detect the formation of a precipitate, both the solution being used and the reagent must be clear (transparent, but not necessarily colorless). Precipitation is accomplished by adding the specified amount of reagent to the solution and stirring well. Stir both in a circular direction and up and down. When precipitation appears to be complete, centrifuge to separate the solid. Before removing the supernatant liquid with a dropper or by decanting (pouring off), add a few drops more of the reagent to check for complete precipitation. If more precipitation occurs, add a few more drops of reagent, centrifuge, and test again.

    Centrifuging

    A centrifuge is used to separate a precipitate from a liquid. Put the test tube containing the precipitate into one of the locations in the centrifuge. Place another test tube containing an equal volume of water in the centrifuge location directly opposite your first test tube. This procedure is extremely important; it must be followed to maintain proper balance in the centrifuge. Otherwise, the centrifuge will not function properly and may be damaged.

    Turn on the centrifuge and let it run for at least 30 seconds. Turn the centrifuge off and let it come to a complete stop without touching it. Stopping the centrifuge with your hand is not only dangerous, but is likely to stir up your precipitate. The precipitate should settle to a compact mass at the bottom of the test tube. The liquid above the precipitate (the supernatant) should not have any precipitate suspended in it. If it does, centrifuge again. The supernatant can then be poured off (decanted) into another test tube without disturbing the precipitate. All of the liquid should be decanted in a single pouring motion to avoid resuspending the precipitate. An eye dropper or a dropper with an elongated tip may also be used to draw off the supernatant.

    Washing a Precipitate

    After a precipitate has been centrifuged and the supernatant liquid decanted or drawn off, there is still a little liquid present in the precipitate. To remove any ions which might interfere with further testing, this liquid should be removed with a wash liquid, usually distilled water. The wash liquid must be a substance which will not interfere with the analysis, cause further precipitation, or dissolve the precipitate. Add the wash liquid to the precipitate, stir well, centrifuge, and decant the wash liquid. The wash liquid is usually discarded. Precipitates should be washed twice for best results.

    Transferring a Precipitate

    Sometimes you will want to divide a separated and washed precipitate into two portions, in order to carry out two additional tests. To transfer part of the precipitate to another test tube, add a small amount of distilled water to the precipitate, stir the mixture to form a slurry, and quickly pour half of the slurry into another container. Do not use a spatula. This could contaminate your sample.

    Heating Solutions

    Test tubes containing reactions mixtures are never to be heated directly over an open flame. If a solution is to be heated, it should be placed in a test tube and suspended in a beaker of boiling (or in some cases only hot) water. It will be convenient to keep a beaker of water hot throughout the laboratory period. If hot water is required in a procedure, it should be distilled water heated in a test tube suspended in the beaker of boiling water. Do not use water directly from the beaker ­­ it may be contaminated.

    Evaporating a Solution

    Sometimes it is necessary to boil a solution to reduce the volume and concentrate a species or drive off a volatile species. To boil a liquid, place it in a small porcelain casserole or evaporating dish and heat it on a wire gauze with a small flame. Watch it carefully and do not overheat it. Generally, you do not want to heat to dryness as this might decompose the sample. Stir the solution during the evaporation. Do not try to evaporate a solution in a small test tube. It will take much longer and the contents of the tube may be ejected if the tube is heated too strongly.

    Spatulas

    Never place a metal spatula in a solution. It may dissolve and cause contamination. If you need to manipulate a solid, use a rubber policeman on a stirring rod.

    Cleaning Glassware

    Cleanliness is essential for a successful procedure. All apparatus must be cleaned well with soap and a brush, rinsed with tap water, and finally rinsed with distilled water.

    Sulfide Ion

    In any procedures involving sulfide ion, thioacetamide (\(\ce{CH3CSNH2}\)) should be used as the source of sulfide ion. Upon heating in water (or acidic or basic solution), thioacetamide decomposes to \(\ce{CH3CO2}\)­, \(\ce{NH4^{+}}\), and \(\ce{H2S}\) (or \(S_2^{-}\)­ in basic solution):

    \[\ce{CH3CSNH2 + 2 H2O ­­­-> CH3CO2­ + NH4+ + H2S }\]