Identifying Ions in Solution: Solubility Equilibria
SEMIMICRO TECHNIQUES: Equipment,
Chemicals and Operations
Adapted from Professor Jim Bert's Chem 115 course, Arizona
State University
Several simple laboratory techniques and skills will be required to effectively identify unkown aqueous ions from a list of possible ions. The scale of the operaions will be reasonabe: not very large and not extremely small, but somewhere in between: a "semimicro" scale, which will involve about 12 mL of solutions and addition of drops of reagents using eye droppers and pasteur pipettes, or addition of solids in amounts of less than a gram. Reaction vessels will be standard, small test tubes. Techniques and skills for reagent/ solution handling and working with volumes of this scale are outlined below. Approximations are normal, but must be within a reasonable precision and accuracy.
Whenever it is necessary to use water in any procedure including cleaning/washing , you MUST use distilled or deionized water. Ordinary tap water is not completely pure and may introduce additional ions that will contaminate your sample or provide erroneous test resultts.
When obtaining reagent solutions from the reagent bottles on the carousels, always dispense the solution with the dropper that is 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. Droppers are typically in the range of 10 to 15 drops per milliliter, pasteur pipettes about 20 drops per millitliter.
Solid reagents when used in the analyses are typically used in amounts of less than 1 gram. Unless an exact amount is required, an approximation is acceptable. If for example the procedure required 0.1 g (100 mg) a small amount on the tip of a spatula would be ok. It might be 0.15 g or 0.20g. The caveat is that is that it be enough, so it is better to us amount slightly more than called for. CAUTION: 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.
In any procedures involving sulfide ion, thioacetamide (CH3CSNH2) must be used as the source of sulfide ion. Upon heating in water or acidic or basic solution, thioacetamide decomposes to CH3CO2, NH4+, and H2S (or S2 in basic solution):
CH3CSNH2 + 2 H2O > CH3CO2 + NH4+ + H2S
Caution: H2S is extremely hazardous. Use of thioacetamide is to be done ONLY in the fume hood!
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 maintain stirring rods is to prepeare several rods and to clean them immediately after use by rinsing them with distilled/deionized water from your squeeze bottle and then place them in a beaker of clean distilled water. Any slight contamination will be highly diluted in the beaker's water. It is advisable to refill the beaker at the start of each lab period.
The control of acidity/basicity of a solution will at times be critical to the correct result. 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.
In order to detect the formation of a precipitate, both the solution being used and the reagent must be clear (transparent [clear], 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 (the clea,r liquid portion of the mixture) 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.
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.
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.
On occasion, a precipitate will need to be divided into two portions, separated and washed, 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.
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.
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.
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.