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Laboratory Techniques: Qualitative Inorganic Analysis Techniques

 

Introduction

 

Some people seem to think that a chemist is capable of analyzing practically any substance or mixture merely by looking at a sample or by performing a few simple tests. However, those who study chemistry know that analysis is not that easy. At present, there are 106 known elements that combine in many ways to form an immense number of compounds, each with its own characteristic set of properties. As a result, a combination of very sophisticated techniques is often required to analyze most samples.

 

Nonetheless, the method of qualitative Inorganic analysis provides a simple procedure for separating and identifying approximately one-fourth of the known metallic elements from mixtures of inorganic compounds, without the need for elaborate procedures or expensive instruments. In addition, some qualitative inorganic analysis schemes include procedures for the analysis of a number of commonly encountered anions.

 

The most common method for separating components in the usual qualitative inorganic analysis scheme is selective precipitation, although some ions can be separated by conversion to gases or by formation of complexes. Selective precipitation can be used to separate approximately 30 metal ions and nearly a dozen anions. This method involves standard laboratory equipment such as beakers, burners, casseroles, crucibles, flasks, and test tubes. After separation, the individual ions can be identified using colors, odors, crystalline appearance, and solubility characteristics.

 

Several other techniques are used regularly in qualitative inorganic analysis and also, with some adjustments, in other laboratory work. Frequently these specific operations are described only in general terms, for example: "heat the solution in a test tube,' "evaporate the solution to dryness," "mix the reagents until a precipitate forms,' and "separate the precipitate and wash it thoroughly." Such general directions lead you to ask the following practical questions: How can a solution be heated in a test tube without the solution boiling out of the test tube? How can I do an evaporation without spattering the sample or burning it? How can reagents be mixed most effectively? How can I separate precipitates from the solutions in which they form? How do I wash a precipitate?

 

The answers to these and other practical questions concerning the techniques used in qualitative inorganic analysis are given in the sections that follow. Study these directions before beginning your laboratory work, and keep them handy for quick review when your analysis procedure specifies a particular technique. Sometimes an experienced laboratory instructor may suggest alternative ways to perform some of the operations described; there is more than one acceptable way to carry out some of these operations.

An important general point is to avoid carelessness in your laboratory work. Experience has shown that reasonably careful laboratory technique is required d the qualitative inorganic analysis scheme is to be a success.

 

Some Preliminary Suggestions

 

Preparing for the Laboratory

 

It is very important to prepare for laboratory work before actually going into the laboratory. Such procedures as heating solutions and allowing precipitates to form and settle can require considerable time. You should plan to do other work during these waiting periods. Study all of the procedures you expect to use. If the procedure lists an unfamiliar technique, reread the appropriate section in this module. Make the necessary preparations in your laboratory notebook for recording observations and other data. All this should be done before you enter the laboratory. Laboratory work periods are scheduled for performing analyses and generally do not allow time for study or preparation.

 

Keeping Your Laboratory Notebook

 

The necessity of keeping a complete, up-to-date, honest record of laboratory work in a laboratory notebook cannot be overemphasized. Inorganic analysis involves too many ions and too many tests for you to rely upon your memory as your source of information. For example, in many analysis schemes, metal ions that form insoluble sulfides in acid solution are separated into one group for analysis. The complete analysis of this group of eight ions may require as many as 25 tests. The result may be four black precipitates, three yellow precipitates, a red precipitate, an orange precipitate, and a colorless solution, if all eight ions are present in the sample. Frequently, however, some ions are not present in the sample. This complicates the situation, because some of the nine precipitates will not be found. When the analysis is completed and a report is being prepared, questions such as the following might be raised: Was the yellow precipitate lead (II) chromate, or was it cadmium sulfide? Was the black precipitate the result of a test for bismuth, for mercury, or for tin? Was the colorless solution a positive test for tin or a negative test for cadmium? These questions are difficult to answer without accurate laboratory notes.

 

A further problem is that some analyses cannot be completed in one laboratory period. As much as a week may pass before an analysis is resumed. A week is too long a period over which to remember the identities of solutions and/or precipitates in eight or ten test tubes. Such problems can be resolved quite simply. Record all of your observations in your notebook, and label all of your samples and containers.

 

Use a bound notebook, rather than loose pages or paper, for entering your observations and comments. A bound notebook is much less likely to be lost, and the binding keeps the pages in the proper sequence. Many experimental results can be recorded in tabular form, so you should consider using tables where possible, such as for summaries of the results of your analyses.

 

Develop the habit of recording observations promptly. Describe your observations and comments in sufficient detail, so that you can reconstruct each step of the analysis later. Write down what you actually see or smell rather than what you expect to observe. If one of your analytical tests does not work properly, record the results and label them "faulty." Such an entry may be important at a later stage of the analysis. If you need help from your laboratory instructor at some point, a well-kept notebook will be helpful in determining the sequence of tests that you performed and in reviewing your conclusions resulting from your observations.

 

Label all containers that hold solutions or precipitates. The best devices for labeling are either glass-marking pencils or small squares of paper and rubber bands. Gummed labels can be used, but they can dry out and peel off the container. If you use a letter or numerical system, you should use the same labeling system with the corresponding entries in your notebook.

 

Your laboratory instructor will tell you exactly how to make entries in your notebook.

 

Avoiding Sample Contamination and Cleaning Equipment

 

Neat, careful, orderly laboratory work is essential in qualitative inorganic analysis to avoid sample contamination. Samples can become contaminated in subtle ways. For example, tap water contains such ions as Ca2+, Mg2+, Fe3+, HCO, and Cl-, so a container rinsed with tap water may contain unwanted ions. For this reason, only distilled or de-ionized water should be used in the analysis. All equipment should be cleaned regularly and rinsed with distilled or de-ionized water for further use. In the remainder of this module, the term "distilled water' refers to either distilled or de-ionized water.

 

Allow some time at the end of each work period for cleaning all equipment. Dirty equipment is much easier to clean immediately after use. Quickly scour the equipment with a test tube brush dipped in warm water containing dishwashing detergent. Rinse the equipment a few times with tap water to eliminate the suds, and then rinse several times with small amounts of distilled water. Tap water is used for the detergent solution and the first rinses because the distilled water supply is generally limited. Wipe dry the outside surfaces of equipment. Store the equipment in an inverted position to allow the inside surfaces to drain dry. Contamination can be introduced into flasks and beakers if their inside surfaces are wiped dry. By following this procedure, your equipment will be clean, dry, and ready for use during the next work period.

 

The laboratory bench top is also a notorious source of unwanted contaminants. Therefore, your working area must be kept clean. Equipment that will contact the sample, such as stirring rods, capillary pipettes, and droppers, should be placed on a clean towel or sheet of paper at the edge of your working area. This practice reduces the likelihood of contamination and makes such equipment readily available when needed.

 

Using Chemicals and Discarding Chemical Waste

 

Many chemicals are toxic and/or corrosive. Be especially careful of ammonia, thioacetamide, and concentrated acids, but consider all chemicals as potentially harmful. Immediately wash off any chemical that contacts your skin using plenty of water, whether or not you feel a burning sensation. Wear safety goggles at all times to avoid harming your eyes with spattered chemicals.

 

Carefully read all container labels. Solutions can be made in a large range of concentrations. Sometimes more than one concentration of the same solution is used in a single experiment. Use of the wrong concentration could lead to unexpected results. Some solids can exist as strips, wire, granules, or powder, and sometimes the use of the correct form of solid is extremely important. Also, some substances have names that appear to be the same but are not identical. Compare, for example, sodium nitrate and sodium nitrite, tin (IV) chloride and tin (II) chloride, or hydrochloric acid and perchloric acid. Be absolutely certain that you are using the correct substance and the correct concentration.

 

While you are performing the analyses, you will have to discard some solid items such as matches, indicator paper, and filter paper. If your workspace is not near a waste container for paper, use a beaker labeled "waste paper" as a personal waste jar. This practice will save a lot of trips back and forth to the end of the laboratory bench or across the room. The beaker should be emptied at the end of each work period into a container designated by your laboratory instructor.

 

Occasionally, you will break glassware. You must clean up the broken glass immediately and discard it, following the directions of your laboratory instructor.

 

Federal law now requires that all chemical waste be handled according to established regulations. This law requires segregating waste according to hazard categories and dealing with each category in the prescribed fashion. Some waste can be poured directly into the drain, but much of it cannot. Discarded mercury compounds qualify as hazardous waste materials, although discarded silver compounds can be processed and recycled. Be sure to follow the directions of your laboratory instructor for discarding solutions and residues from each experiment.

 

At the end of each laboratory session, it is important to wash your hands thoroughly with detergent or soap. This action will reduce the risk of skin irritation from chemicals or ingestion of traces of chemicals remaining on your hands.

 

 

Making Observations

 

In most cases, conclusions about the presence or absence of ions in a solution are based on formation of precipitates, color changes, flame tests, or evolution of gases.

 

 

Precipitate Formation

 

A solid that forms in a solution when solutions of reagents are mixed is called a precipitate. Formation of a precipitate makes a solution appear cloudy. Precipitates may range from coarse particles to gelatinous or fluffy solids to a suspension of particles so fine that individual particles cannot be seen. Milk is a suspension. The formation or disappearance of precipitates gives important information about whether or not certain ions are present in a solution.

 

Color Changes

 

A solution is a clear mixture of one or more solutes in a solvent such as distilled water. The word "clear" means that no precipitate is present to make the solution appear cloudy. The term "clear" does not indicate anything about the color of the solution. A solution without cloudiness is clear, but a solution without color is colorless. Sodium chloride dissolved in water is a clear, colorless solution, but copper(II) sulfate dissolved in water is a clear, blue solution. A solution cannot be cloudy and colorless.

 

If a solution changes color when reagents are mixed, a reaction has occurred. A change of color must be interpreted carefully. For example, formation of a cloudy, green solution may indicate any one of the following possibilities:

 

1.      a green precipitate in a colorless liquid,

2.      a white precipitate in a green liquid,

3.      a green precipitate in a green liquid,

4.      a blue precipitate in a yellow liquid, or

5.      a yellow precipitate in a blue liquid.

 

Consequently, you should separate the solids from the liquids before making decisions about the meanings of color changes. The liquid remaining after a precipitate has been removed is called the supernatant liquid. Supernatant liquids are clear but may be colored. Techniques for separating the precipitate from the supernatant liquid are discussed later in this module.

 

Flame Tests

 

The presence of certain cations can be confirmed by flame tests. A flame test is performed by dipping a special wire into a solution of the cation and then holding the wire in a Bunsen burner flame. Sodium Ions give a bright yellow flame, barium ions a green flame, and potassium ions a purple flame. Other cations also have characteristic flame colors.

Figure 1 The flame test

The wire should be a piece of platinum or nichrome sealed in the end of a piece of glass tubing. A small loop is formed at the end of the wire, and the tubing is used for holding the wire. The wire is first cleaned by dipping it in 6M HCl and then heating it in a Bunsen burner flame as shown in Figure 1. Note that the wire is not placed in the hottest part of the flame but at the edge. Repeat the cleaning procedure until no color appears when the wire is placed in the flame. Then dip the wire in the test solution, heat the wire in the flame as before, and note any color. For comparison, always repeat this test with a solution known to contain the suspected cation.

 

Alternate flame test: Place a few drops of the solution to be tested near the edge of a clean, dry watch glass and hold it near the air intake of the burner. Clean the wire by dipping It Into 6M HCI and heating in the flame until it gives no color to the flame. While the clean wire is still glowing, touch it to the solution on the watch glass. As the solution is vaporized, some of it will be drawn into the air intake and give color to the entire flame. Note the color for comparison with solutions of known cations. (Bare, W.D., Bradley, T., Pulliam, E., J. Chem. Educ. 1998, 75, 459)

Figure 2 Detecting odors

 

 Evolution of Gases

 

The presence of certain anions is indicated by the evolution of a gas. These reactions occur frequently when an acid is added to a solid. Many gases are colorless, but their presence can be detected by their odor or the appearance of bubbles. To detect the odor, wait until vigorous bubbling has subsided, hold the test tube near your nose, but not pointed at your face, and waft vapors toward your nose as shown in Figure 2. Do not place the test tube directly under your nose and Inhale.

 

A few gases, such as NO2, are colored. To see these clearly, hold the test tube against a white background, and note any color in the space above the solution.

 

Measuring Reagents

Liquids

Qualitative inorganic analysis procedures require the addition of liquid reagents in either drop or milliliter amounts. Actual drops vary in size, but the drops referred to in the procedures of the experiments in this series are assumed to have a volume of 0.05 ml. Many standard medicine droppers deliver drops of approximately this volume; thus there are about 20 such drops per ml. The droppers should be calibrated to make sure that they deliver approximately 20 drops per ml. However, note that capillary pipettes deliver much smaller drops. When a procedure specifies the addition of a certain number of drops, use a medicine dropper rather than a capillary pipette. All additions of liquids in amounts of up to two or three milliliters should be made with droppers rather than with a graduated cylinder. The addition of larger amounts of solutions is seldom called for, but graduated cylinders are sufficiently accurate for measuring volumes larger than 3 ml.

 

Solids

 

Amounts of solid substances can be measured by using a triple-beam balance, a centigram balance, or a top-loading balance that is sensitive to at least 0.05 g. Weighing by difference is the best procedure to use when measuring solids. This procedure consists of determining the mass of the solid and its container, pouring out the solid until the required amount has been removed, and then weighing the container and remaining solid. Chemicals should always be placed on the balance in an appropriate container. Never put chemicals directly on the balance pan. Sometimes the exact amount of solid to be added is not important. This is indicated by directions such as, "Add some solid..."or "Add approximately...." In such cases, you can estimate the amount of solid you are adding. Such estimates are simplified if you know the approximate mass of a heaping spatula-full of some common solids, such as ammonium nitrate or ammonium sulfate. For example, if you know that a spatula can hold about 0.15 g of a common inorganic salt, then the direction to add approximately 0.3 g of ammonium sulfate can be followed by adding two heaping spatula-fulls of the salt.

 

Estimations using spatulas and droppers can save considerable time, provided that a supply of clean droppers and a clean spatula are readily available.

 

 

 

Transferring Reagents

 

Because of the nature of qualitative inorganic analysis, it is of utmost importance to avoid contaminating either the sample or the reagents used. The following techniques are designed to maintain the purity of all chemicals.

 

Liquids

Figure 3 Transferring a liquid using a dropper

Figure 4 Pouring a liquid

 Take an appropriate container to the reagent shelf where the liquid is kept. Make the actual transfer at a nearby sink to avoid spills on the laboratory bench, shelf, or floor. If the general supply bottle is equipped with a dropper, use the dropper, but be sure that it never touches the container or its contents, as shown in Figure 3.

 

If the general supply bottle is equipped with a stopper, the stopper should either be held during the transfer or placed on its flat top. Do not lay the stopper on its side on the laboratory bench or shelf. The proper technique is shown in Figure 4. Do not put your dropper or capillary pipette into the general supply bottle. Pour chemicals from the general supply bottle into a labeled container for personal use. Be sure the appropriate stopper is returned to the supply bottle. Use a clean dropper to dispense liquids from your labeled container.

 

Solids

Figure 5 Pouring a solid

 

Take an appropriate container to the reagent shelf where the general supply chemicals are kept. Solids are somewhat more difficult to transfer than are liquids, so a wide-mouth container such as a beaker is preferable. Make the transfer at or near the reagent shelf.

 

During the transfer, do not contaminate the stopper. Hold it or lay it on the laboratory bench, as shown in Figure 4. You should never use your spatula in the general supply bottle. Solid chemicals are most easily poured by tipping the general supply bottle. Slowly rotate the bottle back and forth about an imaginary axis passing through the top and bottom of the bottle, as shown in Figure 5. Mere tipping of the bottle alone often causes large pieces of solid to fall out of the bottle, leading to spills. Be sure the right stopper is returned to the general supply bottle. Do not interchange stoppers between bottles.

 

Mixing Reagents

 

Students often underestimate the importance of thoroughly mixing reagents. Reactions cannot occur unless reagents are mixed. Mixing of reagents is a difficult operation because most reactions are carried out in narrow test tubes. You should not try to mix chemicals by holding a cork or your thumb over the top of the test tube while shaking it. This may cause contamination of the sample. Instead, the following mixing techniques are best.

 

A stirring rod rinsed with distilled water can be used for mixing. Both vertical and rotary movement of the rod should be used for the most satisfactory results. If the test tube is almost full, be sure that its contents will not overflow when the stirring rod is inserted.

 

For a test tube that is less than half full, use the following mixing method. Grasp the top of the test tube with the thumb and forefinger of one hand, and sharply tap or snap the test tube several times near the bottom with your other forefinger. It may seem an unlikely technique, but it produces no splashing and works quite efficiently.

 

For centrifuge tubes, which are tapered at the bottom, or for test tubes that are more than half full, efficient mixing, can be accomplished as follows. Draw up a portion of the solution into a clean capillary pipette. Then, squirt the solution back into the tube near the bottom. This procedure should be repeated at least two or three times. You may also pour the liquids back and forth from one test tube to another to achieve mixing, but you must take care to avoid spilling the solution during the transfer.

 

If you need to mix liquid chemicals with water, always add the concentrated chemical to water, rather than vice versa. This procedure keeps the new solution dilute at all times and avoids many accidents. Usually additions should be made slowly, using small amounts of the reagent being added. It is especially important to add acid to water because of the heat generated by the dilution.

 

If you need to dissolve a solid in a liquid, add the solid to the liquid, rather than vice versa. Solids should be added in small amounts with stirring, except under special circumstances.

 

Determining the Acidity of a Solution

 

An operation that must be performed quite often in qualitative inorganic analysis is the determination of whether a solution is acidic or basic. For this purpose a variety of kinds of indicator paper are available in narrow strips less than 1 cm wide. These indicator papers are saturated with either a single indicator, as with litmus paper, or with a mixture of indicators, as with indicator papers that have several colors over a wide range of pH values.

 

Litmus paper is used when you want to know whether a solution is either acidic or basic, without needing to know the specific pH. Litmus paper is red in acidic solutions and blue in basic solutions. If, however, the procedure requires a solution with a specific pH (within a narrow range), then you should use special indicator papers. The choice of paper depends upon the application. If, for example, you need a solution with the acidity of a 0.3M HCl solution (pH 0.5), then you should use methyl violet paper, because it is yellow-green at this acidity. There are also mixed indicator papers that change colors over narrow pH ranges.

 

Do not dip the indicator paper directly into the solution in a test tube for the following reasons:

 

1.      The strips are generally too short to reach the solution in the test tube,

2.      the sample may become contaminated by dissolved indicator and paper fibers,

3.      such a procedure would test mainly the acidity of the droplets at the top of the test tube, rather than the acidity of the entire solution, and

4.      the use of a whole strip for only one test is inefficient.

 

An acceptable method for testing the acidity of a solution with indicator paper is:

 

1.      Dip a clean, dry glass stirring rod into the thoroughly mixed solution,

2.      stir vigorously,

3.      withdraw the rod carefully to avoid touching the inside walls of the test tube, and

4.      touch the end of the rod to one spot on the indicator paper.

 

In this way, a 4- or 5-cm strip of indicator paper can be used for several tests, and there is less risk of solution contamination.

 

Heating Solutions

 

Liquids in test tubes are best heated by holding the tube with a test tube clamp and placing the test tube in a 250-ml beaker of boiling water. A 250-ml beaker is small enough to permit the test tubes to remain upright during heating. Many tests require the heating of solutions, so get in the habit of heating water in a beaker at the beginning of each work period. Keep the water hot throughout the work period. Do not let the water level in the beaker get too low. Keep the beaker one-fourth to two-thirds full. If tap water leaves a scale on the beaker as the water evaporates, you can remove the scale at the end of the work period by adding water to the beaker, cooling the water for a short time, and carefully adding a small amount of dilute hydrochloric acid to the water.

 

Larger amounts of liquids in beakers and flasks are best heated by placing the container on a ceramic-centered wire gauze on a ring stand, supporting the container with a clamp. Heating liquids for evaporation is described in the next section.

 

Evaporating a Solution

 

Qualitative inorganic analysis procedures frequently require reducing the solution volume or evaporating a solution to dryness. These techniques need considerable attention and caution must be taken to avoid sample loss by spattering, generation of toxic or unpleasant fumes, or decomposition caused by overheating. However, if proper methods are used, evaporation can be accomplished without sample loss. When acids or other solutions that emit toxic or unpleasant fumes must be evaporated, the procedure must be performed under a fume hood.

 

The most convenient container to use for evaporation is a porcelain casserole. However, the casserole handle is short and quickly becomes hot while the casserole is being heated. The handle can be extended and kept cooler by slipping a two-inch length of heavy-wall rubber tubing over about half of the handle. Do not push the rubber tubing too close to the casserole bowl, or the rubber may get hot and melt or burn. In some cases, crucibles and beakers can be used for evaporation, as described below.

 

Figure 6 Holding a beaker with crucible tongs

 As a heat source for evaporation, a microburner is best, if it is available. The microburner should be adjusted so that the flame is about 12 mm. If a microburner is not available, a Bunsen burner can be substituted, as long as it is adjusted to produce a small flame. If necessary, use a screw clamp on the burner tubing to make this adjustment. Small amounts of liquid can be evaporated by placing the liquid in a casserole under an infrared lamp, but this method is rather slow.

 

When evaporating a solution in a casserole, constantly swirl the casserole while heating. Avoid getting the liquid too hot, or it will boil vigorously and spatter out of the casserole. If the solution is being evaporated to dryness, remember that the casserole retains quite a bit of heat. Thus, you should stop heating the casserole before evaporation is complete. The heat retained by the casserole will then complete the process. It is a good idea to have withdrawn a little of the liquid being evaporated in a dropper or capillary pipette before beginning the evaporation. Then, if the casserole seems to be overheating, you may add one or two drops of the liquid, to prevent baking of the residue. However, it is better to stop the heating early enough to avoid the need for addition of the extra liquid.

 

Figure 7 Evaporating a solution using an air bath

 If the solution is in a small beaker, and the liquid needs only to be partially evaporated, you can hold the beaker with crucible tongs over a burner flame and the swirl the beaker and its contents gently in the flame. It is crucial to hold the beaker property, with both jaws of the tongs outside the beaker. To grip only one side of the beaker with one of the jaws of the tongs inside the beaker is a sure way to contaminate the sample. The proper way to hold a container with crucible tongs is shown in Figure 6. The beaker should be less than half full at the beginning of the heating, and the evaporation should not be carried to dryness when using a beaker. Crucibles can also be used for evaporation by following this method.

 

A crucible can be used for evaporation in an air bath, which can be constructed from a beaker and a wire triangle, as shown in Figure 7. This method gives uniform, but slow, evaporation, although care must be taken to avoid getting the beaker too hot and breaking it.

 

Forming a Precipitate

 

Most of the tests in qualitative inorganic analysis involve the formation of precipitates and their complete separation from the remaining supernatant liquid. These operations are necessary to ensure that a given ion is precipitated completely, so that the ion does not interfere with later tests. The precipitate needs to be completely free of the solution, for the same reason. This section and the four that follow describe the methods that will best assure success in this aspect of inorganic analysis.

 

When you anticipate the formation of a precipitate, slowly add the precipitating agent, using a dropper or capillary pipette. Mix the solutions constantly, using one of the methods described in the section on mixing reagents. Formation of large particles is favored if you warm the solution in a hot water bath.

 

Always check for completeness of precipitation. After heating the mixture, let the precipitate settle in the test tube, or centrifuge H necessary. Add another one or two drops of precipitating agent. If more precipitate forms, add additional amounts of precipitating agent and reheat the solution. Check again for completeness of precipitation. Avoid adding too much precipitating agent; large excess can sometimes dissolve the precipitate by forming a complex ion.

 

A few comments must be made about the precipitating agent, thioacetamide. This substance is used as a precipitating agent for more than half of the metal ions in the usual qualitative analysis scheme. It reacts with water to supply sulfide ion for precipitation of metal sulfides. The reaction in water is

 

Although thioacetamide is safe to use when handled prudently, you should be aware of its less desirable properties. First, thioacetamide has exhibited carcinogenic properties in some animal feeding studies. Consequently, it is prudent to avoid skin contact with a thioacetamide solution and to wash your hands thoroughly with soap or detergent after using this solution. Second, both thioacetamide and the product H2S are toxic, so it is prudent to use small amounts and to work under a fume hood, if possible. Finally, H2S has a foul odor that is described as a “rotten-egg smell”, and this odor is minimized if small amounts of thioacetamide are used under a fume hood.

 

Using the Centrifuge to Separate Precipitates

 

Once a precipitate has formed, it must usually be separated from the remaining solution. Although this can be done by filtration, it is often faster and just as effective to use a centrifuge. A centrifuge is a device that holds test tubes or other similar containers and spins them in order to separate the components into two distinct phases. The mixture of solid and liquid is rotated at high speed, which exerts as much as 300 to 400 times the force of gravity on the precipitate. The force causes the precipitate to settle quickly and pack at the bottom on a test tube. The remaining supernatant liquid can be separated from the precipitate by decanting or by using a capillary pipette. The precipitate is then usually washed thoroughly to remove traces of supernatant liquid. The great advantage of using the centrifuge is the short time required for a precipitate to settle to the bottom of the test tube. In most cases, the centrifuge does not have to be run at top speed for more than about 1 min, although some light, fluffy precipitates may require several minutes for separation. The following precautions should be observed when using the centrifuge.

Mixtures are ordinarily centrifuged in test tubes. However, because of the terrific force exerted on the test tubes, you should examine them carefully to make sure that they are free of any flaws before using them in the centrifuge. If there is only a tiny amount of precipitate in the test tube, you may prefer to use a centrifuge tube that is tapered at the bottom.

 

The head of the centrifuge revolves at speeds in excess of 1 000 rpm, so your hands and clothes must be kept clear of the spinning parts. The centrifuge must be carefully balanced at all times. Two test tubes should always be inserted into the centrifuge, one containing the solution being centrifuged and the other containing an equal volume of water. The tubes should be placed directly opposite each other in the centrifuge head. Centrifuge heads usually have four or more compartments for test tubes. When you are sharing the centrifuge with someone else, be sure that the tubes are properly labeled so that they can be identified after the centrifuge stops spinning.

 

Occasionally a test tube will shatter in a centrifuge because of the intense force placed on it. If this happens, do not immediately peer into the centrifuge to check the damage. The likelihood in such a case of being cut by f lying glass particles is high if you open the centrifuge before it has completely stopped. Nothing can be done until the centrifuge has stopped.

 

Many centrifuges must be allowed to coast to a stop after the motor has been turned off. Some centrifuges have mechanical brakes or can be stopped by hand pressure. Be extremely careful if you use your hand to slow the centrifuge head. The head should be slowed gradually to avoid mixing the precipitate into the solution again, injuring your hand, or damaging the centrifuge.

 

Removing Supernatant Liquid from a Precipitate

 

In some cases involving precipitates such as silver chloride or lead(II) sulfate, centrifugation packs the precipitate so tightly in the bottom of the test tube that the supernatant liquid can be separated from the precipitate by pouring off the liquid into another test tube. This procedure is called decanting. However, precipitates such as sulfides or hydroxides are usually packed less firmly. In such cases, a capillary pipette is required for removal of the supernatant liquid.

 

The bulb is squeezed before the pipette is inserted into the supernatant liquid, and the liquid is removed in small portions.

 

If a precipitate does not centrifuge completely or to be stirred up when you are using a pipette to remove the supernatant liquid, you may isolate the precipitate as follows. Twist a small piece of cotton wadding, and insert it into the tip of a clean dropper. Do not stuff the wadding down into the barrel of the dropper. Leave a tuft of the wadding protruding from the dropper. Draw the supernatant liquid through the cotton, which will trap any loose precipitate. Remove the wadding and release the clear supernatant liquid into a clean test tube. Repeat as necessary. This procedure filters precipitate particles from the supernatant liquid.

 

The supernatant liquid should always be retained unless you are absolutely positive you will have no further use for it. Additional dissolved ions are often contained in the liquid, and the liquid is sometimes used for later tests. Be very sure that this is not the case before you discard this liquid.

 

Washing a Precipitate

 

After the supernatant liquid has been removed from the precipitate, there will still be an appreciable amount of solution clinging to the wet precipitate' Because the remaining solution may contain ions that could interfere with later tests, the last traces must be removed by washing the precipitate. Washing is also necessary for retaining soluble ions in the supernatant liquid that are to be tested for later. If a fraction of the soluble ions is lost with each precipitation, there may be insufficient ions in solution for the later tests to be successful.

 

The wash liquid is usually distilled water, although dilute hydrochloric acid and other solutions are sometimes used. Experience has shown that several washings with small portions of wash liquid are more effective than one washing with a large amount. The procedure for washing follows.

 

Add a small amount of wash liquid to the packed precipitate. Use a stirring rod to break up the precipitate. Thoroughly mix the precipitate and the wash liquid. Centrifuge again and decant or withdraw the wash liquid as before. The first wash liquid is often added to the supernatant liquid from the first centrifugation, because the wash liquid may contain an appreciable amount of soluble ions to be tested for later. Repeat this washing procedure at least two or three times. These later washings are often discarded.

 

Transferring a Precipitate

 

If possible, plan your laboratory work so that you can avoid transferring precipitates from one container to another. This procedure is not very efficient. However, sometimes precipitation is carried out in such a large volume of solution that the mixture must be centrifuged in more than one test tube. In this case, after centrifugation, the precipitates should be recombined before continuing the analysis. To do this, transfer the precipitate as follows.

 

Add distilled water to the precipitate you wish to transfer. If necessary, use a stirring rod to break up the packed precipitate and form a suspension, but be certain to use distilled water to rinse off any precipitate clinging to the rod, before removing the rod from the test tube. Then, transfer the precipitate by drawing small portions of the suspension into a dropper and quickly depositing them into the other container. Carefully clean the dropper after you have transferred all of the precipitate.

 

Summary

 

If you use the techniques discussed in this module, you will significantly increase the chances that you will complete your qualitative inorganic analyses efficiently and successfully. One final observation: Generally, analysis procedures are quite detailed and often tell exactly how much reagent to add and what to do for each test. However, there are some variations in solution concentrations and other conditions from one laboratory to the next. Thus, there is no substitute for an alert, thinking mind that questions the reasons for each step in the procedure, is capable of interpreting observations, and can make appropriate adjustments when confronted now and then with an unfamiliar situation.


 

 

 

 

Name

Section

Date

 

 

Pre-Laboratory Assignment

 

1. Name two sources of sample contamination.

 

 

 

 

 

2. List five general rules for handling chemicals.

 

 

 

 

 

3. What is the most important general rule to follow when discarding chemicals and solutions?

 

 

 

 

 

4. What is the best procedure for weighing solids?

 

 

 

 

 

5. List four precautions regarding the use of general supply bottles of chemicals.

 

 

 

 

 

6. What is the correct procedure for mixing an acid solution with water?

 

 

 

 

7. List four reasons for not dipping litmus paper or pH paper into solutions.

 

 

 

 

 

8 What should you do at the beginning of each ry session?

 

 

 

 

 

9. What test should be performed after each precipitation?

 

 

 

 

 

10. List six precautions to be followed when using the centrifuge.

 

 

 

 

 

12. Why must precipitates be washed after the supernatant liquid has been removed?