Substance Composition Reagent Result Conclusion Precip. Supernat. 1 Unknown- G I, II, III Possible GI, II, III D Clear residue -- -- -- 2 Residue from I Possible NH4+ 6M NaOH D Litmus red to blue Confirm NH4+ -- -- 3 Unknown solution Possible Ag, Pb, GII, III 6M HCl White ppt Ag or Pb Ag or Pb G II, III 4 White ppt from 3 AgCl / PbCl2 DI H20 D Ppt washed -- -- Possible Ag 5 White ppt from 4 AgCl/PbCl2 1M K2CrO4 Yellow ppt Pb2+ present PbCrO4 -- UNKNOWN # _________________
GENERAL LABORATORY DIRECTIONS
This course in qualitative analysis has two principal objectives. One of these is to give you the reasons for the analytical procedures and results in terms of the theory of ionic equilibria, especially that relating to weak electrolytes, solubility products, complex ions, and oxidation-reduction. The other objective is the practical one of introducing the sights and smells of chemical reactions and of teaching you careful laboratory manipulation, critical observation, and logical interpretation of observed results.
Classification of the metals into Analytical Groups
Qualitative analysis pertains to the identification of the constituents present in a sample of a substance, or a solution. In the qualitative analysis of a solution that may contain any of all of the common metal ions, the first step is that of separating the ions into several groups. Each group contains ions exhibiting a common chemical property that is the basis of the separation of the common metal ions into groups is usually done as outlined briefly here:
THE METALS OF ANALYTICAL GROUP I
When dilute hydrochloric acid is added to a solution containing all of the common metal ions (and ammonium ion), mercury (I) chloride, silver chloride, and lead chloride precipitate. The chlorides of all the other common metal ions are soluble in this acid solution and can be separated from those of Group I by filtration or centrifugation.
THE METALS OF ANALYTICAL GROUP II
After the Group I chlorides have been separated, the solution is made 0.3 M in hydrochloric acid, and the Group II metals are precipitated as sulfides by the addition of hydrogen sulfide to the solution. The precipitate formed consists of the sulfides of lead, bismuth, copper, cadmium, mercury (II), arsenic, antimony, and tin.
THE METALS OF ANALYTICAL GROUP III
After the Group II sulfides have been separated, the solution is saturated with hydrogen sulfide, and then an excess of aqueous ammonia is added to it. Under these basic conditions, the sulfides of cobalt, nickel, manganese, iron, and zinc and the hydroxides of aluminum and chromium are precipitated.
A flow sheet diagramming the separations of the metal ions into the various analytical groups is given in the table below. Specific details for these separations are given in later sections.
Even though rather definite directions are given for each analysis to be carried out, no two analyses will be exactly alike. For this reason, directions should never he followed blindly, to the letter, but with careful thought; procedures should be adapted to the particular problem at hand.
Note that formulas for precipitates are underlined and will be so indicated throughout the lab procedures. Solid materials are customarily indicated in one of three ways: (1) with the symbol (s) for the solid following the formula (this system has been used in the preceding chapters), for example, AgCl(s); (2) with a descending arrow following the formula, for example, AgCl(descending arrow); or (3) by under-lining the formula, for example, AgCl. The latter method is especially well adapted to flow sheets and will be used the following sections in flow sheets and in equations.
In semimicro qualitative analysis volumes of solutions from 1 drop to about 5 mL are employed, and small test tubes, centrifuge tubes, capillary syringes, and medicine droppers are used to carry out the separations and identifying tests.
The solids and solutions called for in the analytical procedures will be available in small (typically l0mL) reagent bottles. If you are to fill reagent bottles from stock bottles for your own use, be sure that your bottles are completely clean before filling them. To avoid mistakes, it is advisable to label each bottle before filling it.
Practically all of the precipitations are carried out in either 4mL Pyrex test tubes or 2mL conical test tubes. Check for completeness of precipitation by adding a drop of reagent to the solution (centrifugate) obtained from the separation of the precipitate. If the addition of more reagent to the solution shows that precipitation is incomplete, separate the mixture and test the second solution for completeness of precipitation.
The precipitating agent should be added slowly, preferably from a medicine dropper, and with vigorous shaking or stirring of the reaction mixture. The formation of larger crystals of the precipitate is favored by warming the solution, and separation of the precipitate should not be attempted before the crystals become large enough to settle.
A slight excess of the precipitating agent is added to reduce the solubility of the precipitate by the common ion effect. However, a very large excess of the precipitating agent should be avoided, since it may actually increase the solubility of the precipitate. For example, in precipitating silver chloride a large excess of Cl will bring about the formation of AgCl2-, and thereby increase the solubility of AgCl. Many precipitates are dissolved, at least partially, by the formation of related complex ions.
Centrifugation of Precipitates
A precipitate may be separated from a liquid in a centrifuge (Fig). Spinning a mixture of solid and liquid at high speed in a centrifuge forces the denser precipitate to the bottom of the containing tube by a centrifugal force that is many times the force of gravity. This accounts for the much shorter time required for settling a precipitate when centrifugation is employed. Colloidal precipitates require longer centrifugation than do crystalline precipitates because of the small size of colloidal particles.
Before centrifuging a test tube or centrifuge tube, prepare another tube to balance the first by filling an empty tube to the same level with water. Insert the tubes in opposite positions in the centrifuge, and turn the machine on. Allow the centrifugation to continue for at least 30 seconds. Turn the machine off and, after the rotation has stopped completely, remove the tubes.
Transfer of the Centrifugate
After centrifugation the precipitate should be found packed in the bottom of the tube. The supernatant liquid, or centrifugate, is separated from the precipitate by holding the tube at an angle of about 30o (Fig) and removing the liquid by slowly drawing it into a capillary syringe. The tip of the syringe is held just below the surface of the liquid. As the pressure on the bulb is slowly released, causing the liquid to rise in the syringe, the capillary is lowered into the tube until all of the liquid is removed. As the capillary approaches the bottom of the tube, the tip must not be allowed to stir up the mixture by touching the precipitate.
Washing of the Precipitate
The precipitate left in the tube after the removal of a supernatant liquid is still wet with a solution containing the ions of this liquid. The precipitate must be washed, usually with water, to dilute the solution adhering to the precipitate. The wash liquid is added to the precipitate, and the mixture is stirred thoroughly. The mixture is then centrifuged to cause the precipitate to settle again. After centrifugation, the washings are removed by a capillary syringe as described earlier. A precipitate is usually washed at least twice. The first wash liquid is ordinarily saved and added to the first centrifugate. If the precipitate must be transferred to another container, the reagent to be used is added, the mixture is well stirred, and then it is poured into the other container. After the precipitate has settled, the supernatant liquid may be employed to remove any precipitate remaining in the centrifuge tube.
Failure to wash precipitates thoroughly is one of the principal sources of error in qualitative analyses.
Dissolution and Extraction of Precipitates
When all or a part of a precipitate is to be dissolved by a reagent, the solvent is added to the precipitate that is in the centrifuge tube and the mixture is stirred. The mixture is then separated by centrifugation, and the operation is repeated using fresh solvent. Often the extraction of a precipitate is more efficient at an elevated temperature.
Heating of Mixtures or Solutions
Whenever it is necessary to heat a mixture in order to bring about a precipitation or to dissolve or extract a precipitate, the test tube or centrifuge tube is placed in a water bath maintained at a suitable temperature. The water in the water bath should be kept hot throughout the work period.
Evaporation It is often necessary to heat a solution to boiling and hold it at the boiling temperature in order to concentrate it or to remove a volatile acid or base, or even to evaporate it to dryness. Evaporation should be carried out in a small casserole or porcelain evaporating dish. The contents of the container should be agitated constantly while the heating continues. The evaporation of solutions contained in small test tubes should be avoided because the contents of the tube may be lost due to overheating.
Because small amounts of contaminants may give rise to erroneous results, all glassware used in the analytical procedures should be thoroughly cleaned before it is used. The cleaning should be done with a brush and some cleansing powder such as a synthetic detergent. The apparatus should then be rinsed first with tap water and finally with distilled water. Test tube brushes and centrifuge tube brushes are available. Medicine droppers, capillary syringes, and stirring rods should be cleaned, rinsed, and stored in a beaker of distilled water.
Known and Unknown Solutions
You should know the details of the analytical procedures before attempting to analyze a solution of unknown composition. Therefore, known solutions containing all the ions of a given group are provided. You should become familiar with the separations and confirmatory tests for the ions of each group by practicing on a known solution before trying to determine the ions present in an unknown solution. You should especially note the quantities of precipitates obtained and the colors of precipitates and solutions as you proceed with the analysis of a known solution. These observations and the equations for the reactions involved should be recorded systematically in a notebook at the time the observations are made.
A typical set of laboratory assignments for knowns and unknowns is given in Appendix P.
QUALITATIVE ANALYSIS LAB NOTESHEET
In qualitative analysis, it pays to be well organized. In this lab packet, you will find several data sheets that you can use to keep your observations organized and understandable. We recommend that you record all observations as soon as each step in the lab has been completed. This way you can be sure of them later as you go back over the lab. The sample table below should give you a good idea of how to proceed:
For your experiment, we will be providing each team with an unknown that may contain any, all, or none of the ions in Qualitative Analysis. We strongly recommend that you run your unknown tests parallel to the identical set of tests on a known solution. This will allow you to see how each ion behaves in the tests and obtain more accurate verification of your unknown results. Keep in mind if you didn't correctly identify all the ions in your unknown, you will be expected to explain why in your report.
Cleanliness, organization, and first rate lab technique are essential to getting good results in this lab.
As you proceed through the tests, the chart below can be used to log ions that you have identified in your unknown.
Unknown- G I, II, III
Possible GI, II, III
Residue from I
6M NaOH D
Litmus red to blue
Possible Ag, Pb, GII, III
Ag or Pb
Ag or Pb
G II, III
White ppt from 3
AgCl / PbCl2
DI H20 D
White ppt from 4
UNKNOWN # _________________
All contents copyrighted (c) 1998
Peter Jeschofnig, Ph.D., Professor of Science, Colorado Mountain College
All Rights reserved
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