Qualitative Analysis
Chemical analysis can be either qualitative or quantitative in character. A qualitative analysis enables us to find out what elements or chemical species are present, while a quantitative analysis tells us how much of each component is present. Quantitative analysis comprises a one- semester course usually taken during the sophomore year. Qualitative analysis is usually offered as part of the laboratory requirement for the second semester of General Chemistry. In this course, six weeks will be devoted to “qual” in lab.
The methods and techniques you will learn are the classical “wet” methods, whereas, modern analytical labs rely on instrumentation almost exclusively. Technicians with little background in chemistry can be readily trained to do routine instrumental analyses without knowing anything about the chemical principles underlying the technique. You, however, upon completion of qualitative analysis will have learned a lot of descriptive chemistry and applied many of the important chemical principles that were introduced in lecture. “Qual” presents a perfect opportunity to study the properties of metal ions in aqueous solution, chemical equilibrium, precipitation reactions, acid-base chemistry, complex ion formation, oxidation-reduction reactions, and much more. You will also sharpen your powers of observation and enhance your ability to critically evaluate data. This may sound like a monumental task, but the great majority of students find “qual” to be their most enjoyable experience in chemistry lab. In fact, no other laboratory experience in general chemistry comes as close to providing a feeling for the flavor of scientific research.
A good research chemist must have a clear understanding of the problem he or she is trying to solve and the methods used to approach the problem. Good chemists are usually good cooks, but the reverse is not necessarily true. Don't take a “cookbook” approach to qualitative analysis, simply following a set of instructions by rote. You most probably will encounter unexpected results, ambiguity, and uncertainty. Only a real familiarity with the chemistry involved in your analysis will allow you to work through puzzling results and be successful.
Your actual laboratory work in qualitative analysis will consist of analyzing a “known” solution containing ten different metal cations. You will also analyze three unknown cation solutions. Following this you will carry out tests to identify the anion in nine “known” sodium salts, as well as two unknown sodium salts. Lastly, all of the laboratory technique and analytical chemistry that you have learned will be integrated when you correctly identify both the anion and cation in an unknown salt. An approximate schedule is:
Week 1 Cation group 1 and 2 known
Week 2 Cation group 1 and 2 unknown
Week 3 Cation group 3 and 4 known and cation group 3 and 4 unknown
Week 4 General cation unknown
Week 5 Anion knowns and two anion unknowns
Week 6 Salt unknown
The Qualitative Analysis Notebook
The notebook must have the pages sewn to the binding. Papers are never removed from a lab notebook. All information must be recorded in ink. If you make a mistake in the notebook, simply cross through it with a single line. If the mistake is still readable, you can see where you goofed and avoid the same mistake later. Record observations in detail. Except for the reactions, all information must be put in the notebook as you go along. Do not write on loose paper.
The notebook is more than a record of what you do in the lab; it can help when doing an unknown. Ignoring the notebook during an unknown causes many errors. For example; the lab manual says that the precipitate confirming the presence of Al3+ is red (brown shades). You will find that description is not accurate. Many students have missed the presence of Al3+ in their unknowns because they look only at the flow chart and not their notebook. When doing an unknown, if you are unsure of something, check your notebook. It will contain the same test carried out on a known solution. Believe your notebook!
It's to your advantage that you record your observations in detail. When describing colors, don't just say “green”. Is it “seasick green”, “light green”, “dark green”, “olive” or “pond scum green”? Estimation as to the amount of a precipitate is also helpful. This can help when faced with very small quantities of precipitates.
When starting a known or an unknown, the first thing to record is the appearance of the solution or solid. If it is an unknown, make some conclusion as to the components of the unknown based on color. Of the ten cations in this scheme, only four are colored. A yellow unknown must contain Fe3+! What else could give the unknown this color?
Set up your notebook as shown below for cation analysis, with the four columns spread across two pages.
Procedure / Observations / Conclusions / ReactionTake 1 mL of known solution / Dark blue-green liquid / Known contains Ag+, Pb2+, Cr3+, Al3+, Fe3+, Mg2+, Ba2+, Cu2+, Ni2+, Zn2+
Add 2d HCl to known; stir & centrifuge / Soln 2A: dark green. Ppt 1A: white crystalline / Ag+ and/or Pb2+ indicated in ppt 1A.
Soln 2A may contain any of the other 8 cations and PbCl42-. / Ag+ + Cl- D AgCl(s)
Pb2+ + 2Cl- D PbCl2(s)
Pb2+ + 4Cl- D PbCl42-(aq)
Important guidelines for keeping a notebook: Before your first day of Qual Lab do steps 1, 2, and 3!
1. Number at least 30 pages in the upper right-hand corner.
2. Include a Title Page with course name and section, your name, phone number and email address (for returning lost notebooks), and your instructor’s name.
3. Include a Table of Contents with the following topics; add page numbers as you do each part.
Group 1 and 2 known Group 1 and 2 unknown
Group 3 and 4 known Group 3 and 4 unknown
General cation unknown
Anion known Anion unknown 1
Anion unknown 2 Salt unknown
4. Refer to all solutions and precipitates by number and letter, i.e., solution 2A or precipitate 1A.
5. Save lots of room for the reactions – there may be 7 or 8 of them when only 1 reagent is added. Use a ruler to draw a horizontal line after each step and its reactions!
6. Always stir and centrifuge before recording observations about precipitates. Colorless and clear do not mean the same thing. (A blue solution may be clear—i.e., transparent.)
7. Do not recopy anything in your notebook. Get in the habit of recording notes neatly. The notebook must be an original contemporaneous record. Copying observations into the notebook from scratch paper is definitely forbidden--poor science. On the other hand, summarizing results after completing a known and unknown series is encouraged.
8. Your notebook may be collected and graded at any time (unannounced) during the semester.
Colors In The Qualitative Analysis Scheme For Cations:
It is useful to memorize the colors used for identifying ions and precipitates.
When two or more colors are present, the darker one will mask the lighter one.
Simple ions: Ag+, Al3+, Mg2+, Ba2+, Zn2+, & Pb2+ colorless
Fe3+ yellow Cr3+ dark blue
Cu2+ pale blue Ni2+ pale green
Complex ions: Ag(NH3)2+, Zn(NH3)42+, Zn(OH)42+, & Al(OH)4- colorless
Cu(NH3)42+ deep blue CrO42- yellow
I3- golden FeSCN2+ blood red
Ni(NH3)62+ light violet
Precipitates: AgCl, PbCl2, Al(OH) 3, Mg(OH)2, BaSO4 White
Pb(OH)2 tannish-white, very pale tan
Mg(NH4)PO4 white, crystalline
CuI tan or white Fe(OH) 3 brown
Cr(OH) 3 blue-gray Ni(OH)2 pale-green
PbCrO4 bright yellow Al(OH) 3. aluminon[1] cherry-red lake
BaCrO4 pale yellow Mg(OH)2. magneson[2] blue lake
Ni(DMG)2[3] pinkish red
Laboratory Techniques:
The objective of qualitative analysis is the identification of the ions present in a sample. The usual procedure is to run a carefully selected series of reactions on a solution containing all the ions of a particular group. This solution is called a “known” solution or “control.” The behavior of an unknown, which contains one or more of the ions of the group, is then compared with that of the known. The occurrence of certain reactions and the absence of others allows us to determine which ions are present in the unknown. Such identification of ions requires careful laboratory work.
The procedures that we will use in the laboratory are on a semimicro scale. Semimicro procedures involve considerably smaller volumes of solutions than are normally employed in ordinary laboratory work; solution volumes are normally on the order of 1 ml, and solution concentrations are on the order of 0.1 M. Semimicro methods have the advantages of requiring only small amounts of chemicals and of permitting rapid separation procedures. When working on the semimicro level, most reactions can be performed in small test tubes that are about 3-inches high (10 x 75 mm) and hold 3 milliliters. Solutions are separated from precipitates by centrifuging the solid to the bottom of the tube and then carefully removing the liquid with a dropper. Tedious filtrations are avoided.
Standard operating procedures for semi-micro qualitative analysis are discussed below. These procedures and techniques are generally used throughout the qualitative analysis scheme and will not be described in detail when they occur.
1. Cleanliness: If a known or unknown sample comes in contact with any surface that has not been thoroughly cleaned, it will be contaminated and the analytical results will be unreliable. It is important that all glassware be kept clean (not necessarily dry) during qual lab. Bring in a clean dish towel to cover your work area. Label two beakers,“clean” and “dirty”, and use these for stirring rods, droppers, test tubes, etc. Always rinse glassware twice with deionized water and use only deionized water whenever instructions call for water. When a sample is contaminated it should be discarded.
2. Labeling: Label all solutions and precipitates immediately with the designations used in the lab procedure; i.e., 2A, 3B, etc. Use these same labels in the lab notebook.
3. Dispensing Solutions: Volumes of solutions can usually be estimated. You should have a plastic dropper with a 1 mL mark etched into it for your convenience. It is also helpful to determine how many drops from your glass droppers are equal to 1 mL (usually around 20 drops). You should also be aware of the level of a milliliter of liquid in your 10 x 75 mm test tubes so that you can readily estimate that volume.
When adding reagents from dropper bottles (either your own or those in the hoods) be careful not to contaminate the dropper: do not put the tip into your solution or touch the inside of the test tube with the dropper tip. And always place the dropper back into the reagent bottle immediately. Never lay the dropper on a contaminated bench top.
If a reagent bottle does not have a dropper, pour a small amount into a clean test tube and use one of your own droppers to dispense it from the test tube. Never place your own dropper into a reagent bottle that is used by others.
4. Testing Acidity/Basicity: Usually when instructions call for the addition of acid or base to a solution it is important to add enough to change the solution from acidic to basic or vice versa. A litmus test is used to detect this change. Blue litmus changes to red in acid, and red litmus changes to blue in base. When adding acid or base to your sample, stir with a clean stirring rod. Do not dip litmus paper into your solution in the test tube since this may contaminate your sample. Use the stirring rod to transfer a drop of the solution to a small piece of litmus paper and note when the color change occurs.
5. Heating Solutions: Keep a small beaker of water on a hot plate with a boiling stone in the beaker to prevent bumping. To heat a sample, place your test tube in the beaker of hot water.
6. Precipitation of Solids: To accomplish a precipitation, add the indicated amount of precipitating agent to the solution and stir well with a clean stirring rod. Heat the solution in a water bath if so directed. Some precipitates form slowly and must be given time to form completely. When precipitation is believed to be complete, centrifuge the sample. Generally, before the liquid above the solid is removed, it is tested for complete precipitation. This test is accomplished by adding another drop of the precipitating reagent. If the precipitation is not complete, additional precipitate will form in the liquid. In such a case, add a few more drops of precipitating reagent, stir thoroughly, centrifuge, and then retest for complete precipitation.
Testing a solution for completeness of precipitation
7. Centrifugation: This is the principle method used in qualitative analysis to separate a solid from a liquid. A centrifuge spins the sample at high speeds which forces the precipitate to the bottom of the tube. It is very important to balance the centrifuge contents before turning it on. Tubes placed in the centrifuge must be symmetrically distributed. If necessary, use a tube with a similar volume of water to balance another tube. Most precipitates will settle when centrifuged for 30-60 seconds at medium speed.
8. Removing Supernatants: The clear solution above the precipitate after centrifugation is called the supernatant. Removal of the supernatant can be accomplished by decanting or by using a capillary pipet. Decantating involves merely pouring the liquid off into another test tube without disturbing the precipitate. However, if the precipitate is very light it may not be possible to decant successfully. In this case, the liquid may be transferred to a clean test tube using a long, thin dropper called a capillary pipet. It is usually not necessary to remove the liquid entirely.