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Robert Reeves Marlborough School 250 S. Rossmore Avenue Los Angeles, CA 90004
Simple Identification of Inorganic Salts Ronald I. Perkins Greenwich High School, 10 Hillside Road, Greenwich, CT 06830
In today’s world of sophisticated instrumentation, easy, inexpensive methods of analysis are often overlooked. If an unknown sample is known to contain one of a limited number of substances, a few simple tests sometimes can identify it. The alternative, sending the sample to an analytical laboratory, costs both time and money. A chemist, using knowledge of descriptive chemistry, often can solve the problem of analysis quickly and inexpensively. The Analysis
In this experiment students are asked to identify the contents of a number of vials. The substance in each vial is one of the two compounds listed on the label. To decide which compound is present, the student will need to make a few simple tests. Five of these methods of analysis are described below. 1. Appearance is often enough to differentiate between two compounds. A substance may be deliquescent and appear wet. It may have a characteristic crystalline shape or color. For example, hydrated transition metal tons often have distinctive colors, while group IA and IIA metal ions are colorless. The physical appearance of over 5000 commonly used inorganic compounds are listed in the “Physical Constants of Inorganic Compounds” section of a CRC Handbook of Chemistry and Physics.1 2. A flame test can be used to identify certain ions. To perform this test, prepare a concentrated solution (or paste, if the sample is insoluble) by mixing a few crystals of the unknown with a few drops of 6 M hydrochloric acid. Next, clean the end of a platinum or Nichrome wire by dipping it into 6 M hydrochloric acid and then thrusting it into the edge of a Bunsen burner flame. The following ions give a characteristic flame color:
barium calcium copper lithium potassium sodium strontium
yellow-green orange-red emerald-green carmine
violet yellow scarlet
3. Many salts react with water to change its pH, a process called “hydrolysis”. Consequently, one salt can sometimes be distinguished from another by placing a few grains of it on a piece of moistened litmus paper and noting the color of the paper near the
sample.
Anions of weak acids (acetate, carbonate, sulfite, etc.) react with water to form basic solutions. An example is provided in eq 1. F" + HOH
HF + OH"
(1)
On the other hand, the ammonium ion and most hydrated metal ions react to form acidic solutions (see eqs 2 and 3)
NH/ + Fe(H.,0)/+
HOH
+ HOH
-
NH3 + H30+
(2)
—
Fe(H20)50H2+ + H30+
(3)
1 CRC Handbook of Chemistry and Physics', CRC Press: Boca Raton, FL, 1969; pp B82-8176.
258
Journal of Chemical Education
Some ions do not react with water to form acidic or basic solutions. The following ions do not hydrolyze:
alkali metal ions alkaline earth metal ions other than Be2+ anions of strong acids such as Cl", Br~, I", NO3", CIO3”, and C104-.
Using these rules for hydrolysis, a solution of ammonium chloride, NH4C1, is expected to be acidic due to hydrolysis of the ammonium ion, NH4+. A solution of calcium acetate, CafCsHgCkk, is expected to be basic due to hydrolysis of the acetate ton, C2H3O2". The salt calcium chloride, CaCk, should not affect the color of litmus be-
neither the calcium ion nor the chloride ion hydrolyzes. For salts in which both ions hydrolyze, for example, NH4C2H3O2, it is not possible to predict the acidity or basicity of the solution without knowing the degree of hydrolysis of each ion. When predicting whether a given salt will hydrolyze, students may find it helpful to draw a line through ions in the formula that do not react with water. The remaining ion determines whether a solution of the salt will be acidic or basic. Consider, for example, NaaCOj. Since the carbonate ion hydrolyzes, a hydrogen ion from the water will combine with the carbonate ion, leaving an excess of OH". This results in a basic solution. 4. A few simple reactions may be useful in identifying unknown salts. (a) An acid reacts with a carbonate or bicarbonate to produce carbon dioxide gas. (b) An acid reacts with a sulfite or a bisulfite to produce sulfur dioxide gas. The sharp odor of sulfur dioxide gas is easy to recognize. It is the characteristic odor of a smoldering match. (c) A base, such as 6 M sodium hydroxide, reacts with the ammonium ion to produce ammonia gas, which can then be detected with moist litmus paper. To test for the ammonium ion, place a few crystals of the unknown in one of the depressions of a spot plate. Add a few drops of 6 M NaOH, and then hold a piece of moist red litmus paper over the depression, being careful that the paper does not contact the solution. If the ammonium ion is present in the sample, the ammonia gas generated will turn the litmus paper blue. (d) Some ions can be precipitated using a solution of chloride or hydroxide ion. Check the solubility data in Table 1. 5. The fact that one material is relatively insoluble in water and the other soluble may be enough to differentiate between two possible compounds. Solubility information is contained in the “Physical Constants of Inorganic Compounds” section of the CRC Handbook of Chemistry and Physics.1 It may, however, be enough to refer to the general solubility data provided in Table 1. cause
Preparation of Materials The samples should be chosen so that students are forced into using a variety of methods of identification. A list of suggested compounds can be found in Table 2. For each unknown, place about 5 g of one of the compounds into a vial and label with both names. A vial of each of these 35 unknowns is sufficient for a class of 25 students. Students should have access to 100-mL dropper bottles of 6 M HC1 and 6 M NaOH along with vials of red and blue
Table 1.
'
Ion
Solubility
no3-
soluble soluble soluble soluble soluble soluble insoluble insoluble insoluble
C2H302Na+ K+
cr S042" P043OH" s2~
Table 2.
2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13.
14. 15. 16. 17. 18. 19.
20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.
Water
Except in Combination with:
Ag+, Hg/+, Pb2+
Date_
Simple Identification of Inorganic Salts Materials: 6 M HCI, 6 M NaOH, red and blue litmus paper, platinum or Nichrome wire, Bunsen burner, spot plate, stirring rod, spatula, and vials of unknown samples. Procedure: For each of 20 unknown salts in a numerical sequence, use a simple test to determine which of the two compounds listed on the label is correct. Briefly explain your choice.
Ba2+, Sr2+, Hg2+, Pb2+, Ag+ ions of group IA and NH4+
Unknown
Explanation
Identity
ions of group IA, Ca2+, Ba2+, Sr2+ ions of group IA, IIA, and NH4+
Suggested List of Unknowns
calcium acetate or sodium chloride strontium nitrate or potassium carbonate ammonium sulfate or aluminum sulfate sodium phosphate or sodium bromide sodium chromate or sodium dichromate potassium permanganate or chromium(lll) nitrate copper(lf) carbonate or copper(ll) chloride ferric ammonium sulfate or ferric chloride lead(ll) acetate or lead(ll) chloride magnesium sulfate or sodium sulfite sodium carbonate or sodium acetate barium nitrate or potassium nitrate silver nitrate or sodium bromide iron(lll) chloride or iron(lll) oxide chromium(lll) chloride or copper(ll) nitrate potassium bromide or magnesium sulfate ammonium chloride or sodium acetate lead(ll) nitrate or sodium nitrate zinc sulfate or magnesium nitrate sodium hydrogen carbonate or barium nitrate lithium nitrate or strontium chloride potassium ferricyanide or potassium ferrocyanide barium sulfate or magnesium sulfate nickel(ll) chloride or copper(ll) chloride potassium chloride or sodium chloride silver chloride or lead(ll) nitrate zinc sulfate or magnesium chloride calcium sulfate or barium sulfate potassium iodide or potassium fluoride lithium chloride or sodium bromide sodium hydrogen sulfite or sodium acetate cobalt(ll) nitrate or copper(ll) sulfate magnesium phosphate or cesium phosphate aluminum nitrate or potassium nitrate calcium chloride or magnesium sulfate
litmus paper (one set/six students). Each student needs either a Nichrome or platinum wire and a spot plate. In addition, several copies of the CRC Handbook of Chemistry and Physics1 should be available. A sample student record sheet is provided in the figure.
Prelab Information
Prior to the actual laboratory work, it is advisable to explain and demonstrate each of the “simple methods of analysis”. Show the students how to use the “Physical Constants of Inorganic Compounds” section of the CRC Handbook of Chemistry and Physics.1 2. Demonstrate techniques for getting a good flame test. Suggest that didymium glasses used by glassblowers are 1.
Name_
Possibilities
Unknown 1.
In
Solubility Data for Ions
Sample student reference sheet.
better than cobalt glass for eliminating the flame color of sodium ions when detecting the flame color of potassium ions.2 The strontium flame color tends to be bright red and to disappear quickly; the lithium flame is dull red and lingers.
Test a few salt solutions with litmus to explain the concepts of hydrolysis. Demonstrate that many tests can be performed using the same piece of litmus paper. This concept of hydrolysis is often difficult for students. 4. Demonstrate some of the simple ion tests. Emphasize that the test for the ammonium ion requires that the moist litmus paper be suspended over the basic solution. In this test many students mistakenly test the solution rather than the vapors. 5. Explain that a solubility comparison requires that equal, small amounts of salt be added to equal amounts of 3.
water.
Finally, caution students about keeping all reagents and test samples from contamination. Students should remove only a few crystals from the unknown containers at a time and should not return excess to the container. Acknowledgment This experiment was developed at the Institute for Chemical Education, University of Wisconsin—Madison. Richard McQueen of Clayton Valley High School, Concord, CA, and Richard Piotrzkowski of Greenwich High School, Greenwich, CT, field tested this experiment and as a result offered many suggestions. 2
Wood, W. F. J. Chem. Educ. 1984, 61, 1024.
Volume 64
Numbers
March 1987
259
