Mar oorougn Scnaal 250 S Rossrnore Avnnle

Los Ange ar. CA 90004

Simple Identification of'lnorganic Salts Ronald I. Perkins Greenwich High School, 10 Hillside Road, Greenwich, CT 06830

I n today's world of sophisticated instrumentation, easy, inexpensive methods of analysis are often overlooked. If an unknown sample is known t o contain one of a limited num- ber of substances, a few simple tests sometimes can identify it. The alternative, sending the sample t o an analytical lab- oratory, costs both time and money. A chemist, using knowl- edge of descriptive chemistry, often can solve the problem of analysis quickly and inexpensively.

The Analysis

In this experiment students are asked t o identify the con- tents of a number of vials. T h e substance in each vial is one of the two compounds listed on the label. T o decide which compound is present, the student will need t o make a few simple tests. Five of these methods of analysis are described below.

1. Appearance is often enough to differentiate between two com- pounds. A substance may be deliquescent and appear wet. It may have a characteristic crystalline shape or color. For example, hydrat- ed transition metal ions often have distinctive colors, whilegroup IA and IIA metal ions are colorless. The physical appearance of over 5000 commonly used inorganic compounds are listed in the "Physi- cal Constants of Inorganic Compounds"section of a CRC Handbook of Chemistry and Physics.'

2. A flame test can he 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 afew drops of 6 M hydrochloric aeid. Next, dean 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 yellow-green calcium orange-red copper emerald-green lithium carmine potassium violet sodium yellow strontium scarlet

3. Many salts react with water to change its pH, a process called "hydrolysis". Consequently, one salt can sometimes he distin- guished 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 uther hand, thp ammmium ion and most hydrated mrtal ions wart t o form acidic rdutions (see rqs 2 and 3,

NH,' + HOH - NH, + H80+ (2)

Fe(H,O)? + HOH - Fe(H20),0H2+ + H,Ot (3)

' CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton. FL. 1969; pp B82-8176.

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-, NOs-, C103-, and

Clod-.

Using these rules for hydrolysis, asolution of ammonium chloride, NHICI, is expected to be acidic due to hydrolysis of the ammonium ion, NH4+. A solution of calcium acetate, C B ( C ~ H ~ O ~ ) ~ , is expected to he basic due to hydrolysis of the acetate ion, C2H302-. The salt calcium chloride, CaC12, should not affect the color of litmus he- cause neither the calcium ion nor the chloride ion hvdrolvzes. For ,~~ ~, ~~

salts in which both i ~ m s hydmlgm, fop rx~mplr , NHIC2H ,0., i t i9

nut puasiblc t o predict rheardiry or hnsicityof thesvlution without k w w i n g the degree of hydndysis grf each )on.

\VItrn pred;cting wheth~r R gwen snlt will hgdrolgze, studenu may find i t helpful todrsualine thrmahions in the furmula that do not react with water. The remaining ion determines whether a solu- tion of the salt will he acidic or hasie.

Consider, for example, Na2C03. Since the carbonate ion hydro- lyzes, 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 he useful in identifying unknown salts.

(a) An aeid reacts with a carbonate or bicarbonate to produce carbon dioxide gas.

(h) 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.

(dl Some ions can he precipitated using a solution of chloride or hydroxide ion. Check the solubility data in Tahle 1.

5. The fact that one material is relatively insoluble in water and the other soluble may he enough to differentiate between two possi- ble compounds. Solubility information is contained in the "Physical Constants of Inorganic Compounds" section of the CRC Handbook of Chemistry and Physics.' It may, however, he enough to refer to the general solubility data provided in Tahle 1.

Preparation ot Materials

The samples should he chosen so t ha t students are forced into using a variety of methods of identification. A list of sueeested comoounds can be found in Table 2. For each u&-nown, place about 5 g of one of the compounds into a v i d and label with both names. A vial of each of these 35 un- knowns is sufficient for a class of 25 students.

Students should have access t o 100-mL dropper bottles of 6 M HCI and 6 M NaOH along with vials of red and hlue

258 Journal of Chemical Education

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