Experiment 10 – Combined Qualitative Analysis of Cations (Part 1)
Qualitative Analysis of Group I Cations (Ag+, Pb2+, Hg22+)
small test tubes centrifuge
hot water bath
1 M K2CrO4
Standard solution containing Ag+, Pb2+, Hg22+ ions Solution of unknown Group I ion(s)
The Group I Cations are the metal ions that are insoluble in the presence of halides (Cl-, Br-, I-). Because the chlorides of Pb2+, Ag+, and Hg22+ are insoluble, they may be precipitated and separated from the cations of groups II, III, and IV by the addition of HCl. The following equations represent the reactions that occur:
Pb2+(aq) + 2Cl- (aq) —> PbCl2 (s) white Ag+(aq) + Cl- (aq) —> AgCl (s) white Hg22+(aq) + 2Cl-(aq) —> Hg2Cl2(s) white
A slight excess of HCl is used to ensure complete precipitation of the cations and to reduce the solubility of the chlorides by the common-ion effect. However, a large excess of chloride must be avoided, because both AgCl and PbCl2 tend to dissolve by forming soluble complex anions:
PbCl2 (s) + 2Cl- (aq) —> PbCl4 2-(aq) AgCl (s) + Cl -(aq) —> AgC12-(aq)
PbCl2 is appreciably more soluble than either AgCl or Hg2Cl2. Thus, even when PbC12 precipitates, a significant amount of Pb2+ remains in solution and is subsequently precipitated with the group 2 cations as the sulfide PbS. Because of its solubility, Pb2+ sometimes does not precipitate as the chloride, because either its concentration is too small or the solution is too warm.
Lead Lead chloride is much more soluble in hot water than in cold. It is separated from the other two insoluble chlorides by dissolving it in hot water. The presence of Pb+ is confirmed by the formation of a yellow precipitate, PbCrO4, upon the addition of K2CrO4:
Pb2+ (aq) + CrO42- (aq) —> PbCrO4(s) yellow
Mercury(I) Silver chloride is separated from Hg2Cl2 by the addition of aqueous NH3. Silver chloride dissolves because Ag+ forms a soluble complex cation with NH3:
AgCl(s) + 2NH3(aq) —>Ag(NH3)2+(aq) + Cl-(aq)
Mercury(I) chloride reacts with aqueous ammonia in a disproportionation reaction to form a dark gray precipitate
Hg2Cl2(s) + 2NH3(aq) → HgNH2Cl (s) + Hg (l) + NH4+(aq) + Cl-(aq)
Although HgNH2Cl is white, the precipitate appears dark gray because of a colloidal
dispersion of Hg(l).
Silver To verify the presence of Ag+, the supernatant liquid from the last reaction is acidified and AgCl re-precipitates if Ag+ is present. The acid decomposes Ag(NH3)2+ by neutralizing NH3 to form NH4+. It is necessary that the solution be acidic, or else the AgCl will not precipitate and Ag+ can be missed.
Ag(NH3)2+(aq) + 2H+(aq) + Cl-(aq) —> AgCl(s) + 2NH4+(aq)
Waste Disposal: Dispose of all wastes in the heavy metal containers. PROCEDURE:
You will perform 2 parallel analyses: one standard sample containing Ag+, Pb2+, and Hg22+ ions and one unknown sample containing a combination of these ions to identify. Prepare a test tube with the standard solution containing all three cations of group I, and another with an unknown solution. Do all of the reactions in parallel, following the same procedures for the unknown as with the known. Record in your notebook the reagents used in each step, your observations, and the equation for each precipitation reaction.
Precipitation of Group 1 Cations
Measure out 10 drops (0.5 mL) of the test solution into a small (10 mm x 75 mm) test tube. Add four drops of 6 M HC1, stir thoroughly, and then centrifuge.
When using the Centrifuge: Fill another test tube of the same size with the same volume of water, and place both in the centrifuge across from each other to counterbalance their weight. Close the cover and centrifuge for about 30 sec. Counterbalancing sample weights is very important. If this is not done properly, the test tubes inside could break and the centrifuge will start shaking violently. Wait for the centrifuge rotor to come to a complete stop before opening the lid and removing the test tubes.
Test for completeness of precipitation by adding one drop of 6 M HCl to the clear supernate. If the supernate turns cloudy, this shows that not all of the group 1 cations have precipitated; add another two drops of 6 M HCl, stir, and centrifuge. Repeat this process until no more precipitate forms. All of the Group I cations must be precipitated or else they will slip through and interfere with subsequent group analyses. Decant (pour off) the supernate (liquid above the precipitate) and dispose of it in the designated waste receptacle. Wash the precipitate by adding five drops of cold distilled water and stirring. Centrifuge and discard the liquid.
Separation and Identification of Pb2+
Add 15 drops of distilled water to the precipitate and place the test tube in a warm- water bath. Stir using a stirring rod and heat for 1-2 mins. Quickly centrifuge and decant the hot supernate into a clean test tube. Repeat this procedure two more times, combining the supernates, which should contain Pb2+ if it is present. Save the precipitate for the following separation of Ag+ and Hg22+ procedure. Add three drops of 1 M K2CrO4 to the supernate. The formation of a yellow precipitate, PbCrO4, confirms the presence of Pb2+ .
Separation and Identification of Ag+ and Hg22+
Add 10 drops of 6 M NH3 to the precipitate from the previous procedure.
The formation of a dark gray precipitate after 5 seconds centrifugation
indicates the presence of mercury. The precipitate is very fine and can
appear as a thin border lace on the periphery of the white precipitate.
Sometimes it is visible if you look directly from the neck against the bottom of the test tube, as shown in the picture.
Centrifuge and decant the clear supernate into a clean test tube.
To confirm the presence of Ag+ add several drops of 6 M HNO3 to the decantate. Swirl the solution and test its acidity. Continue to add HNO3 dropwise until the solution is acidic. A white precipitate confirms the presence of Ag+.
In your conclusion, report the ions present in your unknown and which reactions (equations) were used to verify their presence.
Experiment 10 – Combined Qualitative Analysis of Cations (Part 2)
Qualitative Analysis of Group IV and Group V Cations (Ba2+, Ca2+, NH4+, Na+)
test tubes centrifuge Nichrome wire Bunsen Burner
Red Litmus Paper
1 M K2CrO4
6 M Acetic acid, HC2H3O2 6 M H2SO4
1 M K2C2O4 (potassium oxalate)
6 M NaOH
0.2 M NaCl
Standard solutions of Ba2+, Ca2+, NH4+, Na+ ions Unknown Solution of ions
You will test two samples simultaneously: A standard solution containing Ba2+, Ca2+, NH4+, and
Na+ ions and an unknown sample which contains a combination of these ions. The unknown sample may contain all, none or any mixture of the four ions above.
Chemistry of Group IV and V Cations: Ba2+, Ca2+, NH4+, Na+
Group IV cations form insoluble phosphates while Group V is made up of the soluble cations that cannot be precipitated from solution. The cations we will consider in these groups are the Group IV ions Ba2+ and Ca2+ and the Group V ions NH4+ and Na+. Because their chlorides and sulfides are soluble, these ions do not precipitate with groups I, II, or III. Sodium ions are a common impurity and were even introduced (as was ammonium ion) in some of the reagents that are used in the analysis of groups I, II, and III. Hence, in the analysis of a general unknown mixture, tests for these ions must be made on the original sample before performing the group analysis.
Because barium chromate, BaCrO4 (Ksp = 1.2 x 10-10), is less soluble than calcium chromate, CaCrO4 (Ksp = 7.1 x 10-4), Ba2+ can be separated from Ca2+ by precipitation as the insoluble yellow chromate salt:
Ba2+(aq) + CrO42-(aq) → BaCrO4(s) yellow
BaCrO4 is insoluble in the weak acid HC2H3O2, but it is soluble in the presence of the strong acid HCl. After BaCrO4 is dissolved in HCl, a flame test is performed on the resulting solution. A green-yellow flame is indicative of Ba2+. Further confirmation of Ba2+ can be achieved by precipitation of BaSO4, which is white.
Waste Disposal: Dispose of all waste in Inorganic Waste Record all observations in your lab notebook.
Separation and Identification of Ba2+
Place 7-10 drops of the solution in a small test tube. Add eight drops of 6 M acetic acid, HC2H3O2, and one drop of 1M K2CrO4 and mix. The formation of a yellow precipitate indicates the presence of Ba2+.
Centrifuge and save the decantate for the subsequent procedure to test for calcium.
Dissolve the precipitate with 6 M HC1 and perform a flame test as described below. To perform the flame test, obtain a piece of platinum or Nichrome wire. Clean the wire by dipping it in 12 M HC1 that is contained in a small test tube, and heat the wire in the hottest part of a Bunsen burner flame. Repeat this operation until no color other than orange is seen when the wire is placed in the flame, several cleanings may be required before this is achieved. Then dip the wire into the solution to be tested and place the wire in the flame. A pale green flame confirms the presence of Ba2+. If the concentration of Ba2+ is very low, you may not detect the green color.
As further confirmation of barium, add 10 drops of 6 M H2SO4 to the solution on which the flame test was performed. A white precipitate confirms the presence of Ba+2
Test for Ca2+
Make the decantate from the barium test basic to litmus with 15 M NH3. If a precipitate forms, centrifuge and discard the precipitate. Add seven drops of 1 M K2C2O4 (potassium oxalate) and swirl. The formation of a white precipitate indicates the presence of calcium ion. Should no precipitate form immediately, warm the test tube briefly in the warm-water bath and then cool.
Additional evidence for Ca2+ is obtained from a flame test. Centrifuge the precipitate, dissolve it in 6 M HCl and then perform a flame test. A transitory red-orange color that appears when the wire is first placed in the flame and later reappears somewhat more red as the wire is heated is characteristic of the calcium ion. If the concentration of Ca2+ is very low, you may not observe the red color.
Test for Na+
The flame test for sodium is very sensitive, and traces of sodium ion will impart a characteristic yellow color to the flame. Just about every solution has a trace of sodium and thus will give a positive test. On the basis of the intensity and duration of the
yellow color, you can decide whether Na is merely a contaminant or present in substantial quantity. Using a clean wire, perform a flame test on your original (untreated) unknown. To help you make a decision as to the presence of sodium, run a flame test on distilled water and then on a 0.2 M NaCl solution. Compare the tests.
Test for NH4+
Place 2 mL of each of the original untreated solutions (standard and unknown) in separate l00-mL beakers. Moisten a piece of red litmus paper with water and stick it to the convex side of a small watch glass. Add 2 mL of 6 M NaOH to each beaker and cover the beaker with the watch glass, convex side down. (The litmus paper must not come into contact with any NaOH.) Allow the covered beaker to stand for 1-2 minutes. A change in the color of the litmus paper from red to blue confirms the presence of ammonium ion.
Your conclusion should include the identity of the ions present in your unknown and all the observed results that lead you to your conclusion.
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