Cations - Qualitative Analysis

Cation Identification Tests

• Generally Soluble Cations

§  Ammonium Ions: Take a small amount of the material to be tested and place it in a 50-mL beaker. Add 6 M NaOH and smell cautiously. The odor of ammonia indicates the presence of ammonium ions. If you do not smell ammonia, warm the beaker and again smell the emitted vapors. The liberated ammonia will also change the color of a moistened strip of red litmus paper held at the entrance of the test tube.

NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O

This test is very reliable. It should be performed whenever the generally soluble cations, NH₄⁺, Na⁺, and K⁺, are suspected.

§  Sodium Ions: The most common method of identification of Na⁺ is the flame test. Sodium imparts a brilliant, long lasting, yellow flame that masks colors from other ions. The test may be performed on a small sample of the unknown treated with concentrated HCl or a few drops of solution unknown treated with concentrated HCl. The flame should be bright and it should last as long as that of 0.1 M NaCl. Sodium is a common impurity and traces will be found in almost any unknown. You must learn to distinguish between an unknown that has sodium ion as the cation and an unknown that has sodium ion as an impurity.

§  Potassium Ions: The most common method of identification of K⁺ is the flame test. The test may be performed on a small sample of the unknown treated with concentrated HCl or a few drops of solution unknown treated with concentrated HCl. The violet flame is not intense but it is clearly visible in the absence of sodium ions. Cobalt glass filters yellow light from sodium impurities and allows the violet flame to be seen. Do not confuse the glowing wire for the potassium flame.

2.      Cations That Form Insoluble Chlorides

§  Silver Ions: Although Ag⁺, Pb²⁺, and Hg₂²⁺ all form insoluble white chlorides, Ag⁺ is the only one of these cations that forms an ammonia complex. Therefore, AgCl dissolves readily in aqueous NH₃. When the resulting solution is acidified with HNO₃, AgCl reprecipitates.

AgCl(s) + 2NH₃(aq) → Ag(NH₃)²⁺(aq) + Cl^-(aq)
Ag(NH₃)²⁺(aq) + Cl^-(aq) + 2H₃O⁺ → AgCl(s) + 2NH₄⁺ + 2H₂O

Add 3 M HCl dropwise to the solution being tested. If a white precipitate is formed, centrifuge and remove the supernatant liquid. Add 6 M NH₃ solution to the precipitate. If the precipitate dissolves, add 6 M HNO₃. Formation of a white precipitate indicates Ag⁺.

§  Lead Ions: Although PbCl₂ is insoluble at room temperature, its solubility is increased dramatically at higher temperatures; it dissolves readily in boiling water. Pb²⁺₃COO)₂. The addition of chromate ion to this lead acetate solution yields a precipitate of yellow lead chromate. also forms an insoluble white sulfate, which dissolves in a solution containing acetate ion due to the formation of the weak electrolyte, Pb(CH

Pb²⁺(aq) + SO₄^2-(aq) → PbSO₄(s)
PbSO₄(s) + 2CH₃COO^-(aq) → Pb(CH₃COO)₂(aq) + SO₄^2-(aq)
Pb(CH₃COO)₂(aq) + CrO₄^2-(aq) → PbCrO₄(s) + 2CH₃COO^-(aq)

To the solution to be tested add 3 M HCl dropwise. (A large excess of HCl must be avoided because of the formation of the soluble chloro complex, PbCl₄^2-.) Centrifuge and remove the supernatant from the white precipitate (PbCl₂). Add hot water to the precipitate and stir. If the precipitate dissolves, Pb²⁺ is indicated. Add 3 M H₂SO₄ to the hot solution. Centrifuge and remove the supernatant liquid from the white precipitate (PbSO₄). To the precipitate add 3 M NH₄(CH₃COO) and stir. If the white precipitate was PbSO₄, it will dissolve. To confirm, add a few drops of 0.5 M K₂CrO₄ to the resulting solution. A yellow precipitate of PbCrO₄ indicates the presence of Pb²⁺.

§  Mercury(I) Ions: When Hg₂Cl₂ is treated with aqueous NH₃ a reaction occurs in which free mercury and amidochloromercury(II) are formed.

Hg₂Cl₂(s) + NH₃(aq) → Hg(l) + HgNH₂Cl(s) + HCl(aq)

The HgNH₂Cl is a white solid, while the Hg in a finely divided state appears black. The resultant mixture is gray to black.
Add 3 M HCl to the solution to be tested for Hg₂²⁺. If a white precipitate forms, centrifuge and remove the supernatant liquid. To the precipitate, add 6 M NH₃ and stir. The appearance of a gray to black precipitate is positive for Hg₂²⁺.

3.      Cations That Form Insoluble Sulfates Identification tests for Pb²⁺ and Ag⁺ (Ag₂SO₄ is moderately soluble) are described above (Cations that form Insoluble Chlorides). Ba²⁺, Sr²⁺, and Ca²⁺ form moderately soluble sulfates.
The alkaline earth ions Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ are one of the best examples of a periodic relationship among the elements of a family. Solubilities of their compounds are graduated nicely and the separations (qualitatively) can be accomplished readily. Flame tests are very important.

§  Barium Ions: Barium ions can be identified by precipitation of its insoluble yellow BaCrO₄²⁺ or Sr²⁺ are present they will also precipitate in the presence of high concentrations of CrO₄^2-. However, the chromates of Ca²⁺ and Sr²⁺ are moderately soluble; their precipitation can be prevented by addition of acetic acid. This weak acid provides sufficient hydronium ions to lower the CrO₄^2-₄ and SrCrO₄ in solution but to allow the BaCrO₄ to precipitate. salt. If Ca concentratiion enough to keep CaCrO

2CrO₄^2-(aq) + 2H⁺(aq) → Cr₂O₇^2-(aq) + H₂O

The flame test on the solid chromate is important for confirmation.
To about 1 mL of solution add 10 drops of 6 M CH₃COOH. Then add a few drops of 0.5 M K₂CrO₄ solution. The appearance of a yellow precipitate indicates the presence of Ba²⁺. To confirm, dissolve the precipitate in concentrated HCl and flame test.

§  Strontium Ions: Strontium can be identified, in the absence of calcium, by precipitating its sulfate. To the solution add 0.1 M H₂SO₄ dropwise. The formation of a finely-divided, crystalline, white precipitate indicates the presence of Sr²⁺. (Ba²⁺ must be absent, of course.) To confirm, dissolve the precipitate in concentrated HCl and flame test.

§  Calcium Ions: If Ba²⁺ and Sr²⁺ are absent, Ca²⁺ may be precipitated as the oxalate from neutral or alkaline solutions. Test the acidity of the solution with litmus paper. If it is acidic, add 3 M NH₃ until basic. Then add 0.2 M (NH₄)₂C₂O₄²⁺. Confirm by adding a few drops of concentrated HCl and flame testing. solution. The formation of a white precipitate indicates the presence of Ca

4.      Cations That Form Ammonia Complexes

§  Cadmium Ions: Cadmium forms a yellow precipitate with sulfide ion either from a neutral solution containing free Cd²⁺ or from an ammoniacal solution of Cd(NH₃)₄²⁺. Since most sulfides are insoluble, and many of them are black, the presence of other metal ions may make it difficult to detect the yellow color of CdS. Therefore, separations must be as complete as possible before testing for Cd²⁺.

Cd²⁺(aq) + S^2-(aq) → CdS(s)
Cd(NH₃)₄²⁺(aq) + S^2-(aq) → CdS(s) + 4NH₃

To a solution of Cd²⁺ or to a solution thought to contain Cd(NH₃)₄²⁺ add 0.1 M Na₂S solution dropwise. The formation of a yellow precipitate confirms the presence of Cd²⁺.

§  Copper(II) Ions: The very distinct deep blue color of the copper ammonia complex can be used to identify Cu²⁺. This identification can be carried out in the presence of other cations which form either colorless ammonia complexes or white precipitates. Thus, Zn²⁺, Cd²⁺, Al³⁺, among others, will not interfere.
In relatively dilute solutions the color of the ammonia complex may not be intense enough to give an unqualified identification, and some other test for confirmation must be used. Cu²⁺ forms a very insoluble reddish-brown hexacyanoferrate(II).

2Cu²⁺(aq) + Fe(CN)₆^4-(aq) → Cu₂Fe(CN)₆(s)

Other cations that react with this reagent to form highly colored precipitates must be absent (Co²⁺ and Fe³⁺ for example). Acidify the test solution with acetic acid. Then add a few drops of 0.1 M potassium hexacyanoferrate(II) solution (K₄Fe(CN)₆). A red-brown precipitate confirms the presence of Cu²⁺.

§  Nickel(II) Ions: Nickel(II) is one of the easiest cations to identify. Ni²⁺ forms a red precipitate with dimethylglyoxime in a buffered acid solution. Palladium(II) is the only other cation which forms a precipitate with this reagent. However, a few other cations can interfere. Cobalt(II) preferentially forms a dark brown solution with dimethylglyoxime, and excess reagent must be used in its presence.
Acidify the solution to be tested with 6 M CH₃COOH. Then add about one mL of 0.2 M NaOOCCH₃ solution. Add dimethylglyoxime solution dropwise. A bright red precipitate is positive for Ni²⁺.

§  Zinc Ions: Zinc forms one of the few insoluble white sulfides. It is precipitated from a solution of the ammonia complex. Small traces of cations that form dark colored sulfides will obviously interfere.
Add an excess of 3 M NH₃ to the test solution, so that any zinc present is in the form of Zn(NH₃)₄²⁺. Then add a few drops of 0.1 M Na₂S solution. A white precipitate indicates the presence of Zn²⁺.

5.      Cations That Form Amphoteric Hydroxides

§  Aluminum Ions: Aluminum is generally identified by making use of the amphoteric property of its hydroxide and the red color of the "lake" AlOH₃ forms with the reagent, aluminon. Aluminon is a dye (an organic molecule, usually fairly large, that absorbs visible light). As the Al(OH)₃ precipitates the dye is adsorbed on the Al(OH)₃ particles. The adsorption of the dye is called "laking." Aluminum is a fairly common impurity and care must be taken that trace quantities are not reported. Since most laboratory manipulations are carried out in glass containers, silica gel, which physically resembles aluminum hydroxide, is also a common impurity.
Adjust the pH of about 1 mL of the test solution (with 3 M NaOH and 3 M HNO₃) to precipitate the hydroxide. Centrifuge the mixture. Remove the mother liquor with a capillary pipet and wash the precipitate with distilled water. Centrifuge the mixture. Remove the mother liquor with a capillary pipet and wash the precipitate with distilled water. Centrifuge the mixture and remove the mother liquor with a capillary pipet. These repeated washings remove other ions from the precipitate. Dissolve the precipitate in 3 M HNO₃. If any precipitate does not dissolve in the nitric acid, remove the supernatant to a clean test tube and discard the residue. Add two drops of aluminon reagent (avoid any excess). Add 3 M NH₃(aq) until the solution is basic. Centrifuge. A red, gelatinous precipitate (sometimes called a red lake) indicates Al³⁺.
Any precipitate that remains after the addition of the nitric acid is probably silica gel, SiO₂•xH₂O. Silica gel is present in many solutions; it is leached from glass containers. Any silica gel present must be removed before the addition of the aluminon and the ammonia because silica gel will also give a red lake.
Do not confuse traces of red-brown ferric hydroxide for the red lake. Other precipitates will also form colors with the reagent. The supernatant liquid will be an intense blue-purple color if too much reagent has been added. This color has nothing to do with the presence of aluminum. The color of the reagent is sensitive to changes in pH, (the reagent is an acid-base indicator).

§  Chromium(III) Ions: Chromium can be taken through a series of colored tests which leaves no doubt as to its identity. Chromium(III) forms a steel green hydroxide which dissolves in excess strong base to give a deeply green colored solution of the hydroxy complex. Treating this complex with 3% hydrogen peroxide gives the yellow solution of the chromate ion, which upon acidification with dilute nitric acid gives the orange color of dichromate. Treatment of the cold solution of dichromate with 3% hydrogen peroxide gives the intense blue color of a peroxide of chromium. (The actual composition of this peroxide is not known, but it is believed to have the empirical formula CrO₅.) This peroxide readily decomposes to the pale violet color of the original hydrated chromium(III) ion. In low concentrations of dichromate the blue color is fleeting, and attention must be focused on the test tube during the addition of the hydrogen peroxide to avoid missing the color change.

Cr(OH)₄^- (green) --(H₂O₂)--(OH^-)→ CrO₄^2- (yellow)
CrO₄^2- --(H⁺)→ Cr₂O₇^2- (orange)
Cr₂O₇^2- --(H₂O₂)--(HNO₃)→ CrO₅ (blue)→ Cr(H₂O)₆³⁺ (violet)

The following color changes are all indicative of Cr³⁺. Add an excess of 6 M NaOH to about one mL of test solution. To this green solution add 10 drops of 3% H₂O₂. Heat the test tube in the water bath until the excess H₂O₂ is destroyed as indicated by the cessation of bubbles. Acidify the yellow solution with 3 M HNO₃. Cool the resulting orange solution in an ice bath. To the cooled solution add a drop or two of 3% H₂O₂ and observe the immediate fleeting blue color.

§  Tin(IV) Ions: Sn⁴⁺ is most conveniently identified by reduction of Sn⁴⁺ to Sn²⁺ with iron. The Sn²⁺ solution is treated with HgCl₂ solution, whereupon Sn²⁺ is oxidized to Sn⁴⁺ and, simultaneously, HgCl₂ is reduced to Hg₂Cl₂ (a silky, white precipitate). The Hg₂Cl₂ is further reduced by Sn²⁺ to Hg, which appears black.

Sn⁴⁺(aq) + Fe(s) → Sn²⁺(aq) + Fe²⁺(aq)
Sn²⁺(aq) + 2HgCl₂(aq) → Sn⁴⁺(aq) + Hg₂Cl₂(s) + 2Cl^-(aq)
Sn²⁺(aq) + Hg₂Cl₂(s) → Sn⁴⁺(aq) + 2Hg(l) + 2Cl^-(aq)

Add some concentrated HCl to the solution to be tested for Sn⁴⁺. Place an iron brad (or small iron wire) in this solution and heat in a water bath for 5 minutes. Take the clear solution (filter if necessary) and add HgCl₂ solution dropwise. The appearance of a silky, white precipitate, which then turns black, confirms the presence of tin.

6.      Other Cations

§  Manganese(II) Ions: Manganese is easily identified by oxidation of Mn²⁺ to purple MnO₄^- using sodium bismuthate (NaBiO₃). Heat must be avoided to prevent the decomposition of permanganate ion to brown, insoluble manganese dioxide. Chloride ion must be absent, because it reduces permanganate ion to either manganese dioxide or manganese(II) depending upon the conditions.
Acidify the test solution with 3 M HNO₃. Add solid NaBiO₃ and stir. Centrifuge. If the supernatant has the characteristic purple color of MnO₄^-, Mn²⁺ was present.

2Mn²⁺ + 14H⁺ + 5NaBiO₃ → 5Bi³⁺ + 5Na⁺ + 7H₂O + 2MnO₄

§  Bismuth(III) Ions: Bismuth(III) forms a highly insoluble hydroxide which upon treatment with the hydroxy complex of tin(II) is immediately converted to free bismuth, a black precipitate.

3Sn(OH)₄^2-(aq) + 2Bi(OH)₃(s) → 2Bi(s) + 3Sn(OH)₆^2-(aq)

Precipitate Bi³⁺ from the test solution with 3 M NaOH and centrifuge the precipitate. Then, to a solution of tin(II) chloride add with stirring 6 M sodium hydroxide until the precipitate of tin(II) hydroxide which first forms just redissolves. This solution is then added dropwise to the precipitate of bismuth(III) hydroxide. The rapid formation of a black color confirms bismuth.

§  Iron(III) Ions: The Fe³⁺ ion is readily identified in a dilute nitric acid solution through the blood red color of its thiocyanate complex. A large excess of reagent should be avoided.

Fe³⁺(aq) + SCN^-(aq) → Fe(SCN)²⁺(aq)

Acidify the solution with 3 M HNO₃. Then add a few drops of 0.1 M NH₄SCN solution. The solution turns red if Fe³⁺ is present.

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