Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

AS Qualitative Analysis – Ions Recognition

This is a list of some qualitative laboratory tests that can be used to identify unknowns in lab. These are simple tests leading to the identification of a few anions and cations and some common gases. Also included here is flame test information for selected cations.

    • Testing for anions
    • Testing for cations
    • Flame test for cations
    • Tests for some common gases
  • Test for cations using aqueous sodium hydroxide

    To an aqueous solution of the sample, add aqueous sodium hydroxide a little at a time (until a change is observed) until it is present in excess (no further change is observed)
    Observe the colour of precipitate and the solubility of the precipiate in excess.
    If there is no precipitate, warm the mixture and test for ammonia gas.A diagram to illustrate the test for cations using sodium hydroxide is as follows:

    To an aqueous solution of the sample, add aqueous ammonia a little at a time (until a change is observed) until it is present in excess (no further change is observed) Observe the colour of precipitate and the solubility of the precipiate in excess.
    A diagram to illustrate the test for cations using sodium hydroxide is as follows:

    Test for ALUMINUM 3+ IONS

     TEST FOR CALCIUM +2 IONS

     TEST FOR IRON 3+ IONS

    TEST FOR IRON 2+ IONS

     TEST FOR LEAD 2+ IONS

    TEST FOR COPPER 2+ IONS

    TEST FOR AMMONIUM IONS<

Tests for Anions

Anion Symbol
Test
Results
Bromide
Br
Add silver nitrate solution to a solution of substance in dilute nitric acid Pale yellow precipitate, dissolves slightly in ammonia solution.
Carbonate
CO32-
a)Add dilute hydrochloric acid to the substance.

b)Add drop of phenolphthalein to a solution of substance.

Carbon dioxide gas is given off.

Turns bright pink (HCO3 turns light pink).

Chloride
Cl
Add silver nitrate to a solution of substance in dilute nitric acid. Thick white precipitate dissolves in diluted ammonia solution.  
Bromide
Br-
Add silver nitrate solution to a solution of substance in dilute nitric acid Pale yellow precipitate, does not dissolve in diluted ammonia solution but DOES dissolve in concentrated ammonia solution
Iodide
I-
Add silver nitrate to a solution of substance in dilute nitric acid.

Add lead nitrate to a solution of substance in dilute nitric acid.

Pale yellow precipitate, does not dissolve in ammonia solution.

Bright yellow precipitate will form.

Nitrate
NO3
Add iron(II) sulfate solution followed by concentrated sulfuric acid to the solution Brown ring forms at the junction of the two liquids.
Sulfate
SO42-
Add solution of barium chloride to the solution. White precipitate, does not dissolve in dilute hydrochloric acid.
Sulfite
SO32-
Add solution of barium chloride to the solution. White precipitate, does dissolve in dilute hydrochloric acid.
Sulfide
S2-
Add lead(II) ethanoate solution to the solution. Black precipitate

Tests for Cations

 

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Cation Symbol
Test
Results
Aluminum
Al3+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

White precipitate that dissolves as more sodium hydroxide solution is added.

White precipitate that does not dissolve as more ammonia solution is added.

 
Ammonium
NH4+
Add sodium hydroxide solution to a solution of the substance and gently heat. Ammonia gas is given off.
Calcium
Ca2+
Add dilute sulfric acid to a solution of the substance

Also try flame test

White precipitate formed.
Copper (II)
Cu2+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

Also try flame test

Pale blue precipitate that dissolves as more sodium hydorxide is added.\.

Pale blue precipitate, changing to deep blue solution as more ammonia is added.

Iron(II)
Fe2+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

Pale green precipitate formed.

Pale green precipitate formed.

Iron(III)
Fe3+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

Red-brown precipitate formed.

Red-brown precipitate formed.

Lead(II)
Pb2+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

Or: add dilute sodium or potassium iodide

White precipitate, that does dissolve as more sodium hydroxide is added.

White precipitate that does not dissolve as more ammonia is added.

Bright yellow precipitate form

           

 

 

 

Magnesium
Mg2+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

White precipitate, that does not dissolve as more sodium hydroxide is added.

White precipitate that does not dissolve as more ammonia is added.

Zinc
Zn2+
a)Add dilute sodium hydroxide solution to a solution of the substance.

b)Add dilute ammonia solution to a solution of the substance.

White precipitate, that dissolve as more sodium hydroxide is added.

White precipitate that dissolve as more ammonia is added.

http://chewtychem.wiki.hci.edu.sg/file/view/cation_test_with_NaOH.jpg/214972380/528×276/cation_test_with_NaOH.jpg
The qualitative analysis of anions involved two main types of tests.
The first type produces precipitate while the second type liberates a gas.
There are altogether 5 anions that you are required to test and identify, namely, sulfate, chloride, iodide, nitrate and carbonate ions.

Anion Tests involving the Production of Precipitate.

Anion present Test to be conducted Observation Video
Chloride ion Add dilute nitric acid followed by aqueous silver nitrate.

OR

Add dilute nitric acid followed by aqueous lead (II) nitrate

White precipitate of AgCl is formed

White precipitate of PbCl2 is formed.

Iodide ion Add dilute nitric acid followed by aqueous silver nitrate.

OR

Add dilute nitric acid followed by aqueous lead (II) nitrate

Pale yellow precipitate of AgI is formed.

Bright yellow precipitate of PbI2 is formed.

sulfate ion Add dilute nitric acid followed by aqueous barium nitrate
OR
Add dilute hydrochloric acid followed by aqueous barium chloride
In both bases, white precipitate of BaSO4 is formed.

Anion Tests involving the production of gases:

Anion present Test to be conducted Observation Video
Carbonate ion Add dilute nitric/hydrochloric acid. Pass the gas into limewater (calcium hydroxide solution) Brisk effervescence occured. A white precipitate is formed in the limewater when the gas is passed through limewater. The gas is carbon dioxide gas
Nitrate ion Add aqueous sodium hydroxide, followed by a few pieces of aluminum foil/zinc powder/Devarda’s alloy and warm the mixture gently. A pungent and colourless gas evolved which turned moist red litmus blue. The gas is ammonia

 

Flame  Tests
Metal Symbol
Flame Color
Barium
Ba
Yellow-green
Calcium
Ca
Red
Copper
Cu
Green
Lead
Pb
Blue
Lithium
Li
Pink
Potassium
K
Lilac
Sodium
Na
Orange

http://chewtychem.wiki.hci.edu.sg/file/view/cation_test_with_NaOH.jpg/214972380/528×276/cation_test_with_NaOH.jpg

Tests to identify gases
Gas Symbol
Test
Results
Carbon dioxide
CO2
bubble gas through limewater (calcium hydroxide solution) Turns limewater cloudy.
Hydrogen
H2
Put a lighted splint into a sample of the gas. Burns with a “popping” sound
Oxygen
O2
Put a glowing splint into a sample of the gas. Splint relights.

Anion – An ion with a negative charge, formed when an atom gains electrons in a reaction (it now has more electrons than protons). Non-metals tend to form anions.

Cation – An ion with a positive charge, formed when an atom loses electrons in a reaction (it now has more protons than electrons). Hydrogen and metals tend to form cations.

 

Specific Tests

Cation Identification TestsGenerally 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.
  1. 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+ also forms an insoluble white sulfate, which dissolves in a solution containing acetate ion due to the formation of the weak electrolyte, Pb(CH3COO)2. The addition of chromate ion to this lead acetate solution yields a precipitate of yellow lead chromate.

      Pb2+(aq) + SO42-(aq) → PbSO4(s)
      PbSO4(s) + 2CH3COO(aq) → Pb(CH3COO)2(aq) + SO42-(aq)
      Pb(CH3COO)2(aq) + CrO42-(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, PbCl42-.) 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+.

  2. Cations That Form Insoluble SulfatesIdentification 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 BaCrO4 salt. If Ca2+ or Sr2+ are present they will also precipitate in the presence of high concentrations of CrO42-. 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 CrO42- concentratiion enough to keep CaCrO4 and SrCrO4 in solution but to allow the BaCrO4 to precipitate.

      2CrO42-(aq) + 2H+(aq) → Cr2O72-(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)2C2O4 solution. The formation of a white precipitate indicates the presence of Ca2+. Confirm by adding a few drops of concentrated HCl and flame testing.
  3. 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) + S2-(aq) → CdS(s)
      Cd(NH3)42+(aq) + S2-(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)64-(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+.
  4. 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)3precipitates 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)→ CrO42- (yellow)
      CrO42- –(H+)→ Cr2O72- (orange)
      Cr2O72- –(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.

  5. Other Cations
    • Manganese(II) Ions:Manganese is easily identified by oxidation of Mn2+ to purple MnO4using 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)42-(aq) + 2Bi(OH)3(s) → 2Bi(s) + 3Sn(OH)62-(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.

  1. Anion Identification Tests
      • Carbonate Ions:The most characteristic reaction of carbonate is the formation of carbon dioxide upon treatment with acid:

        CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l)

        The colorless, odorless carbon dioxide can be identified by bubbling it through a saturated solution of barium hydroxide, with which it forms a white precipitate of barium carbonate.

        CO2(g) + Ba2+(aq) + 2OH(aq) → BaCO3(s) + H2O(l)

        Assemble a gas-liberation apparatus from a small test tube and a section of bent tubing. Dissolve or suspend a portion of your compound in a small amount of water and place it in the small test tube. Add about 0.5 mL of 6 M HCl and quickly fit the tube into the small test tube, allowing the gas liberated to bubble into a 6″ test tube of saturated Ba(OH)2 solution. The formation of a white precipitate in the large test tube (if the gas liberated is odorless) is a positive test for carbonate. It is imperative to test the gas-liberation apparatus by adding HCl to Na2CO3.

      • Sulfide Ions:When treated with nonoxidizing acids (HCl, CH3COOH) sulfides react to liberate H2S gas (rotten-egg odor). If the sulfide is very insoluble liberation of the gas may require concentrated acids (indeed some sulfides, HgS, CuS, are so insoluble that dissolution requires special treatment). The gas is generally identified by its odor and its precipitation of colored sulfides of various metal ions. Sulfides or hydrogen sulfide also are oxidized to elemental sulfur and sulfate by oxidizing agents such as permanganate, nitric acid, sulfuric acid, Fe(III), etc.

        3H2S(aq) + 2H+(aq) + 2NO3(aq) → 2NO(g) + 4H2O + 3S(s)

        Acidify a sample with 6 M hydrochloric acid and warm. Cautiously smell the gas evolved and also test the gas with a piece of filter paper moistened with lead acetate solution. A foul smelling gas which turns lead acetate paper black constitutes a positive sulfide test.

      • Sulfate Ions:Sulfate is conveniently identified by precipitation of BaSO4. Other insoluble barium salts contain anions of weak acids (CO32-, SO32-and PO43-). Precipitation of these anions is prevented by acidifying the solution.Acidify the test solution with 6 M HCl, and add a few drops of 0.2 M BaCl2 solution. A white precipitate indicates the presence of SO42-.
      • Nitrate Ions:The most notable feature of the chemistry of the nitrate ion is its oxidizing ability as illustrated by the following reactions:

        3Fe2+(aq) + 4H+(aq) + NO3(aq) → NO(g) + 2H2O + 3Fe3+(aq)

        In the last reaction the nitrogen oxide reacts with excess Fe2+ to give the brown complex ion Fe(NO)2+. It is the formation of this brown complex that is used to identify NO3 (called the brown ring test).

        Acidify about 2 mL of the test solution with 3 M H2SO4 and then dissolve one-half spatula full of solid FeSO4.7H2O in the acidified solution. Cool the solution and then carefully introduce about 0.5 mL of concentrated H2SO4 by allowing it to flow down the side of the tilted test tube. Allow the solution to sit undisturbed so that the sulfuric acid forms a definite layer. The formation of a brown color at the interface of the layer constitutes a positive test for nitrate.

      • Phosphate Ions:The precipitation usually used to identify phosphate is the formation of yellow ammonium molybdophosphate from ammonium molybdate in acidic solution.

        12MoO42- + 3NH4+ + PO43- + 24H+ → (NH4)3[P(Mo12O40)] + 12H2O

        Acidify the sample with concentrated nitric acid and add several drops in excess. Then treat the solution with ammonium molybdate reagent and warm. The formation of a yellow crystalline precipitate confirms the presence of phosphate.

      • Chloride, Bromide, and Iodide Ions:All three of these anions form insoluble silver salts. Although the precipitates are of different colors (AgCl white, AgBr cream, AgI yellow) the colors are difficult to distinguish, and confirmatory tests are necessary.Silver chloride, the most soluble of the three, dissolves readily in 6 M NH3 solution because of formation of the ammonia complex. Furthermore, when the solution of the ammonia complex is acidified, AgCl reprecipitates. Neither AgBr nor AgI will dissolve readily in 6 M NH3, a much higher concentration of NH3 being required to form the complex.

        Cl(aq) + Ag+(aq) → AgCl(s)
        AgCl(s) + 2NH3(aq) → Ag(NH3)2+(aq) + Cl(aq)
        Ag(NH3)2+(aq) + Cl(aq) + 2H+(aq) → AgCl(s) + 2NH4+(aq)

        Bromide and iodide are usually identified by oxidation to the free elements with chlorine. The elements thus formed are extracted into carbon tetrachloride and identified by their color.

        2Br(aq) + Cl2(g) → Br2(g) + 2Cl(aq)
        2I (aq) + Cl2(g) → I2(s) + 2Cl(aq)

        • Chloride:Acidify the test solution with 3 M HNO3. Then add several drops of 0.1 M AgNO3. If a white precipitate forms, centrifuge and remove the supernatant. To the precipitate add 6 M NH3 with stirring. If the precipitate dissolves, add 6 M HNO3 to the solution. A white precipitate will form if the original test solution contained Cl.
        • Bromide and Iodide:Acidify the sample with several drops of 6 M HCl and add 4-5 drops of carbon tetrachloride. Then add about 0.5 mL of chlorine water and shake. Appearance of an orange-brown carbon tetrachloride layer indicates the presence of bromide. Formation of a purple layer indicates iodide.