Anion Analysis

QUALITATIVE ANALYSIS

IDENTIFICATION OF THE ANION

PREPARATION OF THE ANION TEST

SOLUTION

Preparation of the Unknown

FROM THE SOLUBILITY TEST DONE IN PART A…

If sample is soluble in water:

Dissolve a pinch of the sample in 2 mL water

Add a few drops of 3M Na2CO3

No precipitate: Repeat step 1 and use as anion test solution.

W/ precipitate: Proceed to step 2.

FROM THE SOLUBILITY TEST DONE IN PART A…

If sample is insoluble in water or formed a precipitate w/ Na2CO3:

Place a pinch of sample in a micro test tube

Add 3 mL of 3M Na2CO3

Heat with stirring in a water bath for about 5 minutes

Centrifuge and decant

Use decantate as the anion test solution

ANION ANALYSIS

Preliminary Tests

TEST WITH SULFURIC ACID

Add 1 drop of concentrated sulfuric acid to a small bi of solid sample (1/4 size grain of rice)

Watch out for formation of product (a gas or a substance different in color from the sample)

If no evident reaction, warm the test tube gently.

TEST WITH SULFURIC ACID

Anions that react with cold sulfuric acid:ANIONS VISIBLE RESULT REACTION EQUATION

Carbonate Effervescence CO23- + H20 → CO2 + H20

Oxalate Evolution of CO and CO2C2O4

2- + 2H+ → H20 + CO + CO2

Sulfide Rotten egg odor S2- + H+ → H2S

Chloride Pungent odor; Blue litmus to red

Cl- + H2SO4 → HCl + HSO4-

Bromide Reddish brown solution KBr + H2SO4 → HBr + HSO4- + K+

2KBr + 2H2SO4 → Br2 + SO2 + SO42- + 2K+ + 2H2O

Iodide Violet powder – starch powder blue

3I- + 2H2SO4 → I3- + SO42- + 2H20 + SO2

I- + H2SO4 → HI + HSO4-

9I- + 4H2SO4 → 3I3- + S + 3SO4

2- + 4H2O

12I- + 5H2SO4 → 4I3- + H2S + 4SO4

2- + 4H2O

Chromate Orange- red solution 2CrO42- + 2 H+ ↔ Cr2O7

2- + H2O

Nitrite Evolution of brown fumes NO2- + H+ → HNO2

2 HNO2→ H2O + N2O3

3 HNO2→ HNO3 + 2 NO + H2O

2NO + O2→ 2 NO2

TEST WITH SULFURIC ACID

Anions that react with hot sulfuric acid:

ANIONS

VISIBLE RESULT REACTION EQUATION

Nitrate Reddish brown vapor NO2; pungent acid vapor of nitric acid with fumes

2CrO42- + 2 H+ ↔ Cr2O7

2- + H2O

Acetate Vinegar-like odor NO2- + H+ → HNO2

2 HNO2→ H2O + N2O3

3 HNO2→ HNO3 + 2 NO + H2O

2NO + O2→ 2 NO2

THE CHLORIDE GROUP TEST

Add 1 drop of 0.5 AgNO3 to 5 drops of test solution

If a precipitate forms it could be one of the following:

Anion Precipitate

CNS- White

CO32- White

Br- Cream colored

I- Light yellow

PO43- Yellow

CrO42- Red

S2- Black

THE CHLORIDE GROUP TEST

Centrifuge and discard solution.

Wash residue with 5 drops of distilled water. Discard wash liquid.

**If all dissolves CNS-

, Cl-

, Br-

, I-

, S-

2

are absent**If not all dissolves, one or more of these could be present

THE CHLORIDE GROUP TEST

Centrifuge and discard solution.

Add 6 M ammonia solution dropwise until red litmus turns blue.

Yellow precipitate: PhosphateRed precipitate: Chromate

THE SULFATE GROUP TESTAdd 3M NH3 dropwise to 5 drops of test solution until the solution test alkaline to litmus

Add 2 drops 0.3M BaCl2 and two drops of 0.3M CaCl2

**No precipitate: STOP! No sulfate group present (SO42

-

, CrO42

-

, PO43

-

, C2

O42

-

)**Yellow precipitate: CrO42

-

**Non-yellow precipitate: proceed to next step

THE SULFATE GROUP TESTA

d

d

6

M

H

C

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s

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l

u

ti

o

n

u

n

til

b

l

u

e

lit

m

u

s

t

u

r

n

s

r

e

d

Add five more drops

If precipitate dissolves, SO42

-

is absent.

ANION ANALYSIS

Confirmatory Tests

ANION CONFIRMATORY TEST

CNS-

I-

Br-

Cl-

S2-

CO32- Omit if no effervescense observed in test with

Sulfuric Acid

SO42-

CrO42- Omit if unknown solution is colorless

PO43-

C2O42-

CH3COO-

NO3-

S2O32-

NO2-

MnO4- Omit if unknown solution is colorless

Summary/Notes on Confirmatory Tests

CNS-

Add 1 drop of 0.1 M Fe(NO3)3 to 5 drops of test solution

Blood-red solution confirms presence of CNS-

Discussion:

The chemical equation for this confirmatory step is:

CNS-(aq) + Fe3+ (aq) → FeSCN2+ (aq)

The addition of Fe(NO3)3 causes the CNS- ion to react with the Fe3+ ion to form a dark red complex ion in the form of FeSCN2+.

I-

Acidify 6 drops of the test solution with 6M HNO3

Add 2 drops of 0.1M Fe(NO3)3 and 10 drops of CCl4. Shake.

**No purple layer: No iodide; proceed to Bromide test**Purple layer: Iodide is present

Add fresh CCl4 and Fe(NO3)3 until violet colors dissappear

Use aqueous layer for Bromide test

I-

Discussion:

In solutions, I- ions are easily oxidized. A water sol’n of KI, for example, turns yellow with time because the ion is readily oxidized by the air. In the confirmatory test, the I- ion was oxidized by the Fe+ ion, which is a moderately strong oxidizing agent under acidic solutions. Also, I- is easily extracted with CCl4.

This is the chemical equation involved in the I- confirmatory test:

2I-(aq) + 2Fe+(aq) → I2 + 2Fe2+(aq)

Br-

5 drops of test solution from procedure 2 (Iodide test) + equal volume of 16M HNO3

Submerge test tube in boiling water for 30 seconds and cool in cold water

Add 2 drops 0.02M KMnO4 + 3 drops CCl4 and shake

If the CCl4 layer becomes yellowish or reddish brown, Br- is present in the sample.

Discard the CCl4 layer and then extract the remaining Br- by adding fresh CCl4 and shaking.Use aqueous layer for Bromide test

Br-

Discussion:

The chemical equation for the confirmatory test is:

10 Br-(aq) + 2MnO4

-(aq) + 16H+

(aq) → 5Br2(aq) + Mn2+(aq) +

8H2O

The MnO4- ion oxidizes Br- ion to

elemental Br2 which is soluble in the CCl4 layer. This is why it could be extracted easily through the CCl4 layer. The oxidation results to the reddish brown layer.

Cl-

Acidify the solution from confirmatory test of Br- with 6M HNO3

Add 10 to 15 drops of water + 2 drops AgNO3

If there is a white precipitate, Cl- is present

Discussion:

The chemical equation for this step is:

Cl-(aq) + Ag+(aq) → AgCl(s)

In the test, the Cl- ion reacted with the Ag+ ion from the AgNO3 and formed an insoluble salt, AgCl.

S2-

In a small beaker, add 10 drops of 6M HCl solution to a small bit of the original sample

Moisten a small piece of filter paper with PbOAc sol’n and place it on the convex side of the watch glass

Place the watch glass over the beaker with the moist paper inside the beaker

Warm the beaker gently and watch the paper

If the paper turns black or silvery black in color, S- is present in the sample.

S2-

Discussion:

H2S(g) + Pb2+(aq) → PbS(s) + 2H+

Some sulfides dissolve in non-oxidizing acids like HCl forming solutions which contain H2S. Hydrogen sulfide occurs as a gas at room temperature and is only slightly soluble in water. However, it is not a very stable substance. It could be easily oxidized by air or oxidizing agents like HNO3, Fe3+, Cr2O7

2-, NO2-, Pb2+.

CO32-

Put a small bit of the sample into a test tube and add 3 drops 3M HCl

Quickly drop a medicine dropper with short glass tube into the Ba(OH)2 and insert the drop of Ba(OH)2 into the test tube such that the rubber bulb covers the top of the test

tube

Warm the bottom of the test tube (but not to boiling)

If the droplet of Ba(OH)2 becomes more cloudy or white, CO32-

is present

CO32-

Discussion:

The chemical equation for this confirmatory step is:

CO2(g) + Ba2+(aq) → BaCO3(s) + H2O

Generally, carbonate salts are insoluble in water except those of the alkali metals and ammonium ion. However, all carbonates dissolve in solutions of acids because of the formation of carbonic acid. Addition of acids to solid carbonates and carbonate solutions causes effervescence due to the evolution of gas.

The cloudy or white observation in the droplet of Ba(OH)2 is actually the barium carbonate precipitate, BaCO3(s)—formed from the contact of CO2 gas with the Ba(OH)2 solution.

SO42-

To 3 drops of test solution, add 6M HCl drop by drop until the solution turns acidic

Add four more drops

Add a drop of BaCl2.

A white ppt indicates the presence of SO42-

Discussion:

The chemical reaction is: SO42-

(aq) + Ba2+(aq) → BaSO4(s)

Sulfate ion is very unreactive compared to the other anions. It is not readily reduced or oxidized. However, sulfate ion undergoes ppt reaction with Ba2+ or Pb2+ ion.

In this confirmatory test, the SO42- ion reacts with the Ba2+ ion

forming a white ppt which is insoluble n sol’ns of strong acids or any other common reagent.

CrO42-

Add 1M HNO3 drop by drop to 3 drops of test sol’n in a test tube, until it turns acid in litmus test.

Add 3 drops of 3M HNO3 and enough either to make a layer above the water, ¼ to ½ inch thick

Add 3 drops of 3% H2O2 sol’n, shake once quickly.

A blue color in the solution indicates the presence of chromate.

CrO42-

Discussion:

The chemical reactions involved in the confirmatory test are:

2CrO42- + 2H+ ↔ Cr2O7

2- + H2O

Cr2O72-

(aq) + 4H2O2(aq) + 2H+(aq) → 2CrO5(aq) + H20

In the analysis of anions, chromate ion is the only colored anion—bright yellow in aqeous sol’ns. Most chromates are insolube in water but will dissolve in strong acids. Under acidic conditions, it reacts to form the dichromate ion which is orange in nature.

In the confirmatory test, the Cr2O72- ion reacts with

peroxide to form a dark blue solution still containing the peroxide. However, the chromium peroxide formed from the reaction of the dichromate with peroxide is very unstable. Its color fades quickly from dark blue to a somewhat violet color of the Cr3+ ion.

PO43-

Add 5 drops of 16 M HNO3 + 3 drops of ammnioum molybdate reagent to five drops of test solution

Warm in water bath

If a yellow precipitate forms, phosphate is present in the solution.

Discussion:

The equation below demonstrates the chemical reaction for ths confirmatory test:

H3PO4- (aq) + 12 MoO4

2- (aq) + 3 NH4+ (aq) + 22 H+ (aq)→ (NH4)3PO4 · 12

MoO3 (s) + 12 H2O

Phosphates of the alkali metals are soluble. Most other phosphates, such as FePO4, CrPO4, BiPO4, Ca3(PO4)2 and AgPO4 are only sparingly soluble. Phosphate ion also forms a bright yellow precipitate with ammonium molybdate—ammonium phosphomolydate.

C2O42-

Add a drop of 6 M CH3COOH and 2 drops of 0.2 M CaCl2 solution to 2 drops of test solution

If a precipitate forms, it may be calcium oxalate.

Centrifuge and wash once with drops of water.

Centrifuge again and discard wash water.

To the precipitate, add the ff:2 drops H2O + 2 drops of 0.02 M KMnO4 + 2 drops 6 M H2SO4

If the color of the permanganate fades w/in 30 sec, oxalate is present.

C2O42-

Discussion:

Oxalates, though unresponsive in neutral and basic solutions, tend to slowly oxidize in acidic mediums. This oxidation process is due to strong oxidizing agents like MnO4

- ion. The reaction converts the oxalate ion to carbon dioxide and bleaches the purple permanganate ion.

The reaction is:

5 C2O42- (aq) + 2 MnO4

- (aq) + 8 H+ (aq)→ 10 CO2 (g) + 2 Mn2+ (aq) + 4 H2O

CH3COO-

Place a small amount of original sample in a small beaker

Add 2 drops of 1:1 ratio of sulfuric acid and water

If the smell of the mixture is like vinegar, then acetate is present. If not proceed to the succeeding steps.

Add 2 drops of alcohol and stir well.

Warm gently but not to boiling. AFTER heating, smell cautiously

If the same sweetish odor of ethyl acetate can be detected coming from the sample as from the control, then the sample contains acetate.

CH3COO-

Discussion:

One of the natures of acetate is that it has the odor of vinegar whenever H2SO4 is added or any solid containing acetate. Also, warming the solution could also enhance the odor.

NO3-

Place of 2 drops of test solution in a 5 mL test tube. Add 10 drops of conc H2SO4 + a grain of FeSO4.

Let stand for 1- 2 minutes.

A brown ring indicates the presence of nitrate sample.

Discussion:

Nitrate ions only respond to a specific test reagent, consisting of acidified iron (II) sulfate. Nitrate ion will oxidize iron (II) to iron (III), and brown nitrogen (IV) oxide gas will evolved. The chemical reactions are a follows:

3Fe2+ (aq) + NO3

- (aq) + 4 H+ (aq) → 3Fe3+ (aq) + NO (aq) + 2 H2O

(l)

Fe2+ (aq) + NO (aq) ↔ Fe(NO)2+ (aq)

S2O32-

5 drops of test solution + 3 drops 3 M HCl

If a cream/whitish precipitate formed, thiosulfate.

Discussion:

Thiosulfates are unstable in acidic solutions. They will decompose to sufite and sulfur as shown below:

S2O32- (aq) + 2 H+ (aq) → SO2 (g) + S (s) + H2O

NO2-

5 drops of test solution + 6 M HOAcAdd drop by drop until it is acidic to litmus.

Add 5 drops of 0.1 M K3Fe(CN)6

Let stand for a minute then add 1 drop of FeCl3

A deep red precipitate indicates the presence of nitrite in the sample.

Discussion:

The following are the chemical reactions involved in the confirmatory test:

HNO2 (aq) + Fe2+ (aq) + H+ (aq) → Fe3+ (aq) + NO (aq) + H2O

NO (aq) + Fe2+ (aq) ↔ Fe(NO)2+ (aq)

Nitrites when subjected to acidic solutions are converted to nitrous acid, HNO2.

MnO4-

2 drops test solution + 2 drops 0.02 M Na2C2O4 + 2 drops 6 M H2SO4

The disappearance of the color of the solution within 30 seconds is the indication of the presence of the permanganate ion.

Discussion:

Oxalates, though unresponsive in neutral and basic solutions, tend to slowly oxidize in acidic mediums. This oxidation process is due to strong oxidizing agents like MnO4

- ion. The reaction converts the oxalate ion to carbon dioxide and bleaches the purple permanganate ion.

The reaction is:

5 C2O42- (aq) + 2 MnO4

- (aq) + 8 H+ (aq)→ 10 CO2 (g) + 2 Mn2+ (aq) + 4 H2O

CONCLUSION AND RECOMMENDATIONS

Qualitative Analysis

Qualitative analysis is an effective culminating activity for chemistry laboratory subjects. It includes and combines both laboratory techniques and chemical reactions which will test one’s knowledge of the subject involved.

In the experiment that we are about to perform, water baths, centrifugation, and decantation are the three main processes which we will repeatedly execute. The three given methods will help us determine the cations and anions present in the unknown. They will also receive some aid from the numerous chemicals that are to be added to the unknown. A change in color and the evolution of gas are just some of the indicators that we are to take note of. This will give us a clearer and more specific idea of the solution that we are to be assigned with. It will narrow the wide array of choices that are possible for identity of the unknown. Lastly, performing the experiment accurately and systematically will pilot towards a more successful analysis and recognition of the given.

-THE END-