Systematic Analysis of Anions | Chemistry for EmSAT Achieve PDF Download

Systematic Analysis of Anions

• What is an Anion?

  An anion is an ion with more electrons than protons, giving it a negative charge. When an atom gains electrons, it forms an anion. The number of electrons gained determines the negative charge on the anion. For instance, gaining one electron results in a one-unit negative charge.

• Importance of Anion Analysis

  Anion analysis helps identify the types and quantities of anions present in a given sample. Traditional chemistry techniques, such as colorimetric methods, are used to analyze anions in samples. Modern methods like anion exchange chromatography allow for both separation and quantification of individual anions present in samples.

• Anion Analysis Techniques

  Colorimetric methods and anion exchange chromatography are commonly used to analyze anions. Anion exchange chromatography, for example, not only separates ions in samples but also quantifies individual anions. This method can identify multiple anions in a single run within 10 to 30 minutes.

Systematic Analysis of Anions

• Introduction to Anion Analysis:

  Anions are formed through the partial or complete neutralization of acids by bases in inorganic salts. In a salt, the anion comes from the acid, while the cation comes from the base.

• Significance of Preliminary Examination:

  Preliminary examination results provide crucial hints regarding the potential presence of specific cations or anions in a sample.

• Experiment Objective:

  The aim is to identify the anionic radicals present in an inorganic salt mixture through various testing procedures.

• Theoretical Framework:

  Qualitative analysis focuses on detecting and identifying acidic and basic radicals within inorganic salts. These salts originate from reactions between acids and bases or acidic oxides with bases or basic oxides.

Theory of Qualitative Analysis

Qualitative analysis involves identifying and detecting acidic and basic radicals in inorganic salts. These salts result from the reaction of acids or acidic oxides with bases or basic oxides.

Examples of Reactions:

• CO2 + 2NaOH → Na2CO3 + H2O
• NaOH + HCl → NaCl + H2O
• 2NaOH + H2SO4 → Na2SO4 + 2H2O
• KOH + HNO3 → KNO3 + H2O

Organic Compounds

Many organic compounds are crystalline solids due to their specific shapes and oppositely charged ions known as radicals.

Fundamental Principles in Salt Analysis:

• Solubility Product:

The solubility product represents ion concentrations raised to the power of the ion count in the equation. It defines the electrolyte dissociation when a solution is saturated at a given temperature.

Solubility equilibrium is a form of dynamic equilibrium between solid compounds and their solution.

• Common Ion Effect:

The common ion effect influences the solubility of slightly soluble salts when they are in the presence of a common ion. It reduces the solubility of the salt.

Experimental Procedures

• Apparatus Setup

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• Procedure

  • Preliminary Reactions

    When conducting preliminary reactions, if the salt appears colorless, it indicates the absence of Fe2, Ni2, Fe3, and Co2 ions.

  • Appearance

    The appearance of the salt can provide clues about its composition. For instance:

    • If the salt appears green, it may contain Ni2, Fe2, or Cu2 ions.
    • If the salt appears brown, it might contain Fe2.
    • If the salt appears pink, it could contain Mn2 or Co2.
    • If the salt appears blue, it may contain Cu2.

  • Action of Heat

    When heat is applied to the salt in a test tube, observations such as:

    • The release of a colorless gas with a pungent odor, turning red litmus paper blue, possibly indicating the presence of NH4 salt.
    • Formation of reddish-brown vapors that turn acidified ferrous sulfate brown, suggesting the presence of NO3- ions.

Chemical Analysis Techniques

• NO3- Test: Nitrate ions can be detected using specific chemical reactions. When exposed to certain conditions, they exhibit distinct characteristics.

Flame Test

Perform the following steps to observe the flame colors produced by different metal salts:

• Crimson Red Flame: This color indicates the presence of a specific metal ion in the compound.
• Bluish Green Flame: Another color that signifies a different metal ion.
• Brick Red Flame: This color is characteristic of yet another metal ion.

Each metal ion produces a unique flame color, aiding in their identification.

Identification of Anions from Volatile Products

By following specific steps, anions in chemical compounds can be identified:

• Dilute H2SO4 Test: This test involves adding dilute sulfuric acid to a salt sample and observing the resulting effervescence and gas properties.

For example, the carbonate anion (CO32-) reacts in a distinctive manner, producing odourless gas turning lime water milky.

Chemical Reactions Summary

Anions Identification

• Adding either 1 or 2ccs of dilute H to a small portion of salt in a test tube and gently warming it can help identify different anions.
• Brisk effervescence produces an odourless, colourless gas that turns lime water milky, indicating the presence of Carbonate anion (CO32-).
• A colourless gas with a rotten egg odor that turns lead acetate black paper suggests the existence of Sulphide anion.
• Another colourless gas with a burning sulfur smell, turning acidified dichromate green, signifies the presence of Sulfate anion.
• A reddish-brown gas with a fishy odor, turning acidified brown ferrous sulphate, indicates the presence of Nitrate anion.
• A colourless gas with a vinegar-like smell may suggest the presence of the Acetate anion.
• No distinct observations point to the absence of the aforementioned anions.

Action of Concentrated H2SO4

• To identify bromide anion, add 2-3 ccs of Concentrated H2SO4 to a small amount of salt in a test tube, gently heat it, and observe reddish-brown vapors that turn moist red paper fluorescent.

Action of Concentrated Sulfuric Acid (H₂SO₄)

• Observation 1: When concentrated sulfuric acid is added to a salt and gently heated:
  • Reddish-brown vapors are produced, turning moist red paper fluorescent. This indicates the possible presence of the bromide anion.
• Observation 2: A colorless gas with a pungent smell forms dense white fumes when a dipped glass rod is placed in ammonium hydroxide (NH₄OH) solution:
  • This may suggest the presence of the chloride anion.
• Observation 3: Violet-colored vapors turn blue or violet starch paper:
  • This could indicate the presence of the iodide anion.
• Observation 4: Reddish-brown vapors turn brown ferrous sulfate paper into the acidified form:
  • This reaction might suggest the presence of the nitrate anion.

Chemical Tests for Anions

• Observation for Absence of Anions:
  • No characteristic observations indicate the absence of specific anions.
• Test 6: Concentrated H2SO4 with Cu Turnings
  • Procedure: Mix Cu bits with salt in a test tube, add H2SO4, and heat.
  • Result: Reddish-brown gas turns acidified ferrous sulfate paper brown, indicating the presence of nitrate anion.
• Test 7: Concentrated H2SO4 with MnO2
  • Procedure: Mix MnO2 with salt, add H2SO4, and heat gently.
  • Result: Greenish-yellow gas turns blue starch iodide paper, possibly indicating the presence of chloride anion.

Chemical Reactions Summary

Test for Various Anions

• Add manganese oxide (MnO) to a small amount of salt in a test tube, followed by concentrated hydrochloric acid (HCl). Apply gentle heat.

  A greenish-yellow gas turns blue or starch iodide paper.

  Reddish-brown vapors can be produced, turning moist red litmus paper fluorescent red, possibly indicating the presence of bromide anions.

  Violet vapors can be observed, turning starch paper violet or blue, suggesting the presence of iodide anions.

  No distinctive colored vapors indicate the absence of the mentioned anions.

Reaction with Sodium Hydroxide (NaOH)

• Add 10% NaOH solution to a small amount of salt in a test tube, then gently warm the mixture.

  Colorless gas with a pungent odor is released, which forms dense white fumes when a glass rod dipped in hydrochloric acid (HCl) is introduced, possibly indicating the presence of ammonium ions.

  No specific gas is released from ammonium, indicating its absence.

Confirmatory Anion Testing Overview

• Ammonium ion is not present during the confirmatory tests.

Sodium Carbonate Extract Method

• Confirmatory tests are conducted for sulfide (S^2-), carbonate (CO₃^2-), sulfite (SO₃^2-), acetate (CH₃COO^-), and nitrite (NO₂^-) anions.
• Water-soluble salt: Perform tests using the water extract.
• Water-insoluble salt: Utilize sodium carbonate extract for testing.
• Confirmation of CO₃^2- can be achieved using either an aqueous salt solution or solid salt due to the presence of carbonate ions in the sodium carbonate extract.
• Water extraction involves dissolving the salt in water.

Sodium Carbonate Extract Preparation

• Take 1 gram of the salt in a porcelain dish or boiling tube.
• Mix approximately 3 grams of solid sodium carbonate with 15 mL of distilled water.
• Heat the mixture for up to 10 minutes, then filter, cool, and collect the filtrate as the sodium carbonate extract.

Silver Nitrate Test

• Add dilute HNO₃ to the sodium carbonate extract until effervescence ceases.
• Introduce a few drops of AgNO₃ solution in excess (2 to 3 drops).
• Presence of NH₄OH indicates a curdy white soluble precipitate, potentially signifying the chloride anion.

Silver Nitrate Test

• Add dilute nitric acid to the sodium carbonate extract until effervescence stops.
• Then, add a few drops of silver nitrate solution in excess (2-3 drops).
• Observation: Formation of a curdy white soluble precipitate.
• Possible identification: A pale yellow precipitate in NH4OH, sparingly soluble, indicating the bromide anion.
• Possible identification: Insoluble yellow colored precipitation in NH4OH, possibly indicating the iodide anion.
• If no characteristic precipitate forms, it suggests the absence of the mentioned anions.

Barium Chloride Test

• Add a solution of BaCl2 to around 1-2 ccs of the extract following acetic acid neutralization and CO2 boiling.
• Then, introduce dilute hydrochloric acid to the precipitated portion mentioned earlier.
• Observation: Formation of an insoluble white precipitate in the presence of HCl acid, indicating the presence of SO42- anion.

Qualitative Analysis of Anions

• Test for Sulfate (SO₄^2-)

  Add BaCl₂ solution to a small portion of the solution after neutralizing with acetic acid and boiling off CO₂.

  Add dilute HCl to the precipitate formed.

  Formation of a white precipitate indicates the presence of SO₄^2-.A white precipitate soluble in HCl suggests the presence of sulfite ions (SO₃^2-).No precipitate formation indicates the absence of sulfate and sulfite ions.

• Lead Acetate Test

  Add lead acetate solution to a small portion of the solution after acidification and cooling.

  Formation of a white precipitate soluble in excess ammonium acetate confirms the presence of sulfate ions.

• Ferrous Sulphate Test (Brown Ring Test)

  Add dilute H₂SO₄ followed by FeSO₄ to the solution.

  Observe the formation of a brown ring at the liquid junction when concentrated H₂SO₄ is added, indicating the presence of nitrate ions (NO₃^-).

Chemical Tests for Anions

• Ferrous Sulphate Test (Brown Ring Test):
  • Add dilute H2SO4 drop by drop to a small amount of the extract until effervescence ceases.
  • Follow by adding a few excess drops and then introduce 2-3 drops of freshly prepared FeSO4.
  • Place the test tube at a slant and carefully add Concentrated H2SO4 without mixing.
  • A brown ring at the liquid junction indicates the presence of the nitrate anion (NO3-).
  • Absence of the brown ring signifies the absence of the nitrate anion.
• Ferric Chloride Test:
  • Take a small amount of extract in a test tube and add neutral FeCl3 solution.
  • If necessary, divide and filter the solution into two parts.
  • Observation of a deep red color confirms the presence of the acetate anion (CH3COO-).

Chemical Tests for Anions

Acetate Anion Test

• Add dilute HCl:
  • Observation: Red coloring disappears
  • Result: CH3COO- confirmed
• Add water and boil to the 2nd part:
  • Observation: Reddish brown precipitate forms
  • Result: CH3COO- confirmed

Calcium Chloride Test

• Incorporate the diluted HNO3 to the white and hot ppt:
  • Result: Precipitate dissolves

Ethyl Acetate Test

• Add some drops of ethanol to the pinch of salt in a test tube, followed by H2SO4:
  • Procedure: Gently heat and cool down
  • Observation: Pleasant, fruity odor
  • Result: Anion acetate presence confirmed

Ethyl Acetate Test Summary

• Ethyl Acetate Test Procedure:
  • Add a few drops of ethanol to a pinch of salt in a test tube.
  • Follow by adding 1-2ccs of H2SO4.
  • Gently heat the mixture and then allow it to cool down.
  • Observe the presence of a pleasant and fruity odor.
  • Confirmation of the presence of the acetate anion.
• Observations and Inferences:
  • Carbonate Anion (CO32-):
    • CO2 gas evolves vigorously with dilute sulfuric acid, turning lime water milky.
  • Sulphide Anion (S2-):
    • When sodium nitroprusside is added, it appears violet or purple.
  • Sulfite Anion (SO32-):
    • A white precipitate forms with barium chloride solution, dissolving in sulfur dioxide gas. It also dissolves in dilute hydrochloric acid.

Chemical Reactions of Anions

Sulfite Anion (SO3^2-)

• A white precipitate forms with a barium chloride solution.
• This precipitate dissolves in sulfur dioxide gas and dilute hydrochloric acid.

Sulfate Anion (SO4^2-)

• Take 1 mL of salt water extract in sodium carbonate or water.
• Add BaCl2 solution after acidification with dilute hydrochloric acid.
• An insoluble white precipitate forms in concentrated HCl or HCl, confirming the presence of sulfate ions.

Nitrite Anion (NO2^-)

• Add drops of potassium iodide solution and starch solution, acidified with acetic acid.
• A blue color appears as a result of the chemical reaction.

Nitrate Anion (NO3^-)

• Take 1 mL of salt solution in a test tube and add 2 mL of a concentrated solution.
• Mix thoroughly with H2SO4 solution and cool the mixture under tap water.
• Introduce freshly prepared ferrous sulfate without shaking on the sides of the test tube.
• A dark brown ring forms at the junction of the two solutions, indicating the presence of nitrate ions.

Chemical Reactions of Anions

• Nitrate Anion (NO3-)
  • Take 1 mL of salt solution in a test tube in water.
  • Add 2 mL of a concentrated solution.
  • Mix thoroughly with H2SO4 solution.
  • Cool the mixture under tap water.
  • Add freshly prepared ferrous sulfate without shaking on the sides of the test tube.
  • A dark brown ring can be formed at the junction of the two solutions.
• Chloride Anion (Cl-)
  • Take 0.1 g of salt in a test tube.
  • Add a pinch of manganese dioxide and 3-4 drops of concentrated sulfuric acid.
  • Heat the reaction mixture.
  • Observation: Greenish yellow chlorine gas is produced, detected by its bleaching action and strong odor.
• Bromide Anion (Br-)
  • Take 0.1 gm of salt in a test tube with a pinch of MnO2.
  • Add concentrated sulfuric acid and heat it with 3-4 drops.
  • Observation: Evolution of intense brown fumes.

Chemical Tests for Anions

• Iodide Anion (I^-)

  Take 0.1 gm of salt in a test tube with a pinch of MnO₂. Add concentrated sulphuric acid and heat it; observe the evolution of intense brown fumes.

  Take 1 mL of the salt solution in water in a test tube. Add 2 mL of H₂SO₄ solution, mix thoroughly, and cool the mixture. Add freshly prepared ferrous sulfate without shaking. Look for a dark brown ring at the solution junction.

• Phosphate Anion (PO₄^3-)

  Acidify the salt solution or sodium carbonate extract in water with concentrated HNO₃. Add ammonium molybdate solution, heat to boil, and observe a precipitate of canary yellow.

Chemical Anion Tests

Chemical Laboratory Safety Precautions

• Label Checking: Always read and understand the labels on chemical bottles before use. Avoid using any unlabeled reagents as they can be hazardous.
• Vapor Awareness: When smelling vapors or chemicals, ensure caution. Gently fan vapors towards your nose to avoid inhaling them directly.
• Protective Gear: Wear hand gloves, an apron, and eye protection when working in a chemical laboratory to safeguard yourself from potential hazards.
• Chemical Handling: Avoid unnecessary mixing of chemicals and reagents. Never taste any chemicals and refrain from disposing of sodium metal in regular bins or sinks.
• Acid Dilution: Always pour acid into water for dilution purposes. Never add water to acid as it can lead to dangerous reactions.
• Test Tube Safety: Exercise caution when heating test tubes. Never point a test tube containing reagents or undergoing heating towards anyone to prevent accidents.
• Work Environment: Maintain cleanliness in your workspace. Dispose of papers and glass in designated bins and not in sinks. Keep your work area organized and clean.
• Chemical Usage: Use reagents in moderation. Excessive use not only wastes chemicals but also harms the environment. Always follow guidelines for the correct quantity.
• Hazard Awareness: Be cautious around flammable substances, explosive compounds, electrical appliances, toxic gases, glassware, hot materials, and flames. Understand the risks associated with these items and handle them with care.
• Hygiene Practices: Wash your hands thoroughly after completing laboratory work to prevent the spread of chemicals and maintain personal hygiene.

Systematic Analysis of Anions

• Testing for Sulfide Ions
  • When dilute sulfuric acid is added to a salt, it produces hydrogen sulfide (H2S), recognizable by its rotten egg smell.
  • The presence of sulfide ions can be confirmed by the formation of a black spot of arsenic sulfide (Ag2S) when sulfide ions react with silver foil.
  • For example, when sulfide ions (S²⁻) are added to silver foil, a reaction occurs leading to the formation of black arsenic sulfide.
• Understanding the Common Ion Effect
  • The common ion effect describes the impact on equilibrium when a common ion is added to a solution containing that ion.
  • Typically, the common ion effect decreases the solubility of a substance in the solvent.
  • It specifically refers to the reduced solubility of ions in precipitates when soluble compounds with a common ion are added to the solution.
  • For instance, when a soluble compound with a common ion is added to a solution, it decreases the solubility of the original salt.

Chemical Analysis Techniques

Testing for Carbonate Ion Presence

• Carbonate ions can be detected in solid compounds or solutions by adding a dilute acid like hydrochloric acid.
• When a dilute acid is added to a compound containing carbonate ions, bubbles of carbon dioxide are released.
• The release of carbon dioxide can be confirmed by passing the gas through limewater, turning it milky due to the formation of calcium carbonate.

Iodine Reaction with Starch

• Adding iodine to a starch solution results in a color change to blue-black due to the formation of a triiodide complex.
• The interaction with amylase molecules causes this color change as iodine molecules slip into the coil of amylase molecules, forming the complex.
• The blue-black color arises from the formation of intermolecular charge transfer complexes in the presence of triiodide ions.

Sequence of Anionic and Cationic Analysis

• Anionic analysis precedes cationic analysis to prevent interference between the two types of ions.
• Separating anions first allows for a more accurate and efficient identification of cations in a given sample.
• For example, if both are analyzed together, certain cations might react with anions, leading to inaccurate results.
• By prioritizing anionic analysis, the subsequent cationic analysis can be conducted with higher precision.

Anionic Analysis Precedes Cationic Analysis

• Anionic analysis is typically conducted before cationic analysis to ensure accurate results.
• Certain anions like oxalate, phosphate, and fluoride can disrupt the detection of group 3 cations such as chromium, iron, and aluminum.
• Detecting these interfering anions is crucial before proceeding with cation analysis to avoid erroneous results.

Importance of Anionic Analysis

• Identifying anions like oxalate, phosphate, and fluoride is essential before cationic analysis to prevent misinterpretation of results.

Example Scenario

• For instance, if oxalate ions are present in the solution during cation analysis, they can form insoluble precipitates with certain cations, leading to inaccurate conclusions.