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Test for Manganous Radical

General Aim of Manganous Radical

Detection of the presence of manganese ions as a base radicals in inorganic salts such as manganese chloride.

 

Method of Manganous Radical

Detection of the presence of manganese as a base radical using specific chemical reagents.

Theoretical Background / Context

• Manganese is the 25^th abundant element on earth. 
• Its name comes from the Latin word ‘magnes’ which means a magnet owing to its magnetic properties. 
• Manganese is one of the few elements that humans need daily. 
• Manganese was first isolated from the reduction of manganese compounds such as manganese dioxide. 
• Manganism which is manganese poisoning could result from exposure to manganese dust or fumes for a long duration.

The abundance of Manganese in Nature: 

• Manganese is considered to be the 25^th most abundant element in the earth's crust. 
• Manganese cannot be found easily in nature as a free metal. 
• However, it is found as a mineral in the form of oxides, silicates, or carbonates. 
• Manganese could be extracted from those ores. 
• It is also found in the ocean floor in the form of nodules that are considered to be large lumps of metallic ores.

Properties and Uses of Manganese: 

• Manganese is a hard, brittle, gray-white metal when found in its pure form. 
• It is commonly known for being used as an alloying agent in steel manufacturing. 
• It enhances the resistance of steel to impact. 
• Long ago, Romans have used manganese in manufacturing their weapons and strong equipment for war due to its hardness. 
• Moreover, Romans used manganese as a pigment.

Manganese Compounds: 

• Manganese is a highly reactive metal so it possesses a high ability to react with different elements to produce various manganese containing compounds with various uses and functions. 
• This is attributed to manganese’s valence electron that allows a diversity of reactions. 
• In biological systems, manganese is a mandatory element in the composition of vitamin B1.

• Manganese, like many transition metals, has various oxidation states that could exceed 5 stable oxidation states. 
• Each oxidation state possesses a distinctive color. Therefore, it is commonly used as a chemical reagent or indicator. 
• For instance, it is commonly found in the laboratory as potassium permanganate KMnO₄ that is used as a strong oxidizing agent. 
• In addition, MnO₂ catalyzes the decomposition of H₂O₂ as well as the production of oxygen gas on a lab scale. 
• Furthermore, manganese is incorporated in some inorganic salts such as manganese chloride whose molecular formula is MnCl₂.

Manganese Oxidation Test:

The manganese oxidation test is used to identify the oxidation state of manganese ions in a given compound or solution.

It relies on observing the characteristic color change associated with the transition between different oxidation states of manganese — for example, from the purple permanganate ion (Mn⁷⁺) to the nearly colorless Mn²⁺ ion.

 Preparation of Manganese Chloride: 

• Manganese chloride could be prepared through the reaction of manganese (IV) oxide with concentrated hydrochloric acid. 
• This reaction has been used also for the preparation of chlorine

MnO₂ + 4 HCl  → MnCl₂ + 2H₂O + Cl₂ ↑

• Furthermore, on the laboratory scale, manganese chloride can be prepared through the reaction of manganese metal or manganese carbonate with hydrochloric acid.

Mn + 2 HCl + 4 H₂O → MnCl₂.4H₂O + H₂

MnCO₃ + 2 HCl + 3 H₂O → MnCl₂.4H₂O + CO₂

• Manganese chloride hydrates are water soluble forming acidic solutions of pH 4 containing the metal aqua complex [Mn(H₂O)₆]²⁺. 

Properties and Uses of Manganese Chloride:

• Manganese chloride is a white powdered salt when it is dehydrated. Its color turns pink upon its hydration.
• Anhydrous manganese (II) chloride is used to initiate the synthesis of various manganese compounds. 
• For instance, manganocene is prepared through reacting MnCl₂ with sodium cyclopentadienide solution in THF.

MnCl₂ + 2 NaC₅H₅ → Mn(C₅H₅)₂ + 2 NaCl

• Manganese chloride reacts with typical organic ligands, so that manganese (II) gets oxidized by air to give Mn (III) complexes such as [Mn(EDTA)]⁻, [Mn(CN)₆]³⁻, and [Mn(acetylacetonate)₃]. 
• For example, triphenylphosphine forms a liable 2:1 adduct as shown below.

MnCl₂ + 2 Ph₃P → [MnCl₂(Ph₃P)₂]

• Manganese chloride is commonly used in dry cell batteries manufacturing. 

It is also used as a precursor for the antiknock compound methylcyclopentadienyl manganese tricarbonyl.

 

Principle of Work

• In manganese ion detection, manganese ion in manganese chloride is detected through some identification and confirmatory tests for manganous radical.
• The manganese radical is among the fourth group of basic radicals in which ammonium sulfide along with ammonium hydroxide and ammonium chloride are used as group reagents.
• During the manganese(II) chloride test, salt solubility in water will be tested. 
• Then the behavior of the salt will be tested with ammonium sulfide followed by a confirmatory test for manganese which will be performed using sodium hydroxide and ammonia for the manganese radical confirmation in salts 

First: Physical Appearance Test: In this test, a sample of the manganese chloride salt is tested for its color, odor, texture, etc.

Second: Solubility Test: In this test, a sample of the manganese chloride salt is tested for its solubility in water. 

Third: Ammonium Sulfide Test: 

It depends on the fact that sulfide anions can displace chloride ions in manganese chloride salt forming insoluble manganous sulfide as a pink precipitate through manganese precipitation reactions as the following reactions:

(NH₄)₂S + MnCl₂ → 2 NH₄Cl + MnS ↓ (Pink ppt.) 

Fourth: Sodium Hydroxide Test: 

• Sodium Hydroxide solution is added to aqueous manganese chloride leading to the precipitation of manganese hydroxide as a white precipitate that changes into brown with time due to its oxidation. 
• Both precipitates are not soluble in the excess reagent.

 MnCl₂ + 2NaOH → 2NaCl + Mn(OH)₂ ↓ (White ppt.)

Mn(OH)₂ + 2H₂O + O₂ → Mn(OH)₃ ↓ (Brown ppt.) 

Fifth: Ammonia Test (Manganese confirmatory test):

• Ammonia solution is added to aqueous manganese chloride leading to the precipitation of the manganese hydroxide as a white precipitate due to its low solubility product. 
• This manganous radical test within the inorganic analysis of manganese, shows that upon being exposed to air, the white precipitate changes into brown:

MnCl₂ + 2NH₄OH → NH₄Cl + Mn(OH)₂ ↓ (White ppt.)