Chapter 3. Sodium hydrogen carbonate, lithium carbonate, magnesium, calcium, barium and boron compounds

Sodium hydrogen carbonate

Natrii hydrocarbonas

NaHCO₃

Appearance and solubility

White crystalline powder, odorless, stable in dry air, efflorescent in moist air. Soluble in water, practically insoluble in 95 percent alcohol.

Identification

Identification reactions of ions for sodium cation and hydrogen carbonates

1. Flame test. Reaction of sodium cation.
2. Yellow precipitate formation with zinc uranyl acetate.

Na⁺ + Zn²⁺ + [(UO₂)₃(CH₃COO)₈]^2- + CH₃COOH + 6H₂O → NaZn[(UO₂)₃(CH₃COO)₉] ∙ 6H₂O↓ + H⁺

3. Carbon dioxide bubble appearance after the addition of diluted mineral acids:

HCO₃^- + H⁺ → CO₂↑ + H₂O

The precipitate of calcium carbonate is formed after the evolved carbon dioxide is passed through the limewater:

CO₂↑ + Ca(OH)₂ → CaCO₃↓ + H₂O

Experimental proof of the distinction between hydrogen carbonates and carbonates.

1. By means of the phenolphthalein indicator:

CO₃^2- + H₂O → HCO₃^- + OH^-, alkaline medium, pink color of the indicator;

HCO₃^- + H₂O → CO₂ + H₂O + OH^-, as well as:

HCO₃^- ⇄ CO₃^2- + H⁺, weak alkaline medium, colorless indicator.

2. Carbonates form a white precipitate with magnesium sulfate at once, while hydrogen carbonates form a white precipitate with magnesium sulfate only after boiling:

4Na₂CO₃ + 4MgSO₄ + 4H₂O → 2MgCO₃ ∙ 2Mg(OH)₂ ∙ 2H₂O↓ + 4Na₂SO₄ + 2CO₂↑

2NaHCO₃ → boiling → Na₂CO₃ + H₂O+ CO₂↑

Purity

Sodium carbonate content is determined through the ignition test. Loss on ignition is not less than 36.6 percent. In case sodium carbonate content is higher, loss on ignition will be lower:

2NaHCO₃ → ignition → Na₂CO₃ + H₂O+ CO₂↑

Permissible impurities: chlorides, sulfates, iron, calcium, arsenic.

Impermissible impurities: heavy metals, ammonium salts.

Content of difficultly soluble carbonates of heavy metal: 0.3 percent.

Assay

Direct acidimetry. Titrant: 0,1Н HCl. Indicator: methyl orange.

NaHCO₃ + HCl → NaCl + H₂O+ CO₂

At the endpoint the coloring changes from yellow to red.

Lithium carbonate

Lithii carbonas

Li₂CO₃

Appearance and solubility

White light powder. Slightly soluble in water, very slightly soluble in 96 percent alcohol.

Identification

Lithium ion determination

1. Lithium salt, soaked in HCl, colors the flame in carmine red.
2. White precipitate is formed with disodium hydrogen phosphate:

2Na₂HPO₄ + 3Li₂CO₃ → 2Li₃PO₄↓ + 2Na₂CO₃ + H₂O + CO₂↑

3. Formation of red complex with isoquinoline that exhibits a blue fluorescence in UV light.

Carbonate ion determination

Carbon dioxide bubbles appear after the addition of weak acids:

Li₂CO₃ + 2HCl → 2LiCl + H₂O + CO₂↑

The precipitate of calcium carbonate is formed after the evolved carbon dioxide is passed through the limewater:

CO₂↑ + Ca(OH)₂ → CaCO₃↓ + H₂O

Assay

Back acidimetry. Indicator: methyl orange.

Li₂CO₃ + 2HCl(excess) → 2LiCl + H₂O + CO₂↑

The obtained solution is boiled in order to remove carbon dioxide. The remained acid is titrated with NaOH solution:

HCl + NaOH → NaCl + H₂O

Magnesium sulfate heptahydrate

Magnesii sulfas

MgSO₄×7H₂O

Appearance and solubility

While powder or colorless prismatic crystals, efflorescent in air. Freely soluble in water, very soluble in boiling water, practically insoluble in alcohol.

Identification

Magnesium ion determination

1. Formation of magnesium ammonium phosphate white precipitate:

MgSO₄ + Na₂HPO₄ + NH₃ → MgNH₄PO₄↓ + Na₂SO₄

2. Magnesium cation forms a stable chelate complex with 8-hydroxyquinoline that represents yellowish-green crystals, pH 8.0-13.0:

Sulfate ion determination

Sulfates form a white precipitate, insoluble in acids and alkalis, with soluble barium salts:

SO₄^2- + Ba²⁺ → BaSO₄↓

The reaction is carried out in the presence of a treating agent, hydrochloric acid, that is needed for all other barium salts to dissolve (sulfites, carbonates, etc.).

Purity

The limit of acidity and alkalinity is determined in the drug by titrimetry (titrant - 0.01M NaOH solution, indicator - phenolphthalein). The impurities of chlorides, iron, heavy metals, arsenic are determined according to the current edition of State Pharmacopeia of the Russian Federation.

The specific impurities for this drug are manganese compounds. In order to define them, concentrated sulfuric acid is added to the solution (the medium), as well as silver nitrate (the catalyst) and ammonium persulphate for the oxidation of manganese compounds to permanganate: