Chapter 2. Analysis of inorganic drug substances. Halogen inorganic compounds. Hydrogen peroxyde. Sodium nitrite. Sodium thiosulfate

Most inorganic drug substances are electrolytes, and their identification, assay and purity control include identification of ions: cations and anions.

Hydrogen peroxyde

Hydrogen peroxide (H₂O₂) is a colorless liquid with boiling point of 152 °C.

Drug substances containing hydrogen peroxide:

• Concentrated solution of hydrogen peroxide (perhydrol) (Solutio hydrogenii peroxydi concentrata), hydrogen peroxide content 30%;
• Solution of hydrogen peroxide (Solutio hydrogenii peroxy didiluta), hydrogen peroxide content 3%;
• Magnesium peroxide (Magnesiiperoxydum) - magnesium peroxide and magnesium oxide mixture (MgO₂, MgO).

Identification

Acidic properties

Hydrogen peroxide exhibits weak acidic properties:

Н₂О_{2 ↔} Н⁺ + НО₂^-

Oxidation-reduction properties

Hydrogen peroxide can act as both oxidizer and reducer.

Oxidation reaction with a more powerful oxidizer, such as potassium permanganate:

2KMnO₄ + 5H₂O₂ + 3H₂SO_{4 →} 2MnSO₄ + K₂SO₄ + 8H₂O + 5O₂

Reduction reaction. For example, with potassium iodide:

H₂O₂ + 2KI + H₂SO_{4 →} I₂ + 2H₂O + K₂SO₄

Specific reaction of perchromate formation

Interaction of hydrogen peroxide with potassium dichromate in the presence of ether leads to formation of blue perchromic acid.

Perchromic acid composition depends on reaction conditions (temperature, pH, hydrogen peroxide concentration):

Those compounds containing peroxide chain are highly unstable.

Interaction of hydrogen peroxide with acids

Hydrogen peroxide salts are unstable. Under the influence of inorganic acid solutions they decompose, forming hydrogen peroxide:

BaO₂ + H₂SO_{4 →} BaSO₄↓ + H₂O₂

Assay

Direct method: permanganometry.

Titrant is potassium permanganate.

Method doesn’t use indicator, titration end-point is identified by pink color:

2KMnO₄ + 5H₂O₂ + 3H₂SO_{4 →} 2MnSO₄ + K₂SO₄ + 8H₂O + 5O₂

Quantification of hydrogen peroxide is also performed with indirect iodometric method:

H₂O₂ + 2KI + H₂SO_{4 →} I₂ + 2H₂O + K₂SO₄

Forming iodine is titrated with solution of sodium thiosulfate:

I₂ + 2Na₂S₂O_{3 →} 2NaI + Na₂S₄O₆

Hydrogen peroxide is unstable and during storage with access to light or in the presence of catalyzers it can decompose forming water and oxygen:

2H₂O_{2 →} 2H₂O + O₂↑

Sodium benzoate is added to hydrogen peroxide solution as a stabilizer. Sodium benzoate stabilizer can be quantified with acidimetric titration, using hydrochloric acid as a titrant (indicator - mixture of methyl orange and methyl blue) in the presence of ether, which is needed to remove forming benzoic acid.

Hydrogen peroxide drugs are stored in a dry place, protected from light.

Hydrogen peroxide solution is used as an antiseptic, deodorizing and depigmenting agent. For external use only.

Perhydrol is used for preparation of 3% hydrogen peroxide solution.

Magnesium peroxide is used as an antacid and antiseptic drug.

Halogen derivatives

Inorganic halogen containing drug substances are divided into two groups:

1. free (molecular) halogens - iodine (crystalline iodine and iodine spirituous solution);
2. hydrochloric acid and drug substances containing salts of hydrohalogenic acids (potassium and sodium chlorides, bromides and iodides, sodium fluoride).

Iodine and its drugs

Iodine (from Greek “iodos” - violet) was discovered in 1811 by French pharmacist B. Courto is in seaweed ash.

Table 2.1. Iodine

Identification

Sublimation

Several iodine crystals are heated in dry test tube: purple vapors and formation of dark-blue metallic deposits on the upper colder part of the tube can be observed.

Reaction of iodine with starch

A very sensitive reaction, in the presence of even trace amounts of iodine starch obtains blue color.

Oxidative properties

Iodine, being a free halogen, has stronger oxidative properties, than chlorine. Specifically, iodine can oxidize sulfides, sulfites and thiosulfates:

I₂ + 2Na₂S₂O_{3 →} 2NaI + Na₂S₄O₆

Purity

When obtained from seaweeds, iodine can contain dangerous impurity of cyanogen iodide (ICN), which forms from plant carbon and nitrogen due to incomplete ashing of plant material.

Cyanogen iodide impurity is tested with “Prussian blue” formation
-Fe₄[Fe(CN)₆]₃↓. To perform this reaction, iodine is discolored with solution of sulfurous acid: