September 30, 2023

Assay of Dapsone

Assay of Dapsone

BP607P Medicinal Chemistry III Practical: I Preparation of drugs Sulphanilamide * 7-Hydroxy, 4-methyl coumarin * Chlorobutanol * Triphenyl imidazole * Tolbutamide * Hexamine II Assay of drugs Isonicotinic acid hydrazide * Chloroquine * Metronidazole * Dapsone * Chlorpheniramine maleate * Benzyl penicillin III Microwave irradiation technique Synthesis of Phenytoin by Microwave * Synthesis of Aspirin by Microwave IV Drawing structures and reactions using chem draw®


To determine the percentage purity of a given sample of the tablet.


Dapsone is a diamino diphenyl sulphone. It is assayed by direct diazotization titration. The free primary amino group present in dapsone is diazotized by nitrous acid and hydrochloric acid to form arene diazonium compound. The end point can be determined by using external indicator i.e. starch iodide paper, during titration with sodium nitrite an aromatic primary amine is diazotized.

After the end point the drug solution containing a slight excess of nitrite comes in contact with starch iodide paper. It oxidizes iodide into iodine which gives blue colour with starch indicating the end point.


Assay of Dapsone


Preparation of 0.1M Sodium nitrite solution:

7.5g of sodium nitrite was dissolved in sufficient water to produced 1000ml.

Standardisation of 0.1M Sodium nitrite:

0.3g of sulphanilic acid was dissolved in 50ml of 2M hydrochloric acid, 3g of potassium bromide was added, cool in ice and titrate with 0.1M sodium nitrite solution using starch iodide paper as external indicator.

Each ml of 0.1M Sodium nitrite= 0.01732g of C6H7NO3S.

Assay of Dapsone tablet:

20 tablets were weighed and powered. Tablet powder equivalent to 0.25gm dapsone was weighed and dissolved in a mixture of 15ml of 2M hydrochloric acid.

The solution was cooled to about 15oC and carry out sodium nitrite titration using starch iodide paper as an external indicator.

The endpoint is immediate appearance of blue colour.

Each ml of 0.1M sodium nitrite= 0.01241g of C12H12N2O2S.


The Molarity of 0.1M sodium nitrite=
The percentage purity of given dapsone tablet was found to be=

What is diazotization reaction?

Diazotization reaction, also known as diazotization, is a chemical reaction that involves the conversion of a primary aromatic amine (containing an -NH2 group attached to an aromatic ring) into a diazonium salt. This reaction is achieved by treating the primary aromatic amine with sodium nitrite (NaNO2) in the presence of an acid, typically hydrochloric acid (HCl).

The diazotization reaction proceeds through several steps:

Formation of Nitrous Acid: In the presence of an acid, sodium nitrite (NaNO2) reacts with water (H2O) to produce nitrous acid (HNO2):
NaNO2 + HCl → HNO2 + NaCl

Diazotization: The nitrous acid then reacts with the primary aromatic amine (Ar-NH2) to form a diazonium salt (Ar-N2+X-), where X represents the counterion, usually chloride (Cl-) or tetrafluoroborate (BF4-):
HNO2 + Ar-NH2 → Ar-N2+X- + 2H2O

Isolation and Utilization: The diazonium salt is typically isolated as a solid or used directly in further reactions. Diazonium salts are versatile intermediates and can undergo various reactions, including coupling reactions, diazo-coupling reactions, and other substitution reactions, to produce a wide range of organic compounds, such as azo dyes, aromatic compounds, and pharmaceuticals.

Diazotization reactions have significant importance in organic synthesis and the preparation of various organic compounds. The resulting diazonium salts are highly reactive and can be used for the introduction of functional groups onto aromatic rings, leading to the synthesis of diverse organic molecules. Additionally, diazonium salts can undergo reactions like Sandmeyer reactions, Schiemann reactions, and Gomberg-Bachmann reactions, which further expand their synthetic utility.

starch iodide paper preparation

Starch iodide paper, also known as iodine test paper, is used to detect the presence of certain oxidizing agents, such as chlorine and peroxides. Here’s how you can prepare starch iodide paper:

Materials needed:
Starch powder (cornstarch or potato starch)
Distilled water
Iodine solution (typically potassium iodide and iodine dissolved in water)
Filter paper or absorbent paper strips
Glass rod or brush for application


Prepare the starch solution: Take a small amount of starch powder (around 1-2 grams) and mix it with a small amount of distilled water in a beaker or container. Stir the mixture well until the starch is completely dissolved.

Prepare the iodine solution: In a separate container, prepare a dilute iodine solution. This is typically done by dissolving potassium iodide (KI) and iodine (I2) crystals in distilled water. The concentration of the iodine solution can vary depending on the desired sensitivity of the test paper.

Mix the solutions: Pour the iodine solution into the starch solution while stirring continuously. Continue stirring until the mixture is well mixed and appears dark blue in color. The presence of iodine reacts with starch to form a deep blue complex.

Paper preparation: Cut filter paper or absorbent paper into small rectangular strips or squares, depending on the desired size of the test papers. Using a glass rod or brush, evenly apply the starch iodide solution onto the paper. Make sure the entire surface is coated but avoid excessive saturation.

Drying: Allow the starch iodide paper to air dry completely. This can be done by placing the prepared paper strips on a clean, flat surface or by hanging them in a well-ventilated area. Ensure that the papers are fully dry before storage or use.

Note: It’s important to handle iodine and iodine solutions with care as they can stain and are toxic if ingested or inhaled. Take appropriate precautions such as wearing gloves, working in a well-ventilated area, and storing the solutions in tightly sealed containers.
Once the starch iodide paper is prepared and dry, it can be used by simply exposing it to the substance being tested. If the substance is an oxidizing agent, it will react with the iodine present in the paper, resulting in a color change. The appearance of a blue or dark blue color indicates the presence of an oxidizing agent.

Third Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise

T Y B Pharm Sem VT Y B Pharm Sem VI
BP501T Medicinal Chemistry II TheoryBP601T Medicinal Chemistry III Theory
BP502T Industrial Pharmacy TheoryBP602T Pharmacology III Theory
BP503T Pharmacology II TheoryBP603T Herbal Drug Technology Theory
BP504T Pharmacognosy II TheoryBP604T Biopharmaceutics and Pharmacokinetics Theory
BP505T Pharmaceutical Jurisprudence TheoryBP605T Pharmaceutical Biotechnology – Theory
BP506P Industrial Pharmacy I PracticalBP606T Quality Assurance Theory
BP507P Pharmacology II PracticalBP607P Medicinal chemistry III Practical
BP508P Pharmacognosy II PracticalBP608P Pharmacology III Practical
BP609P Herbal Drug Technology Practical

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