Sahar Mohammed Shakir Abdul Hafeedh Hameed, Summaries of Medical Biochemistry

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Assistant Lecturer Sahar Mohammed Shakir Assistant Lecturer

Abdul Hafeedh Hameed

● Chem. formula, C 6 H 8 O 6 M.Wt, 176.13 gm /mol

● It is a naturally occurring organic compound. ● White to slightly yellowish crystals or powder. ● It dissolves well in water to give mildly acidic solutions. ● In the dry state, it is reasonably stable to air. ● In solution, it’s rapidly oxidized in the presence of air , the reaction is catalyzed by traces of some metals especially copper. ● If the medium is alkaline on exposure to air and light, it gradually darkens.

Name of experiment : Assay of Ascorbic Acid Aim of experiment : Determination of the amount of Ascorbic acid in an unknown sample by Redox titration.

1- In a glass stoppered - iodine flask , dissolve the unknown powder of

ascorbic acid in 10 mL D.W., then add 50 mL of concentrated HCl

.2- Cool with constant swirling till ascorbic acid dissolves.

3- Run in 30 mL of M/100 KIO 3 solution from a burette , cool , add

5 mL of CHCl 3.

4- Stopper the flask and shake vigorously.

5- Continue titration alternatively adding small volume

of M/100 KIO 3 & shake vigorously until the layer of CHCl 3 is just discharged, ( colorless ).

6- Calculate the weight of ascorbic acid by using the chemical factor.

In our analysis today ascorbic acid is titrated with 0.01 M KIO 3 primary standard solution under strongly acidic medium. Potassium iodate is a powerful oxidizing agent , but the course of the reaction is governed by the conditions under which it is employed.

Each 1 mL of M / 100 KIO 3 is equivalent to 0. 003523 g of ascorbic acid.

2 C 6 H 8 O 6 + HIO 3 + HCl 2 C 6 H 6 O 6 + 3 H 2 O + ICl 2 moles 1 mole

Iodate solutions are best expressed in terms of molarity

instead of the more usual normality, since KIO 3 solution is often used in the titration of solution containing both iodide and free iodine. The equivalence of KIO 3 in it's reaction with KI defers from it's equivalence when reacted with the free iodine.

In Andrew’s titration, the medium must be strong acidic

, since in moderate & low acidity the reaction between KIO 3 and the reducing agent stops at the stage when the iodate is reduced to iodine, (The reaction is not completed).

While in strong acidic medium, HCl at or above 5 M,

(Andrew’s Condition ), the free iodine is further reduced

to iodine monochloride.

1 - For each of the following molecules; Write the molecular formula, Draw the chemical structure & Mention the oxidation state of iodine atom in the molecule: a- Iodic acid. b- Potassium iodate. c- Hydroiodic acid. d- Potassium iodide. e- Free iodine. f- Iodine monochloride.

2 - How could you prepare 1 L of 0. 01 M KIO 3 standard solution Knowing that the molar mass of KIO 3 is 214 g/mol.

3 - Prepare a solution of KIO 3 from a 0. 25 M primary standard solution , using a 10 mL volumetric pipette & a calibrated 250 mL volumetric flask & Find the molarity of the prepared solution.

7 - What is the purpose of using the following in our assay experiment: a- CHCl 3? b- Concentrated HCl? c- Glass – stoppered iodine flask?

8 - Describe the color changes observed throughout the titration and explain them with the aid of equations. Give, also, the overall equation of the reaction.

***** J. Mendham , R. C. Denney , J. D. Barnes , M. Thomas , Oxidation with potassium iodate , Vogel's Textbook of Quantitative Chemical Analysis, 6 th^ edition, 2000.

***** Samira Finjan Hassan , Amer Nadem , May Mohammed Jawad , Assay of Ascorbic Acid , A Laboratory manual on Practical Medical Chemistry for 4th^ year students , University of Baghdad , College of Pharmacy , Department of Pharmaceutical Chemistry , 2010.