Methodical instruction
for the students of pharmaceutical faculty
LESSON 16 (practical – 6 hours)
THEME: Redox Titration. Determination of contents of H2O2, KI.
AIM: to learn theoretical bases of a redox titration method; classification of methods; an essence of permanganatometry, dichromatometry, iodometry, iodimetry, iodatometry; to learn to define the content of hydrogen peroxide, potassium iodide, ascorbic acid by redoximetry.
PROFESSIONAL MOTIVATION OF STUDENTS
Oxidation-reduction titration is pharmacopoeia’s method. The method of redox titration is used most widely. Application of these methods can unite with physical methods of fixation of end point of titration. Therefore the future pharmacist should own knowledge of the method of redox titration.
The method of permanganatometry gives the chance quickly and exact to define content of Н2О2, iron, sodium nitrite, carboxylic acids, and compounds with hydrazo-groops R – NH – NH – R. The high oxidation-reduction potential of pair MnО4–/Mn2+ is the big advantage of permanganatometry. One more essential advantage is possibility of equivalence point fixation without indicators. Among lacks is impossibility of preparation of primary standard solution of КMnО4.
THE SELF-PREPARATION PROGRAM
1. Redoxometry: an essence, oxidimetry and reducimetry.
2. Requirements to reactions in a method of redoxometry.
3. Classification of redox-methods (oature of titrant).
4. Permanganatometry: an essence, titrant, stanardization of titrant, the indicator, titration conditions, practical application. Advantages and lacks.
5. Dichromatometry: an essence, titrant, stanardization of titrant, the indicator, titration conditions, practical application. Advantages and lacks.
6. Iodimetry: an essence, titrant, stanardization of titrant, the indicator, titration conditions, practical application. Advantages and lacks.
7. Iodometry: an essence, titrant, stanardization of titrant, the indicator, titration conditions, practical application. Advantages and lacks.
8. Iodatometry: an essence, titrant, stanardization of titrant, the indicator, titration conditions, practical application. Advantages and lacks.
TESTS AND REAL-LIFE SITUATIONS FOR SELF-ASSESSMENT
1. For definition of the content of active chlorine in potable water use a method:
A. Acid-base titration.
B. Dichromatometry.
C. Iodometry.
D. conductometry.
E. Permanganatometry.
2. The content of hydrogen peroxide without application of indicators can be defined by method of redox titration:
A. Chloriodіmetry.
B. Dichromatometry.
C. Cerimetry.
D. Iodimetry.
E. Permanganatometry.
3. At definition of oxidizers by a iodometry method as titrant use starch. We observe in the end point:
A. Occurrence of dark blue colouring.
B. Disappearance of dark blue colouring.
C. Occurrence of green colouring of a precipitatin.
D. Disappearance of green colouring of a solution.
E. Disappearance of brown-orange colouring of a solution.
4. Which substances are used in iodimetry as working solutions for difenition in return titration of reducers?
A. KMnО4, KI.
B. K2Cr2O7, Na2S2O3.
C. I2, KI.
D. I2, Na2S2O3.
E. K2Cr2O7, I2.
1. Normality of potassium permanganate in the sour medium is equal 0,02200 mol/L. Calculate titer of this solution on: Н2С2О4; Fe; Н2О2?
2. To calculate molarity, normality, titer and titer on Fe (II) and As (III) of potassium bichromate solution which are received dissolution shot of 2,4515 g potassium bichromate in 500 mL of water.
3. Which volume of 0,1 mol/L solution potassium permanganate it is necessary to take for preparation of 100 mL solution with normality of 0,05 mol/L?
4. Calculate shot of sodium oxalate for preparation of 100 mL standard solution with normality of 0,05 mol/L which is used for standardization of potassium permanganate solution. Calculate titer of sodium oxalate solution.
5. Calculate weight of potassium bichromate shot for preparation of 250 mL a solution of with normality 0,05 mol/L.
6. What mass part of iron in a preparation if after shot dissolution (weight 0,1400 g) in sulphatic acid without air access on titration of the received solution spends 5,85 mL of potassium permanganate solution with normality 0,1000 mol/L?
7. How many grammes of calcium contain in 250,0 mL of CaCl2 solution, if after addition 40,00 mL of 0,1000 mol/L (NH4)2C2O4 to 25,00 mL of this solution and separation of CaС2O4 precipitate, on titration of ammonium oxalate excess are spent 15,00 mL of 0,02000 mol/L (normality) solution of KMnО4?
8. For permanganatometric definition of magnesium peroxide MgО2 was prepared 10 mL of solution in which contain 0,2045 g, on titration of the prepared solution have spent 19,75 mL of a standard potassium permanganate solution with titer on magnesium peroxide 0,002816 g/mL. Calculate weight and mass fraction magnesium peroxide in a preparation.
9. For an establishment of titer sodium thiosulphate is took of shot 0,1125 g of copper to which add solution of KI. Iodine which was allocated, an investigated solution is titrated by sodium thiosulphate (18,99 mL). Calculate: titer of this solution; titer of solution Na2S2O3 on Cu.
10. Which shot of K2Cr2O7 is necessary to take for an establishment of titer of 0,1 mol/L Na2S2O3 solution if for preparation of a primary standard K2Cr2O7 solution is a volumetric flask (200 mL) and pipette for selection of aliquot (10.00 mL). The recommended volume of Na2S2O3 for titration of iodine which will be allocated, spend 25,00 mL.
11. How many grammes of Cl2 contain in 1 L of chloric water if on titration of iodine which is allocated afret action of 25 mL KI, 20,10 mL of 0,1100 mol/L sodium thiosulphate solution are used?
12. How many grammes HCl contain in 250 mL of a solution chloridic acids if 24,00 mL of a solution are spent for titration of iodine which was allocated after addition 25,00 mL of mix KIO3 + KI solution of 0,02100 mol/L sodium thiosulphate?
13. To define a mass fraction of ascorbic acid, if for titration of 0,3568 g shot is spent 19,95 mL of 0,1008 mol/L iodine solution (
). Equation of reaction:
ANSWERS TON THE SELF-ASSESSMENT
Tests: 1. С, 2. E., 3. B., 4. D.
Tasks: 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13.
REFERENCES:
1. The lecture’s material.
2. David Harvey. Modern Analytical Chemistry // www.mhhe.com
METHOD OF IMPLEMENTATION OF PRACTICAL WORK
Work 1. Preparation and standardization of a solution of 0,02 mol/L (0,1 N) potassium permanganate.
Dissolve 3.2 g of potassium permanganate in 1000 ml of water, heat on a water bath for 1 hour, cool and filter through a sintered-glass filter.
Ascertain its exact concentration immediately before use in the following manner. To 20 ml of the solution add 2 g of potassium iodide and 10 ml of 1M sulphuric acid and titrate with 0.1M sodium thiosulphate VS using 1 ml of starch solution, added towards the end of the titration, as indicator. Each ml of 0.1M sodium thiosulphate VS is equivalent to 3.161 mg of KMnO4.
Store protected from light.
Concentration of secondary standard solution of KMnO4 calculates using this formula:
Work 2. Definition of hydrogen perioxide in drugs.
Definitions is based on reaction
5H2O2+2MnO4– + 6H+ = 5O2+2Mn2++ 8H2O
Apparently from the equation, H2O2 in this reaction is a reducer and is oxidised to oxygen:
H2O2 – 2e = O2+2H+
Accordingly:
g/mol
Dilute 10.0 g to 100.0 ml with water R. To 10.0 ml of this solution add 20 ml of dilute sulphuric acid R. Titrate with 0.02 M potassium permanganate until a pink colour is obtained.
1 ml of 0.02 M potassium permanganate is equivalent to 1.701 mg of H2O2 or 0.56 ml of oxygen.
Content of hydrogen perioxide in drugs calculates using this formula:
Hydrogen peroxide solution (3 per cent) contains not less than 2.5 per cent m/m and not more than 3.5 per cent m/m of H2O2 (Mr 34.01).
Work 3. Definition the maintenance of ascorbic acid in a dragee of 0,05 ascorbic acid
0,74 g a powder of the pounded dragees place in a conic flask spaciousness of 250 ml, add 80 ml of water, free from carbon dioxyde and 10 ml of sulphatic acid, dissolve, then add 1 ml of 1 % starch solution and titrate by 0,05 mol/l iodine solution before reception of proof blue-violet colouring.
This titration repeats three times. By results of titrations define the maintenance of ascorbic acid in a drug, considering on average weight of a dragee.
Content of ascorbic acid in dragee calculates using this formula:
The content of acid ascorbic in one dragee should be 0,045 – 0,055 g.
BASIC LEVEL OF KNOWLEDGE AND SKIILS
A STUDENT MUST KNOW:
1. Essence of redoxmetry, types of reactions which are applied, requirements to them.
2. Classification of methods redox titration.
3. An essence of permanganatometry, dichromatometry, iodometry, iodimetry, iodatometry and their application.
A STUDENT MUST BE ABLE:
1. To prepare and standardise of solutions potassium permanganate, potassium bichromate, iodine, sodium thiosulphate, potassium iodate.
2. To define the contain of reducers and oxidizers by permanganatometry, dichromatometry, iodometry, iodimetry, iodatometry.
