2. Oxidation-Reduction Titrations1
Application of
Oxidation-Reduction Titrations
Hassan.F.Askal, Ph.D.

3. Importance of Redox Reactions in Biological systems and Pharmaceutical Applications2
Oxidizing agents act on human tissues by one of the following mechanisms:
Germicides through liberation of oxygen in the tissues such as hydrogen peroxide
Bactericidal against anaerobes as KMnO4 (for wounds) or Cl2 (for dental therapy) through denaturing the proteins by direct oxidation.
Oxidation can have deleterious effect on the cells and tissues of human body because it produces reactive oxygen and nitrogen species, such as OH, NO, NO2 and alkoxyl radicals RO which damage cells and other body components.
Antioxidants (reducing agents) can counteract the effect of the reactive oxygen and nitrogen species and protect the other compounds in the body from oxidation.

4. Importance of Redox Reactions in Biological systems and Pharmaceutical Applications3
Typical antioxidants include vitamin A, C and E; minerals such as selenium; herbs such as rosemary.
Antioxidants act by one or more of the following mechanisms:
Reaction with the generated free radicales can counteract the effect of the reactive oxygen and nitrogen species
Binding the metal ions needed for catalyzing the formation of reactive radicales
Repairing the oxidative damage to the biomolecules or induction of the enzymes that catalyze the repair mechanisms.

5. Redox Reactions and Biological Action4
Redox Reactions and Biological Action
Redox reactions are included not only in inorganic or organic drug analysis but also in the metabolic pathways of drugs.
Catabolic pathways: oxidation of C-atoms of carbohydrate or lipid or protein
Anabolic pathways: reduction of C-atoms of carbohydrate or lipid or protein
Phase-I Metabolic Pathways (Functionalization reactions)
I- Oxidative reactions
Oxidation of aromatic compounds → hydroxy aromatic compounds
Oxidation of olefins → Epioxides
Oxidative dealkylation

6. Redox Reactions and Biological Action5
Redox Reactions and Biological Action
Examples for the oxidative dealkylation pathways:
N-dealkylation, R-NH-CH3 → R-NH2 + CH2O
Deamination, R-CH2-CH2-NH2 → NH3 + R-CO-CH3
N-oxide formation, R3 - N → R3 N→O
N-hydroxylation. R2NH → R2N-OH
O-dealkylation R-O-CH3 → R-OH + CH2O
S-dealkylation, R-S-CH3 → R-SH + CH2O
S-oxidation, R2S → R2S=O + CH2O
Desulphuration R2C=S → R2C=O

7. ROH (oxidized product)
Redox Reactions in Biological System 6
NADPH + flavoprotein reductase
P-450-(Fe3+)
RH─P-450-(Fe3+)
RH─P-450-(Fe2+)
RH─P-450-(Fe2+)
O2- RH
(Parent drug)
H2O
ROH (oxidized product)

8. Redox Reactions in Biological System7
Example of redox reaction in biological systems ethyl alcohol can be oxidized in liver into acetaldehyde
II- Reductive reactions
Reduction of aldehydes and ketones
Reduction of nitro and azo compounds
Miscellaneous reductive reactions

9. Properties of Oxidizing Agents8
1. Potassium permanganate (KMnO4)
2. Potassium dichromate (K2Cr2O7)
3. Ceric sulphate Ce(SO4)2
4. Iodine (I2)
5. Potassium iodate (KIO3)
6. Sodium thiosulphate (Na2S2O3)
7. Potassium bromate (KBrO3)
8. Bromate-bromide mixture (KBrO3/KBr)

10. 1. Potassium permanganate (KMnO4)9
Powerful oxidant E° = 1.52 V.
In acid medium:
can oxidize: oxalate, Fe2+, Ferrocyanide, As3+, H2O2, and NO2.
MnO4 + 8H+ + 5e → Mn2+ + 4H2O
In alkaline medium:
MnO4 e → MnO42
H2SO4 is the suitable acid since HCl reacts with it: KMnO4 + 16HCl → 2 KCl + 2MnCl2 + 5Cl2 + 8 H2O
Zimmermann’s reagent prevents reaction of KMnO4 with Cl.
Not primary standard, may contain MnO2 which catalyses auto-oxidation that is accelerated by light and acids MnO4 + 2H2O → 4 MnO O OH
It is prepared after removal of MnO2 through glass wall (filter paper contains organic matter can decompose permanganate).
Self indicator.
Standardized by: 1- Oxalate Fe Arsenious oxide.

11. 2. Potassium dichromate (K2Cr2O7)10
2. Potassium dichromate (K2Cr2O7)
Less strong than KMnO4, E°= V.
Primary standard (highly pure and stable).
Not easily reduced by organic impurities in water.
Not reduced by HCl in dil. solution.
Not self indicator; since its orange colour is reduced to the green (Cr3+).
Used for determination of Fe2+ (Cl does not interfere); ferroin indicator.

12. 11
3. Ceric sulphate Ce(SO4)2
More powerful oxidant than KMnO4 and K2Cr2O7.
Reacts in acid medium, it has 2 E° according the type of acid In HNO3 medium E°=1.61V. and in H2SO4 E° = 1.45V
In neutral or alkaline medium its hydroxide or basic salt is ppt.
Not a primary standard (difficult to obtain pure)
Not affected by HCl or Cl on cold.
It can be used as self indicator, because its reduction product is colorlessbut it is better to use an internal redox indicator such as ferroin.
Standardized as KMnO4.

13. 4. Iodine (I2) 12 Slightly soluble in water and volatile.
KI added to increase solubility and decrease volatility (triiodide).
Titrations must be applied in stoppered flasks
Standard I2 prepared by:
Dissolving it in water after addition of KI (iodide is easily oxidized to iodine, specially in acid medium I + O2 + 4H+  2I2 + 2H2O
Mixture of iodate and iodide, solution is very stable in neutral on acidification I2 liberated.
IO3 + 5I + 6H+  3I2 + H2O
Standardized by titration with Na2S2O3 using starch as indicator.
I2 + 2Na2S2O3  2NaI + Na2S4O6 (sod. tetrathionate)
I I → I3 (triiodide ion)

14. 5. Potassium iodate (KIO3)13
5. Potassium iodate (KIO3)
Stronger oxidant than iodine.
It is obtained very pure.
It is more practical to prepare molar solution; due to variations in Eq. wt. according to the medium:
IO3 + 5I H+ = 3I H2O (0.1 N HCl) Eq. wt. = MW/5
IO3 + 2I H+ = 5H H2O (4-6N HCl) Eq.wt. = MW/4

15. 6. Sodium thiosulphate (Na2S2O3)14
6. Sodium thiosulphate (Na2S2O3)
Not used as primary standard, its solution is unstable; decomposed by CO2 and certain type of bacteria.
S2O32  SO32 + S
Decomposition avoided by: using recently boiled and cold distilled water and addition of Na2CO3 or borax.
Eq. wt. = M.w./2

16. 7. Potassium bromate (KBrO3)15
7. Potassium bromate (KBrO3)
Primary standard purity.
Powerful oxidizing agent in acid medium. Its solution is stable. BrO3 + 6H+ + 6e ↔ Br  + 3H2O E° = 1.44 V
Mainly used :
(a) For determination of strong reductants as: As+3, Sn+2, Sb+3 & [Fe(CN)6]4
BrO3 + 3H3AsO3  Br + 3H3AsO4
At end point, first excess BrO3 reacts with Br   Br2 which decomposes the indicator.
(b) As a source of standard bromine solution:
Bromine solution is a mixture of BrO3 and Br  when it is acidified gives Br2.
BrO3 + 5Br  + 6H+  3Br H2O

17. Uses of bromine solution16
Determination of primary aromatic amines, phenols, quinolines, acetylsalicylic acid, sulphanilic acid, etc.
BrO3 + 5 Br  + 6 H+  3 Br2 + 3 H2O d a r k O H + 3 B 2 , 4 6 - T i b o m p h e n l P
Known ex.
standard
The excess Br2 is determined by the reaction with iodide:
Br2 + 2I  I2 + 2 Br  & I2 + 2 Na2S2O3  Na2S4O6 + 2 I
Chloroform is added to dissolve TBP & as indicator.

18. Uses of bromine solution17
Uses of bromine solution
2- Indirect determination of some cations such as Mg+2, Al+3
Mg+2 + C9H6NOH (oxine) → Mg(C9H6OH)2 + 2H+
The ppt is separated, dissolved in acid
Mg(C9H6NO) H+ → 2C9H6NOH + Mg+2
2C9H6NOH + 4Br → 2C9H5ONBr2 + 4 HBr
Known excess standard
+ 2KI
→ 2KBr + I2
≠ Standard Na2S2O3
Using starch as ind.

19. Uses of bromine solution18
Uses of bromine solution
3- Determination of unsaturation:
─CH=CH─ + Br2 → Br ─CH─CH─Br HBr
Known excess standard
+ 2KI
→ 2KBr + I2
≠ Standard Na2S2O3
using starch as ind.
Determination of unsaturation in oils and fats using bromine-dioxane.
Dioxane Dioxane tetrabromide

20. Examples of Applications of
Redox Titrations 19
Free elements
a. Metallic and reduced iron b. Zinc powder c. Sulphur d. Free halogens
2. Determination of peroxides:
a. H2O2 b. ZnO2 c. Higher oxides of heavy metals.
3. Determination of cations
A. Determination of iron:
1. Ferrous Ferric Substances that reduce Fe+3 to Fe+2
4. Substances that oxidize Fe+2 to Fe+3
5. Ferro and ferricyanides
B. Determination of copper salts
C. Determination of HgCl2
D. Determination of cations that form insoluble oxalates.

21. Examples of Applications of
Redox Titrations 20
4. Determination of Anions
A. Determination of soluble oxalates
B. Determination of sulphide, sulphite, thiosulphate,
sulphate & persulphates.
C. Determination of halides, chlorate and hypochlorite.
D. Determination of nitrite and nitrate
5. Determination of aldehydes.
6. Determination of moisture content (Karl-Fischer reagent)
7. Bromometric determination of phenolic compounds.
Determination of organic pharmaceutical compounds
9- Analysis of mixtures.

22. St. KMnO4 (self indicator) in the presence of Zimmermann reagent
Free elements 21
a. Metallic iron (Fe)
It can be determined by dissolving it in ferric chloride solution and the produced ferrous chloride formed is titrated with standard permanganate in the presence of Zimmermann reagent.
Iron oxides do not interfere. ≠
St. KMnO4 (self indicator) in the presence of Zimmermann reagent

23. St. KMnO4 (self indicator) in the presence of Zimmermann reagent
Free elements (cont.) 22
b. Zinc powder (Zn)
is determined by reaction with ferric sulphate and the produced ferrous iron, is acidified with dilute H2SO4 and titrated with standard KMnO4.
This method determines the free zinc and not zinc oxide, because the oxide will not reduce ferric salt
+ H+ ≠
St. KMnO4 (self indicator) in the presence of Zimmermann reagent

24. Stand. I2 (Starch indicator)
Free elements (cont.) 23
c. Sulphur (S)
Sulphur is converted by refluxing with Na2SO3 to Na2S2O3 which is then titrated with standard iodine solution.
I2 + 2Na2S2O3  2NaI + Na2S4O6
The excess Na2SO3, which also reacts with I2, is converted by addition of formaldehyde into formaldehyde bisulphite. ≠
Stand. I2 (Starch indicator)

25. (Using starch as indicator)
Free elements (cont.) 24
d. Free halogens
Iodine can be determined by direct titration with sodium thiosulphate solution. Bromine or chlorine displaces iodine from potassium iodide.
I2 + 2Na2S2O3  2NaI + Na2S4O6 (sod. tetrathionate)
Br2 + 2KI  I2 + 2KBr
Cl2 + 2KI  I2 + 2KCl ≠
Stand. Na2S2O3
(Using starch as indicator)

26. (Using starch as indicator)
2. Determination of peroxides 25
1. Hydrogen peroxide
A. as reducing agent
By direct  KMnO4. (self indicator)
2MnO4 + 5H2O2 + 6H+ → 2Mn+2 + 5O2 + 3H2O
By direct  Ce+4 (using ferroin indicator) Ce+4 + H2O2 → 2Ce+3 + O2 + 2H+
B. as oxidizing agent (iodometrically) H2O2 + 2H+ + 2I → I2 + 2H2O
2. Zinc peroxide
ZnO2 + 2H+ → H2O2 + Zn  KMnO4 or + KI  I2  S2O3-2. ≠
Stand. Na2S2O3
(Using starch as indicator)

27. 2. Determination of peroxides (cont.)26
Organic peroxides
1. Carbamide peroxide
Topical antiseptic and disinfectant solution
H2N-CO-NH2..H2O2 → H2N-CO-NH2 + H2O2
is assayed for H2O2 content iodometrically
2. Hydrous benzoyl peroxide
Keratolytic and keratogenic agent for acne.
It can be determined iodometrically. (C6H5COO)2 + 2I + 2H+  2C6H5COOH + I2

28. 3. Determination of oxides27
1. Higher oxides of manganese and heavy metals as MnO2, PbO2, Pb3O4
a. Iodometrically
MnO2 + 4HCl  MnCl2 + Cl2 + 2H2O
Cl2 + 2KI  I2  S2O32
b. Indirect titration with reducing agents
MnO Fe H+  Mn2+ + 2Fe H2O
MnO C2O H+  Mn2+ + 2CO H2O
2MnO2 + 2AsO H+  2Mn2+ + 2AsO H2O
Known excess standard ≠
Stand. KMnO4 (self indicator)

29. 3. Determination of oxides (cont.)28
Oxides of As or Sb (As2O3, As2O5, Sb2O3 and Sb2O5)
As2O5 and Sb2O5 can oxidize I  I2 or
I2 oxidizes As2O3 and Sb2O3  As2O5 and Sb2O5
depending on the pH of the medium:
In presence of much acid as HCl or HI,
The Eº of As5+/As3+ or Sb5+/Sb3+  (it oxidizes I  I2)
As2O3 + 2I2 + 2H2O ← As2O5 + 4I + 4H+
Sb2O3 + 2I2 + 2H2O ← Sb2O5 + 4I + 4H+
In slightly acid or neutral medium,
The Eº of As5+/As3+ or Sb5+/Sb3+  (I2 oxidizes As2O3 and Sb2O3  As2O5 and Sb2O5)
(iodometric) ≠
Stand. Na2S2O3
(starch as indicator)
(Iodimetric)

30. 3. Determination of oxides (cont.)29
Addition of mild alkali (NaHCO3) keep the oxidation going on.
As2O3 ≠ 2I2 + 2H2O → As2O5 + 4I + 4H+
Sb2O3 ≠ 2I2 + 2H2O → Sb2O5 + 4I + 4H+
using starch as indicator
Na2CO3 or NaOH (pH>8) can not be used as they react with iodine forming hypoiodite and iodate which react with both oxides to form iodinum ions
I2 + OH  IO + I
3IO  IO3 + 2I self oxidation reduction
NaHCO3 →

31. Br2 + methyl orange → irreversible oxidation (bleaching color)
3. Determination of oxides (cont.) 30
As2O3 and Sb2O3 can be determined by direct reaction with potassium bromate in acid medium  As2O5 and Sb2O5 using methyl orange as irreversible redox indicator
3As2O3 ≠ 2BrO3 → As2O5 + 2Br 
3Sb2O3 ≠ 2BrO3 → Sb2O5 + 2Br 
BrO3 + 5Br  + 6H+→ 3Br2 + 3H2O
Br2 + methyl orange → irreversible oxidation (bleaching color) H+

32. 3. Determination of oxides (cont.)31
PbO can be determined by dissolving the sample in glacial acetic acid  lead acetate and precipitating it as lead oxalate
PbO + CH3COOH → Pb (CH3COO)2 + 2H+
Pb (CH3COO)2 + H2C2O4 → Pb C2O4  + 2 CH3COOH
Filter and wash the ppt
Known excess standard ≠
Stand. KMnO4 (self indicator)

33. 4. Determination of Cations
A. Iron 32
I. Ferrous salts: ferrous sulphate, ammonium sulphate (Mohr’s salt), carbonate, ferrous sulphide,
Fe+2  KMnO4 after addition of Zimmermann’s reagent, self indicator.
Fe+2  K2Cr2O7 after addition of H3PO4 and internal redox (diphenylamine) or external indicator.
Fe+2  Ce+4 , irreversible methyl red indicator.
Fe+2  I2 in presence of F or PO43 using starch

34. 4. Determination of Cations
A. Iron (cont.) 33
II. Ferric salts: by three methods
Ferric (Fe3+) reduced to (Fe2+) by pre-reductants
1- SnCl2 + Fe+3 → SnCl4 + Fe+2  KMnO4
after addition of Zimmermann’s reagent, self indicator
SnCl2 + 2HgCl2 → Hg2Cl2  + SnCl4
2- Zn and H2SO4:
2Fe+3 + Znº → Zn+2 + 2Fe+2  KMnO4
H2SO4 accelerates the reduction by Znº and the unreacted Znº is removed by filtration
3- Amalgamated Znº (Znº + HgCl2). Excellent reducing agent; (Jones reductor).

35. 4. Determination of Cations
A. Iron (cont.) 34
Iodometrically: Fe+3/Fe+2 system E°= 0.77
I2/2I system E°= 0.54
You must  difference between the 2 systems
either by  I conc. by addition of xss I or  I2 conc. by extraction with immiscible solvent as CHCl3 or CCl4.
Fe I → Fe+2 + I2  S2O3-2 using starch
Direct titration with titanous chloride
using methylene blue (irreversible) or thiocyanate as indicators (the end point is colorless due to disappearance of either the blue color of MB or the red color of Fe(SCN)3 which is formed at the beginning)
FeCl3 ≠ TiCl3  FeCl2 + TiCl4
standard

36. 4. Determination of Cations
A. Iron (cont.) 35
III. Reductants that reduce Fe+3 → Fe+2 :
1- SnCl2 + Fe3+ → SnCl4 + Fe2+  KMnO4
Znº + 2Fe3+ → Zn Fe2+  KMnO4
Feº + 2Fe3+ → 3Fe2+  KMnO4
IV. Oxidants that oxidize Fe+2 → Fe+3 :
1- K2S2O8 + 2Fe2+ + 2H+ → 2Fe KHSO4
2- KClO3 + 6Fe H+ → 6Fe3+ + KCl + 3H2O
3- MnO Fe H+ → 2Fe3+ + Mn2+ + 2H2O
Persulphate
Chlorate
Known excess standard ≠
Stand. KMnO4 or K2Cr2O (self indicator) (Ferroin ind.)

37. 4. Determination of Cations e.g. Sulphite or sulphide or sod. peroxide
A. Iron (cont.) 36
V. Ferrocyanide
By direct titration  KMnO4 or  Ce4+
1- 5[Fe(CN)6]4-  MnO4- +8H+→ 5[Fe(CN)6]3- + Mn2++3H2O
(self indicator)
2- [Fe(CN)6]4-  Ce → [Fe(CN)6]3- + Ce3+
(ferroin indicator)
Ferricyanide
1- By iodometrical titration
2[Fe(CN)6]3- + 2I- → 2[Fe(CN)6]4- + I2  S2O3-2 (starch)
we add H2SO4 and ZnSO4 (to precipitate Zn2[Fe(CN)6]) to ↑↑ the E° of [ferri]/[ferro] to oxidize iodide to iodine.
2- [Fe(CN)6]3- + pre-reductants → [Fe(CN)6]4-  MnO4-
Remove xss
e.g. Sulphite or sulphide or sod. peroxide

38. 4. Determination of Cations B. Determination of copper salts37
Iodometrically: Cu+2/Cu+ system E°= 0.15
I2/2I system E°= 0.54
Expected
Actually
2Cu I → 2CuI2
The instability of CuI2 results in formation of Cu2I2 ppt which  Cu+ → ↑ E° of Cu+2/Cu+ system to become able to oxidize I → I2
I2 oxidize Cu+ → Cu+2
The reverse occurs due to instability of CuI2
→ Cu2I2  + I2  S2O3-2
unstable
using starch
We add KSCN… Why?
In presence of tartarate or citrate I2 oxidizes Cu+ to cu+2 as these anions form stable complexes with Cu+2

39. 4. Determination of Cations C. Determination of HgCl238
HgCl2 is reduced firstly to Hg° by HCHO in Ca(OH)2 medium.
Hg2+ + HCHO + 3OH-  Hg° + HCOO- + H2O
Hg° + I  HgI2 
+ 2I-  [HgI4]2-
known
Excess stand
red ppt
colorless complex ≠
Stand. Na2S2O3
(starch as indicator)

40. 4. Determination of Cations Ca2+ + H2C2O4 → Ca C2O4  + xss. H2C2O4
D. Cations form insoluble oxalates. 39
(Ca2+, Ba2+ Sr2+, Mg2+, Cd2+, Bi3+, Zn2+, Ni2+, Co2+ & Pb2+)
Ca H2C2O4 → Ca C2O4  xss. H2C2O4
(i) The washed precipitate is dissolved in dil. H2SO4 and ≠ KMnO4 at 60°C. or
(ii) The excess oxalic acid in the filtrate and washing is back titrated with standard KMnO4 at 60°C.
known
Excess stand
Filter and
wash the ppt
then follow one of the 2 ways
Oxidizing substance such as K2S2O8, KClO3, NO3-, MnO2 can be determined by treatment with a known excess oxalic acid and the residual oxalic acid is then back titrated with standard KMnO4.

41. 5. Determination of Anions
A. Soluble oxalates. 40
Soluble oxalates  KMnO4 or Ce+4 in presence of sulphuric acid and heating to 60°C.
The reaction is slow at start becomes rapid after formation of reduction product (Mn+2 or Ce+3).

42. Sources of errors in iodine titrations41
Sources of errors in iodine titrations
(A) Errors due to iodine (I2)
I2, being volatile, is subjected to loss. So, iodimetric and iodometric titrations should be carried out in a glass-stoppered flask.
Iodine may undergo changes to hypoiodite and iodide
I2 + H2O  HIO + HI
(B) Errors due to iodide ion
Iodides are easily oxidized by atmospheric oxygen to iodine, specially in acid medium.
4I- + O2 + 4H+  2I2 + 2H2O
(C) Errors due to starch
Starch may be decomposed by micro-organisms, these decomposition products give non-reversible reddish colour with iodine that masks the true end point.

43. Sources of errors in iodine titrations42
Sources of errors in iodine titrations
Also glucose which may result from hydrolysis of starch can give error as it is reducing agent, Boric acid may be added as a preservative or better starch solution is freshly prepared.
(D) Errors due to thiosulphate
Thiosulphate is unstable being slowly decomposed with the precipitation of sulphur:
S2O32- + H+  HSO3- + S
Decomposition may be caused by CO2 (in distilled water) or by bacterial action. Therefore, the solution should be prepared with recently boild and cooled dist. water and Na2CO3 is added to prevent bacterial activity (pH = 9-10).

44. 5. Determination of Anions
b. S2-, SO32-, S2O32-,SO42- & S2O82- . 43
Direct titration
Sulphide S2 ≠ I2 → 2I + Sº 
Use dilute soln. to decrease the inclusion of I2 by Sº
Thiosulphate 2S2O32 ≠ I2 → S4O62 + 2I
Back titration.
3. Sulphite SO32 + I2 + H2O → SO42 + 2I + 2H+
S2 + I → 2I Sº 
4. Sulphate SO42 + BaCrO4 → BaSO4 + CrO42 (filtrate)
2CrO42 (filtrate) + 2H+  Cr2O72 + H2O
Known xss. standard ≠
St. S2O32 (starch indicator)
Iodometrically
+2KI  I2  S2O32 (starch)

45. 5. Determination of Anions
C. Determination of halides, ClO3 and ClO 44
Chlorates (ClO3) & Hypochlorites (ClO)
Iodometrically
Chlorate
Hypochlorite ≠
Stand. Na2S2O3
(starch as indicator)

46. 5. Determination of Anions C. Determination of halides (cont.)45
e.g.1: Chlorine in bleaching powder
+ 4H+  Cl2 + Ca H2O
Acetic acid used for acidification not HCl, since calcium chlorate present as a result of hypochlorite decomposition will react slowly with I and liberate iodine.
e.g.2: N-chloro-organic compounds as water disinfectant, these in water release hypochlorous acid (HOCl) which is the active germicidal species.
ClO + 2I + 2H+  Cl + I2 + H2O (iodometrically)
(calcium hypochlorite
Ca(OCl)2
basic chloride).
CaCl2.Ca(OH)2.H2O

47. 5. Determination of Anions C. Determination of halides (cont.)46
Organically combined iodine compound
Thyroxine, diiodohydroxyquinoline & chiniofon These are converted into iodide by:
A. Iodoxyl
B. Thyroxine
I + 3Br2 + 3H2O → IO3 + 6Br + 6H+
Znº + gl. HOAc Reflux
ZnI2 filter, cool ≠ IO3-
ignition at 700°C K2CO3 I
xss Br2 removed by phenol.
+ xss I  I2  S2O32
C. Oxygen flask combustion method
Iodide determined iodometrically.

48. Oxygen flask combustion method47
Oxygen flask combustion method
The compound is wrapped in a filter paper attached to pt. wire and sealed in the stopper of a special oxygen filled flask containing dilute Na2S2O5 (sodium metabisulphite) solution.
Combustion is complete within 30 sec at 1200°C.
The resulting iodine is absorbed (reduced) by Na2S2O5 forming iodide

49. 5. Determination of Anions d. Determination of nitrites and nitrates48
Nitrites
Known excess standard ≠
Stand. Fe2+ (self indicator)
nitrite sample is added to an excess of acidified KMnO4 soln. (the tip of the pipette containing the nitrite solution should be below the surface of the liquid during addition in order to stop volatility of the unstable nitrous acid, liberated from acidified nitrite solution).
Nitrates
Known excess standard ≠
Stand. KMnO4 or K2Cr2O (self indicator) (Ferroin ind.)

50. St. S2O32 (starch indicator)
Determination of Aldehydes 49
e.g. formalin (formaldehyde), glucose, and lactose.
2R-CHO + I NaOH → 2R-COOH NaI
Sucrose after acid hydrolysis (inversion) of sugar
C12O22O11 + H2O → H+→ C5H11O5-CHO + C5H11O5CO
(glucose) (fructose)
I NaOH → NaOI + NaI + H2O C5H11O5-CHO IO → C5H11O5-COOH + I
Change in oxidation number = 2 (Glucose  Sucrose)
Kn. Xss. St.
└→ acidification → I2 ≠
St. S2O32 (starch indicator)
Gluconic acid

51. Determination of moisture content (Karl-Fischer reagent)50
Reagent: I2 + SO2 + anhyd. CH3OH and anhyd. pyridine.
Sample containing moisture + reagent until appearance of yellow color of the xss iodine.
SO2 + I2 + H2O → SO3 + 2HI

52. Bromometric determinations
(Bromine substitution method) 51
Compound
substitution products
Reaction time (min)
Phenol 45
Salicylic acid 45
Aspirin 45
NaOH

53. substitution products
Bromine substitution method (cont.) 52
Compound
substitution products
Reaction time (min)
p-Chlorophenol
45 (4ºC)
Oxine 30
Phenylephrine HCl 15

54. substitution products
Bromine substitution method (cont.) 53
Compound
substitution products
Reaction time (min)
Resorcinol 3
Hexylresorcinol 10

55. Miscellaneous Applications54
Miscellaneous Applications
Vitamin C (ascrobic acid) Iodimetrically
Polyhydroxy Alcohols
e.g. glycerol can be determined iodometrically:
3C3H8O3 + 7K2Cr2O7 + 28H2SO4 → CO2 + 40H2O + 7Cr2(SO4) + 7K2SO4
Known excess standard
+ 2KI
→ I2
≠ Standard Na2S2O3 (chloroform)

56. ≠ ≠ Mercaptans (Thiol) compounds Generally 2RSH ≠ I2 → RS-SR + 2HI 55
Dimercaprol (BAL) 55
Mercaptans (Thiol) compounds
Generally 2RSH ≠ I2 → RS-SR + 2HI
e.g., Dimercaprol, Thioglycollic acid, D-penicillamine; can be determined iodimetrically. ≠
dimercaprol ≠
Thioglycollic acid

57. Analysis of Mixtures 56
Acetic & formic acids (Total ≠ OH ─ [ph.ph.] & formic ≠ MnO4─)
Oxalic acid & sulphuric acid (Total ≠ OH ─ [ph.ph.] & oxalic ≠ MnO4─)
Phenol & salicylic acid (Total ≠ bromometrically & salicylic acid ≠ OH ─ [ph.ph.])
I2 & KI (Total I2/I by Andrew's & I2 ≠ S2O3 2─ )
Ammonium oxalate & ammonium chloride (Total by formol ≠ OH ─ [ph.ph.] & oxalate ≠ MnO4 ─)
Ferrous oxalate & oxalic acid (protoxalate) (Total ≠ MnO4 ─ & the produced Fe3+ pass through Jones red. ≠ MnO4 ─ or iodometrically)

58. Analysis of Mixtures (cont.)57
Persulphate & potassium acid sulphate (persulphate + xss ferrous ≠ permanganate and acid sulphate by ≠ st. NaOH using ph.ph.
Glucose & sucrose
Home work
Lead oxide & lead acetate mix.(lead subacetate)
Cl─, Br ─ & I ─ mixture (Fractional oxidation)
Sulphide & thiosulphate mixture
Ferrous fumarate or ferrous gluconate
Iron ammonium citrate
Tartar emetic (Antimony potassium tartarte)
Arsenicals
Chloramine-T (as hypochlorite)

59. Thank You
Hassan.F.Askal