CHLORIODOMETRY

titrant iodine monochloride ICl

When potassium iodate is used as an oxidising agent in the presence of an excess of concentrated hydrochloric acid, the reduction product is iodine monochloride, where iodine is formally in the +1 oxidation state.

OHKClIClHClKIKIO 23 33362

11 g KI

7.9 g KIO3

200 ml HCl

H2O up to 1 L c(1/2 ICl) = 0.1 M

This reaction has been used for the determination of many reducing agents.

Standardization of ICl

КClIexcessКIICl 2)(

NaIOSNaIOSNa 22 6422322

)()( 32221 OSNanICln

ClIIICl 2

Condition of titration: рН ≤ 7

At the equivalence point:

compoundbluestarchI2

ClIeICl 2 ВEIICl

80.00

/

+1 -1

½ ICl

Scheme:

Examples:

Determination of SnCl2 (direct titration)

][3 622 SnClHHIHClIClSnCl

HClIIClHI 2

)()( 21

221 IClnSnCln

HClHIOHCIClOHC 666686

HClIIClHI 2

)()( 21

68621 IClnOHCn

Determination of Vit С

compoundbluestarchI2

compoundbluestarchI2

)(3)(22 2222 leftoverIClHgClHgIexcessIClClHg

КClIleftoverIClexcessКI 2)()(

NaIOSNaIOSNa 22 6422322

Determination of mercury (I) salts (back titration)

)()()( 32221

2221 OSNanIClnClHgn

Advantages of the method:

1. The solution of ICl is more stable, than solution of iodine.

2. This method can be applied for the determination of more reducing agents as well as organic compounds.

IODATOMETRIC TITRATION

(titrant sol. KIO3)

3610

23 09.1366 KIOВEOHIHeIO

3510

2221

3 20.1365 KIOВEOHIHeIO

3410

23 14.1364 KIOВEOHIHeIO

Potassium iodate is a powerful oxidizing agent, but the course of the reaction is governed by the conditions under which it is employed. The reaction between potassium iodate and reducing agent in solutions of moderate acidity (0,1-2 M HCl) stops at the stage when iodate is reduced to iodine.

With a more powerful reductant the iodate is reduced to iodide:

In more strongly acid solution (3-6 M HCl) reduction occurs to iodine monochloride, and it is under these conditions that is most widely used.

These reactions has been used for the determination of many reducing agents.

Examples of the determinations when KIO3 is reduced to KI:

......Re 3 IacidKIOd

....23 IHIKIO

Scheme of titration

compoundcolouredbluestarchI2

Determination of SnCl2

OHSnClHKIHClKIOSnCl 26232 3][31233

OHKClIHClexcessKIKIO 223 3636)(5

)()( 221

361 SnClnKIOn

Determination of hydrazine

OHKINKIOHN 22342 62323

OHKClIHClexcessKIKIO 223 3636)(5

)()( 4241

361 HNnKIOn

Определение KBr

)(612512)(210 32223

0

leftoverKIOОHKClIBrHClexcessKIOKBrt

OHKClIHClexcessKIleftoverKIO 223 3636)(5)(

NaIOSNaIOSNa 22 6422322

)( 351 KIOn

Examples of the determinations when KIO3 is reduced to I2:

OHKClIClHClKIKIO 23 33362

2IKClKIICl

OHKClIClHClKIOI 232 356

Determination of KI

)()( 341

21 KIOnKIn

Examples of the determinations when KIO3 is reduced to ICl:

In the initial stage of the reaction free iodine is liberated: as more titrant is added, oxidation proceeds to iodine monochloride, and the dark colour of the solution gradually disappears.

Preparation of a solution of KIO3

Potassium iodate has a purity of at least 99,9 percent. It is a standard compound and its solution can be prepared by the direct method of preparation.

BROMATOMETRIC TITRATION

(titrant sol. KBrO3)

ВEOHBreHBrO BrBrO 45.1366 /0

23 3

Potassium bromate is a powerful oxidizing agent which is reduced smoothly to bromide:

At the end of the titration free bromine appears:

OHBrHBrBrO 223 3365

The presence of free bromine, and consequently the end point, can be detected by its yellow colour, but it is better to use indicators such as methyl orange or methyl red. These indicators have their usual colour in acid solution, but are destroyed irreversible by the first excess of bromine. In this case such indicators are named irreversible indicators.

1. With all irreversible indicators the destruction of the indicator is often premature to at slight extent: a little additional indicator is usually required near the end point. 2. Direct titrations with bromate solution in the presence of irreversible indicators are usually made in sulfuric acid solution, the concentration of which should be at least 1-2 M.

Peculiarities of bromatometric titrations

.....Re 23 OHKBrHClKBrOd

OHKClBrHClKBrOKBr 223 36365

Scheme of titrations:

IndofndistructioleirreversibBrInd 2

At equivalence point

Examples of determinations:

OHSbClHKBrHClKBrOSbCl 2633 3][393

OHKClBrHClKBrOKBr 223 36365

)()( 361

321 KBrOnSbCln

1. Determination of SbCl3

432332 62923 AsOHKBrOHKBrOOAs

OHKClBrHClKBrOKBr 223 36365

)()( 361

3241 KBrOnOAsn

OHNKBrKBrOHN 22342 63223

OHKClBrHClKBrOKBr 223 36365

)()( 361

4241 KBrOnHNn

2. Determination of As2O3 and arsenic (III) salts

3. Determination of hydrazine

OHClNNHCKBrHClKBrONHNHHC 2563256 6][32323

OHKClBrHClKBrOKBr 223 36365

)()( 361

25641 KBrOnNHNHHCn

OHNKBrCOOHNHCKBrOCONHNHNHC 22453245 332323

OHKClBrHClKBrOKBr 223 36365

)()( 361

24541 KBrOnCONHNHNHCn

4. Determination of phenilhydrazine

5. Determination of izoniazide

Preparation of a solution of KBrO3

Potassium bromate is readily available in a high state of purity, the product has an assay value of at least 99,9 percent. The substance can be dried at 120-150°C, in anhydrous, and the aqueous solution keeps indefinitely. It can therefore be employed as a primary standard. It is only disadvantage is that one-sixth of the relative molecular mass is a completely small quantity.

361 KBrO

BROMOMETRIC TITRATION

(titrant sol. KBrO3 + KBr)

OHKClBrHClKBrOexcessKBr 223 3636)(5

......2BrAnalyte

Various substances cannot be oxidized directly with potassium bromate, but react quantitatively with an excess of bromine. Acid solutions of bromine of exactly known concentration are readily obtained from a standard potassium bromate solution by adding acid and an excess of bromide:

IndofndistructioleirreversibBrInd 2

This method can be used to analyze many unsaturated organic compounds.

OHKClBrHClKBrOexcessKBr 223 3636)(5

Determination of streptocide

)()( 361

228641 KBrOnSONHCn

SO2H2N NH2 + 2Br2 NH2

Br2

Br2

SO2H2N + 2HBr2)

OHKClBrHClexcessKBrOKBr 223 3636)(5

KBrIexcessKIleftoverBr 2)(2)( 22

NaIOSNaOSNaI 22 6423222

)()()( 322361

61 OSNanKBrOnacidsalisylicn

Determination of salicylic acid

OH

COOH

+3Br2(exces, cant. cun)

OH

BrBr

Br

+ CO2 + 3HBr + Br2 (rest)2) (excess) (leftover)

Salicylic acid can be substituted rapidly and quantitatively with bromine produced from bromate and bromide in acid solution. The determination involves treating salicylic acid with an excess of potassium bromate and potassium bromide, when bromination is complete the unreacted bromine is then determined by adding excess potassium iodide and back titrating the liberated iodine with standard sodium thiosulphate

Determination of magnesium salts

N

OH Mg2+ + 2 =

N

O

Mg

2

+ 2H+ 1.

2. The precipitate should be washed and dissolved in HCl acid

N

O

Mg

2

+ 2H+ = N

OH

2 + 2Mg2+

Analyte is treated with 8-hydroxyquinoline (oxine) at pH 5 to precipitate magnesium oxinate:

3. 8-hydroxyquinoline is titrated by bromometric method

OHKClBrHClKBrOexcessKBr 223 3636)(5

N

OH

+2Br2 = N

OH

Br

Br +2HBr

)()( 3612

81 KBrOnMgn

DICHROMATOMETRIC TITRATION

(titrant sol. K2Cr2O7)

ВEOHCreHOCr 33.172614 0

2

32

72

Potassium dichromate is not such a powerful oxidizing agent as potassium permanganate, but it has several advantages over the latter substance. It can be obtained pure, it is stable, and it is therefore an excellent primary standard. Standard solution of exactly known concentration can be prepared by weighing out the pure dry salt and dissolving it in the proper volume of water. Furthermore, the aqueous solutions are stable indefinitely if adequately protected from evaporation. Potassium dichromate is used only in acid solution (even HCl can be used), and is reduced rapidly at the ordinary temperature to a green chromium (III) salt.

72261 OCrK

To ascertain the end point of a dichromate titration a redox indicator should be employed which gives a strong and unmistakable colour change. Suitable indicators for use with dichromate titrations include N-phenylanthranilic acid (0,1% solution in 0,005 M NaOH) and diphenylamine (0,2% aqueous solution)

Determination of CH3OH

OHCOSOKSOCrSOHOHCHOCrK 2242342423722 6)(4

)()( 361

72261 OHCHnOCrKn

Determination of FeSO4

OHSOFeSOKSOCrSOHFeSOOCrK 234242342424722 7)(3)(76

)()( 472261 FeSOnOCrKn

OHCNFeKSOKSOCrSOHCNFeKOCrK 263423424264722 7])([64)(7])([6

]))([()( 6472261 CNFeKnOCrKn

Determination of K4[Fe(CN)6]

Examples of determinations:

OHOHCSOKSOCrSOHOHCOCrK 26664234242686722 73)(43

OHISOKSOCrSOHKIOCrK 224234242722 73)(76

)()( 68621

72261 OHCnOCrKn

At equivalence point:

Determination of Vitamin С

Ind = starch

CERIMETRIC TITRATION

(titrant sol. Ce(SO4)2)

ВECeeCe 45.1034

Cerium (IV) sulphate is a powerful oxidizing agent. It can be used only in acidic solutions, best in 0,5 M or higher concentrations. The solution has an intense yellow colour, and in hot solutions which are not too dilute the end point may be detected without an indicator.

The advantages of cerium (IV) sulphate as a standard oxidizing agent

1. Cerium (IV) sulphate solutions are remarkably stable over

prolonged periods. They need not to be protected from light, and may be even be boiled for a short time without appreciable change in concentration.

2. Cerium(IV) sulphate may be employed in the determination of reducing agents in the presence of a high concentration of HCl.

3. Cerium(IV) solutions in 0,1 M solution are not too highly coloured to obstruct vision when reading the meniscus in burettes and other titrimetric apparatus.

4. In the reaction of cerium (IV) salts in acid solution with reducing agents, the simple change is assumed to take place. With permanganate a number of reduction products are produced according to the experimental conditions.

24234224 )()(2)(2 ISOKSOСeexcessKISOСe

NaIOSNaIOSNa 22 6422322

)())(( 32224 OSNanSOCen

Standardization of cerium (IV) sulphate (1 variant)

The most trustworthy method for standardizing cerium (IV) sulphate solution is application of iodometric titration. Cerium (IV) sulphate is reacted with an excess of potassium iodide, producing iodine. The iodine that is produced is then titrated against sodium thiosulphate, to find how much iodine was produced by the reaction of cerium (IV) sulphate with potassium iodide. Once the amount of iodine has been found, the amount of the cerium (IV) sulphate solution can be calculated.

24234242224 2)()(2 COSONaSOСeOCNaSOСe

)())(( 42221

24 OCNanSOCen

Standardization may also be carried out with sodium oxalate. It is possible to carry out a direct titration.

Standardization of cerium (IV) sulphate (2 variant)

Ind = N-phenylanthranilic acid

Examples of determinations:

Determination of H2O2

2

3

22

4 222 OHCeOHCe

)())(( 2221

24 OHnSOCen

Determination of izoniazide

)())(( 37641

24 ONHCnSOCen

N

C

O

NH NH2

N

COOH

+ N2 + 2Ce2(SO4)3 + 2H2SO44Ce(SO4)2 + H2O+ =

Ind = ferroine

NITRITOMETRIC TITRATION

(titrant sol. NaNO2

Determination of reducing agents (e.g FeSO4)

OHSONaONSOFeSOHONNaFeSO 242

2

342422

3

4 22)(222

)()( 24 NaNOnFeSOn

Determination of oxidizing agents (e.g. KMnO4)

OHSOKONNaMnSOSOHONNaKMnO 2423

5

4422

3

4 352352

)()( 221

451 NaNOnKMnOn

OHNOHeNO 22 2

ВENONO

20.10

/2

OHNOHeNO 232 2ВE

NONO94.00

/ 23

The diazotization titration is nothing but the conversion of the primary aromatic amine to a diazonium compound. This process was first discovered in 1853 and was applied to the synthetic dye industry. The reaction mechanism was first proposed by Peter Griessin. In this method, the primary aromatic amine is reacted with the sodium nitrite in acidic medium to form a diazonium salt. This method is first used in the determination of dyes.

DIAZOTIZATION TITRATION

OHNaClClNNArHClNaNOArNH KBr

2

][

22 2][2

Conditions 1. Acidic solution, because diazo compounds are stable only in

acidic solution. 2. Low temperature. 3. Slow titration. 4. Presence of catalyst – KBr. 5. Indicator – tropeoline 00 (red solution turns yellow).

The principle involved in this method is that the primary aromatic amine present in the sample reacts with the sodium nitrite in the presence of acid such as hydrochloric acid to obtain a diazonium salt.

Preparation and standardization of NaNO2 solution

Standardization (1 variant)

)()( 22463 NaNOnNHHCSHOn

NH2

SO3H

+ NaNO2 + 2HCl

N

SO3H

N

Cl + NaCl + 2H2OKBr

Sodium nitrite is not a standard compound. Firstly the solution is prepared with approximately concentration and then it is standardized.

Sulfanilic acid

)(35235)(2 4242344224 leftKMnOOHSOKNaNOMnSOSOHNaNOexcessKMnO

OHISOKMnSOSOHexcessKIleftoverKMnO 22424424 3523)(10)(2

NaIOSNaIOSNa 22 6422322

)()()( 221

322451 NaNOnOSNanKMnOn

Standardization (2 variant)

Determination of streptocide

)()( 22286 NaNOnSONHCn )()( 221

228621 NaNOnSONHCnor

SO2 NH2H2N + NaNO2 + 2HClKBr

SO2H2N N N Cl + NaCl + 2N2O