chloriodometry titrant iodine monochloride icl fileexamples: determination of sncl 2 (direct...
TRANSCRIPT
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.
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.
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.
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
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
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
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
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)