Titrations Involving Iodine

Redox titrations (Tables 16-2 and 16-3) are available for many analytes with iodine (I2, a mild oxidizing agent) or iodide (F, a mild reducing agent). 

For example, vitamin C in foods and the compositions of superconductors (at the opening of the chapter) can be measured with iodine. When a reducing analyte is titrated with iodine (I2), the method is called iodimetry, lodometry is the titration of iodine produced when an oxidizing analyte is added to excess F. Iodine is usually titrated with standard thiosulfate solution. 

I2 is only slightly soluble in water (1.3 mM at 20°C), but its solubility is enhanced by complexation with iodide  

Add SO2 (or H2SO3 or HSO3 or 803 ) to excess standard I3 in dilute acid and back titrate unreacted I3 with standard thiosulfate. 

Precipitate and wash metal sulfide. Dissolve in 3 M HCl with excess standard 13 and back titrate with thiosulfate. 

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Two important sources of error in titrations involving iodine are (a) loss of iodine owing to its appreciable volatility and (b) acid solutions of iodide are oxidised by oxygen from the air ... 

Discussion. One of the most useful titrations involving iodine is that originally developed by Winkler18 to determine the amount of oxygen in samples of water. The dissolved oxygen content is not only important with respect to the species of aquatic life which can survive in the water, but is also a measure of its ability to oxidise organic impurities in the water (see also Section 10.103). Despite the advent of the oxygen-selective electrode (Section 16.36) direct titrations on water samples are still used extensively.19... 

Numerous analytical procedures are based on redox titrations involving iodine. Starch8 is the indicator of choice for these procedures because it forms an intense blue complex with iodine. Starch is not a redox indicator it responds specifically to the presence of I2, not to a... 

A good example of a back titration involving iodine and thiosulfate is the assay of resorcinol in Resorcinol Solution BP. Resorcinol is an antiseptic that was widely used in the past, although less so now. The assay of resorcinol involves a quantitative electrophilic aromatic substitution reaction using bromine as the reagent, as shown in Figure 6.4. 

More commonly, titrations involving iodine are performed with a suspension of starch as an indicator. The deep blue color that develops in the presence of iodine is believed to arise from the absorption of iodine into the helical chain of j8-amylose (see Figure 20-2), a macromolecular component of most starches. The closely related a-amylose forms a red adduct with iodine. This reaction is not easily reversible and is thus undesirable. In commercially available soluble starch, the alpha fraction has been removed to leave principally j8-amylose indicator solutions are easily prepared from this product. 

This indicator is used for titrations involving iodine. Starch forms a not very reversible complex with I2 that is a very dark-blue color. The color reaction is sensitive to very small amounts of iodine. In titrations of reducing agents with iodine, the solution remains colorless up to the equivalence point. A fraction of a drop of excess titrant turns the solution a definite blue. 

Reflect and Apply You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation-reduction titration involving iodine. You get a starch indicator from the stock-room. Why do you need it ... 

One further useful indicator employed in redox titrations involving iodine is starch, or more synthetic equivalent materials. The starch forms a blue-black complex with iodine, which is rendered colorless when all the iodine has been removed. 

OXIDATION AND REDUCTION PROCESSES INVOLVING IODINE I0D0METRIC TITRATIONS... 

In most direct titrations with iodine (iodimetry) a solution of iodine in potassium iodide is employed, and the reactive species is therefore the tri-iodide ion 13. Strictly speaking, all equations involving reactions of iodine should be written with 13 rather than with I2, e.g. 

Iodometry is a titrimetric method involving iodine. It is an indirect method because the product of the reaction of ST with I" (I2) is titrated. 

Theory Iodine in aqueous solution acts as an oxidizing agent which forms the basis of assay methods involving direct titration with iodine. Thus, we have ... 

These titrations involve displacement of iodine from iodide by a stronger oxidising agent followed by titration of the displaced iodine with sodium thiosulphate. 

As an illustration of the use of electrode potentials, consider the classical method of analysis of copper in brass, which involves dissolving the weighed sample in nitric acid to obtain Cu2+(aq), adjusting the pH to a weakly acidic level, allowing the Cu2+ to react completely with excess potassium iodide to form iodine and the poorly soluble Cul, and then titrating the iodine with sodium thiosulfate solution that has been standardized against pure copper by the same procedure ... 

The requirements of the US armed forces are covered by a military specification (Ref la) which contains the following criteria (1) form — soft crystals (2) color — white (3) Pb as Pb sulfate, 43.0% min (4) salicylate by Na thiosulfate titration of iodine liberated in a reaction involving an excess of Br not used to initially brominate the salicylic acid sample, 55.4% min ... 

Redox titrations find numerous applications in environmental analysis. Iodo-metric titration involving the reaction of iodine with a reducing agent such as thiosulfate or phenylarsine oxide of known strength is a typical example of a redox titration. This method is discussed separately in the next section. Another example of redox titration is the determination of sulfite, (S032-) using ferric ammonium sulfate, [NH4Fe(S04)2]. 

Iodometric titration involves the reaction of iodine with a known amount of reducing agent, usually sodium thiosulfate (Na2S203) or phcnylarsine oxide (PAO). Starch solution is used as an indicator to detect the end point of the titration. Thus, the exact amount of iodine that would react with a measured volume of sodium thiosulfate of known strength is determined. From this, the concentration of the analyte in the sample, which is proportional to the amount of iodine reacted with thiosulfate or PAO, is then calculated. 

Coulometric titration procedures have been developed for a great number of oxidation-reduction, acid-base, precipitation, and complexation reactions. The sample systems as well as the electrochemical intemediates used for them are summarized in Table 4.1, and indicate the diversity and range of application for the method. An additional specialized form of coulometric titration involves the use of a spent Karl Fischer solution as the electrochemical intermediate for the determination of water at extremely low levels. For such a system the anode reaction regenerates iodine, which is the crucial component of the Karl Fischer titrant. This then reacts with the water in the sample system according to the... 

Potassium periodate has been used in place of iodate for various titrations involving the iodine-chloride, iodine-bromide, and iodine-cyanide end points. Reaction of periodate with iodide involves transfer of an oxygen atom rather than an electron, followed by rearrangement of the structure of the IO4" ion. In the oxidation of the active oxidant appears to be 104 . An intermediate... 

Figure 28C represents a titration with two platinum wire electrodes and involving iodine in solution being titrated with sodium thiosulphate (I2 by 8203 ). Here the thiosulphate reaction... 

Titrimetric proceedures for the determination of KjEefCN) in alkaline media, also using hydrazine sulphate, have been desaibed. Determination of hydrazine and its derivatives is also important. A back-titration technique involving iodine monochloride in 5M-HC1 as the oxidant is simpler and of wider applicability than previous methods. ... 

Ascorbic acid, CgHgOs (vitamin C), is a reducing agent. The amount of this acid in solution can be determined quantitatively by a titration procedure involving iodine, I2, in which ascorbic acid is oxidized to dehydroascorbic acid,... 

SO2 can also bind with phenolic compounds. In the case of proanthocyanic tannins, a solution of 1 g/1 binds with 20 mg/I of SO2 per liter. The combinations are significant with anthocyanins. These reactions are directly visible by the decoloration produced. The combination is reversible the color reappears when the free sulfur dioxide disappears. This reaction is related to temperature (Section 8.5.2) and acidity (Section 8.5.1), which affect the quantity of free SO2. The SO2 involved in these combinations is probably titrated by iodine along with the free SO2. In fact, due to their low stability, they are progressively dissociated to reestablish the equilibrium as the free SO2 is oxidized by iodine. 

Though iodimetric titration of iron (III), which involves the reaction of iron (III) with acidified iodide and titration of the liberated iodine with standard thiosulphate, has been recommended in British Pharmacopoeia, 1968, the most frequent [20] difficulties are encountered due to recurring end-points, as the reaction is very slow near the stoichiometric end-point. Extraction of the liberated iodine into chloroform or carbon tetrachloride has been recommended to push the reaction to completion [20]. However, such titrations involving two immiscible phases are always cumbersome and the endpoint is often carried over. 

Ana.lytica.1 Methods. Various analytical methods involve titration with oxidants, eg, hexacyanoferrate (ferricyanide), which oxidize dithionites to sulfite. lodimetric titration to sulfate in the presence of formaldehyde enables dithionite to be distinguished from sulfite because aldehyde adducts of sulfite are not oxidized by iodine. Reductive bleaching of dyes can be used to determine dithionite, the extent of reduction being deterrnined photometrically. Methods for determining mixtures of dithionite, sulfite, and thiosulfates have been reviewed (365). Analysis of dithionite particularly for thiosulfate, a frequent and undesirable impurity, can be done easily by Hquid chromatography (366). 

The water content should be and is increasingly determined during RM preparation by Karl Fischer titration. The principle of this method is that it quantifies water selectively by measuring the consumption of iodine. During the titration, iodine oxidizes methylsulfite, formed from methanol and sulfur dioxide in a first step, to methylsulfate under stoichiometric involvement of water. Complete reac-... 

Perhaps the most obvious method of studying kinetic systems is to periodically withdraw samples from the system and to subject them to chemical analysis. When the sample is withdrawn, however, one is immediately faced with a problem. The reaction will proceed just as well in the test sample as it will in the original reaction medium. Since the analysis will require a certain amount of time, regardless of the technique used, it is evident that if one is to obtain a true measurement of the system composition at the time the sample was taken, the reaction must somehow be quenched or inhibited at the moment the sample is taken. The quenching process may involve sudden cooling to stop the reaction, or it may consist of elimination of one of the reactants. In the latter case, the concentration of a reactant may be reduced rapidly by precipitation or by fast quantitative reaction with another material that is added to the sample mixture. This material may then be back-titrated. For example, reactions between iodine and various reducing agents can be quenched by addition of a suitably buffered arsenite solution. 

The amount of hypochlorite ion present in bleach can be determined by an oxidation-reduction titration. In this experiment, an iodine-thiosulfate titration will be utilized. The iodide ion is oxidized to form iodine, I2. This iodine is then titrated with a solution of sodium thiosulfate of known concentration. Three steps are involved ... 

Theory Chlorinated lime reacts with acetic acid to produce a mole each of calcium acetate, hydrochloric acid and hydrochlorous acid. The two acids interact to give water and chlorine, and the latter reacts with HI to liberate iodine that can be estimated by titrating with 0.1 N sodium thiosulphate solution. The various reactions involved may be expressed as given below ... 

One way to determine the vitamin C content of a sample is to titrate it with an iodine/iodide solution. The diagram below shows the reaction involved. 

The Baeyer-Villiger process for conversion of open-chain ketones to esters, or cyclic ketones to lactones by a peracid involves an intermediate a-hydroxyperester (235) step, as shown in equation 83. Determination of 235 in the reaction mixture involves selective reduction of the peroxyacid with diphenyl sulfide and reduction of 235 with excess iodide, followed by titration of the liberated iodine. The presence of various transition metal ions may affect the determination by accelerating the final step of the synthetic process, thus... 

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