Titration of iodine

Industrial Hquid chlorine is routinely analy2ed for moisture, chlorine, other gaseous components, NCl, and mercury foUowing estabHshed procedures (10,79). Moisture and residue content in Hquid chlorine is determined by evaporation at 20°C foUowed by gravimetric measurement of the residue. Eree chlorine levels are estimated quantitatively by thiosulfate titration of iodine Hberated from addition of excess acidified potassium iodide to the gas mixture. 

Thiosulfate titration of iodine is limited to an iodine concentration of 7.5 fig/mL (69). The use of organic solvents such as benzene, toluene, chloroform, and carbon tetrachloride as indicators in the titration of iodine have been proposed (70—72). These procedures increase the sensitivity of the titration so that 6.0 fig/mL of iodine can be detected, although a sensitivity of 2 fig/mL has been claimed (73). 

Arsenious oxide, trivalent antimony (73), sulfurous acid (74), hydrogen sulfide (75), stannous ion, and thiocianate (76) have been recommended for the titration of iodine. However, none of these appears to have a greater sensitivity for the deterrnination of minute quantities of iodine than thiosulfate. Organic compounds such as formaldehyde (77), chloral hydrate (78), aldoses (79), acetone (70,80), and hydroquinone have also been suggested for this purpose. 

The quantitative conversion of thiosulfate to tetrathionate is unique with iodine. Other oxidant agents tend to carry the oxidation further to sulfate ion or to a mixture of tetrathionate and sulfate ions. Thiosulfate titration of iodine is best performed in neutral or slightly acidic solutions. If strongly acidic solutions must be titrated, air oxidation of the excess of iodide must be prevented by blanketing the solution with an inert gas, such as carbon dioxide or... 

The direct iodometric titration method (sometimes termed iodimetry) refers to titrations with a standard solution of iodine. The indirect iodometric titration method (sometimes termed iodometry) deals with the titration of iodine liberated in chemical reactions. The normal reduction potential of the reversible system ... 

The great merit of starch is that it is inexpensive. It possesses the following disadvantages (1) insolubility in cold water (2) instability of suspensions in water (3) it gives a water-insoluble complex with iodine, the formation of which precludes the addition of the indicator early in the titration (for this reason, in titrations of iodine, the starch solution should not be added until just prior to the end point when the colour begins to fade) and (4) there is sometimes a drift end point, which is marked when the solutions are dilute. 

Only freshly prepared starch solution should be used. Two millilitres of a 1 per cent solution per 100 mL of the solution to be titrated is a satisfactory amount the same volume of starch solution should always be added in a titration. In the titration of iodine, starch must not be added until just before the end point is reached. Apart from the fact that the fading of the iodine colour is a good indication of the approach at the end point, if the starch solution is added when the iodine concentration is high, some iodine may remain adsorbed even at the end point. The indicator blank is negligibly small in iodimetric and iodometric titrations of 0.05M solutions with more dilute solutions, it must be determined in a liquid having the same composition as the solution titrated has at the end point. 

Thiosulfate-fixing solutions, 19 213 Thiosulfate titration, of iodine, 14 367, 368 Thiosulfuric acid, 23 669 Thiotungstates, 25 385 Thiourea, 12 690... 

The amperometric titration of iodine with sodium thiosulphate using two indicator electrons. 

The peroxide value (PV) of an oil or fat is defined as the quantity of peroxide oxygen present in the sample. This classical iodometric method is a volumetric analysis based on the titration of iodine released from potassium iodide by peroxides in a biphasic system using a standardized thiosulfate solution as the titrant and a starch solution as the indicator (see Background Information, discussion of peroxide value). This method will detect all substances that oxidize potassium iodide under the acidic conditions of the test, therefore the purity of the reagents is critical. 

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 ... 

Back-titration of iodine with standard sodium thiosulfate. Iodine is added to oxidize sulfide under acidic conditions. 

Write equations for the titration of iodine to iodine monochloride, using permanganate, dichromate, and periodate as oxidants. 

According to Mr L. Parry, the most accurate and practical methods are, titration of SbjOj with iodine in alkaline solution, and titration of iodine liberated by action of SbClj on KI in HCl solution. 

One way to do this is to determine the AG° values for each half-reaction and then add them to determine the value of AG° for the complete redox reaction. Let us examine one of the steps of the Winkler method for determining dissolved oxygen (DO) as an example of this technique. The last step of the Winkler method is the titration of iodine (Igoq)) to iodide (I ) using thiosulfate, 8203 , which is itself oxidized to tetrathionate, 8406 . The reaction takes place in acid solution. The two half-reactions are... 

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. 

Table 16-3 lodometric titrations Titrations of iodine (actually I3) produced by analyte... 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

The detected equivalence point (endpoint) of the titration depends on the detection method and differs considerably between the titration of iodine with thiosulphate and back titration of surplus thiosulphate with iodine-iodide solution (Grasshoff, 1981). 

Iodine has also been used for the disinfection of swimming pool water. The concentration range in which it shows strong bactericidal, virucidal, or amoebicidal action is 5-50 mg/cm, leaving a 0.2-0.6 mg/dm residual iodine concentration. Traces of iodine or iodide ions are found in raw waters. The iodine is an essential trace element for humans. The adult daily requirement of iodine or iodide is 80-150 xg. Iodide content of the drinking water is sometimes checked to decide the amount of supplement needed. The iodide is usually supplied as a table-salt additive in areas where it is needed. As a matter of fact, the determination of iodine or iodide content of water samples is not needed very often. However, the titration or back titration of iodine content is an everyday trick in iodometric analytical procedures. 

Titration of iodine with thiosulfate or phenylarsin oxide titrant is an everyday task in iodometric analysis. The iodine content of water samples, however, is much lower than the detection limit of this titration, even with amperometric endpoint location. Some of the highly sensitive electrometric inverse methods have been successfully used. 

Titration of iodine with 0-lN sodium arsenite must be conducted in the... 

Another method for titration of iodine when iodide or other halogen salt is absent (as in Simple Solution of Iodine) is by keeping the solution in contact with zinc dust for a few minutes interaction occurs and soluble zinc iodide is formed. After filtration from the excess of zinc the iodide may be titrated with 0 05M potassium iodate or 0 1N silver nitrate. 

The so-called dead-stop method of end-point detection is a special case and one which is of considerable importance in pharmaceutical work. Two identical platinum electrodes are placed in the solution to be titrated and a current of a few millivolts is applied across them. The end-point of the titration is shown by a sudden change in the current flowing between the two electrodes. In the titration of iodine with a thiosulphate solution both electrodes are depolarised during the titration and hence a current flows through the cell but at the end-point only one of the electrodes remains depolarised and the current ceases to flow. This has given rise to the term dead-stop end-point to describe this type of titration. In practice, however, the converse usually applies no current passes until the equivalence-point when depolarisation of the electrodes occurs and a current flows, as shown by a sudden permanent deflection on a galvanometer in series. Details of a suitable circuit and procedure are as follows ... 

The thiosulphate titration of iodine may be completed using a variety of electrochemical end-point detectors (a good account is given by Knowles and Lowden, Analyst, 78 159, 1953) but we have found that none of these methods gives a noticeably better precision than does the classical starch end point when used with proper illumination. The true stoichiometric end point is a little after the starch end point but this error is barely significant in marine work. 

The solution was studied by UV-visible spectrometry and conductometric redox titrations of iodine in mixtures of potassium persulfate and chlorosulfonic acid. Similar studies of the bromine oxidation of iodine, potassium iodide and iodic acid in chlorosulfonic acid indicated the formation of the dibromoiodine cation (Br2U) as a stable entity in solution. ... 

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