Iodine/iodide colors

It is the amylose component of starch that gives the blue color when KI/I2 solution is added. To study the iodine-iodide color of amyloses of different d.p. values, maltodextrin-amylose molecules, with various avg. d.p. values from 6 to 568 were prepared by Bailey and Whelan [62], using phosphorylase, a-D-glucopyranosyl-1-phosphate, and maltohexaose. The colors of the various sized maltodextrins (1 mg) were observed when 10 1 (w/w) KI/I2 solution was added. The first color to be observed was faint red for avg. d.p. 12 a red-purple color was observed for avg. d.p. 31 a purple color was observed for avg. d.p. 40 and a blue color was observed for avg. d.p. 45. The increase in the blue value was linear as a function of avg. d.p. up to avg. d.p. 60 the absorbance at 645 nm then slowly increased and reached a maximum at avg. d.p. of 400. The intensity of the iodine/iodide color in the low molecular weight range was dependent on the concentration of the iodine. When the concentration of the iodine was increased 10-fold, the intensity was increased 50% [62]. 

This reaction undergoes conversion in one sequence of consecutive elementary reaction steps and so only one propagating front is formed in a spatially distributed system [68]. Depending on the initial ratio of reactants, iodine as colored and iodide as uncolored product, or both, are formed [145]. 

In a glass-stoppered, wide-mouth bottle cooled by ice water are placed 35 g. (0.42 mole) of thiophene (p. 73) and 50 cc. of benzene (Note 1). With constant shaking (Note 2), and cooling when necessary, 75 g. (0.35 mole) of yellow mercuric oxide and 109 g. (0.43 mole) of iodine are added alternately in small amounts during a period of fifteen to twenty minutes. The yellow mercuric oxide changes to crimson mercuric iodide. The mixture is filtered, and the residue is washed with three 25-cc. portions of ether. The ether-benzene filtrate is shaken with a dilute solution of sodium thiosulfate to remove excess iodine and then dried over 5 g. of calcium chloride and filtered. The ether and benzene are removed by distillation on a steam bath (Note 3), and the residue is fractionally distilled under reduced pressure. Iodothiophene distils at 73°/15 mm. 8o-8i°/20 mm. 90-94°/34 38 mm. The yield is 63-66 g. (72-75 per cent of the theoretical amount) (Note 4). If the iodothiophene is still colored by traces of iodine, the color may be removed by shaking with a small amount of mercuric oxide. 

Solutions of iodides, upon the dropwise addition of chlorine TS, liberate iodine, which colors the solution yellow to red. Chloroform is colored violet when shaken with this solution. 

Nitrites yield brown-red fumes when treated with diluted mineral acids or acetic acid. A few drops of potassium iodide TS (15%) and a few drops of 2 N sulfuric acid added to a solution of nitrite liberate iodine, which colors starch TS blue. 

The method the iodine reagent to obtain the blue color of starch contains 0.2% (w/v) iodine in a 2.0% (w/v) potassium iodide solution [172]. Usually a solution with 10-times this concentration is prepared and then diluted 1 10 immediately before use. The analysis is carried out by the addition of 25 pL of the diluted iodine/iodide reagent to 50 pL of each sample in a microtiter plate well and the absorbance is measured at 600-620 nm, using a microtiter plate colorimeter. The starch triiodide blue color is most stable under acidic conditions (pH 1-2) and will not form under alkaline conditions (pH 8 and higher). 

The tan-brown complex a-li (formed in the absence of iodide) has been studied by x-ray diffraction. " In this case, only the iodine atoms can be located by x-ray diffraction means alone, but consideration of possible means of packing the a-dextrin molecules indicates that each iodine molecule is enclosed by a cyclic dextrin molecule (see Fig. 28). It may be noted that, in this case, the iodine molecules do not approach each other closely there is no opportunity for cooperative effects or interaction with iodide ion such as is observed in the highly colored iodine-iodide complexes. The... 

The formation of these charged complexes in solution has been used as the basis for an electrophoretic separation of the Schardinger dextrins. Beckmann and Forster also found that complex formation with a-dextrin enhances approximately 2J- -fold the ultraviolet absorption maxima in iodine-iodide solutions at 290 and 350 m/i. It is probable that the colored complexes of iodine with methyl ethers and with the tosyl and mesyl esters of the Schardinger dextrins are also inclusion compounds of the same general type. 

Iodide colors a solution of PtCU red to brown (sensitive to 0.3 mmol Pt) (Fe , Cu , and other oxidants interfere) and may precipitate black PU4. Excess of KI forms K2[Ptl6], brown, shghtly soluble, and unstable enough that platinum may be determined volumetrically by treating [PtCle] with excess r and titrating the liberated iodine with thiosulfate, which shifts the Pt -Pl equilibrium completely toward reduction (as the O2 and acidic CO2 three paragraphs above shift it toward oxidation) ... 

The usefulness of the fundamental reaction of direct and indirect lodometries has its roots, at least in part, in the fact that it is accompanied by the disappearance or appearance of the yellow-brown color in the solution due to tri-iodide ions. Tri-iodide ions are their proper indicator. Hence, its equivalence point detection is particularly easy. In some difficult cases, starch may be used. The partitioning of iodine into an organic phase at the equivalence point may also be used. Finally, some internal indicators of intermediary standard potential values such as variamine blue (E° 0.60 V) may also be used. The difficult cases are those in which the tri-iodide coloration is masked by that of the solution. 

The enzymatic polymerization of G-l-P using 6 and 9 as primers was performed to produce the two types of amylose-grafted polyacetylenes (7) and (10) (Schemes 5a and b). The complex formation with iodine is a well-known characteristic property of amylose [23]. The colorless solutions of 7 and 10 in DMSO turned to violet after the addition of a standard iodine-iodide solution to the polymer solution, as the same color change in the complex formation of amylose with iodine. Values of the A-max of UV-Vis spectra of the iodine... 

Some laboratories measure the absorbance at a specified wavelength using a spectrophotometer and convert the absorbance reading to the iodine or platinum-cobalt color scale using a linear calibration plot. These calibrations are, strictly speaking, only valid when the absorbance spectrum of the colored impurity of interest is exactly the same as that of iodine/iodide or of platinum/cobalt solution. 

Recovery of the wopropyl alcohol. It is not usually economical to recover the isopropyl alcohol because of its lo v cost. However, if the alcohol is to be recovered, great care must be exercised particularly if it has been allowed to stand for several days peroxides are readily formed in the impure acetone - isopropyl alcohol mixtures. Test first for peroxides by adding 0-6 ml. of the isopropyl alcohol to 1 ml. of 10 per cent, potassium iodide solution acidified with 0-6 ml. of dilute (1 5) hydrochloric acid and mixed with a few drops of starch solution if a blue (or blue-black) coloration appears in one minute, the test is positive. One convenient method of removing the peroxides is to reflux each one litre of recovered isopropyl alcohol with 10-15 g. of solid stannous chloride for half an hour. Test for peroxides with a portion of the cooled solution if iodine is liberated, add further 5 g. portions of stannous chloride followed by refluxing for half-hour periods until the test is negative. Then add about 200 g. of quicklime, reflux for 4 hours, and distil (Fig. II, 47, 2) discard the first portion of the distillate until the test for acetone is negative (Crotyl Alcohol, Note 1). Peroxides generally redevelop in tliis purified isopropyl alcohol in several days. 

Iodine compounds are important in organic chemistry and very useful in medicine. Iodides, and thyroxine which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds. Potassium iodide finds use in photography. The deep blue color with starch solution is characteristic of the free element. 

PuUy hydroly2ed poly(vinyl alcohol) and iodine form a complex that exhibits a characteristic blue color similar to that formed by iodine and starch (171—173). The color of the complex can be enhanced by the addition of boric acid to the solution consisting of iodine and potassium iodide. This affords a good calorimetric method for the deterrnination of poly(vinyl alcohol). Color intensity of the complex is effected by molecular weight, degree of... 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

The crude bromide (10 g) is dissolved in 120 ml of acetone and 12.3 g of anhydrous potassium acetate, 3.8 ml of glacial acetic acid and 5.88 g of sodium iodide are added in that order. The mixture is refluxed with agitation for 4 hr the initial yellow iodine color fades completely during this time. Water (200 ml) is added and the acetone removed by distillation at reduced pressure, whereupon the product separates as bulky fibers. After chilling, the product is collected, washed with water and dried to give 9.4 g (98 %) of 5a,6) -dichloro-3i ,17a,21-trihydroxypregnan-20-one 21-acetate mp 192-195° (dec). 

In order that the reaction may proceed rapidly it is important to shake the mixture thoroughly after adding the iodine solution. When this is done the iodine compound is formed completely within one minute. With thymol it affords thymol di-iodide. In order to make sure that any iodine wnich may have entered into the hydroxyl-group is again liberated, care should be taken that a little hydriodic acid is always present hence the addition of the potassium iodide solution before the exc ess of iodine is titrated back with thiosulphate. Titration can only be regarded as complete when the blue coloration does not return in 10 minutes. 

Papier-fabrik, /. paper factory, paper mill, -fabrikant, m. paper maker, -fabrikation, /. paper making, -farbe, /. paper color, -filter, n. paper filter, -flftche, /. paper sur-faee, surfaee of the paper, -gam, n. paper yarn, paper twine, -handel, m. paper trade stationery, -handlung, /. stationer s shop, -holz, n. pulpwood. -jod, n. a solution of iodine and potassium iodide for test paper, papierkaschiert, a. paper-covered, paper-baeked (as metal foil). 

A number of methods have been proposed for the detection of rancidity. The determination of active oxygen consists of dissolving the fat in a suitable medium such as chloroform and acetic acid, adding potassium iodide, and titrating the liberated iodine with a standard thiosulfate solution (16, 20). This is perhaps the most widely used method at the present time. Another procedure which has been proposed for the detection of peroxides employs ferrous ammonium sulfate and ammonium thiocyanate in acetone. The resulting red color of ferric thiocyanate is measured spectrophotometrically, and is said by the authors to yield more reproducible results than do the usual titration methods (21). 

When iodine dissolves in organic solvents, it produces solutions having a variety of colors. These colors arise from the different interactions between the I2 molecules and the solvent (Fig. 15.21). The element is only slightly soluble in water, unless I ions are present, in which case the soluble, brown triiodide ion, I,, is formed. Iodine itself has few direct uses but dissolved in alcohol, it is familiar as a mild oxidizing antiseptic. Because it is an essential trace element for living systems but scarce in inland areas, iodides are added to table salt (sold as iodized salt ) in order to prevent an iodine deficiency. 

FIGURE 15-21 Solutions of iodine in a variety of solvents. From left to right, the first three solvents are tetrachloromethane (carbon tetrachloride), water, and potassium iodide solution, in which the brown I. ion forms. In the solution on the far right, some starch has treen added to a solution of I,. The intense blue color that results has led to the use ot starch as an indicator for the presence of iodine. 

First there is chlorination of nitrogen to yield reactive N-chloro derivatives, which oxidize iodide to iodine in the next step. Finally oxidation of the o-toluidine probably yields colored quinonoid toluidine derivatives. 

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