Iodate absorbance

The analyses of periodate and iodate mixtures is readily accomplished colorimetrically at 222.5-230 m u (in the ultraviolet region).263-273 The method utilizes very low concentrations of reactants, but is useless when the reactants or products have structures which absorb light in this region of the ultraviolet absorption spectrum. Also, certain undesirable side-reactions are catalyzed by ultraviolet light.273... 

Wong [10,11] has studied this in further detail and found that carrying out the titration at pH 2 yields a true concentration of total residual chlorine after correction for naturally occurring iodate. The effectiveness of sulfamic acid in this method for removal of the nitrite interference is shown in Fig. 4.1. In this experiment, all the solutions contained 30 pmol/1 nitrite, and about 0.5 pmol/1 of iodate. The absorbance of the solution at 353 nm decreased with increasing amounts of added sulphamic acid. A constant absorbance was recorded when 3 ml or more of 1% (w/v) sulphamic acid was added to the solution, and this absorbance was identical with that in a sample containing the same amount of iodate and no nitrite. A concentration of nitrite of 30 pmol/l is unlikely to occur in estuarine water and seawater ... 

Figure 4.1. Absorbance of tri-iodide at 353 nm formed from a fixed concentration of iodate in the presence of 30 xmol/l nitrite upon the addition of various volumes of 1% (w/v) sulphamic acid N denotes the case where neither nitrite nor sulphamic acid was present in the solution. From [8]...

In the iodate detection system (ASTM D1552), the sample is burned in a stream of oxygen at a sufficiently high temperature to convert about 97% by weight of the sulfur to sulfur dioxide. The combustion products are passed into an absorber containing an acidic solution of potassium iodide and starch indicator. A faint blue color is developed in the absorber solution by the addition of standard potassium iodate solution. As combustion proceeds, bleaching the blue color, more iodate is added. The sulfur content of the sample is calculated from the amount of standard iodate consumed during the combustion. 

Iodine is present in the environment predominantly in the oxidation states —1 (1, iodide) and - -5 (lOs", iodate). Reduction of lOs" to 1 occurs at pe = 13.3 at pH 5 and pe° = 11.3 at pH 7. Hence 1 is expected to predominate in the soil solution except in oxic alkaline soils (Whitehead, 1984). However Yuita (1992) found predominantly IO3 in acid Japanese soils contaminated with iodine the concentrations in solution were some 20 times those of 1 and I2. On flooding the soils, the total concentration of 1 in solution increased 10- to 50-fold, predominantly as I. The concentrations of sorbed 1 were not measured, but both lOs and 1 are expected to be bound to organic matter and oxides and hence their concentrations in solution are expected to increase with reductive dissolution reactions. Further, for a given concentration in solution, 1 is more rapidly absorbed by plants than IO3 (Mackowiak and Grossl, 1999). Hence flooding is expected to increase accumulation in plants both through increased solubility and increased absorption. 

The contents of the ampule are diluted in water to a final volume of 50 mL. A 1-mL sample is then taken for the assay. To this sample 1.5 mL 0.667% (w/v) rhodanine in methanol is added. After exactly 5 min. 1 mL 0.5N KOH is added. After 2.5 min. water is added to a final volume of 25 mL. The absorbance is read at 520 nm after a 5-10 min. incubation. A standard curve is made by reaction of gallic acid in 0.2N sulfuric acid with the rhodanine solution. Hagerman and Butler (1989) argued that this assay is more suitable than the potassium iodate assay for the determination of hydrolysable tannins, although it has to be kept in mind that the rhodanine assay is sensitive to any gallic acid ester, including those in non-tannin compounds. 

The intensity of the color developed is proportional to the amount of phenan-throline complex formed, which is proportional to the concentration of sulfite in the sample. After removing the excess ferric iron with ammonium bifluoride, the absorbance is measured at 520 nm. The concentration of S032- in the sample is calculated from a sulfite standard calibration curve. Because S032- solutions are unstable, the concentration of working standard is accurately determined by potassium iodide-iodate titration before colorimetric measurement. 

In environmental waters, the most important oxidation states are iodide ( — 1) and iodate ( + 5). Most published methods for the analysis of radioiodine aim only to convert all species to one chemical form in order to determine a total concentration value for the particular nuclide of interest. However, some specialist methods designed for the analysis of the stable element such as that recently described by Woittiez et al. (1991) for the determination of iodide, iodate, total inorganic iodine and charcoal-absorbable (organic) iodine in seawater could presumably be adapted to provide information about the speciation of radioiodine as well. More difficult to adapt would be techniques such as polarography which have been useful in the measurement of the iodide/iodate system (e.g. Liss et al., 1973). 

Mullins and Kirkbright (38) reported separation of the UV absorbing anions iodate, nitrite, bromide, nitrate and iodide using a micellar mobile phase containing hexadecyltrimethylammonium chloride above its CMC. Figure 2 illustrates this separation with two different concentrations of micellar reagent. Increasing the concentration of hexadecyltrimethylammonium chloride decreases the retention time (38) on the column. 

The substance decrepitates. 2. The substance deflagrates. 3. The substance fuses and is absorbed by the charcoal, or forms a liquid bead. 4. The substance is infusible and incandescent, or forms an incrustation upon the charcoal. Crystalline salts, e.g. NaCI, KC1. Nitrates, nitrites, chlorates, perchlorates, iodates, permanganates. Salts of the alkalis and some salts of the alkaline earths. Apply test (ft) below. 

The oil phase, Soltrol 130, a refined kerosene, was doped with iodated oils of similar molecular structure. The dopants are strong photoelectric absorbers and increase the accuracy of saturation determination by increasing the X-ray attenuation. The refined, nearly single-component, oil was also used to insure complete first-contact miscibility. Because this is a single-component oil, multiple contact developed miscibility is not observed below the miscibility pressure. 

Reagent III required rapid acidification upon very slow acidification with either ozone or iodate added, iodine losses have been observed. Similar losses occur upon addition of standard iodine to the alkaline reagent before acidification, regardless of the rate of acidification, but not when iodine is added after acidification. Apparently in the former cases iodine is released at the surface of acid drops and absorbed into the surrounding alkali, where the loss occurs. With rapid acidification of samples the iodine extinction coefficient for reagent III agrees well with expected values. 

In Japan another process is used in which the iodine formed by chlorine oxidation is absorbed as polyiodide on an anionic ion exchanger. Desorption with alkali yields concentrated iodide- and iodate-containing solutions which are worked up to elemental iodine. 

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