Errors with iodine

It has been reported 2d) that the spectrophotometric determination of the enthalpies of adduct formation for sulfur donors with iodine produce the same result (within experimental error) in cyclohexane or CCI4, but these donors with various acids invariably give low results in CCI4 by a constant amount when measured calorimetrically 10). Treating the problem as one involving competing equilibria, we can describe the interaction between the base and CCI4 as ... 

They also confirm the observation that errors may arise in the rate coefficients for the system due to the comparable rates of reaction of thiosulphate with iodate and with iodine. The reaction rates depend on ionic strength quite strongly, in the direction predicted from theory, at ionic strengths less than 0.1 . At higher ionic strength, the change may be in the opposite sense . 

It has been claimed (25) that 2,3-diphenyl-5,6-dihydropyrazine when heated with acetic or benzoic anhydride gives a derivative of the 1,4-dihydropyrazine ring system, but this has been shown by Chen and Fowler (1562) to be in error [the products have been shown to be 1,4-diacetyl(or dibenzoyl)-5,6-diphenyl-l,2,3,4-tetrahydropyrazine (26, R = Me or Ph), respectively, together with 23-diphenyl-pyrazine]. Hexamethyl-2,3-dihydropyrazine cannot undergo oxidation to a pyrazine, but it rapidly dimerized in air in the presence of hydrochloric acid to give compound (27, X = Cl) (also prepared as the iodide with iodine in ether) (1550). 

Even when errors in true free sulfur dioxide content due to dilution or oxidation are avoided, the direct iodometric determination of sulfurous acid and sulfites is subject to error. Eolthoff and Menzel (1929) point out that an appreciable error occurs when sulfite solutions are titrated with iodine owing to oxidation by air and point out that correct results can be obtained only when the sulfurous acid or sulfite solution is allowed to flow into a solution of iodine. The titration of sulfurous acid or bisulfites is... 

The yield of iodine, which has also been evaluated as a function of [I ] [IJiv] concentrations and [H+], indicates that no uranium(v) is produced. Support for the absence of any species of this type is also given from the effects of iron(ra) on the rate, the rate constants being identical to within experimental error with those where Fe is absent. It is of interest to note that [as in the case of cerium(iv) oxidations] although in general the reactions of Cr i with metal ions have either a small positive or no hydrogen-ion dependences, exceptions are provided in the cases of U v Npiv and Pu v. The effect is considered to arise largely because an aquo-ion e.g. U +) is oxidized directly to an oxo-cation (UO ) in the reaction... 

Only a small amount of potassium iodate is needed so that the error in weighing 0.14-0.15 g may be appreciable. In this case it is better to weigh out accurately 4.28 g of the salt (if a slightly different weight is used, the exact molarity is calculated), dissolve it in water, and make up to 1 L in a graduated flask. Twenty-five millilitres of this solution are treated with excess of pure potassium iodide (I g of the solid or 10 mL of 10 per cent solution), followed by 3 mL of IM sulphuric acid, and the liberated iodine is titrated as detailed above. 

Experiments by Freund and Spiro/ with the ferricyanide-iodide system showed that the additivity principle held within experimental error for both the catalytic rate and potential when the platinum disk had been anodically preconditioned, but not when it had been preconditioned cathodically. In the latter case the catalytic rate was ca 25% less than the value predicted from adding the current-potential curves of reactions (15) and (16). This difference in behavior was traced to the fact that iodide ions chemisorb only on reduced platinum surfaces. Small amounts of adsorbed iodide were found to decrease the currents of cathodic Fe(CN)6 voltam-mograms over a wide potential range. The presence of the iodine couple (16) therefore affected the electrochemical behavior of the hexacyanofer-rate (II, III) couple (15). 

Fig. 5 Scatterplot derived from the CSD reporting the N- I-C angle (deg) vs. the N- I distance (A) for crystal structures containing intermolecular N- I contacts only error-free and non-polymeric structures containing single-bonded iodine atoms and showing no disorder with R < 0.06 are considered. The scatterplot clearly demonstrates the high directionality of the N- I XB...

The error introduced by Reaction 3 was further diminished by flushing out much of the oxygen from the refluxing system by boiling the contents before adding the potassium iodide. Crystals between 2 and 5 mm. were used because smaller ones had a tendency to stick to the walls of the wet condenser. Early in this work it was noticed that crystals of potassium iodide which were not washed down were often partly oxidized to iodine, which worked down into the flask and caused high results. Thus it was important to wash all the potassium iodide down the condenser with water. Crystals of potassium iodide were used rather than a solution because this avoided the possibility of such a solution being oxidized to contain triiodide ion. 

The determination is done by oxidation of the -SH group with 0.1N iodine solution in NaHCC>3 solution visually with respect to strength or potentiometrically (the equivalence point is difficult to recognize) or in NaOH solution potentiometrically, which is more favorable. The error of the method for a weight of lOmg amounts is approximately 0.5%. 

Thus, errors in all the heats, apart from the precisely known water value, are decreased ra-fold in the required heat. The hydrolysis heat A//2 is always much less than At and can be measured less precisely. The reactions involved in reactions 2, 3, and 6 of Table II have been used in the CODATA evaluation of Aif,(F q)), although in theorv reactions 7 and 8 should lead to more accurate values. In practice this may not be true. Thus the fluorination of iodine produces some IF7, together with the bulk of IF5, and errors in estimating the mixture can cause uncertainty in the final value of AZf/IF ). The hydrolysis heat for the reaction... 

For the entire set of data, the standard deviation for prediction of log Kc is 0.011 or about 27% of Kc. However, there is evidence of systematic error in series 3 and series 8, and from run to run in series 1. This error does not appear to be associated with variables such as solution composition or temperature. It may have resulted from errors in standardization of iodine solution or in calibration of the SO2 analyzer (for series 8). 

Series 8 in combination with earlier series was intended to provide data on the effects of total anion concentration. The results are internally consistant with the correlation, having a standard deviation of about 15% around the mean error. However the measured values of PSO2 were about 40% lower than the general correlation. An SO2 analyzer, rather than iodine titration, was used to determine SO2 gas concentration from the saturator. The analyzer was calibrated with dry SO2/N2 span gas. In later experiments it was shown that humid gas gives a lower analyzer response. With constant fraction neutralization increased anionic concentration increases PSO2 because pH decreases faster than effective bisulfite activity. 

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