Iodine coulometric

The generation of iodine coulometrically at the anode has an extensive application in the Karl Fischer (KF) technique of water determination. The current... 

The iodine number of fats and oils provides a quantitative measurement of the degree of unsaturation. A solution containing a 100% excess of IGl is added to the sample, reacting across the double-bonded sites of unsaturation. The excess IGl is converted to I2 by adding KI. The resulting I2 is reacted with a known excess of Na2S203. To complete the analysis the excess 8203 is back titrated with coulometrically generated I2. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

The Karl Fischer procedure has now been simplified and the accuracy improved by modification to a coulometric method (Chapter 14). In this procedure the sample under test is added to a pyridine-methanol solution containing sulphur dioxide and a soluble iodide. Upon electrolysis, iodine is liberated at the anode and reactions (a) and (b) then follow the end point is detected by a pair of electrodes which function as a biamperometric detection system and indicate the presence of free iodine. Since one mole of iodine reacts with one mole of water it follows that 1 mg of water is equivalent to 10.71 coulombs. 

Applications The coulometric Karl Fischer titration is a widely used moisture determination method (from ppm to 100%). In the presence of water, iodine reacts with sulfur dioxide through a redox process, as follows ... 

The water (moisture) content can rapidly and accurately be determined in polymers such as PBT, PA6, PA4.6 and PC via coulometric titration, with detection limits of some 20 ppm. Water produced during heating of PET was determined by Karl Fischer titration [536]. The method can be used for determining very small quantities of water (10p,g-15mg). Certified water standards are available. Karl Fischer titrations are not universal. The method is not applicable in the presence of H2S, mercaptans, sulfides or appreciable amounts of hydroperoxides, and to any compound or mixture which partially reacts under the conditions of the test, to produce water [31]. Compounds that consume or release iodine under the analysis conditions interfere with the determination. 

Verhoef and co-workers suggested omitting the foul smelling pyridine completely and proposed a modified reagent, consisting of a methanolic solution of sulphur dioxide (0.5 M) and sodium acetate (1M) as the solvent for the analyte, and a solution of iodine (0.1 M) in methanol as the titrant the titration proceeds much faster and the end-point can be detected preferably bipoten-tiometrically (constant current of 2 pA), but also biamperometrically (AE about 100 mV) and even visually as only a little of the yellow sulphur dioxide-iodide complex S02r is formed (for the coulometric method see Section 3.5). 

As stated previously, the iodine titrant is generated electrochemically in the coulometric method. Electrochemical generation refers to the fact that a needed chemical is a product of either the oxidation halfreaction at an anode or the reduction half-reaction at a cathode. In the Karl Fischer coulometric method, iodine is generated at an anode via the oxidation of the iodide ion ... 

The critical datum is not a buret reading, as it was in the case of the volumetric method. Rather, the amount of iodine used is determined coulometrically by computing the coulombs (total current over time) needed to reach the end point. The coulombs are calculated by multiplying the current applied to the anode-cathode assembly (a constant value) by the total time (seconds) required to reach the end point. The modern coulometric titrator automatically computes the amount of moisture from these data and displays it. 

In the volumetric method, the titrant is added from an external reservoir. In the coulometric method, the titrant (iodine) is generated internally via an electrochemical reaction. 

There are two types of Karl Fisher titrations volumetric and coulometric. Volumetric titration is used to determine relatively large amounts of water (1 to 100. ig) and can be performed using the single- or two-component system. Most commercially available titrators make use of the one-component titrant, which can be purchased in two strengths 2 mg of water per milliliter of titrant and the 5 mg of water per milliliter of titrant. The choice of concentration is dependent on the amount of water in the sample and any sample size limitations. In both cases, the sample is typically dissolved in a methanol solution. The iodine/SCVpyridine (imidazole) required for the reaction is titrated into the sample solution either manually or automatically. The reaction endpoint is generally detected bivoltametrically. 

Coulometric titration is used to determine relatively low concentrations of water (10. ig to 10 mg) and requires two reagents a catholyte and an anolyte (the generating solution). The iodine required for the reaction is generated in situ by the anodic oxidation of iodide. 

The iodine then reacts with the water that is present. The amount of water titrated is proportional to the total current (according to Faraday s law) used in generating the iodine necessary to react with the water. One mole of iodine reacts quantitatively with 1 mol of water. As a result, 1 mg of water is equivalent to 10.71 C. Based on this principle, the water content of the sample can be determined by the quantity of current that flows during the electrolysis. For this reason, the coulometric method is considered an absolute technique, and no standardization of the reagents is required. 

In the coulometric version of the instrument, the iodine necessary for reaction is generated electrochemically by applying electrical pulses to the electrode. In this case, a modified Karl Fischer reagent is used which contains iodide instead of iodine. An iodide solution is in contact with the anode in one compartment of the electrolytic cell, which has a diaphragm between the anodic and cathodic regions (Fig. 19.11). Thus, the volume of reagent, which is the indicator in the classical titration method, is replaced by a more precise quantity of current, obtained coulometrieally. 

Here is a coulometric procedure for analysis of total sulfite in white wine. Total sulfite means all species in Reaction (A) and the adduct in Reaction (B). We use white wine so that we can see the color of a starch-iodine end point. 

An automated constant-current coulometric system employing electrogenerated iodine for assay of ascorbic acid or sodium ascorbate has been reported [38]. Using this apparatus, 25 samples of 30 mg could be determined in 2.5 h with an accuracy and precision of 0.3%. The automated system demonstrates accuracy and precision that are equivalent to or exceed the manual USP method, is significantly more rapid, and eliminates the need for preparation, standardization, and storage of titrant. 

Adhami and Herlem789 have carried out a coulometric titration at controlled potential of iodine in fluorosulfuric acid and have shown that iodine is quantitatively oxidized to I2+ by removal of one electron per mole of iodine. 

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... 

In coulometric analyzers, S02 serves to reduce a solution of bromine or iodine. These elements can then be detected to give an indication of the S02 content of the sample air stream. The required supply of reagents for coulometric analyzers is usually less than the reagent requirements of either colorimetric or conductiometric analyzers. 

The approved variations [14] in the Karl Fischer method include volumetric titration methods to either a visual (excess iodine or addition of an indicator) or volta-metric endpoint detection method. The visual or voltametric endpoint methods usually require 30-40 mg of sample for analysis for freeze-dried biological products containing from 1.0% to 3.0% residual moisture. Coulometric Karl Fischer instruments generate the iodine from potassium iodide for water titration at the electrodes. Only 10-20 mg of freeze-dried sample is required for accurate analysis. 

The quantity of sodium thiosulfate in each chamber is titrated by a coulometric production of iodine from the iodide solution with a current which passes through a separate pair of production electrodes. This method of determining sodium thiosulfate has been used for some time by Ehmert ( ) in a manual method for ozone determination. 

From Faraday s constant, 1 y of iodine (1 y = 10 gram) is obtained for an electrolysis current of 38 / a. in 20 seconds. This coulometric method does not need calibrated solutions. The sensitivity is high enough to detect the equivalent with an accuracy to 10 gram of iodine in 10 ml. of solution. Higher accuracy is possible. 

The iodine generated coulometrically or present in the reagent can be reduced by oxidizable species such as ascorbic acid, ammonia, thiols, Tl, Sn ", In, hydroxyl amines, and thiosulfite. This results in consumption of F and water determinations that are too high. Phenolic derivatives and bicarbonates also cause reduction of F. 

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