Iodine, volumetric determination

Volumetric Determination op Iodine by the Method op Leipert and Munster1... 

Instead of making a direct volumetric determination of the hydrosulphite it is possible to modify the process by estimating volumetrically the product of a primary reaction. For example the hydrosulphite solution may be submitted to atmospheric oxidation and the resulting acidity determined with standard alkali,2 or a mercuric salt may be reduced, the mercury produced being estimated subsequently by the addition of standard iodine solution and titration of the excess of iodine one molecule of hydrosulphite is equivalent to an atom of mercury and therefore to two atoms of iodine.4 Similarly, instead of the gravimetric estimation of silver as described above, the latter may be redissolved in nitric acid and determined volumetrically.5... 

Quadrivalent cerium salts may be volumetrically determined by arsenious acid in aqueous sulphuric acid solution using a manganese salt as catalyst, with potassium iodate present as a promoter.2 Excess of arsenious acid is used and back-titrated with permanganate. If nitric acid is present in place of sulphuric acid, an alkali chloride and a trace of iodine are used to promote the action of the manganese salt. Direct potentiometric titration with arsenious acid may also be employed. 

Aqueous solutions of osmium tetroxide are readily reduced by the introduction of practically any metal except those known as the precious metals.3 Thus zinc, silver, mercury, etc., effect the precipitation of metallic osmium from acidulated solutions in a very pure form. In the last-named ease an amalgam is produced from which the osmium is obtained by distilling off the mercury. Ferrous sulphate and stannous chloride4 also reduce the tetroxide solutions, but hydrogen,5 sulphur and selenium 6 appear to have no action under ordinary conditions. Sulphur dioxide reduces the solution to osmium sulphite, whilst potassium iodide reduces it to dioxide with liberation of iodine—a reaction that may be utilised in the volumetric determination of osmium.7... 

Tetrathionates are obtained by treatment of thiosulfates with iodine as in the reaction used in the volumetric determination of iodine mentioned earlier. 

Volumetrically tellurium may be determined by several methods (1) Tellurous acid is oxidized to telluric by permanganate either in acid solution 3 or alkaline solution.4 (2) Tellurous acid in hydrochloric acid solution is reduced by an excess of standard stannous chloride, according to the reaction TeCl< + 2 SnCU — Te + 2 SnCL. The excess stannous chloride is determined iodometrically.5 (3) Telluric acid may be reduced to tellurous by the use of potassium bromide in sulfuric acid solution, thus H2TeO< + 2 HBr = H2TeO + H20 + Br2. The bromine is distilled into a solution of potassium iodide, the liberated iodine being determined by standard thiosulfate.6... 

Kuorre, Volumetric Determination with Iodine, Z. angew. Chem., 1888. 

Although tin can be precipitated by various complexing agents e.g. dithizone and then transformed to the dioxide which is weighed, the process is lengthy and other metal impurities interfere. The volumetric determination with iodate is adequate but it is simpler and quicker to determine Sn(II) by titration with standard iodine solution. 

For the estimation of benzaldehyde, Eipper proposed a volumetric modification of the bisulphite process, the aldehyde being shaken with a measured volume of a standard solution of bisulphite, and the excess of bisulphite titrated back with iodine solution at a low tempe/atnrer Dodge found this give fairly accurate results, and recommends the iollowing method of carrying out the determination. About 0 15 gram... 

The sample is acidified with sulfuric acid. The bromide content is then determined by the volumetric procedure described by Kolthoff and Yutzy [21 ]. In this procedure the buffered sample is treated with excess sodium hypochlorite to oxidise bromide to bromate. Excess hypochlorite is then destroyed by addition of sodium formate. Acidification of the test solution with sulfuric acid followed by addition of excess potassium iodide liberates an amount of iodine equivalent to the bromate (i.e., the original bromide) content of the sample. The liberated iodine is titrated with standard sodium thiosulfate. 

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. 

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. 

The detection and determination ot the perchlorates.—The perchlorates give no precipitates with silver nitrate or barium chloride soln. cone. soln. give a white crystalline precipitate with potassium chloride. Unlike all the other oxy-acids of chlorine, a soln. of indigo is not decolorized by perchloric acid, even after the addition of hydrochloric acid and they do not give the explosive chlorine dioxide when warmed with sulphuric acid unlike the chlorates, the perchlorates are not reduced by the copper-zinc couple, or sulphur dioxide. Perchloric acid can be titrated with —iV-alkali, using phenolphthalein as indicator. The perchlorates can be converted into chlorides by heat and the chlorides determined volumetrically or gravimetrically they can be reduced to chloride by titanous sulphate 28 and titration of the excess of titanous sulphate with standard permanganate they can be fused with zinc chloride and the amount of chlorine liberated can be measured in terms of the iodine set free from a soln. of potassium iodide and they can be... 

Some of the volumetric methods described above may also be adapted to the electrometric determination of arsenic. Such methods have been described for titration of arsenites with ceric sulphate,9 iodine in the presence of sodium bicarbonate,10 chloramine (p-toluene-sulphone chloramide),11 alkaline potassium ferricyanide solution,12 potassium bromate13 or potassium iodate14 in the presence of hydrochloric acid, silver nitrate15 (by applying a secondary titration with 01N alkali to maintain the desired low H+-ion concentration), and with... 

The corresponding sodium salts, which are cheaper, serve equally well. Excess of potassium bromide and hydrochloric acid is added to a solution of the substance to be estimated and the bromate, in the form of a volumetric solution (usually N/5), is run in. The end point (shown by the presence of free bromine) is detectable, quite sharply, by spotting on starch-iodide paper as external indicator. Alternatively the end point may be determined by adding a known amount of bromate in excess, then potassium iodide, and titrating the liberated iodine with thiosulphate. 

The determination may also be carried out volumetrically, the distillate being collected in a known volume of N/20-iodine solution, in which the excess of iodine is subsequently determined by titration with thiosulphate (see Wine, section 20, 1). 

Volumetric Method, i gram of the minium is treated with 2 4 grams of potassium iodide and 30 grams of crystallised sodium acetate dissolved in a little water, 5 c.c. of glacial acetic acid being then added and the liquid shaken until the lead dioxide is completely dissolved. The solution is diluted to about 100 c.c. and the separated iodine determined by means of standard sodium thiosulphate solution in presence of starch paste I X 0 94193 = PbOj. 

In 100 c.c. of the filtrate the thiosulphates are determined volumetrically by means of standard iodine solution and are calculated as sodium thiosulphate in 100 parts of the substance (see also Vol. I, p. 108). 

Assay Dissolve about 1.2 g of sample, previously dried at 105° for 3 h and accurately weighed, in about 50 mL of water in a 100-mL volumetric flask, dilute to volume with water, and mix. Transfer 10.0 mL into a 250-mL glass-stoppered flask, add 40 mL of water, 3 g of potassium iodide, and 10 mL of 3 10 hydrochloric acid, and stopper the flask. Allow to stand for 5 min, add 100 mL of cold water, and titrate the liberated iodine with 0.1 N sodium thiosulfate, adding starch TS near the endpoint. Perform a blank determination (see General Provisions), and make any necessary correction. Each milliliter of 0.1 N sodium thiosulfate is equivalent to 3.567 mg of KI03. 

Volumetric Method (Bunsen). This method depends on the determination of the quantity of iodine freed by chlorine when brought into contact with a solution of potassium iodide. 

According to Froboese, it is preferable to determine, either gravimetrically or volumetrically, the carbon dioxide formed in the above reaction rather than to titrate the iodine. 

Volumetrically, platinum may be determined by adding El to a solution of platinic chloride or alkali chlorplatinate. The iodine liberated is titrated with thiosulfate, one molecule of platinic chloride liberating a molecule of iodine. This method is not extensively used. 

The 21 formed in the second reaction is determined either by visual chemical titration with a reagent such as sodium thiosulfate in the presence of a suitable endpoint indicator or by amperometric, coulometric, or photometric titration methods. The most sensitive KF methods for the measurement of iodine are coulometric. For both the volumetric-amperometric and coulometric methods the endpoint is detected by a pair of platinum electrodes called the indicator electrodes. An electrical potential (100-400 mV) is applied across the electrodes to balance the circuit and the endpoint is reached when the concentration of I2 ( 50pmoll ) depolarizes the cathode deflecting a galvanometer. The volumetric method measures the amount of standardized reagent necessary to depolarize the platinum electrodes. The coulometric method utilizes, in addition to the indicator electrodes, a second pair of platinum electrodes (generator electrodes) that electrolytically convert the 1 to I2. The current consumed in this process is used to calculate the amount of water using the equation that describes Faraday s laws of electrolysis. 

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