Standardization iodine

Hydrazine hydrate may be titrated with standard acid using methyl orange as indicator or, alternatively, against standard iodine solution with starch as indicator. In the latter case about 0-1 g., accurately weighed, of the hydrazine hydrate solution is diluted with about 100 ml. of water, 2-3 drops of starch indicator added, and immediately before titration 6 g. of sodium bicarbonate are introduced. Rapid titration with iodine gives a satisfactory end point. 

The pH must be kept at 7.0—7.2 for this method to be quantitative and to give a stable end poiut. This condition is easily met by addition of soHd sodium bicarbonate to neutralize the HI formed. With starch as iudicator and an appropriate standardized iodine solution, this method is appHcable to both concentrated and dilute (to ca 50 ppm) hydraziue solutious. The iodiue solutiou is best standardized usiug mouohydraziuium sulfate or sodium thiosulfate. Using an iodide-selective electrode, low levels down to the ppb range are detectable (see Electro analytical techniques) (141,142). Potassium iodate (143,144), bromate (145), and permanganate (146) have also been employed as oxidants. 

The excess Na2S202 is back-titrated with standard iodine solution. The permanganate method is based on the oxidation of Se(IV) to Se(VI). 

Ofner Method. This method is for the determination of invert sugar in products with up to 10% invert in the presence of sucrose and is a copper-reduction method that uses Ofner s solution instead of Fehling s. The reduced cuprous oxide is treated with excess standardized iodine, which is black-titrated with thiosulfate using starch indicator. 

The Reich test is used to estimate sulfur dioxide content of a gas by measuring the volume of gas required to decolorize a standard iodine solution (274). Equipment has been developed commercially for continuous monitoring of stack gas by measuring the near-ultraviolet absorption bands of sulfur dioxide (275—277). The deterrnination of sulfur dioxide in food is conducted by distilling the sulfur dioxide from the acidulated sample into a solution of hydrogen peroxide, foUowed by acidimetric titration of the sulfuric acid thus produced (278). Analytical methods for sulfur dioxide have been reviewed (279). 

Sodium thiosulfate is determined by titration with standard iodine solution (37). Sulfate and sulfite are determined together by comparison of the turbidity produced when barium chloride is added after the iodine oxidation with the turbidity produced by a known quantity of sulfate iu the same volume of solution. The absence of sulfide is iadicated when the addition of alkaline lead acetate produces no color within one minute. 

Assay of hydrogen cyanide can be done by specific gravity or silver nitrate titration. Sulfur dioxide in hydrogen cyanide can be deterrnined by infrared analysis or by reaction of excess standard iodine solution and titration, using standard sodium thiosulfate or by measurement of total acidity by... 

For the preparation of standard iodine solutions, resublimed iodine and iodate-free potassium iodide should be employed. The solution may be standardised against pure arsenic(III) oxide or with a sodium thiosulphate solution which has been recently standardised against potassium iodate. 

C) With a standard solution of iodine. If a standard solution of iodine is available (see Section 10.112), this may be used to standardise the thiosulphate solution. Measure a 25.0 mL portion of the standard iodine solution into a 250 mL conical flask, add about 150 mL distilled water and titrate with the thiosulphate solution, adding 2 mL of starch solution when the liquid is pale yellow in colour. 

In the second method, the hypochlorite solution or suspension is titrated against standard sodium arsenite solution this is best done by adding an excess of the arsenite solution and then back-titrating with standard iodine solution. 

A similar procedure may also be used for the determination of antimony(V), whilst antimony (III) may be determined like arsenic(III) by direct titration with standard iodine solution (Section 10.113), but in the antimony titration it is necessary to include some tartaric acid in the solution this acts as complexing agent and prevents precipitation of antimony as hydroxide or as basic salt in alkaline solution. On the whole, however, the most satisfactory method for determining antimony is by titration with potassium bromate (Section 10.133). 

Soliman and Belal investigated argentimetric (67,68) and mercurimetric (69) methods. Hydralazine precipitates silver from ammoniacal silver nitrate solution. The silver is dissolved with hot nitric acid and titrated with ammonium thiocyanate solution. Alternatively, mercury is precipitated from alkaline potassium mercuric iodide solution. The precipitated mercury is dissolved by adding excess standard iodine solution. The excess iodine is back-titrated with sodium thiosulfate solution after acidifying with acetic acid. 

In another situation, a known excess quantity of standard iodine solution is added in the substance (a reducing agent) to be assayed and then the excess iodine may be titrated with the help of standard sodium thiosulphate solution, such as the estimation of sodium bisulphite ... 

A sample of organolithium compound solution is added to an excess of standardized iodine solution in Et20 the excess iodine is extracted with aqueous KI and titrated with standard thiosulfate solution. Evaluation of the organometallic compound is made according to equation. A possible interference can be expected from the couphng reaction in equation 20. This was shown to be neghgible, as demonstrated spectrofluorometrically for phenyllithium , whereas a low titer was found for n-BuLi, attributable to this side... 

Analytical. Hydrazine content in aqueous solutions is determined by reacting with picryl chloride to form the yellow hexanitro hydrazobenzene, which, with alkali forms red or violet salts that can be measured colorimetrically (Ref 3). More concentrated solution of hydrazine can be titrated with either standard iodine or iodate solutions according to the following equations (Ref 4) ... 

Hydrogen sulphide is sometimes estimated volumetrically by treating a solution containing less than 0-04 per cent, with excess of standard iodine and then after a short period titrating the residual iodine in the usual manner with sodium thiosulphate or arsenious oxide accurate direct titration is not possible 3... 

The sulphites, both normal and acid, are easily oxidised, and in solution readily undergo atmospheric oxidation with the formation of sulphates. The oxidation proceeds more readily in neutral than in acid solution,2 and is accelerated by warming. The change in SOa-content of a solution of potassium metabisulphite (0-1 per cent.) kept in a stoppered bottle, observed by titration at intervals of aliquot portions with standard iodine solution, has been observed to be as follows 3... 

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... 

Excess of standard thiosulphate is added after the action and back-titrated with standard iodine. 

Small amounts of arsenic (as little as 0-00002 g.) may be determined by converting to arsine and absorbing the latter in standard iodine, the residual iodine being titrated.4... 

The combination of ortho metallation and meta nucleophilic acylation was used to prepare a key intermediate in a synthesis of deoxyfrenolicin (42), as outlined in Scheme 11. The complex of anisole is orf/io-metallated with n-butyllithium and quenched with chlorotrimethylsilane the resulting [(o-(tri-methylsilyl)anisole)Cr(CO)3] (43) is then metallated again, converted to the arylcuprate, and coupled with ( )-2-hexenyl bromide to give the complex of l-trimethylsilyl-2-methoxy-3-(2-hexenyl)benzene (44). Addition of the carbanion from the cyanohydrin acetal of 4-pentenal, followed by the standard iodine oxidation and subsequent hydrolysis of the cyanohydrin acetal to regenerate the carbonyl group... 

The air is completely expelled from the apparatus by means of a current of carbon dioxide passed through the drawn-out tube and 50 c.c. of the standard iodine solution introduced into the absorption tube. Through the tapped funnel 100 c.c. of the wine and 2 c.c. of concentrated hydrochloric acid are poured into the flask, which is then carefully heated until half the wine has distilled over, the current of carbon dioxide being maintained meanwhile. The iodine solution, which should still be brown, and the rinsings of the absorption tube are titrated in a beaker with the thiosulphate solution in presence of starch paste. The number of c.c. of iodine solution reduced, multiplied by 0 016, gives the total sulphurous add per litre of the wine. 

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). 

Note. (1) The progress of the oxidation may be followed by iodimetry. In this method unreacted periodate is reduced by arsenite solution in the presence of iodide at about pH 8. Excess arsenite is then determined by back titration with standard iodine solution. 

A measured volume of sample is added to a known quantity of standard iodine solution estimated to be in excess over the amount of analyte (e.g., sulfide) in the sample. The standard iodine solution should contain an excess of potassium iodide. The analyte reacts with iodine. This would cause a lowering of strength of iodine solution after the reaction. The normality of iodine is then determined from titration against a standard solution of sodium thiosulfate. Concentration of analyte in the sample, which is proportional to the amount of iodine consumed, is calculated as follows ... 

A measured amount of standard iodine solution is placed in a 500-mL flask. The amount of iodine should be the excess over the expected quantity of sulfide in the sample. Add distilled water and bring the volume to 20 mL. Add 2 mL of 6 A HC1. Pipette 200 mL of sample into the flask. If the sulfide in the sample was precipitated as ZnS, transfer the precipitate with 100 mL distilled water into the flask. Add iodide solution and HC1. If iodine color disappears, add more iodine standard solution into the flask, until the color remains. Titrate with Na2S203 standard. Before the end point, when the color changes to straw yellow or brown, add a few mL of starch solution and continue titration by dropwise addition of Na2S203 standard until the blue color disappears. Record the volume of titrant added. 

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