Starch, iodine titrations

Transfer 25 ml. of this dilute solution by means of a pipette to a conical flask, and add similarly 50 ml. of Ml 10 iodine solution. Now-add 10% sodium hydroxide solution until the liquid becomes pale yeilow in colour, and allow the solution to stand, with occasional shaking, at room temperature for at least 10 minutes. Then acidify with dilute hydrochloric acid (free from chlorine) in order to liberate the remaining iodine. Titrate the latter w ith Mho sodium thiosulphate solution, using starch as an indicator in the usual way. 

If it is desired to base the standardisation directly upon arsenic(III) oxide, proceed as follows. Weigh out accurately about 0.20 g of pure arsenic(III) oxide into a conical flask, dissolve it in 10 mL of 1M sodium hydroxide, and add a small excess of dilute sulphuric acid (say, 12-15 mL of 0.5M acid). Mix thoroughly and cautiously. Then add carefully a solution of 2 g of sodium hydrogencarbonate in 50 mL of water, followed by 2 mL of starch solution. Titrate slowly with the iodine solution to the first blue colour. Repeat with two other similar quantities of the oxide. 

Suppose 25.00 mL of an aqueous solution of iodine was titrated with 0.0250 M Na2S20,(aq), with starch as the indicator. The blue color of the starch-iodine complex disappeared when 27.65 mL of the thiosulfate solution had... 

Sandri (15) tested three titration methods with good results. One was titration with potassium bromate in the presence of potassium bromide and hydrochloric acid, using a starch-iodine end-point. Another was addition of excess periodic acid-potassium iodide with sodium... 

Often, to make the endpoint of an iodine titration more obvious, an indicator solution that contains starch is added to the solution being titrated. Starch forms a deep blue complex with triiodide. Is", but it is colourless with l . As long as there is unreacted vitamin C in solution, no triiodide ions will be present in solution. Therefore, the blue colour will appear only at the endpoint. 

APHA Method 4500-CL02-B, iodometric titration analysis, measures the concentration of chlorine dioxide in water by titration with iodide, which is reduced to form iodine. Iodine is then measured colorimetrically when a blue color forms from the production of a starch-iodine complex. The detection limit for this method is 20 pg/L (APHA 1998). 

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

Reducing agents can be titrated directly with standard If in the presence of starch, until Reducing agent + I3 -> 31-reaching the intense blue starch-iodine end point (Table 16-4). An example is the iodimetric... 

Cover the flask with a watch glass to exclude air, and put in a dark place for 10 mins. Dilute to ca 350ml with w and titrate with 0.1N std Na thiosulfate soln until the btn color of liberated iodine begins to fade. Add 3-5ml of starch indicator soln(See Note below) and continue the titration until the blue color of the starch iodine disappears, leaving the grn color of chromic... 

An accurately weighted amount of primary standard is dissolved in water containing an excess of potassium iodide. Upon acidification, stoichiometric amounts of iodine are liberated instantly, which are titrated with thiosulfate titrant of unknown strength, decolorizing the blue starch-iodine complex at the end point. With potassium iodate, the ionic reaction is as follows ... 

The blue starch iodine complex has been used from time immemorial as an indicator in aqueous iodometric titrations water is required for development of the blue color. The color sensitivity is decreased with increasing temperature and also with increasing concentration of ethanol [134]. Although one might imagine this to be evidence for a charge-transfer complex,... 

The potentiometric, iodine titration clearly shows the adsorptive powers of both starch fractions, namely, their relative affinities for iodine in a quantitative way. As is well known, the iodine affinity of amylose depends on its chain length, the shorter chains having the lower affinity. Evidently,... 

H.OJ and 5 ml of 10% potassium hydroxide (or sodium hydroxide). Boil the solution gently for 1 h to oxidize Cr to C.f + and to get rid of excess peroxide. Add 10 ml of 6 N H,SO to acidify the solution. After standing in the dark for 5 min, add 20 ml of 10% (w/w) potassium iodide. The solution turns to a dark reddish brown owing to the formation of iodine. Titrate the iodine in the solution with 0.1 N standard solution of sodium thiosulphate until the colour of the solution turns to yellow. Add 1-2 ml of starch solution (1% w/w) as an indicator. The colour of the solution turns to dark blue. Continue titration until the colour just disappears. Record the volume of the standard solution of sodium thiosulphate used (V ). The concentration C (g 1 ) of initial chromium sulphate solution can be calculated by the following equation ... 

In iodometry, the analyte is an oxidizing agent that reacts with I to form L. The L is titrated with thiosulfate, using disappearance of the starch-iodine color for the end point. 

In the analysis of monoquinolineiodine(I) benzoate and dipyridinebromine(I) nitrate, a sample of about 0.1 g. was added to a solution containing 2 g. of potassium iodide and a few drops of hydrochloric acid in 50 ml. of water, and the liberated iodine was titrated with a standard sodium thiosulfate solution in the presence of starch as indicator. In the analysis of dipyridinebromine(I) perchlorate, a 0.2-g. sample was dissolved in 25 ml. of 0.5 N sodium hydroxide solution to which 0.5 g. of potassium iodide had been added. The solution was then acidified with 10 ml. of 2 N sulfuric acid and the liberated iodine titrated immediately with thiosulfate. 

Other types of indicator can be used for other reactions. Starch, for example, is used in iodine titrations because of the deep blue complex it forms. Oxidation-reduction indicators are substances that show a reversible colour change between oxidized and reduced forms. SeeafeoADSORprioNtNDicA-... 

Ingram, M. 1947a. An electrometric indicator to replace starch in iodine titrations of sulphurous acid in fruit juices. J. Soc. Chem. Ind. 66, 50. 

Total SulfuT Dioxide After Alkali Treatment The fact that neutral sodium sulfite does not combine with carbonyl compounds and that the hydroxysulfonic acid compounds are rapidly decomposed on treatment with alkali was used by Ripper (1892) as the basis for the determination of total sulfur dioxide in wine by direct iodine titration. In his method, 50 ml. of wine were pipetted into a 200-ml. flask containing 25 ml. of 1 iV KOH. The mixture was shaken and allowed to stand for 10 to 15 minutes. Then 10 ml. of dilute sulfuric acid (1 + 3) were added, and the solution titrated rapidly with 0.02 N iodine solution to a starch end point which persisted for some time. This method was used as the ofiicial direct titration method for wine in the first edition (1919) of the A.O.A.C. Methods of Analysis in the third (1930) edition it was extended to white grape juice, wine, and similar products (1N NaOH or KOH was used and the solution during standing for 15 minutes was occasionally agitated) hut it was dropped from the fourth (1935) and succeeding editions. Ripper compared his method with the Haas distillation method on ten wines whose SO2 content varied from 42 to 1488 mg. per liter and found the difference between the two to vary from 0 to 5 mg. 

After the expiration of four hours 20 c.c. of 10 per cemt. solution of potassium iodide and 150 c.c. water are added to the contents of the bottle, and the excess of iodine titrated with N/10 sodium, thiosulphate. solution, the whole being well agitated during the titration, which is finished with starch paste as indicator. The blank experiment is titrated in the same manner, and from the amount of thiosulphate required in the blank experiment is deducted the number of c.c. required by the unabsorbed iodine in the other bottle this figure multi-... 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

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