Starch Iodide Paste

Starch Iodide Paste TS Heat 100 mL of water in a 250-mL beaker to boiling, add a solution of 750 mg of potassium iodide (KI) in 5 mL of water, then add 2 g of zinc chloride (ZnCl2) dissolved in 10 mL of water, and while the solution is boiling, add with stirring a smooth suspension of 5 g of potato starch in 30 mL of cold water. Continue to boil for 2 min, then cool. Store in well-closed containers in a cool place. This mixture must show a definite blue streak when a glass rod dipped in a mixture of 1 mL of 0.1 M sodium nitrite, 500 mL of water, and 10 mL of hydrochloric acid is streaked on a smear of the paste. 

Aminosalicylic acid and its salts may be determined either by bromin-ation or by titration with nitrite. The acid itself, official in the t/.5.P., is determined by titration with nitrite, the end-point being detected externally with starch-iodide paste. A more satisfactory method is to determine the end-point electrometrically by the dead-stop technique (see p. 867). A suitable method is as follows ... 

The sodium nitrite volumetric solution is preferably standardised against a specimen of pure sulphanilamide. In the absence of apparatus for the electrometric end-point, starch-iodide paste may be used as external indicator. It is necessary to titrate slowly and to keep the temperature of the reaction mixture below 15°. 

Weigh about 0 5 g into a beaker, add 20 ml of concentrated hydrochloric acid and 50 ml of water and stir until dissolved. Cool to 15°, add about 25 g of crushed ice and titrate slowly with 0 1M sodium nitrite, with vigorous stirring, until the solution, after being allowed to stand for one minute, immediately gives a blue colour when a drop is drawn across the surface of a film of starch-iodide paste. 

In other words, the small excess of HN02 present at the end-point can be detected visually by employing either starch-iodide paper or paste as an external indicator. Thus, the liberated iodine reacts with starch to form a blue green colour which is a very sensitive reaction. Besides, the end-point may also be accomplished electrometrically by adopting the dead-stop end-point technique, using a pair of platinum electrodes immersed in the titration liquid. 

This excess HN02 may be detected by employing either starch iodide strip or paste and designated by the following equation ... 

Dapsone 0.3 g Starch-iodide paper/paste Each ml of 0.1 M NaN02 = 0.01242 g of c12h12n2o2s... 

In 1916, Henri Bordier reported on the action of light on iodine and starch iodide in water. He observed that dilute solutions of 10% tincture of iodine (pale yellow) and three drops of 10% tincture of iodine plus a little starch paste (blue) were both decolorized after a few hours of exposure to light (13) or a few minutes exposure to X rays (14) and that yellow glass had no effect on the reaction. Both solutions were faded by exposure to light and the protective ability of any colored glass could be judged using these solutions by its intermediate color. Thus, we have one of the first... 

In a i-l. beaker 21.2 g. (o.i mole) of d-tolidine is made into a thin paste with 300 cc. of water and dissolved by the addition of 20 cc. (23.6 g., 0.23 mole) of concentrated hydrochloric acid (sp. gr. 1.18), warming a little if necessary. The solution is cooled to 10° with ice, mechanical agitation is started, and 21 cc. (35 g., 0.24 mole) more of concentrated hydrochloric acid added (Note i). This causes partial separation of o-tolidine dihydrochloride. Keeping the temperature at 10-15°, a solution of 14.5 g. (0.2 mole) of gs per cent sodium nitrite in 40 cc. of water is run in rapidly, holding back the last 10 per cent which is added slowly and only as needed to give a distinct positive test for nitrous acid on starch-iodide paper. This excess is maintained for one-half hour. Throughout the diazotization an excess of hydrochloric acid should be present as shown by a strong test... 

Starch-iodide paper. Indicator paper made by dipping paper in starch paste containing potassium iodide. 

In a 5-I. round-bottomed flask are placed 165 g. (i mole) of ethyl -aminobenzoate (Coll. Vol. i, 235), 300 cc. of water, and 204 cc. (2.5 moles) of concentrated hydrochloric acid (sp. gr. 1.19) (Note i). This mixture is warmed on a steam bath for an hour with occasional shaking. The flask containing the resulting white paste of -carbethoxyaniline hydrochloride is placed in an ice-salt bath and cooled to 0°. The mixture is stirred mechanically, and a solution of 72.6 g. (i mole) of 95 per cent sodium nitrite in a minimum quantity of water is run in slowly while the temperature is kept below 7°. The diazotization is complete when a faint positive test for nitrous acid with starch-iodide paper persists for ten minutes. 

Starch-Iodide Solution—Make a paste by adding 9 g of soluble starch to 15 mL of water. Add this mixture, with stirring, to 500 mL of boiling water. Cool the mixture, add 15 g of potassium iodide (KI), and dilute to 1 L with water. 

Add 101 g. (55 ml.) of concentrated sulphuric acid cautiously to 75 ml. of water contained in a 1 htre beaker, and introduce 35 g. of finely-powdered wi-nitroaniline (Section IV,44). Add 100-150 g. of finely-crushed ice and stir until the m-nitroaniUne has been converted into the sulphate and a homogeneous paste results. Cool to 0-5° by immersion of the beaker in a freezing mixture, stir mechanically, and add a cold solution of 18 g. of sodium nitrite in 40 ml. of water over a period of 10 minutes until a permanent colour is immediately given to potassium iodide - starch paper do not allow the temperature to rise above 5-7° during the diazotisation. Continue the stirring for 5-10 minutes and allow to stand for 5 minutes some m-nitrophenjddiazonium sulphate may separate. Decant the supernatant Uquid from the solid as far as possible. 

The assay determination of sulfamic acid is made by titration of an accurately prepared sulfamic acid solution using sodium nitrite solution and an external potassium iodide starch-paste indicator. It is based on the reaction... 

Standard 1/10 N nitrite is used to titrate a solution prepared by dissolving 10—100 mg of sulfamic acid and about 6 mL of (1 + 1) H2SO4 in 300 mL of distilled water at 40—50°C. At the end point, the colorless external potassium iodide starch-paste indicator changes to blue. A 1-mL solution of 1/ION NaN02 is equivalent to 9.709 mg of sulfamic acid. The 1/10 N nitrite titrant solution is standardized using primary standard-grade sulfamic acid. For sulfamate assay determination, the same procedure is used as for sulfamic acid. 

Preparation and use of starch solution. Make a paste of 0.1 g of soluble starch with a little water, and pour the paste, with constant stirring, into 100 mL of boiling water, and boil for 1 minute. Allow the solution to cool and add 2-3 g of potassium iodide. Keep the solution in a stoppered bottle. 

C. F. Schonbein also prepared ammonium hypobromite by a process analogous to that employed for the hypochlorite, and obtained a liquid with similar oxidizing properties. The liquid is assumed to contain a mixture of ammonium hypobromite and bromide. C. F. Schonbein likewise inferred the transient formation of ammonium hypoiodite when iodine water and aqueous ammonia are mixed, whereby the liquid is decolorized. The soln. gave a deep blue coloration with starch paste and potassium iodide, etc., and behaved like analogous soln. of the alkali... 

Add two or three drops of a starch paste and the same volume of a potassium iodide solution to the obtained aqueous ozone solution. What is observed Write the equation of the reaction. 

When small quantities of icdme hare to be found—as, foT instance, in the ashes of plants, the residuary matter left after the evaporation of spring or river waters, and tho like—the methods by which they are to he detected must evidently insure all possible accuracy, if the investigator is anxious to arrive at the truth. Tor ft long time die tests known, such as decomposition of the iodide by sulphuric acid, and addition of starch paste, were found insufficient to determine the minute traces of iodine which, as appears by late researches, are contained in several bodies. Without entering upon the genera] description of all the tests proposed, a few of the more remarkable may be briefly noticed. ... 

By Reduction to Nitrite. This is based on reduction of the nitrate to nitrite by ferrous sulphate and sulphuric add and on the colour reaction of nitrous add with starch paste and iodide. 

Procedure, ioo c.c. of the wine are evaporated in a flask to io c.c., allowed to cool and treated with 6 c.c. of the saturated ferrous sulphate solution and 4 c.c. of concentrated sulphuric acid.1 The flask is connected with a vertical condenser about 50 cm. long and heated carefully over a small flame so that excessive frothing of the mass is avoided. The distillate is collected in two or three well-cleaned tubes, each containing 2-3 c.c. of the iodide-starch paste acidified with 2 drops of dilute sulphuric acid the tubes are inclined so that the distillate flows down the walls. In presence of nitrites, a blue ring forms at the zone of separation between the starch and the distillate. 

Another 100 c.c. of the solution is treated with 5-10 c.c. of concentrated hydrochloric acid and 10 c.c. of potassium iodide]solution, the iodine liberated being titrated with standard sodium thiosulphate solution in presence of starch paste and from the result the chromium, oxide calculated. The alumina is then obtained by difference from the two determinations. 

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