Starch iodine test for

A solution of 0.2-0.5 gm iodine in 100 ml aqueous 0.5% potassium iodide is often used as a reagent for certain polysaccharides in lichens (the I test). The reagent is susceptible to air oxidation and should be renewed when the brownish color fades. Isolichenin, but not lichenin, will give a blue color. The chemistry of the color reaction is probably the same as that for the well-known iodine test for starch. The reaction is reversible (the color disappears upon dilution with water). 

Iodine is also used as a test for starch. When placed on starch (a potato for example), iodine turns the starch a dark blue color. Silver iodide is used in the manufacture of photographic film and paper. It is also used to seed clouds because of its ability to form a large number of crystals that act as nuclei upon which moisture in the clouds condenses, forming raindrops that may result in rain. 

Starch occurs in potatoes, rice and wheat. Glucose, from which starch is polymerised, belongs to a group of simple carbohydrates known as monosaccharides. They are sweet to taste and soluble in water. Starch belongs to the more complicated group of carbohydrates known as polysaccharides. Starch does not form a true solution and it does not have a sweet taste. With iodine it gives an intense blue colour (nearly black), which is used as a test for starch or iodine itself (Figure 15.20). 

Tests for starch and glucose. Test for starch using dilute aqueous iodine and for glucose with Fehling s solution or Benedict s reagent. 

The reaction of starch with iodine is a common identity test for starch. A dilute solution of iodine stains starches a blue to bluish red color. It is believed that the amylose portion complexes with iodine by forming a helix around it. This blue color has been used both as a qualitative and quantitative test for starch in various systems. 

Fig. 17-—Iodine test for Schardinger dextrins. Microscopic appearance typical of the early stage of the decomposition of starch by B. macerans amylase. Under higher power the crystals are seen to be blue hexagons.

Iodine solution. Dissolve 10 g of potassium iodide in about 100 ml of distilled water. To this add 1.3 g well-powdered resublimed iodine and stir until completely dissolved then dilute to one liter. This solution is about 0.01 Af and is used to test for starch. 

Test for Staxen. Tho old and fhmiliar test for starch is the blue color which free iodine produces when brought in contact with it but this is not the only reagent by means of which we can detect the presence of starch in combination with similar bodies. Bromine is nearly as good as iodine, and tannin is smd, in some instances, to be better. A solution of 50 grains tannin in i pint dis-tUled water will answer for making the test. A drop of this tannin solution wUl cause a... 

There are more or less specific color tests for starch (iodine blue) and carrageenans (using methylene blue), and in some instances the action of enzymes is diagnostic and used in reagent kits. A relatively new approach to the assay of food gums and stabilizers, one that shows many advantages in speed and specificity, is enzyme-linked immunosorbent assay, which has a number of variants. 

Starches differ in their chemical composition and, except in rare instances, are mixtures of two structurally different polysaccharides, amylose and amylopectin. The proportions of these present in natural starches depend upon the source, although in most starches amylopectin is the main component, amounting to about 70-80 per cent of the total. An important qualitative test for starch is its reaction with iodine amylose produces a deep blue colour and amylopectin solutions produce a blue-violet or purple colour. 

Hydrolysis by acids. Place 15 ml. of starch solution in a boiling-tube, add I ml. of cone. HCl, mix well and place in a boiling water-bath for 20 minutes. Cool and add 2 drops of iodine solution to i ml. of the solution no blue coloration is produced. On the remainder, perform tests for glucose in particular show that glucosazone can be formed. Neutralise the excess of acid before carrying out these tests. (Note that a more concentrated acid is required to hydrolyse starch than to hydrolyse the disaccharides, such as sucrose.)... 

Recovery of the wopropyl alcohol. It is not usually economical to recover the isopropyl alcohol because of its lo v cost. However, if the alcohol is to be recovered, great care must be exercised particularly if it has been allowed to stand for several days peroxides are readily formed in the impure acetone - isopropyl alcohol mixtures. Test first for peroxides by adding 0-6 ml. of the isopropyl alcohol to 1 ml. of 10 per cent, potassium iodide solution acidified with 0-6 ml. of dilute (1 5) hydrochloric acid and mixed with a few drops of starch solution if a blue (or blue-black) coloration appears in one minute, the test is positive. One convenient method of removing the peroxides is to reflux each one litre of recovered isopropyl alcohol with 10-15 g. of solid stannous chloride for half an hour. Test for peroxides with a portion of the cooled solution if iodine is liberated, add further 5 g. portions of stannous chloride followed by refluxing for half-hour periods until the test is negative. Then add about 200 g. of quicklime, reflux for 4 hours, and distil (Fig. II, 47, 2) discard the first portion of the distillate until the test for acetone is negative (Crotyl Alcohol, Note 1). Peroxides generally redevelop in tliis purified isopropyl alcohol in several days. 

The molecular uniformity of constituting components of a nb/lcb glucan fraction of potato starch was investigated with Sepharose CL 2B (Fig. 16.16) as well as with Sephacryl S-1000 (Fig. 16.17). Therefore, each of the subsequently eluted 3-ml fractions was analyzed on their potential to form inclusion complexes with iodine, a sensitive test for the presence of nb/lcb glucans. Results are shown in Fig. 16.17 in terms of branching index, the ratio of extinction of pure iodine solution and of nb/lcb glucan/iodine complex the higher the index, the more pronounced the nb/lcb characteristics. 

You have used an iodine solution to test for the presence of starch in many laboratory activities. Iodine has a very low solubility in water. When the iodide ion is present, however, the triiodide ion,... 

A) Anon, IEC, NewEdn, 14, 305(1936)(Peroxide formation in ethers and a test for peroxides in ether) [Procedure a) Add 10ml ether (sample to test) to 150ml of 2N sulfuric acid soln, followed by 3 drops of 1% soln of Amm molybdate (a catalyst to favor the liberation of dine) and 15ml of 10% KI soln. Shake well aiiJ allow to stand for 15 mins b) Titrate the liberated iodine with 0.05N Na thiosulfate and shake well after each addn until near discoloration of soln c) Add a few cc of starch soln and continue titration until disappearance of blue color]... 

Starch consists of two main components amylose (insoluble in cold water) and amylopectin (soluble in cold water). Amylose, which accounts for about 20 per cent by weight of starch, has an average molecular weight of over 10. It is a polymer of glucopyranose units linked together through a l,4 -linkages in a linear chain. Hydrolysis of amylose produces maltose. Amylose and iodine form a colour complex, which is blue/black. This is the colour reaction of iodine in starch, a confirmatory test for the presence of starch. 

The acids attack KI and the liberated iodine colors the starch paper c)Koehler Mar-querol(Ref 2) do not recommend the use of Abel s test for NC propellants contg Ca carbonate - Bergmann-Junk test(qv) gives more reliable results... 

I hen iodine is mixed with starch, I the amylose stTands coil around the iodine molecules, forming a starch-iodine complex that has a characteristic dark blue color. The more starch present in a solution, the deeper the blue. The formation of this color is used to identify the presence of starch. In this activity, you will use an iodine solution to test for the presence of amylose and the amylose-digesting action of your saliva. 

A portion of the sample is heated on the water-bath with 8% alcoholic potash until the fleshy mass is dissolved. The liquid is filtered off with the help of a pump and the residue on the filter washed with cold 95% alcohol and then boiled with 96% alcohol, which partially dissolves the glycogen but leaves the starch undissolved. The filtered liquid is evaporated on a water-bath, the residue taken up in a little water and the solution tested for glycogen by means of iodine,... 

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