Effect of iodine on starch

Helpful reaction


In this ar­ti­cle, you will learn how the re­ac­tion of starch and io­dine takes place. This chem­i­cal process is not only very in­ter­est­ing, it is also prac­ti­cal, as it is of­ten use­ful in ev­ery­day life, when we need to find out whether a cer­tain prod­uct con­tains starch.

To start with, let’s de­fine what starch is.

It’s a taste­less, white pow­der re­sem­bling flour in its con­sis­ten­cy, and with the for­mu­la (C₆H₁₀O₅)n – a polysac­cha­ride con­sists of amy­lose and amy­lopectin.

Structure of Amylopectin [Wikimedia]

Starch is the re­sult of a nat­u­ral process – pho­to­syn­the­sis. For plants, it serves as a sup­ply of nu­tri­ents, and for the hu­man body, it serves as a source of im­por­tant car­bo­hy­drates.

Phys­i­cal prop­er­ties of starch

Starch is in­sol­u­ble in cold wa­ter. If you press the pow­der with a spoon, squeez­ing it, you hear a char­ac­ter­is­tic creak, caused by the fric­tion of mi­cropar­ti­cles against one an­oth­er.

Chem­i­cal prop­er­ties of starch

In hot wa­ter (C₆H₁₀O₅)n does not dis­solve ei­ther, but swells up to a thick and vis­cous sub­stance, form­ing a col­loidal mix­ture known as a starch paste. A so­lu­tion of starch in wa­ter is a non-New­to­ni­an flu­id (liq­uid that changes its den­si­ty and vis­cos­i­ty when ex­posed to phys­i­cal force).

If you add acid to wa­ter con­tain­ing starch, for ex­am­ple, H₂­SO₄, you can ob­serve the hy­drol­y­sis process with a re­duc­tion of the molec­u­lar mass of the sub­stance and the for­ma­tion of “sol­u­ble” starch.

Starch con­tains dif­fer­ent polysac­cha­rides in its struc­ture.

Starch is also a mul­tiatom­ic al­co­hol, which forms ethers and es­ters dur­ing cer­tain re­ac­tions – in­ter­molec­u­lar de­hy­dra­tion and ether­i­fi­ca­tion.

Starch is ob­tained in­dus­tri­al­ly from wheat, pota­toes, corn, and rice.

It is also not dif­fi­cult to ob­tain starch in the home.

Ap­pli­ca­tion of starch

Starch is wide­ly used for in­dus­tri­al pur­pos­es. It can be used to ob­tain such sub­stances as: glu­cose, mo­lasses, ethanol.

Starch is also wide­ly used in tex­tile man­u­fac­ture, and for treat­ing fab­rics. In pa­per fac­to­ries, the pow­der is used as a hy­drophilic agent, a ma­te­ri­al that in­creas­es dura­bil­i­ty and im­proves the ty­po­graph­ic qual­i­ties of pa­per. It is also used for man­u­fac­tur­ing medicine and food.

In the house­hold, starch is used by prac­ti­cal­ly ev­ery­one, as it is used to starch cloth­ing, make jel­ly, and pre­pare pastes by mix­ing starch with wa­ter and flour etc.

Starch-io­dine re­ac­tion

Granules of wheat starch, stained with iodine, photographed through a light microscope [Wikimedia]

In this ar­ti­cle, we will use a 5% al­co­hol io­dine so­lu­tion which is used in medicine.

Starch in­ter­acts with io­dine, form­ing in­clu­sion com­pounds, which are called clathrates. This chem­i­cal re­ac­tion was dis­cov­ered back in 1814 by the sci­en­tists Jean Jacques Col­in and Hen­ri-François Gaulti­er de Claubry.

In­clu­sion com­pounds are spe­cial com­pounds in which the mol­e­cules of one sub­stance en­ter the molec­u­lar struc­ture of an­oth­er sub­stance.

In this case, amy­lose mol­e­cules (one of the main polysac­cha­rides of starch) will be the “hosts” and the io­dine mol­e­cules will be the “guests”.

Ex­per­i­ment with starch and io­dine at home

This is a quite sim­ple chem­i­cal ex­per­i­ment which can be car­ried out at home and shown to chil­dren, to in­spire them with a love for chem­istry.

You will need:

  • glass test tube;
  • al­co­hol io­dine so­lu­tion;
  • pinch of starch;
  • luke­warm wa­ter;
  • stir­ring rod.

Pour wa­ter into the test tube and add 4-5 drops of io­dine. Add a pinch of starch and mix well with a rod. As a re­sult will be a dark blue so­lu­tion.

By the way, you can also re­peat this ex­per­i­ment in oth­er ways, for ex­am­ple, add one drop of io­dine to a small mound of starch, and a dark blue patch will ap­pear. You can also drip io­dine on to half a pota­to, as it has a high starch con­tent. If you im­merse a peeled pota­to in cold wa­ter, starch par­ti­cles will ap­pear in the wa­ter af­ter a cer­tain pe­ri­od of time. If you hold a peeled pota­to in your hands, they will also be­come coat­ed with starch.

If you heat a test tube con­tain­ing a so­lu­tion of starch, io­dine, and wa­ter over a chem­i­cal burn­er for some time, the so­lu­tion will turn white and trans­par­ent. This is be­cause the com­pound of io­dine and starch is un­sta­ble, but if you put the test tube in cold wa­ter, a dark blue sed­i­ment will form once more.

When starch is heat­ed to the boil­ing point, it be­gins to break down, and the chains of amy­los­es break, thus form­ing short chains of dex­trins, so the col­or starts to change. Glu­cose does not give any col­or in a re­ac­tion with io­dine.

An in­ter­est­ing fact: amy­lopectin (an­oth­er polysac­cha­ride of starch) gives a pur­ple-red col­or­ing when re­act­ed with io­dine. There is sig­nif­i­cant­ly more amy­lopectin in starch than amy­lose, which gives a blue col­or, but the blue col­or over­rides the red-pur­ple col­or.

The result of reaction between starch and iodine solution [Wikimedia]

Let’s see how the re­ac­tion of starch and io­dine can be use­ful in dai­ly life.

It’s sim­ple: if you have two un­la­beled con­tain­ers of soda and starch at home, and you don’t want to find out which one is which by tast­ing them, add a drop of io­dine.

Some food prod­ucts are forged with starch, be­cause of its vis­cous struc­ture. This es­pe­cial­ly con­cerns hon­ey: you can of­ten find fake hon­ey for sale at mar­kets con­tain­ing a large amount of (C₆H₁₀O₅)n. Once again, you can de­tect starch us­ing the same sim­ple chem­i­cal method with ab­so­lute­ly any food prod­ucts.