Starch indicator
Encyclopedia
Starch
is often used in chemistry as an indicator
for redox titration
s where triiodide
is present. Starch forms a very dark blue
-black
complex
with triiodide which can be made by mixing iodine with iodide (often from potassium iodide
). However, the complex is not formed if only iodine or only iodide (I-) is present. The colour of the starch complex is so deep, that it can be detected visually when the concentration of the iodine is as low as 0.00002 M at 20 °C. The colour sensitivity decreases with increasing temperature (ten times less sensitive at 50 °C), and upon the addition of organic solvents such as ethanol or propanone. During iodine titrations, concentrated iodine solutions must be reacted with some titrant, often thiosulfate
, in order to remove most of the iodine before the starch is added. This is due to the insolubility of the starch-iodine complex which may prevent some of the iodine reacting with the titrant. Close to the end-point, the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch.
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
is often used in chemistry as an indicator
Redox indicator
A redox indicator is an indicator that undergoes a definite color change at a specific electrode potential....
for redox titration
Redox titration
Redox titration is a type of titration based on a redox reaction between the analyte and titrant.Redox titration may involve the use of a redox indicator and/or a potentiometer.-Example:...
s where triiodide
Triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
is present. Starch forms a very dark blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
-black
Black
Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...
complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
with triiodide which can be made by mixing iodine with iodide (often from potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
). However, the complex is not formed if only iodine or only iodide (I-) is present. The colour of the starch complex is so deep, that it can be detected visually when the concentration of the iodine is as low as 0.00002 M at 20 °C. The colour sensitivity decreases with increasing temperature (ten times less sensitive at 50 °C), and upon the addition of organic solvents such as ethanol or propanone. During iodine titrations, concentrated iodine solutions must be reacted with some titrant, often thiosulfate
Thiosulfate
Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...
, in order to remove most of the iodine before the starch is added. This is due to the insolubility of the starch-iodine complex which may prevent some of the iodine reacting with the titrant. Close to the end-point, the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch.