POLARIMETRIC ESTIMATION OF SUGARS

13FET1011- PRATHAMESH KUDALKAR

OPTICAL ACTIVITY• Chiral Carbon

– A chiral carbon is a carbon atom that is attached to four different types of atoms or groups of atoms.

• Chiral Molecule

– A chiral molecule is a molecule containing at least one chiral carbon.

• Optical Activity

– Optical activity is the ability of a chiral molecule to rotate the plane of plane-polairsed light.

OPTICAL ACTIVITY

OPTICAL ACTIVITY• Dextrorotatory (d; (+))

– A substance that rotates plane polarized light in a clockwise direction.

• Levorotatory (l; (-))

– A substance that rotates plane polarized light in a anti-clockwise direction.

OPTICAL ACTIVITY

POLARIMETRY• Every chiral molecule has a specific optical activity which is its

fixed physical characteristic.

• Polarimetry is a sensitive, nondestructive technique for measuring the optical activity exhibited by inorganic and organic compounds.

• It involves measurement of rotation of plane polarized light as it passes through the optically active compounds.

• The amount of optical rotation is determined by the molecular structure and concentration of chiral molecules in the substance.

• The measured rotation is used to calculate the value of solution concentrations.

POLARIMETER

BIOT’S LAW• Biot discovered that the observed rotation is proportional to the length l of the cell or

tube containing the optically active liquid (in decimeter) and the concentration c (in

g/ml)

• Angle of Rotation, α = [α ] * l * c ……. (Biot's law)

• The proportionality constant [α ] is called specific rotation.

• It depends on wavelength and temperature which are usually indicated as subscripts

and superscripts, respectively.

• For example, [α ]D25 denotes the specific rotation for light of the wavelength of the

sodium D-line (589 nm) at 25 °C

SACCHARIMETER

• A saccharimeter is basically a polarimeter with the International Sugar

Scale(ISS).

• It is used for measuring the concentration of sugar solutions.

• Based on the International Sugar Scale (ISS) 100 °S point corresponds to

the optical rotation of the normal solution of pure sucrose.

• The normal sugar solution is defined as 26.0160 g of pure sucrose

weighed in vacuum and dissolved in water at 20 °C to a final volume of

100.000 cm³.

ICUMSA ISS• The ICUMSA International Sugar Scale (ISS) fixes the basis

of a polarimetric method allowing determination of the relationship between the angular degree optical rotation caused by an aqueous solution of a sample and that caused by a pure sucrose solution of prescribed concentration using the same polarized light. The scale is graduated accordingly.

• 100 °S = Pure Sucrose

• 0 °S= Pure Water

• Optical rotation varies linearly with change in concentration.

SACCHARIMETER• Calibration is done using Quartz crystal control plate.

• Selection of Wavelength

– In Visible Region (589nm): Ideally, the compound being measured to dissolve in the solvent used to give a colourless or nearly colourless solution. The solution must be transparent, i.e. it should not contain any suspended particles. Used for refined sugar.

– In NIR Region (880nm): Ideally, the solution being measured must be transparent. The solution can be coloured but should not be too dark. Lead clarification is not required. Used mainly for raw sugar, sugarcane juice etc.

FSSAI Method

• The sample solution is clarified with lead acetate and then lead acetate is dried off.

• Direct reading: The Pb free sample is polarized and the reading is observed. The reading is multiplied by 2 and noted. (P)

• Invert reading: The Pb free sample is inverted using HCl and reading obtained is multiplied by 2.(I)

% Sugar in the sample,

S = 100 (P−I)

132.66−0.0794(13−m)−0.53(t−20)

% Sugar in the sample,

S = 100 (P−I)

132.66−0.0794(13−m)−0.53(t−20)

• Where,

– P = direct reading

– I = indirect reading

– t = Temperature at which readings are made

– m = gm of total solids from original sample in 100 ml inverted solution

APPLICATIONS

• In a sugar factory the main polarimetry applications are the

payment of farmers and quality control. This is the most

important application.

• For quality control, several steps of the process is checked by

Polarimeter.

• Other sugars and pharmaceutical ingredients can also be

analyzed using polarimetry.

Manual Saccharimeter