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Define Refractometry

The refractive index of substances is measured with the help method called as Refractometry. Refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction or refractive index (n) for the substance.

Principle of Refractometry

The main principle involved in refractometry is that the refraction supported the speed of the sunshine that passes within the different mediums.

In Refractometry as light enters into the light denser medium to high denser medium at an angle, that is, with bent. The bent in the light ray is known as the refraction. The relation between the refraction of the light between the air and the medium is given by Snell’s law:

 n = sin i/sin r

According to Snell’s Law we have,

i = Angle of incidence,

r = Angle of refraction,

n = Refractive index of both medium (b) comparative to medium (a).

Light permits more quickly through a space medium than through a substance medium. It has been observed that when a ray of light happens to pass from one medium (a) into another medium (b) it is subjected to refraction. In more optically dense medium the ray travels at a lower speed than in less optically dense medium. It is a common practice to compare the refractive indices of liquids to that of air. various refractometry methods are used.

Fig: The path of light between Two Media a and b

The critical angle is used consistently in refractometry, considering a narrow band of rays, x-y, held near to the boundary between the two media a and b (refer above diagram), and observed at Z, one may see a band of light. This specific band has a sharp edge at y, where the actual ray (y-y) may be seen. However, no rays are to be seen in the y-y′ region. Therefore, we have this

 n=sin i/sin r=sin 90/sin θ=1/sin θ

Thus, a dimension of the critical angle θ may eventually offer the precise refractive index of medium (b). It is pertinent to mention here that the refractive index of a substance is not a static (constant) property of the substance but it alters with (a) wavelength and (b) temperature

Fascinatingly, both specific refraction [n] and molar refraction (R), are temperature independent, should have the similar values for a given substance both in the solid, liquid or gaseous state, and as long as the molecular structure is unchanged.


Unit of Molar Refraction: Refractive index has no dimensions so called as dimensionless quantity, the units of molar refraction are basically those of molar volume, M/p i.e. centimeter cube (Cm3) and (mol-1).The molar refractivity is relatively an additive property.

Atomic Refractivity’s: Atomic refractivity’s may be acknowledged by virtue of:

(a) By structures e.g. double bond, triple bond or by nature of ring structure (3-member/4-member rings).

(b) Individual atoms, e.g. Hydrogen, Carbon, Chloride, Bromine, Iodine and Oxygen. However, ‘Oxygen’ contributes different values for different groups, for example: hydroxyl (-OH), carbonyl (CO) and oxygen (O) moieties.

A few representative refractivity’s and bond contribution are

Atomic Refractivities for Na D-light (λ=589.3 nm)

Sr.noAtomR (cm³ mol-1)
103-Member Ring0.71
114-Member Ring0.48

Samples with different index of refraction will produce different angles of refraction. This helps in the assessment of the compounds’ composition and the purity of the compounds.

Determination of refractive index in Refractometry

There are different methods for the determination of the index of refraction as follows:

1. Becke line method: This method was first proposed by Mittchell and is mainly used for the determination of facetted stones. It is comprised of the microscope with the light field illumination. By using the known substance, the refractive index of the unknown substance is measured.

2. Immersion contrast method: This method involves the determination of the relief and appearance of the girdle and therefore the facet edges when immersed within the known liquid.

3. Direct measurement method: This method involves the measurement of the index of refraction by using the microscope with a verneir scale. This method is more effective for the single refracting stones.

4. Minimum deviation method: This method involves the utilization of the table spectrometer and is that the most accurate method for the measurement of the index of refraction. It requires skill and ideal conditions.

Determination of refractive index of pharmaceutical substance:

A huge number of pharmaceutical substances for example volatile oils, specifically- peppermint oil, oil of lemon (citric acid), and many other seed oil have a fixed range of refractive index. Founded on this physical characteristic it is likely to determine the purity of this volatile oil exactly and accurately.

Essential Materials: refract meter of Abbe, xylene, volatile oil, capillary tubes.

Procedure for Determination of refractive index

In order to achieve precise and accurate measurements the prism case of Abbe’s refractometer is connected to a temperature regulator bath whose temperature is earlier maintained at 25°C.

Open the prism box mildly and keep a few drops of pure volatile oil on the lower prism with the help of a capillary tube and finally close the box. The mirrors are duly adjusted so as to obtain a bright illumination of the field of view.

The knurled knob is turned gradually until the field of view displays a dark and light zone. In case, a coloured-fringe is observed between the two zones it becomes necessary to adjust the Amici prisms carefully to achieve a sharp and black boundary. It is significant to regulate this on the cross hair and lastly the reading of refractive index is noted. Afterward use of the prism box it is opened and washed systematically with a lens cleansing tissue soaked with xylene or acetone.Thus, for certain volatile oils the refractive index, n25/D is as follows;

Refractometry Of Pharmaceutical substance

Sr.noOilsRefractive index
1Lemon Oil1.474-1.476
2Peppermint Oils1.460-1.467
3Clove Oils1.528-1.537
4Aniseed Oils1.553-1.560
5Dill Oils1.481-1.492
6Eucalyptus Oils1.458-1.470


The instrument used for the determination of the index of refraction is understood because the refractometer. There are different refractometers used for the determination of the index of refraction. They are as follows:

1. Traditional handheld refractometer:

The most principle involved during this refractometer is that the measurement of the angle of incidence. It is comprised of the lenses and prisms to project the black line on the glass when the sample is placed between the measuring prism and the plate.

2. Digital hand-held refractometer:

The principle is that the same because the traditional handheld refractometer. The main difference is that the light from the LED light source is focused on the prism. This creates the black line thanks to the refection of the photodiode arrays.

3. Abbes refractometer:

This is often a benchtop refractometer, designed by Ernst Abbe, which provides high accuracy. In this refractometer, the sample is held between the illuminating prism and therefore the refracting prism. A light source is allowed through the illuminating prism and therefore the detector is placed behind the refracting prism.


4. Inline process refractometer:

This is often mainly used for the continual measurement of a fluid flowing through a pipe. This refractometer consists of a sensor placed within the flow of the fluid. This is connected to the control box which gives the digital readout.

Procedure: The liquid having Refractive index is to be determined is placed between the two prisms. The superior face of lower prism has a ground level surface so as to diffuse the light rays in almost every possible direction. The rays travels from the liquid to the upper prism experiences refraction in the normal manner, thus providing a bright area in the eye-piece. The critical ray is originated by virtue of the rays that strike the liquid glass interface at the grazing incidence.

 As a conclusion of these collective effects the ‘field of view’ is denoted as a separate dark and light area which is having a sharp dividing line. The diagram entitles the optical path for the upper prism in Abbe’ Refract meter. When a ray of light passes from the liquid medium and enters the upper prism, it gets refracted by an angle θ between the lower face of the prism and the normal, an angle β between the emerging refracted ray at the upper face and the normal, and finally an angle α between the reflected ray at the upper face and the normal.

Now, based on the 2 constants, which are A and N, for a particular prism and a measurable angle α it is suitable to determine the refractive index of the liquid which is relative to air from Eq. (i).With the support of the Abbe’ refract meter the angle α lying among the normal and the critical ray evolving from the upper surface of the prism may be measured. By the help of the 2 constants A and N (for a specific prism) the angle α has been changed into the refractive index directly and the scale of the instrument has been accordingly calibrated and printed accordingly.

The telescope (F) of the Abbe’ refractometer is stable and the prism box (C) is directly attached to the scale. When C is made to rotate slowly the critical ray (E) falls on the cross hair (H) of the telescope (F). At this stage the value of the refractive index of the liquid (n) can be measured with scale (G). It is, still, important to mention here that the calibration of Abbe’s refractometer may be checked occasionally by making use of standard liquids whose refractive index are specified in the European Pharmacopoeia.

Standard liquid-Refractive index

Sr.noStandard LiquidRefractive Index*
2α-Methyl naphthylamine1.6176
3Carbon Tetrachloride1.4603

APPLICATIONS of Refractometry

1. Used in the determination of the index of refraction of the compounds

2. Used in the determination of the concentration of the compounds

3. Used in the determination of the structure

4. Used in the determination of the critical micelle concentration of the compounds.

5. Some other applications are given below;

 (a) It is possible to work out the molar refractivity’s of various substances experimentally and subsequently comparing their values with theoretical ones as discussed in Lorentz and Lorentz theory.

(b) Supported the fact that molar refractivity is an additive property, it’s going to be utilized to work out for determination of refractivity’s of homogeneous mixtures (as solutions).

Thus, the solution of molar refraction having two components (which are the solute and solvent) given by the expression:

                                                R1, R2 = N1R1 + N2R2 ………………………….Equation (i)

Where, N1 = Mole fraction of the solute,

              N2 = Mole fraction of the solvent,

              R1 = Molar refractivity of the solute, and

              R2 = Molar refractivity of the solvent.

Evidently, from Eq. (i), it’s quite possible to work out the molar refractivity of an unknown solute R1 provided we all know the mole fraction N1 and N2 and also refractivity’s of the solute R2 and the homogeneous solution R1.

Besides, the concentration of the solute in the solution may be determined by employing the following expression, provided the refractivity’s of the solute, the solvent and the solution are known

 (c) Determination of Critical Micelle Concentration (CMC)

In common, substances that procedure micelles in water gives two separate regions in their molecules.

First one isthe hydrophobic entity (caused due to the hydrocarbon chain), and secondly, the hydrophilic entity (causeddue to the polar group). It has been observed that a number of monomers usually hold all the hydrocarbonchains together specifically in the centre of the micelle which are ultimately responsible for minimising thefree energy of the system. Thus, the actual concentration at which the micelles are first observed istermed as the critical micelle concentration (CMC).

Interestingly, the physical features of theconstituents creating micelles afford sharp variations at the CMC. Therefore, a plot of refractive index (RI) Vsconcentration (g/L) must depict a clear change in slope at the CMC.

Materials Required: Butyric acid solution (25%, w/v in DW): 200 ml; volumetric flasks (50 ml).


Prepare exactly following quantity 2.5, 5, 7.5, 10.5, 15 and 20% solutions of butyric acid in H2O-(water) by quantifying suitable volumes (from a standard stock solution of 25% weight/volume) with the help of a burette into 6 volumetric flasks of 50 ml, and lastly making up the volume with distilled water. By means of Abbe’ refract meter quantity the refractive indices of all the above six solutions and also the stock solution (25%) at 25°C. Measure also the refractive index of DW.

Results: After this plot a graph having the several concentrations of butyric acid laterally the abscissa and the refractive indices along the ordinate, two straight lines are obtained which are intersecting each other at the CMC as shown in diagram.



Temperature: it’s inversely proportional to the index of refraction.

Viscosity: It is inversely proportional to the refractive index.

Wavelength: The sodium D-line at 595 nm is that the appropriate wavelength for the determination of the index of refraction.


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