Determine the refractive index of the given liquids (Acetone and Benzene) using Abbe's refractometer and calculate specific and Molar refractions

Determine the refractive index of the given liquids (Acetone and Benzene) using Abbe's refractometer and calculate specific and Molar refractions.

Abbe refractometer Theory:

When a ray of light passes from one medium to another, it suffers a change of direction which is known as refraction. If i is the angle of incidence and r is the angle of refraction, then according to snell's law, the ratio

   sini / sinr = n 

where the n is constant known as the Refractive Index of the liquid. The Refractive index of a liquid can be measured easily with the help of Abbe's refractometer. Knowing the value of the refractive index specific refraction R can be calculated by the relation

                                         

where ρ in the density of the Iqiuid. Again knowing the value of refractive index (n) density (ρ) and molecula: mass (M) the value of molar refraction (Rm) can be calculated by the relation

Apparatus required: Abbe's refractometer, source of light, dropper, and soft hair brush to clean the prisms and lens.

Chemicals required:

Samples of liquids (acetone and benzene), rectified spirit, and distilled water, commercial acetone (for cleaning). 

Procedure:

• Open the prism box 'B', and clean the prism surface with ethyl alcohol then acetone. Let the prism dry.
• Put 2-3 drops of liquid with the help of a pipette between the prisms and press firmly.
• Let the light from the opaque electric lamp fall on the mirror M. Adjust the mirror to reflect maximum light into the prism box.
• Rotate the prism box by moving lever L until the boundary between the shaded and illuminated sections appears in the field of view. Then lever Clock L.
• Turn the compensator knob C to get a sharp boundary between light and shadow.
• Read the refractive index of the liquid on the S scale through the reading lens R.
• Take at least three readings from light to dark and three from dark to light.
• Calculate the average of all readings

 

Observations and calculations:

S.NO.	Liquid	Refractive index (n)		Density (ρ)	Molecular Mass (M)	Specific Refractivity (R)	Molecular Refractivity (Rm)
1.	Acetone	I= …. II=….. III=…. IV=….  V=….	Mean Value	0.7960	58	…………	…………
2.	Benzene	I= …. II=….. III=…. IV=….  V=….	Mean Value	0.8842	78	……………	…………

 Note: Try Desktop Mode for Full View or Rotate Screen Orientation

Precautions:

• Light refracted from the upper surface of the upper prism must not enter the observing telescope.
• The temperature of the prism box must be kept constant by circulating water through the inlet of the water jacket.
• The prism inside the prism box must be cleaned properly before taking readings.
• After inserting the given liquid between the two prisms, immediately close the two. if it is not uniform, the illumination in the field of view will not be visible due to the evaporation of the liquid

Questions & Answers

• Abbe refractometer definition

An instrument used to measure the refractive index of a liquid, by calculating the critical angle for total reflection.

• Abbe refractometer application

The Abbe refractometer serves as a measuring instrument by which you can determine the refractive index. It works for both liquids and solids, as long as they are transparent, with the help of refractometry.

• Abbe refractometer labeled diagram

• Abbe refractometer principle

Abbe refractometer is based on the principle of total internal reflection (TIR).

• Abbe refractometer uses

Refractometers are used to determine the index of refraction of liquid samples, and to measure fluid concentrations. 

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