Instrumental Analysis, ChFE 5503 Refractive Index of a Liquid A. The Refractive Index (R.I.) is an important property of the molecular structure of a liquid. For pure hydrocarbons, one can get an idea of aromatic content of a liquid using R.I. R.I.aromatics 1.50 naphthenes 1.44 saturates 1.37 B. Theory: When a light beam passes from one substance into another, the beam is refracted (bent) so that it travels in a different direction. The extent of refraction depends on: 1) the relative concentration of atoms or molecules 2) the structure of the atoms or molecules A refractometer measures this property (R.I.) using the D line of sodium. N/n = sin i1/sin r1 N = R.I. of prism n = R.I. of liquid N/na = sin i/sin r na = R.I. of air (= 1) since r + rl = 90Ε, sin r = cos r1 = %(1-sin2r1) Hence, n = %(N2 - sin2i) and n is determined from angle i and N R.I. is reported as nD = . C. The R.I. is measured by the critical angle method with a Bausch and Lomb or Abbe Refractometer using monochromatic light. R.I. = (Sine of Angle of Incidence/sine of angle of refraction) as light passes from air into the substance. Refractive Index of a Liquid - 2In the Abbe Refractometer liquid is placed between two prisms, which are rotated to obtain grazing refraction angle. - This is achieved by a focusing lens, which gives a dark field and a light field in the viewpiece. - When the two fields are equal (as determined by a set of crosshairs), the angle is measured. - A calibrated scale gives the R.I. directly for this angle and the R.I. of the prism. Procedure: (ASTM 1218, 1747) 1. Set the bath temperature to 20ΕC. 2. Open the prism and place approximately two drops of the liquid sample on the top of the bottom prism. Use care not to touch the prism surface with the dropper. Common samples are 2-proponol and ethylene glycol. Check with lab instructor. 3. Close the prism case and place the lamp close to the prism. Turn on the lamp. 4. Adjust the hand wheel on the right side of the instrument until the viewer appears light above and dark below the center horizontal line. 5. Adjust the compensator dial on the front until the image in the viewer shows a sharp line between light and dark. 6. Adjust the hand wheel on the right until the horizontal line in the viewer passes between the crosshairs. 7. Depress and hold the switch on the left and read the R.I. value. 8. Clean the prism with acetone and a Kimwipe.Refractive Index of a Liquid - 3 D. Molar Refraction, MrRefraction of light is an additive property, but also depends upon the structural arrangement of atoms in a molecule. From the electromagnetic theory of light, the molar refraction, Mr, is given by: MW(n2-1) Mr = ρ(n2+2) The molar refraction can be calculated from the sum of the individual atomic refractions of the atoms and groups present. This can sometimes be used to determine the structure of an unknown compound whose molecular formula is known. It is not useful for mixtures. Table I. Atomic Refractions Group Mr Group Mr CH2 4.618 Cl 5.967 H 1.100 Br 8.865 C 2.418 I 13.900 Double bond (C=C) 1.733 N (pri-amines) 2.322 Triple bond (C/C) 2.398 N (sec-amines) 2.499 O (carbonyl) (C=O) 2.211 N (tert-amines) 2.840 O (hydroxyl) (OCH) 1.525 CC/N 5.459 O (ethers) (RCOCR) 1.643 Example: CH3 A substance having the analysis C3H6O might be either acetone CO CH3 OR H allyl alcohol C=CCCCOH. H H Determine which of these two chemicals the substance is from the fact that its molar refraction, Mr, is 16.971.Refractive Index of a Liquid - 4 Answer to example: Acetone Allyl Alcohol 3 carbons = 7.254 3 carbons = 7.254 6 hydrogens = 6.600 6 hydrogens = 6.600 1 carbonyl oxygen = 2.211 1 double bond (C=C) = 1.733 1 hydroxyl oxygen = 1.525 CCC 16.065 CCC 17.112 Since the experimentally determined molar refraction agrees more closely with that calculated for ally alcohol, the substance probably is ally alcohol rather than