REFRACTOMETRY

principle

measures the extent to which light is bent (i.e. refracted) when it moves from air into a sample and is typically used to determine the index of refraction (aka refractive index or n) of a liquid sample.

Figure (1)

The refractive index is a unitless number, between 1.3000 and 1.7000 for most compounds, and is normally determined to five digit precision.

Since the density of a liquid usually decreases with temperature, it is not surprising that the speed of light in a liquid will normally increase as the temperature increases. Thus, the index of refraction normally decreases as the temperature increases for a liquid. For many organic liquids the index of refraction decreases by approximately 0.0005 for every 1 °C increase in temperature

Factor effecting refracting index: 1-temperature of the sample . 2-wavelength of light used . Ex.:

The italicized n denotes refractive index, the superscript indicates the temperature in degrees Celsius, and the subscript denotes the wavelength of light (in this case the D indicates the sodium D line at 589 nm).

Whenever light changes speed as it crosses a boundary from one medium into another it is refracted (Figure 1). The relationship between light's speed in the two mediums (vA and vB), the angles of incidence (qA) and refraction (qB) and the refractive indexes of the two mediums (nA and nB) is shown below:

where M is the molecular weight expressed in ml.mol, n it is the refraction index and r the density.

Instrument:

figure (2)

Cross hair:

figure (3) figure(4)

the cross hair should be half dark & half light if not adjust it using the knob.

Why the refracting prism would show a light region to the left and a dark region to the right? Figure(5)

Inspection the Figure(5) shows that light traveling from point A to point B will have the largest angle of incidence (qi) and hence the largest possible angle of refraction (qr) for that sample. All other rays of light entering the refracting prism will have smaller qr and hence lie to the left of point C

What is dispersion?

dispersion would result in the light and dark borderline being in different places for different wavelengths of light (appears as "fuzziness" of the borderline)

How to over come dispersion? "compensating prisms" into the

optical path after the refracting prism. These compensating prisms are designed so that they can be adjusted to correct (i.e., compensate for) the dispersion of the sample

Applications:

Qualitative analysis: in which the refractive index of each element is constant.

Quantitative analysis: we can get the concentration of an element from the refractive index.

HPLC: as a detector. In the testing the purity of a solvent.

POLARIMETRY

POLARIMETRY:

It is the re study of the rotation of light by different substances.

Theory:Light( electromagnetic radiation) is

randomly distributed, it consist of electrical and magnetic vectors perpendicular to each other.

Polarized light is the light which vibrates in one direction and it can be obtained by passing the light through a polarizer.

substances

Optically active(chiral center)

Dextrorotatory

(clockwise)

LevorotatoryAnti-

clockwise

Optically inactive

In optically active cpds. Rotation occurs in a certain angle known as the angle of rotation(α°)Which defined as the number of degrees of rotated plan .

It depends on: 1- temperature.2-concentration.3-solvent (in which the sample is dissolved)4- λN.B: it’s the reading I get from the

instrument.

Specific rotation:

Equation(1)[α] = specific rotation with the unit of

degree*ml/dcm*gm, T = temperature, λ = wavelength of Na= 589.3, α = optical rotation, c = concentration in g/ml, l = optical path length in dm ( usually 2 dm).

N.B: in case of pure liquids we use density D instead of the concentration C .

Specific rotation:

Defined as the no. of degrees of rotation observed when using 1 dcm. tube and 1 gm/ml of the substance.

It depends on:1- temperature.2-solvent.3- λ.

Applications:

Qualitative analysis: in which each material has its own specific rotation( at a specific T and λ).

Quantitative analysis :we can get the concentration from the equation

Molecular rotation:

t t

( M) = (α ) *M / 100

λ λ* Where M is molecular weight.