TASK 3 - REFRACTOMETRY:
QUALITATIVE EVALUATION OF ALCOHOLS AND QUANTITATIVE EVALUATION OF SUCROSE SOLUTIONS

INTRODUCTION

The refractive index (= index of refraction = RI) of a medium is a measure of how much the speed of light (or other waves such as sound waves) is reduced inside the medium. During the travel of light through a medium light changes its speed and direction.

The ratio of the speed of light in vacuum (c) and the velocity of light (ν) in tested substance = (absolute) refractive index (n).

(1)
If the light goes from one substance n1 to the other n2 then we use the relative refractive index = ratio of absolute refractive indexes of both substances:

(2)
Refractometry is the method of measuring refractive index of substances (one of their fundamental physical properties) in order to, for example, assess their composition or purity. A refractometer is the instrument used to measure refractive index ("RI"). Although the refractometers are best known for measuring liquids, they are also used to measure gases and solids, such as glass or gemstones.

RI of a substance is strongly affected by temperature (inversely proportional) and the wavelength of light used to measure it, therefore, care must be taken to control or compensate for temperature differences and wave length. RI measurements are usually reported at a reference temperature of 20 oC (considered as room temperature). A reference wavelength of 589.3 nm (the sodium D line) is the most often used.
MEANING OF REFRACTOMETRY
Refractometry is in general used to identify a chemical substance and/or its concentration in a solution, and to evaluate purity of it. For example, refractometers are frequently used by the grape or kiwifruit growers for testing of sucrose levels in their fruits.
Aim of the task: Refractive index measurement of various alcohols, and identification of 2 unknown samples of alcohols and 3 different concentrations of sucrose.

Refractometer AR3 / AR4



PROCEDURE OF MEASUREMENTS
  1. Open the prism chamber and cover the prism with liquid measured, then close the chamber;
  2. Using the right bottom knob move the grey interface into the center of the cross;


  3. Reading scale has two parts; on the lower one read the degrees for measurements of alcohols (read on 4 decimal places), on the upper one read percentage for measurements of sucrose (read on 2 decimal places; the smallest segment = 0.25%)

    Part of the lower scale (e.g. the correct value is 1.3344 or 1.3387)

    Part of the upper scale (e.g. the correct value is 1.25% or 3.75%)



    The measurements proceed in digestor

  4. Note: Between the measurements always dry the chamber prism with cellulose, rinse with cellulose soaked with ethanol (in case of alcohols) or with deionized water (sucrose solutions), dry again with cellulose, and only then drop another liquid.
PROCEDURE OF THE TASK
  1. Measure concentrations of 3 unknown samples of sucrose (in %) (each 2time);
  2. Measure n of known alcohols: methanol, ethanol, propanol, butanol, isobutanol, pentanol (amyl alcohol), acetone, and octanol (each 2time);
  3. Measure concentrations of 3 unknown alcohols (each 2time);
  4. Analysis of results:
    • calculate the average measured value for all alcohols and sucroses (results insert into the table containing measured values and their average values);
    • try to identify unknown alcohols according to their average value of refractive index by comparison with the measured experimental values of known alcohols (look for the nearest values).

    Table: Refractive index of tested alcohols at 20 oC

    alcohol nD20
    methanol 1.3286
    ethanol 1.3613
    propanol 1.3854
    butanol 1.3993
    isobutanol 1,3958
    amyl alcohol 1.4099
    acetone 1.3591
    octanol 1.4304


VIDEO-CLIP DEMONSTRATING PROCEDURE OF THE TASK

Version for HIGH SPEED connexion to internet!

Version for LOW SPEED connexion to internet


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