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Laboratory practice session 05

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    OPTICAL METHODS FOR MEDICINES QUALITY CONTROL. REFRACTOMETRY

    I. Questions and tasks for discussion:

    1. Define refraction.
    2. Define refractive index.
    3. What is the unit of refractive index?
    4. List out the factors on which the refractive index of a medium depends.
    5. Define angle of incidence.
    6. What is the angle of incidence if a ray of light is incident normal to the surface separating the two media?
    7. Find the velocity of the light in a medium having refractive index n = 2. Speed of light is 300 000 km/sec.
    8. Relative refractive indexes of mediums are n1,2 = 2; n2,3 = 1,5. Find n1,3.
    9. If the ray follows following path while passing from one medium to another, find the refractive index of medium.

    [image]

    II. Lab practice. Determination of magnesium sulfate aqueous solutions concentration by the means of Abbe type refractometer.

    Device of Abbe type refractometer is represented on Fig. 5.1.

    [Fig. 5.1] [A – The appearance of refractometer IRF-454: 1 – ocular, 2 –measuring prism, 3 – light prism, 4 – mirror of illumination scale, 5 – mirror of light illuminating prism, 6 – cylinder removing color, 7 – cylinder displacement of the boundary light-shadow; B – The main working parts of the refractometer: 1 – liquid under investigation, 2 – lighting prism, 3 –measuring prism, 4 – the incident beam, 5 – the field of view]

    Starting the work carry out the following steps:

    1. Open the window of prism block. Rotating the mirrors illuminate the prism with white light.
    2. Customize the refractometer with distilled water. To do this, take up the lighting lens, drop 2–3 drops of distilled water on the measuring prism, close the block.

    WARNING! Do not touch the surface of a prism with a pipette.

    The whole field of the eyepiece should be illuminated uniformly. Unequal illumination, the dark stains on it point to an insufficient amount of introduced liquid.

    Rotate the prism to achieve the appearance of dark field in the eyepiece. The appearance of the dark-field corresponds to the position of the prism in which the light beam undergoes internal reflection from the interface between the prism and the test substance. If the border of dark field is not sharp, then rotate compensator until a sharp boundary of dark field appears. Then move dark field boundary to the intersection of lines. Count the value of the refractive index on the scale.

    1. Open the box and wipe the prism surface with a soft cloth or filter paper.
    2. Place 2–3 drops of the standard solution between the two halves of the prism and tightly compress them. Combine sight buttons with the boundary line separating the light and dark parts of the visual field. Record the scale division of the refractive index, which coincides with the sighting stroke. Measure the refractive index three times for each solution.

    Before each subsequent measurement the prism should be thoroughly cleaned with distilled water and then wiped with filter paper.

    After measuring the refractive index of 4–5 solutions of magnesium sulfate place the results in Table 5.2.

    Plot the dependence of the refractive index n of the solution concentration C (x – values Ci, and Y –values ni). Don’t forget: the value of the refractive index of distilled water = 1,33. Draw a straight line through the data points.

    The measurements results of the refractive indices of magnesium sulfate solutions
    № n1 n2 n3 ncp
    № 1. С =___ %        
    № 2. С =___ %        
    № 3. С =___ %        
    № 4. С =___ %        
    № 5. Сx = Х %        

    [image]

    Find the value K(F) – the refractive index increment (dn/dC = tgγ, where γ – the angle of the slope to the horizontal axis). Substituting the value of K in the formula n = n0 + K · C find the concentration of the solution № 5:

    [equation]

    where n – refractive index of solution № 5,

    n0 – refractive index of water.

    III. Self-test:

    1. Refractometry in pharmaceutical analysis is used for:
      1. identification
      2. purity assessment
      3. assay
    2. The basis of refractometry is the law:
      1. of light absorption
      2. of light refraction
      3. of light scattering
      4. of light polarization
    3. The ratio of light speed in vacuum to its speed in the medium is called:
      1. the relative refractive index
      2. the absolute refractive index
    4. The refractive index depends on:
      1. temperature
      2. the wavelength of light
      3. the length of the cuvette
      4. solution concentration
      5. the nature of the solvent
    5. In the method of refractometry the concentration of sample solution is calculated by the formula:
      1. [equation]
      2. [equation]
      3. [equation]
      4. [equation]
    6. Specify a range of solutions refractive indices, which can be measured in daylight:
      1. 0,2–0,8
      2. 1,3–1,7
      3. 1,8–2,5
    7. Designation of the refractive index by symbol nD20 indicates that:
      1. The measurement was performed at 20 °C.
      2. The measurement was performed at the wavelength of the yellow line of the sodium spectrum.
      3. The measurement was performed at a wavelength of the red line of the hydrogen spectrum.
      4. The measurement was performed at a wavelength range of the blue line of hydrogen.
      5. The measurement was performed at wavelength of violet spectral lines hydrogen.

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