Polarimeter Operation and Calibration

 1.0 OBJECTIVE :

To provide a Standard Operating Procedure for Operation and Calibration of Polarimeter.

2.0 SCOPE :

This procedure is applicable to RAJDHANI make Polarimeter Instrument.

3.0 RESPONSIBILITY :

Quality control Chemist and above

4.0 ACCOUNTABILITY:

Quality Control Head

5.0 PROCEDURE :

5.1 PRECAUTION

To obtain accurate results, great care must be exercised in filling the tube and ensuring that circular disc of tubes are not stained thus causing erroneous results.

Test solution must be clear.

Polarimeter tube should be filled in such a way so as to avoid air bubbles.

Clean the polarimeter tube with tissue paper before taking the reading.

Set the field telescope for better observations.

Rinse the polarimeter tube with acetone and keep it in polarimeter tube box after use.

Ensure that all the parts including optical lens, telescope lens and Polarimeter tube are clean.

Maintain the temperature of solution throughout operation as mentioned in standard test procedure.

5.2 OPERATION

Switch on the instrument and let the sodium lamp (λ = 589.3 nm) elute to its maximum brightness, it will take about 20 minutes.

Set the alignment of the sodium lamp compartment to the optical bench.

When you focus on upper eyepiece, graduations of scale and vertical line (called as reference line) are observed.

The scale in the telescope is divided at 1° intervals from 0 to 360°.

Coincide the reference line with the zero on scales, i.e., upper circular scale of optical rotation.

Now focus the lower eyepiece, circular field with two halves are observed as below,

Note that when the reference line coincide with 0, micrometer drum is also set at 0, then intensity of illumination (I.O.A.) for both side is equal (as shown in above figure).

Open the polarimeter tube compartment and place the polarimeter tube, which is filled with the solvent in which the sample preparation is made.

Clean the side of tubes with tissue paper.

If any air bubble comes then it must be trapped in center of polarimeter tube.

Observe through telescope when the half dark and half light portions of a circle are in equilibrium.

This half dark and half-light portion is to be matched to equal and minimum illumination with a knob by rotating it in clockwise or anticlockwise directions.

If the darker half shadow is to the right, then the test substance is dextrorotatory (rotates the light beam to the right). A darker left shadow indicates a levorotatory substance (rotates the light beam to the left).

This reading is considered as a Blank reading. 

Place the polarimeter tube filled with test solution in tube compartment and view the field by lower eyepiece.

The I.O.A. of both the sides will become unbalanced.

Now turn the scale control knob viewing the field such that I.O.A. become equal for both the sides (similar to the initial position).

The reading corresponding to the reference line on the scale is the direct measurement of Optical Rotation.

If the reference line is between two division of scale, then turn the knurled knob of micrometer drum to coincide the reference line with the adjutant division of the respective scale as below,

The micrometer drum is employed to subdivide the angular scale and I.S.S. scale and itself carries two scales, angular scale on left subdivided at intervals of 0.05 and I.S.S. scale on right subdivided at intervals of  0.1°.

The range of angular scale is 0 to 1.

It may be noticed that movement of the drum from 0 to 1 will cause the line seen in eyepiece to traverse exactly one scale division.

Rotate the micrometer drum knob to bring the index line coincident with a scale line. the true reading is then the scale line concerned plus the drum reading indicated on the appropriate.

For example:

First examination shows index line between scale 17° and 18° upon angular scale, turn the micrometer drum knob to bring the reference line coincident with the 17° division.

The drum scale then read 0.75° on the left hand scale.

True determination is therefore 17.75°.

If the reference line is moved toward the lower value of scale then the micrometer drum reading should be added otherwise subtracted.

The micrometer drum does not act as a fine control to field observed in telescope an does not influence it in any way. it serves only to subdivide the scale once a balance has been obtained.

Calculate the corrected reading by subtracting the blank reading from the sample reading.

After taking the first reading remove the polarimeter tube, discard the contents, rinse it with Test solution and then again fill it with Test solution. 

Take 5 Consecutive readings and calculate the mean reading.

Do not touch the eye piece once adjusted.

5.3 CALIBRATION

Dry 170 gram analytical grade Sucrose at 60°C for one hour in vacuum oven.

Prepare solution of Sucrose with concentrations of 10, 20, 30, 40, 50 gram per 100 ml distilled water respectively.

Transfer the above solutions to Conical Flasks and bring the temperature to 25°C.

Transfer the solution to the 2-dm polarimeter tube within 30 minutes from the time the substance was dissolved and during this interval maintain the solution at 25°C.

Measure the optical rotation of each concentration as per procedure given above.

Take 5 Consecutive readings and calculate the mean reading.

Acceptance criteria: Observed angle of rotation must be within + 0.02 than the standard angle of rotation.

Concentration In gm/100 ml Standard Angle of Rotation At 25° C

               10                            13.33

               20                            26.61

               30                            39.86

                40                            53.06

                50                            66.23

5.4 Calculation

Calculate Specific Optical Rotation using the following formula, dextro-rotation and laevo-rotation being designated by (+) and (-) respectively.