SECTION XLVIII.

ROTATION OF PLANE OF POLARISATION. II.

Saccharimetry.

Apparatus required: Laurent or other polarimeter, sugar, and sodium flame.

With the apparatus used in the previous exercise the eye has been called upon to judge the point at which the minimum light passed through the Nicol's prisms. As the estimation of the exact position for a minimum is difficult and the practical determination of the amount of sugar in a sample is made to depend on its power of rotating the plane of polarisation of light, several instruments have been devised for getting over the difficulty, one of these being the Laurent polarimeter provided. By their means it is possible to estimate the position of extinction to a small fraction of a degree.

The Laurent instrument is arranged as shewn in Figs. 99, 100.

K

F

B

Fig. 99.

The light from the Bunsen burner A, in which a bead of a mixture of equal parts of sodium diborate and common salt is placed, passes through the lens B, the small hole in the diaphragm C, which is covered by a thin plate of bichromate of potash to cut off all but the yellow light, and falls on the lens and Nicol prism D. The plane polarised light which emerges from the Nicol falls on a plate of quartz E, which covers half the field. The quartz plate is cut so that the

optical axis is parallel to the edge which bisects the field. The plane polarised light falling on the plate is decomposed into two rays, one polarised in a plane parallel to the edge of the plate, the other in a plane perpendicular to this edge. The two rays traverse the plate with different velocities, and the thickness of the plate is so arranged that a difference of phase of half a wave-length is produced. The effect of this is, that if the light passing through the uncovered half of the field is polarised, say in the direction CA inclined at an A'BA angle 0 to CB the edge of the quartz plate, then that which has passed through the plate is polarised in the direction CA' such that BCA' = BCA. On looking through the analysing Nicol K of the eyepiece, the two halves of the field will appear unequally bright, unless Fig. 99 a. the principal plane of the analysing Nicol makes equal angles with the directions CA, CA', i.e., is either parallel or perpendicular to CB. If it is parallel to CB the halves are equally bright, if perpendicular equally dark. The dark position is the one made use of, and the instrument is more sensitive the smaller is consistently with sufficient light passing through the apparatus. Generally does not exceed 2°.

Adjust the eyepiece till the dividing line between the fields is seen distinctly when one half of the field is dark and the other light.

The Nicol D is connected to a horizontal moveable arm, shewn in Fig. 100, and may be rotated within certain limits. In order to get the position of greatest sensitiveness, determine first the position of the arm when the two halves of the field are equal in brightness whatever be the position of the Nicol K. This should be done with an empty tube at F. Now rotate D through a small angle not exceeding 2° by means of the moveable arm. It will then be found that the two halves of the field are equally dark for a certain position of K, the lefthand half increasing, the other decreasing, in brightness if K is moved in one direction, and the right-hand half increasing, the other decreasing, if the rotation is in the opposite direction.

By means of the rotating screw G, adjust the vernier to read 0, and by means of the tangent screw H, rotate K till

equality of fields is again produced. The instrument now reads 0 when no active material is present.

Insert at F a tube filled with distilled water, and determine

the reading for equality of the fields, the adjustment being made alternately from opposite sides of the position of equality. This reading should still be 0 if the water was pure, and the glass ends of the tube unstrained.

Now insert a tube filled with a sugar solution containing 10 grams of sugar per 100 c.c. made and clarified according to the instructions, p. 226, and take readings several times as before.

The sample of sugar from which the solution has been derived may contain both cane sugar and invert sugar, and

impurities which will be assumed to be inactive. The cane sugar rotates the plane of polarisation to the right, and the invert sugar to the left. The observed rotation will be due to the difference of these effects.

To determine the amount of each constituent present, we make use of the fact that when cane sugar is heated gently with acid it is converted into invert sugar, so that the whole of the sugar then present in the solution is invert sugar.

From the two observations of the rotation the amount of each constituent present can be calculated.

Take 20 c.c. of the original solution of sugar in a flask, add to it '01 grams of strong hydrochloric acid per gram of sugar present, i.e. in the present case about 2 drops, add water till the volume is 22 c.c., and warm gently for 10 minutes, keeping the temperature about 70° C.

Cool the resulting invert sugar solution by placing the flask for 10 minutes in cold water, and after its temperature has fallen to about 20° C. note the temperature, insert the solution in the tube, and determine the rotation produced.

Let R1 be the rotation observed with the original sugar solution, and let R be that observed with the invert sugar, rotation to the right being considered positive and to the left negative. Let p be the specific rotatory power of cane, p, that of invert sugar, and let a grams of cane sugar and a1⁄2 grams of invert sugar be present in 1 gram of the sample. Then in the first experiment,

The factor 360/342 being due to the change from C2H22O11 to C12H2O12 in inversion of the sugar, and the factor 20/22 due to dilution with acid. Reducing we have

1·1R2 = 2p2 {α, 360/342 + α2}

.(2).

Subtracting (2) from (1) we have

R1-1.1R, (2p1 - 2.105p2) α.

=

The quantity p, has been found to be nearly independent of temperature. Its value is + 6·65 degrees.

p2 is negative and depends on the temperature t. Its value for sugar inverted in the way described is — 2:00 +031 (t −20) degrees.

Record observations and results as follows:

15 Jan., 1896.

Solution containing 10 grams of sample per 100 c.c. Reading with water in tube

Do. sugar solution) 12° 44', 12°50', 12° 50′, 12°44′ .. Rotation at temp. 18° C. ... Reading with invert solution) 3° 20', 4° 0', 3° 35', 3° 45'

.. Rotation at 18° 5' C.

0° 10′ to right.

mean, 12° 47'

12° 37'

...

دو

mean, 3° 40′ to left.

Or the sample contains 96% of cane sugar and 4% of

invert sugar.

Wash out the tubes thoroughly with tap water before putting away the apparatus.