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Refraction
When a straw is placed into a glass of water, the straw appears
bent. Now if a straw is placed in a glass with water containing
dissolved sugar, the straw should appear even more bent (see illustrations).
This phenomenon is known as the principle of light refraction.
Refractometers are measuring instruments which put this phenomenon
of light refraction to practical use. They are based on the principle
that as the density of a substance increases (e.g. when sugar
is dissolved in water), its refractive index (how much the straw
appears bent) rises proportionately. Refractometers were devised
by Dr. Ernst Abbe, a German/Austrian scientist in the early 20th
century.
Principles of Refractometers
There are two detection systems for refractive index: transparent
systems and reflection systems. Hand-held
refractometers and Abbe refractometers use transparent systems,
while digital refractometers use reflection systems.
Transparent Systems
The detection system for hand-held refractometers (transparent
system) is summarized below.
| ( 1 ) |
In the figure below the detection is done by utilizing the
refractive phenomenon produced on the boundary of the prism
and sample. The refractive index of the prism is much larger
than that of the sample. |
| ( 2 ) |
If the sample is thin, the angle of refraction is large
(see "a") because of the large difference in refractive
index between the prism and the sample. |
| ( 3 ) |
If the sample is thick, the angle of refraction is small
(see "b") because of the small difference in refractive
index between the prism and the sample. |
Reflection Systems
The detection system for digital refractometers (reflection system)
will be discussed below.
In the figure below, Light A, being incident from the lower left
of the prism, is not reflected back by the boundary, but exits through
the sample. Light B is reflected by the boundary face to the right,
directly along the prism boundary. Light C, having an incident angle
too large to be let through to the sample side, is totally reflected
toward the lower right of the prism.
As a result, a boundary line is produced dividing light and dark
fields on either side of the dotted line "B' " in the
figure. Since the angle of reflection of this boundary line is proportional
to refractive index, the position of the boundary line between light
and dark fields is caught by a sensor and converted into refractive
index.
About the Brix ( % ) Scale
The Brix (%) shows the concentration percentage of the soluble solids
content in a sample (water solution). The soluble solids content
is the total of all the solids dissolved in the water, including
sugar, salts, protein, acids, etc., and the measurement reading
is the sum total of these. Basically, Brix (%) is calibrated to
the number of grams of cane sugar contained in 100g of cane sugar
solution. So, when measuring a sugar solution, Brix (%) should perfectly
match the actual concentration. With solutions containing other
components, especially when one wants to know the exact concentration,
a conversion chart is necessary.
>Applications of Refractometers
FRUIT
When measuring the sugar concentration of fruit, refractometers
have the following benefits:
| 1 |
The correct time for harvesting can be determined. |
| 2 |
Through sorting and grading control, fruit can be made uniform
for shipping. |
| 3 |
For the improvement of product quality and growing conditions,
sugar concentration is an important indicator. |
FOOD INDUSTRY
By measuring the sugar concentration or soluble solids content (%)
of various food products the following benefits can be obtained
:
| 1 |
Product standardization. |
| 2 |
Measurements of precise mixture proportions for the flavoring
solutions of canned goods, etc. |
| 3 |
Ideal for maintaining exporting and other international
standards. |
| 4 |
Quality comparisons with standard or competitors' goods
can be made easily. |
| 5 |
Product development and improvement can be made from the
data obtained. |
OTHER INDUSTRIAL AND RESEARCH FIELDS
| 1 |
By controlling the appropriate concentration of cutting
oil, corrosion can be prevented and cutting efficiency can
be increased. |
| 2 |
Measurements on animal and plant secretions for improving
varieties and for research on growing and cultivation conditions.
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•Light waves
as it travels. As shown in Figure 1, light may seem to travel
unidirectionally. In actuality light travels in all directions
as shown in Figure 2.
•When light, which waves
in all directions, goes through a grating placed in its course
of travel, only the light wave that oscillates in the direction
parallel to the bars of the grating passes through, Light waves
that oscillate in other directions get blocked by the bars of
the grating. ( Figure 3 )
Such light, which waves in one particular direction, is called
polarized light, and the grating is called a polarizing plate.
•When polarized light
travels through an observation tube filled with a sample solution
that does not make light rotate ( water, for example ) , the light
continues to wave in the same direction even after passing through
the solution. ( Figure 4 )
•In contrast, when it
travels through an observation tube filled with a sample solution
that makes light rotate ( sucrose solution, for example ) , the
light wave begins to rotate as it passes through the solution.
( Figure 5 ) This is called optical rotation.
•Those samples that make
light rotate have a molecular formula that contains asymmetric
carbon ( indicated by "C" ) . Sugar is the most common.
The explanation of the asymmetric carbon can be highly technical.Discussion
on asymmetric carbon will be discussed in a later section.
•Imagine making a light
path by placing a polarizing plate, an observation tube, another
polarizing plate, and a sensor one after another. ( Figure 6 and
7 )
•The path in Figure 6
has an observation tube filled with water, in Figure 7 a sample
solution, such as sucrose solution, that makes light rotate.
•In Figure 6 a certain
amount of light reaches the sensor.
•In Figure
7 the light does not reach the sensor. ( Technically speaking,
in terms of a vector an imperceptible amount of light does reach
the sensor, but let's assume that the light does not reach the
sensor here. )
•When the second polarizing
plate is rotated as shown in Figure 8, the same amount of light
as in Figure 6 now reaches the sensor.
< Conducting Zero-Setting on a Polarimeter
>
•Conduct zero-setting
in the step shown in Figure 6. In the actual adjustment procedure,
the observation tube filled with water is not necessary and zero-setting
is conducted by letting light travel through the air.
•Next, place an observation
tube filled with a sample solution that makes light rotate as
shown in Figure 8.
•Rotate the second polarizing
plate so that the equal amount of light reaches the sensor as
it did when zero-setting was conducted.
•The measured angle of
the rotated polarizing plate is the angle of rotation of the sample
solution.
* Instructions for the POLAX-2L polarimeter are described similarly
as the instructions above. The sensor mentioned here is the same
as your eye (in the instructions for the POLAX-2L.) The steps
of conducting zero-setting on the AP-100 are not exactly the same
as those described here, but both use the same principles.
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