Applications
Determination of phase transitions with
simultaneous video observation
Dr. Matthias Wagner
The melting point is without doubt the thermal value most frequently used to characterize materials. This
fact together with ever-increasing requirements for melting point determination were the two main reasons
why METTLER TOLEDO decided to develop a completely new series of instruments. The new Excellence Melting Point Systems allow substances to be analyzed that could previously not be measured by conventional
melting point instruments. The following article presents a number of different examples.
Introduction
The melting point
The melting point is a characteristic
property of a substance. It is the temperature at which the crystalline phase
changes to the liquid state. A pure substance normally has a sharp melting
point, whereas an impure substance
melts over a temperature range that is
lower than the melting point of the pure
substance. This effect is well known and
called the melting point depression. Some
organic compounds melt and decompose
simultaneously. This makes it difficult to
determine an exact melting point. Melting can also occur over a relatively wide
temperature range. One then refers to
a melting range rather than a melting
point. This effect is especially observed
with polymers.
In general, melting point determination
is used in research and development as
Figure 1.
Intensity curves of
transmitted light
during a typical
melting process:
point A is the start
of melting; B is a
threshold value,
40%; C is the end
of melting for the six
samples that can
be simultaneously
measured.
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well as in quality control to identify and
check the purity of substances.
Melting point detection
Many materials are opaque in the crystalline state but transparent in the liquid
state. This change in optical properties
during melting can be used to determine
the melting point or melting range. The
measurement is performed by heating
the sample in a furnace at a constant
rate and continuously measuring the intensity of light transmitted through the
sample (i.e. the transmittance). When
the transmittance exceeds a predefined
value, the sample is said to have melted.
This well-proven principle is also employed in the new METTLER TOLEDO
melting point instruments.
The new instruments use a camera as a
detector to measure the light intensity of
the sample and LEDs as the light source.
LEDs offer advantages such as lower en-
ergy consumption and longer lifetime
and at the same time generate more homogeneous light.
The large display incorporated in the
instrument allows several observers to
follow the melting process. Two models,
the MP70 and MP90, permit videos to be
transferred via SD card to the computer
and archived. The video format is AVI.
This means that you can replay videos on
the computer with commercially available software. Likewise, the experiment
data and intensity curves stored on the
SD card as an ASCII text file can also
be archived if desired. The file can be
opened and processed in a spread sheet
such as Microsoft Excel.
The instruments simultaneously determine up to four (MP50 and MP70) or six
samples (MP90).
Figure 1 shows typical light intensity
curves measured during a typical melting process. Three points, labeled A, B,
and C, are marked on the curve. These
points are characteristic temperatures
determined in a melting point analysis.
Point A marks the start of melting, point
B a characteristic temperature at which
the transmittance reaches a certain value, and point C the end of melting.
For the melting point determination,
either point B or point C is used. Most
standards define the end of melting (point
C) as the melting point. Point C can also
be automatically evaluated by the new
melting point systems. Points A and C are
used to determine the melting range.
Figure 2.
Video images of
substance during
melting.
Before the start of melting
At the meniscus point
Melting is complete
Visual observation
Standards that govern the determination
of melting point, such as USP <741> or
Ph. EUR 2.2.14, require visual observation of the sample.
Figure 3.
Sample preparation
using the sample
preparation tool.
Figure 4.
The start screen
showing the One
Click™ “UREA”
shortcut key.
This is done using the built-in camera
system. However, the possibilities offered
by the new melting point systems go much
further. Previously, you had to sit in front
of the melting point apparatus during the
measurement and observe the samples
through a lens. If you missed the melting
point, you had no choice but to prepare a
new sample and repeat the measurement.
In the new instruments, the melting process is stored as a video. You can replay the
video as many times as you wish.
Figure 2 displays different images from
such a video. The left image shows the
sample as a white powder in the capillary at the beginning of a measurement.
During melting, the sample consists of a
mixture of liquid and solid crystals. The
liquid forms a column with a meniscus at
the top. This point is called the meniscus
point and is shown in the middle image.
Finally, as the temperature increases, the
remaining crystals melt and the contents
of the capillaries are completely clear.
This is the case at point C in Figure 1 and
at higher temperatures (right image).
Experimental details
An experiment begins with sample preparation. First, the dry substances are finely
ground and filled into capillaries. The
furnace of the instrument can accommodate capillaries with diameters of up
to 1.8 mm.
This satisfies all current standards. Depending on the type of substance you
are measuring, you will have to vary
the filling level and possibly stopper the
capillaries. The filling level is typically
Figure 5.
Method programming.
Figure 6.
The online screen
showing the melting range that was
detected.
2.5 to 5 mm and can be easily checked
using the sample preparation tool (see
Figure 3).
After switching on the instrument, the
start screen appears on the display (Figure 4). The pharmacopeia operation
mode is the default setting in the instrument. In this mode, the temperatures displayed refer to the furnace temperature,
which is measured by a Pt100 temperature sensor. If thermodynamically correct melting temperatures are required,
the instrument setting must be changed.
These temperatures refer to the actual
sample temperature.
Most standards, however, are based on
the pharmacopeia temperatures.
The next step is to define the temperature program (Figure 5). Under “Manual
method”, you set the Operation mode,
Start temperature, End temperature,
Heating rate, insert the capillaries and
start the measurement. In the Operation mode you can select Melting point,
Melting range or Manual determination
of two temperatures (by clicking the appropriate keys).
The start temperature should be 3 to
5 °C lower than the expected start of
melting. The heating rate is typically
1 °C/min. If necessary, you can program
an isothermal wait time for equilibration
before heating the sample and hold the
end temperature isothermally for a certain time after heating is finished. You
can store the method as a “One Click™
short cut key” on the start screen by
clicking “AddToHome”.
This key allows the user to start the entire measurement sequence with just one
click, which is very useful for repetitive
measurements. When an experiment is
running, the “Online” screen appears.
This displays the live video, the status,
the remaining measurement time, and
the temperature.
If you wish, you can display the intensity
curve instead of the image information.
You can also add commentaries about
the measurement or details of the sam-
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11
Applications
ples. The “Stop” key terminates a measurement that is running. The instrument
signals the end of melting with an acoustic tone and simultaneously displays the
measurement results (see Figure 6).
Other applications concerned the determination of the characteristic temperatures of thermochromic substances (i.e.
substances that change color on heating),
as well as the melting of polymers.
Measurements and results
Colored substances
The test substance chosen was red solid
potassium dichromate (K 2Cr2O7). According to literature data, it should melt at
398 °C.
A number of “difficult” substances were
measured to assess how reliably the
new instruments were able to determine
melting points and melting ranges. The
substances chosen for these studies were
expected to cause problems because of
their particular physical or chemical
properties.
They included colored substances, substances that sublime, and substances that
melt with decomposition. Another interesting study dealt with substances that
tend to form bubbles during melting and
whether such substances can be reliably
characterized.
Figure 7.
The melting of
potassium dichromate. The experiments showed that
potassium dichromate and other
colored substances
can easily be determined.
The measurement was started at 395 °C
at a heating rate of 1 °C/min (Figure 7).
Substances that decompose
Sugar was chosen as the test substance.
In practice, the controlled decomposition of sugar is used for the production of
caramel desserts (caramelization).
The measurement was started at 180 °C
at a heating rate 1 °C/min. The melt-
Bubble formation
If the vanillin sample is not carefully
prepared, bubbles often occur during the
measurement and make reliable determination of the melting point difficult.
Vanillin presented no problems with the
new MP90 (Figure 10).
Thermochromism
Thermochromism is the term used to
describe the reversible change in the
color of a substance as its temperature is
changed. A good example that illustrates
this phenomenon is mercuric iodide
(HgI2). On heating, it changes from red to
yellow during a solid-solid transition.
The characteristic temperatures for the
thermochromic transition are the first
occurrence of yellow crystals and the disappearance of the last red crystals. These
temperatures can be determined manually using the set keys.
Potassium dichromate at the beginning of the measurement
After the start of melting at 397 °C
Completely molten at 399.5 °C
Sugar after the start of the measurement: white crystals
At 186 °C, beginning of decomposition: still solid but clearly brown
After the melting point was detected
in all samples: 186.3 °C
Figure 8.
Melting and decomposition of sugar.
The melting process
of substances that
decompose can
easily be measured.
Figure 9.
Caffeine during
melting at 235 °C.
Figure 10.
Vanillin after melting
with bubbles in five
of the six capillaries. Despite this, the
automatic detection
system functioned
properly: the melting point threshold
value (20%) was
83 °C.
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ing point was detected automatically at
186 °C (Figure 8).
Substances that sublime
Caffeine is well known as a substance
that sublimes and tends to cause problems in melting point determination.
The measurements showed that the new
instruments can easily determine the
melting point of caffeine. The melting
point determined for a batch with a certified melting point of with 235.5 °C was
235.6 °C (Figure 9).
The method consisted of heating from
120 °C to 160 °C at 5 °C/min. The start
of the transition was determined as
142.3 °C and the end 150 °C (Figure 11).
Polymers
Many homogeneous crystalline polymers become transparent at their melting point. In such cases, the new melting
point systems can also be used to distinguish between polymers.
As an example, two different polyethylenes, an LDPE and an HDPE, were pre-
Figure 11.
Thermochromism
of mercuric iodide.
The application
shows that thermochromic solid-solid
transitions can also
be measured.
The red crystals of mercuric iodide
at 120 °C
pared in capillaries of 1.75 mm internal
diameter. The capillaries were heated at
5 °C/min from 95 °C (LDPE) and 125 °C
(HDPE) respectively. The melting points
determined in this way were 104 °C and
132 °C (Figure 12).
Liquid crystal transitions
Even liquid crystal transitions can be
studied using the melting point system.
Azoxydianisole is a good example. In
a DSC experiment measured at a heating rate of 20 °C/min, azoxydianisole
first exhibits a phase transition at about
118 °C and then undergoes a transition
to an isotropic liquid at about 135 °C.
In the new melting point system measured at 1 °C/min, the transitions are
observed visually as follows: the yellow
powder melts at about 119 °C but the
liquid remains cloudy. The contents of
the capillaries do not become clear until
about 134 °C. The intensity curve also
The thermochromic transition of
mercuric iodide at 148 °C
Substance is completely yellow
at 155 °C
Figure 12.
Measurement of
LDPE and HDPE.
Polymers can be
qualitatively differentiated using
the new melting
point systems.
LDPE rods prepared in capillaries
LDPE rods after melting – dimensionally stable but transparent
HDPE rods in capillaries
Molten HDPE rods
shows a marked increase at this temperature (Figure 13).
Summary
The new melting point systems offer
many advantages such as:
• High resolution touchscreen for optimal display of videos, intensity curves
and results
• Ease of use including One Click™
short cut keys
• Video recording in reflection coupled
with well-proven automatic measurement in transmission
• High throughput analysis for the
simultaneous measurement of up
to four or six samples
• Manual setting of up to two points
Figure 13.
Measurement of
liquid crystal transitions.
Initial situation: capillaries filled with
azoxydianisole
At 119 °C: The substance has become
a cloudy liquid; individual crystals are
no longer visible
The intensity curve shows a small increase at A, a small
decrease at B and then a marked increase at C when the
substance becomes clear at 132 °C.
At 134 °C: The liquid is now clear
DSC curve of azoxydianisole measured at a
heating rate of 20 °C/min.
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Applications
• Automatic endpoint detection, pharmacopeia mode, and the size of
capillaries ensure compliance with
all current standards
• Video replay on the instrument
• AVI video and experiment data export
via SD card
• Statistical evaluation of the results
of a measurement
These functionalities and many other
features enable the user to quickly obtain
valuable information about the melting
behavior of different materials.
Detailed IQ/OQ documents are available for all instruments. If required, a
METTLER TOLEDO service engineer will
help you with IQ/OQ.
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