3 FP81: Melting Point, Melting Range, Cloud and Boiling ... · PDF fileFP900 System FP81 Melting Point, Melting Range, ... Melting Range, Cloud and Boiling Point Measuring Cell of

FP900 System FP81 Melting Point, Melting Range, Cloud and Boiling Point

97/04 METTLER TOLEDO FP900 3-1

3 FP81: Melting Point, Melting Range, Cloud andBoiling Point Measuring Cell of the FP900 Sys-tem

Content

3.1 What is the FP81 Measuring Cell Used For? .....................................................3-3

3.2 Attaching the Measuring Cell and Switching On ..............................................3-3

3.3 Melting Point and Melting Range..........................................................................3-43.3.1 Measuring principle .....................................................................................................3-63.3.2 Preparing the samples ................................................................................................3-83.3.3 Rules for the most important measurement parameters .........................................3-93.3.4 Preparing the method................................................................................................3-103.3.5 Starting the measurement.........................................................................................3-113.3.6 Results.........................................................................................................................3-11

3.4 Boiling Point.............................................................................................................3-123.4.1 Measuring principle ...................................................................................................3-133.4.2 Preparing the samples ..............................................................................................3-143.4.3 Rules for the most important measurement parameters .......................................3-143.4.4 Preparing the method................................................................................................3-153.4.5 Starting a boiling point determination......................................................................3-153.4.6 Results.........................................................................................................................3-16

3.5 Cloud Point...............................................................................................................3-163.5.1 Measuring principle ...................................................................................................3-173.5.2 Preparing the samples ..............................................................................................3-173.5.3 Rules for the most important measurement parameters .......................................3-183.5.4 Preparing the method................................................................................................3-183.5.5 Starting a cloud point determination........................................................................3-183.5.6 Results.........................................................................................................................3-19

3.6 Temperature Calibration .......................................................................................3-193.6.1 Checking the temperature.........................................................................................3-193.6.2 Recalibrating the temperature ..................................................................................3-20

3.7 Maintenance.............................................................................................................3-21

3.8 Accessories..............................................................................................................3-25

FP81 Melting Point, Melting Range, Cloud and Boiling Point FP900 System

3-2 METTLER TOLEDO FP900 97/04

3.9 Technical Data.........................................................................................................3-26

FP900 System FP81 Melting Point, Melting Range, Cloud and Boiling Point

97/04 METTLER TOLEDO FP900 3-3

3.1 What is the FP81 Measuring Cell Used For?

The FP81 Measuring Cell detects thermo-optical changes of substances. Transparentcapillary tubes of the type used in visual melting point determinations are employed assample holders. Three samples are heated at a defined heating rate while they are illumi-nated by a lamp via a light pipe. Three photosensors continuously measure the intensity ofthe light transmitted through the samples. Depending on the measurement setup and oper-ating mode, this allows objective and reproducible detection of the melting point, the melt-ing range, the cloud point or the boiling point.

Difficulties arising from the nature of the substance appear with highly colored compounds.It is possible that the transmittance of the melted sample is not high enough to obtain re-sults. Critical substances are primarily those with a violet, blue or green color. However,these can be determined with the FP85 Measuring Cell, although with a somewhat loweraccuracy. Substances with slightly cloudy melts pose no problems.

With the Mettler-Toledo FP81 you can also measure substances which melt or boil belowroom temperature by operating the cell in a deep freezer at approx. 20 C.

In this operating instructions Remarks are marked with the symbol .

3.2 Attaching the Measuring Cell and Switching On

Before you switch on the instrument for the first time, ensure that the installation has beenperformed in accordance with the directions given in section 2.14: Startup procedure. Position measuring cell on the left of the control unit. Switch off FP90 Central Processor. If a measuring cell is still attached, unplug its 64-pin

connector. Carefully insert 64-pin connector of the FP81 Measuring Cell in the appropriate socket

at rear of the control unit ensuring correct alignment. The FP81 Measuring Cell has a switch to set the operating mode. Its setting has no

meaning for the FP90 as the operating mode is entered using the keypad (see chap.2.4: The operating mode).

Switch on the FP90; the furnace remains idle (no heating yet). On completion of the self-test, the display REMOVE CAPILLARIES appears (except in the operating mode boil-ing point).

Ensure that the measuring cell does not contain any sample capillaries. Any brokencapillaries must be removed following the directions given in section 3.7 Maintenanceand this acknowledged with F1 CALIB. Automatic calibration of the optical detectionsystem is now performed, in other words the light intensity at each of the three photosensors without inserted capillary tubes is set equal to 100% transmittance.

FP81 Melting Point, Melting Range, Cloud and Boiling Point FP900 System

3-4 METTLER TOLEDO FP900 97/04

During the calibration, no capillaries may be inserted as this would distort thecalibration completely and hence lead to wrong results.

If a light path is still obstructed, however, one of the bars representing the light intensity re-mains below the marked minimum line and the message Not enough light! appears.

Obviously, there is an object in the respective channel that is blocking the light path. Assoon as the foreign body has been removed (see chap. 3.7), the light intensity bar will beabove the minimum.

The same message also appears when the lamp is no longer in optimum adjustment. Ad-justment of the lamp is described in section 3.7: Maintenance.

After the optical calibration, the display has the following appearance, e.g.

Function key

Method number

Temperature unit C25.3

MODE SPECIALOUTPUT

FP81 Melting pointStart temp.RateEnd temp.Choose method: press

10:01:43

Co

T PROG METHOD

11

TimeCell temperature

Measuring cell attachedCurrent measurement parameters

Operating mode

Method:

: 120.0C: 1.0C/min: 180.0C

Fig. 3-1 Display in the main menu. Method 11 in use before the instrument was switched off is againactive. The display of the method number in reverse video indicates the entry state (just key ina method number).

3.3 Melting Point and Melting Range

In organic chemistry the melting point (clear melting point = end of the melting range) pro-vides information on the purity and is used to determine the identity of substances.

The fact that an impure substance shows a different melting behavior than the pure materialis often used in practice to compare substances of different purity.

FP900 System FP81 Melting Point, Melting Range, Cloud and Boiling Point

97/04 METTLER TOLEDO FP900 3-5

140

141T ,Cf

0 1 %

T cf

140.4 0140.2 0.31 %139.6 1.16 %

cImpurity

Fig. 3-2 Graphical representation of the melting point (clear melting point) as a function of the impurityconcentration (mass content). The main component is dimethyl terephthalate, the added impu-rity benzoic acid. The 3 measured points (mean values of 3 single measurements in eachcase) result in the dashed regression line.

As a rule, the melting point is lower when impurities are present (melting point depres-sion), in exceptional cases, e.g. in the formation of solid solutions with a higher-meltingimpurity it is higher. The dependence on the degree of purity can be described by agraphical representation of the melting point as a function of the impurity concentration(Fig. 3-2). In addition to the substance, which must be as pure as possible, artificially im-pure mixtures prepared by grinding in an agate mortar are needed.

The identity test method for the determination of the mixed melting point is based onthe fact that a mixture of two substances usually has a lower melting point than that of eitherof the two pure components. This is always the case when the two sub