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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 and Boiling 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 .......................................................................... 3-4 3.3.1 Measuring principle ..................................................................................................... 3-6 3.3.2 Preparing the samples ................................................................................................ 3-8 3.3.3 Rules for the most important measurement parameters ......................................... 3-9 3.3.4 Preparing the method ................................................................................................ 3-10 3.3.5 Starting the measurement ......................................................................................... 3-11 3.3.6 Results......................................................................................................................... 3-11 3.4 Boiling Point............................................................................................................. 3-12 3.4.1 Measuring principle ................................................................................................... 3-13 3.4.2 Preparing the samples .............................................................................................. 3-14 3.4.3 Rules for the most important measurement parameters ....................................... 3-14 3.4.4 Preparing the method ................................................................................................ 3-15 3.4.5 Starting a boiling point determination...................................................................... 3-15 3.4.6 Results......................................................................................................................... 3-16 3.5 Cloud Point............................................................................................................... 3-16 3.5.1 Measuring principle ................................................................................................... 3-17 3.5.2 Preparing the samples .............................................................................................. 3-17 3.5.3 Rules for the most important measurement parameters ....................................... 3-18 3.5.4 Preparing the method ................................................................................................ 3-18 3.5.5 Starting a cloud point determination ........................................................................ 3-18 3.5.6 Results......................................................................................................................... 3-19 3.6 Temperature Calibration ....................................................................................... 3-19 3.6.1 Checking the temperature......................................................................................... 3-19 3.6.2 Recalibrating the temperature .................................................................................. 3-20 3.7 Maintenance ............................................................................................................. 3-21 3.8 Accessories .............................................................................................................. 3-25
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Page 1: 3 FP81: Melting Point, Melting Range, Cloud and Boiling ...bikeitech.com/DATAS/es_free2/f_496e840fe0f7e.pdf · FP900 System FP81 Melting Point, Melting Range, ... Melting Range, Cloud

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

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

3-2 METTLER TOLEDO FP900 97/04

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

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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.

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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:

: 120.0°C: 1.0°C/min: 180.0°C

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.

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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 substances form aeutectic. This depression also appears even if the two substances should happen to havethe same melting point. In practice, three melting points are determined in a single run,namely a sample of each of the two pure components with the same melting point whoseidentity or dissimilarity has to be established and a mixed sample in proportion approxi-mately 1:1. The melting point depression of the mixed sample is very pronounced when thesamples are not identical and is usually more than 10 °C. With such types of relativelyqualitative investigation, less reliance is placed on the accuracy and reproducibility of theinstrument and more on the other benefits such as the facility to measure three samplessimultaneously, the operating convenience, reliability and digital display. Work is per-formed with a heating rate between 2 °C/min and 10 °C/min. The mixed melting-pointmethod fails only when the two substances form solid solution.

Melting point determinations with amorphous (resin-like or glassy) and polymorphous(2 or more crystal forms with different melting points appearing) substances are critical asthe sample can re-crystallize on warming and hence the measured melting point is not thatof the original substance. Depending on the type of sample preparation, e.g. crystallizationfrom the melt, one or other modification can arise at random. If the melting points of thesemodifications are close to each other, this may lead to an apparently poor reproducibility ofthe results. If this uncertainty is to be excluded with polymorphous substances e.g. by re-crystallization from a certain solvent, which can ensure that a quite specific modification isformed.

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

3-6 METTLER TOLEDO FP900 97/04

3.3.1 Measuring principle

Up to 3 samples can be measured at the same time, for example in the raw material test-ing a sample of perfect quality (reference sample) and two samples of a new delivery.

At the bottom of the FP81 Measuring Cell there is a light source (7) with a light pipe (6),which illuminates the inserted samples (1). Three photo sensors (5) are positioned on theouter wall and convert the residual transmitted light into a proportional electrical signal.While the cell temperature rises linearly at the set heating rate, the transmittance of thesamples is measured continuously.

1

2

3

4 5

7

6

1 Capillary tube with sample

2 Cell body of temperature T

3 Electric heating element

4 Temperature sensor Pt100

5 Photo sensor

6 Light pipe

7 Light source, lamp

c

Fig. 3-3 Schematic cross-section through the FP81 measuring cell, at a melting point determination

The radiant flux to the photosensor, Φex, divided by the radiant flux of the incident lightbeam, Φ in (measured without sample in the calibration), is called the light transmittance τ:

t = Φex /Φ in

If the sample is a fine powder, the individual crystals scatter the incident light beam in all di-rections and thus only a small part reaches the photosensor (5). When the melting point isreached, the crystals coalesce and finally disappear completely leading to a large increasein the transmittance. The final value of the transmittance is in the range of approx. 10 to ap-prox. 110%, depending on the optical properties of the melts (color, refractive index). Val-ues above 100% can appear as the melt can act as a convergent lens in the capillary.

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

97/04 METTLER TOLEDO FP900 3-7

100 %

0

Transmittance, τ

Temperature

A

B

C

20 %1.2%10 s

Start temperature

0.4 %/s Slope

Fig. 3-4 The transmittance curve of a melting substance. A to C corresponds to the melting range, B tothe melting point. Point A corresponds to the temperature at which the transmittance exceedsthe initial value (mean value during the first 10 s of measurement) by 1.2%.Point B is 20%above the initial value. At point C the slope of the transmittance curve decreases below 0.4%/s.

The melting point is detected at an increase of the transmittance of 20% as standard(point B in Fig. 3-4), whereas the melting range extends from A to C.

The melting of a substance needs a great deal of energy, the heat of fusion of organic sub-stances is of the order of 150 J/g. As a result, the temperature in the sample from point Ato point C increasingly lags behind the furnace temperature. With the thermodynamic re-sult output configured as standard, the sample temperature Ts is calculated as follows:

T T fS = - β β = heating rate in °C/min; fA = 0,711; fB = 1,02; fC = 1,15

The correction term increases from point A to point C due to the increasing heat flow.

A pure substance has a melting range of virtually zero. While the results are theoreticallyindependent of the heating rate, the correction factors are somewhat dependent on thesample and the sample preparation and hence the measurement uncertainty increaseswith increasing heating rate.

NUM

123.7CURVE Y CURVE ZCURVE X

FP81 Melting Range 2'57"

14

CX Y Z o

121.3

119.4 120.6 119.9

BAR

Fig. 3-5 Display of the results of the melting range during a measurement. The samples Y and Z havenot yet melting completely.

The other result output is called „following pharmacopoeia“ and corresponds to thetemperature of the heating medium = furnace temperature, T. The results following phar-

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

3-8 METTLER TOLEDO FP900 97/04

macopoeia are higher than the actual (thermodynamic) melting point by the square root ofthe heating rate employed.

To be able to distinguish them, in the pharmacopoeia case a small P appears in front ofthe temperature unit.

The temperature of the FP81 is checked by measuring the melting point of calibrationstandards (see section 3.6).

3.3.2 Preparing the samples

Finely grind coarse crystalline, dry substances in a clean agate mortar.

Pre dry moist substances in a desiccator.

Add powdered samples to sample capillary tubes to a height of 4 ... 6 mm by pressingthe open end of the capillary into the substance and then compacting by tapping the bottomof the capillary on a hard surface. Reproducible compaction of the sample is also possibleby allowing the capillaries to fall to the bench through a glass tube of length approx. 1 m. Ifthe sample does not move to the capillary bottom, tamp it down using a minimum amountof force with the tamping wire supplied.

The fill height can be checked using the lines marked on the sample stand (front right). Forprecision measurements, the optimum fill height of 4 mm should be complied with!

109 I I

Y

Z

8

Fig 3-6 Pulverize sample in agate mortar, add tocapillary and tap down

Fig. 3-7 Checking the fill height

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

97/04 METTLER TOLEDO FP900 3-9

You can measure liquid samples with a melting point >–20 °C after crystallization in theFP81 installed in a deep freezer. Add the liquid sample using a disposable syringe with along needle ensuring freedom from air bubbles. Or warm the capillary and with the openingpointing downward immerse in the liquid. The sample will be sucked up on cooling and canbe moved to the bottom of the capillary by shaking to-and-fro. Rapidly crystallize the sub-stance in a cold bath (e.g. liquid nitrogen, acetone/dry ice) or in an aluminum block withappropriate holes (pre cooled in a deep freezer) so that the crystals formed are as small aspossible. Crystals with a reproducible melting point are formed only under reproduciblecrystallization conditions! Many substances exhibit polymorphism, the different modifica-tions have different melting points.

Fats and waxes should normally not be pre melted in order to avoid „wrong“ measure-ment results owing to the frequent appearance of metastable modifications. Press an opencapillary (e.g. break off the fused end of a normal melting point capillary) into the substanceuntil it is filled to a height of 3 ... 5 mm. Prevent sample running out of the capillary by a layerof sealer 1 mm thick. (e.g. „seal-ease® tube sealer and holder“, cat No. 1050, from BectonDickinson and Company, Rutherford, N.J. 07070) or in an emergency with some chewinggum.

Samples that decompose, sublime or vaporize: When a sealed capillary is heated, thevolatile components evolved by the sample produce an over pressure that inhibits furtherdecomposition or sublimation. Proper and rapid sealing of the capillary in a gas flame ispossible only if the upper part of the capillary is free from sample.

Number of samples: Experience has shown that it is best to measure 3 samples of thesubstance under investigation in parallel. This provides you with information on the homo-geneity of the substance.

3.3.3 Rules for the most important measurement parameters

Heating rate: Usually 1 °C/min. For highest accuracy and non decomposing samples0.2 °C/min. With substances that decompose, 5 °C/min, for exploratory measurements10 °C/min.

Changing the heating rate during the measurement influences the measure-ment results! For this reason, the keys F1 T PROG, F2 HOLD and F4 FASTare inactive during locked operation (for routine measurements).

Start temperature: 3 ... 5 min before the expected melting point (3 to 5 times the heatingrate).

End temperature: If you need a well resolved measuring curve use an end temperaturewhich is approx. 10°C above the expected effect. Other wise take the maximum tempera-ture of the relevant measuring cell as the temperature program automatically stops as soonas all samples are completely melted (stop after event, see section 2.5).

Melting point capillaries: If at all possible, use capillaries from Mettler-Toledo (OrderNo., see Accessories). It is essential to check other capillaries with the enclosed holegauge (1.3 ...1.5 mm).

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3-10 METTLER TOLEDO FP900 97/04

3.3.4 Preparing the method

For determination of the melting point, e.g. of benzoic acid, a suitable method is needed.You have the following possibilities:

• Numeric: (if you know the method number, e.g. No. 90).− Press F6 MENU to show the main menu:

T PROG SPECIALOUTPUTMODE METHOD

Fig. 3-8: The main menu

− Enter method number. It appears in reverse video. Close with ENTER.− From the method listing: via F4 METHOD, F3 LIST and t show the method in re-

verse video and press F4 VIEW, F1 LOAD.

VIEW PRINT

FP81 Melting point 08:13:3701 FP81 Additive (B. Perrenot)

05 FP81 Check of powder 323 U. Groth

81 FP81 Mettler Melting Point 90 FP81 Temp Check Benzoic Acid

Fig. 3-9: Select the desired method from the method list

• Manual method: Redefine the operating mode (F1 MODE, F1 MELT P) and the tem-perature program (F2 T PROG). Benzoic acid has a melting point of 122.4 °C. With aheating rate of 1 °C/min, enter a start temperature lower by approx. 4 °C, that is119 °C.

♣ You can change a method only if the FP90 is not locked (see section 2.12).

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97/04 METTLER TOLEDO FP900 3-11

3.3.5 Starting the measurement

As soon as you have selected or entered a method, press or F1 RUN respectively toenter the sample identification and insert the samples. Ensure that the capillaries are in-serted to the bottom of the 3 measuring slots. The retaining clips press the capillariesagainst the furnace and thus result in some resistance. Do not forget to ensure that thecenter stopper is inserted in the boiling point opening. It protects the photosensors againstdisturbing ambient light.

122.3 RESETDISPLAY

FP81 Melting point 4'48"

90

C

100%

0

X100%

0

Y100%

0

Z o

Fig. 3-10 Display (bar display) during the melting point determination.

The FP90 is in locked operation and hence F1 T PROG, F2 HOLD and F4 FAST are inac-tive.

3.3.6 Results

As soon as all melting points have been determined or the end temperature has beenreached, the statistics of this measurement appear:

120.7RESETDISPLAY

FP81 Melting Point

90

CX Y Z o

Finishe

121.9 122.2 122.1

x=122.1 s=0.15

Fig. 3-11 As soon as the measurement is finished, the mean value (at least 2 measured samplesneeded) and the standard deviation s (of 3 or more measurements only) appear. The statusdisplay above„°C“ shows that the measuring cell is now being cooled to the start temperature.

In this condition, you can still view, e.g. the transmittance curve of measuring slot X usingF3 VIEW. By pressing F5 RESET you reach the main menu to start a new measurementemploying the same method.

The melting range instead of the melting point can be determined by changing the oper-ating mode F1 MODE in the main menu (F6 MENU).

The melting range is outputted as the result without and s (see section 2.8).

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3-12 METTLER TOLEDO FP900 97/04

3.4 Boiling Point

All pure substances that boil without decomposition have a characteristic boiling point.Analogous to the melting point, the boiling point can thus be enlisted to identify a sub-stance. Soluble impurities lower the vapor pressure and thus increase the boiling point.

At the boiling temperature, the liquid phase is in equilibrium with the gas phase. At thestandard boiling point, the vapor pressure of a liquid reaches the standard pressure of101.325 kPa. A boiling point not measured at standard pressure thus has to be corrected.As the atmospheric pressure decreases with increasing height above sea level, the boilingpoint is also lowered (by approx. 0.3 °C per 100 m increase in height).

The correction is based on the Clausius-Clapeyron equation assuming an entropy of va-porization of 94 J mol-1 K-1:

T TT K In

pp

std p

pstd

= +·[ ]

.113

Tstd = boiling point at standard pressure

Tp = boiling point at pressure p

Tp [K] = boiling point at pressure p, expressed in Kelvin

ln = natural logarithm

pstd = standard pressure = 101.325 kPa

p = current pressure during boiling point determination

The constant 11.3 is the quotient of the entropy of vaporization (94 J mol-1 K-1) divided bythe gas constant (8.314 J mol-1 K-1).

In small, clean vessels, a dust- and gas-free liquid can often be heated well above the boil-ing point without boiling. There are no nuclei for the vapor bubbles. On further heating boil-ing often starts in an explosive manner (bumping). This bumping is combated by insertingboiling capillaries and in most cases this allows the boiling point to be determined.

Boiling point determinations with small amounts of sample are less accurate and repro-ducible than the determination of melting points. The main cause is the relatively compli-cated process of boiling.

With mixtures or with highly impure samples, the results must be interpreted with caution: Inthe case of an impure sample, the escape of a low boiling fraction, for instance, is re-corded as the boiling point.

With multiple determinations using one and the same sample, the composition can changefrom determination to determination as a result of the evaporation of low boiling compo-nents: continuously increasing values are thus obtained.

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97/04 METTLER TOLEDO FP900 3-13

3.4.1 Measuring principle

1

2

3

4

5

7

6

1 Boiling point tube with introduced boiling capillary and a sample evolving vapor bubbles

2 Cell body at temperature T

3 Electric heating element

4 Temperature sensor, Pt100

5 Photo sensor

6 Light pipe

7 Light source, lamp

c

Fig. 3-12 Schematic cross-section through the FP81 measuring cell at the boiling point determination

In the FP81, the boiling point is determined using an improved method following Siwobo-loff1). The substance is added to a small tube shaped like a test tube and the troublesomebumping countered by a boiling capillary. The tube is inserted in the center opening of thefurnace. The ascending bubbles reflect the light of the built-in light source and are detectedone by one by the photosensor.

Above the boiling temperature, the gas bubbles escape at an increasing rate, dependingon the temperature difference between the furnace and the sample. The air or gas bubblesthat ascended initially can be clearly distinguished from the uniformly ascending vaporbubbles. The temperature at a bubble frequency of 0.6 Hz is taken as the boiling tempera-ture. If the time to the attainment of a second frequency of 0.85 Hz is < 2 s, bumping is re-ported.

After entry of the atmospheric pressure prevailing at the time of the measurement, the soft-ware of the FP90 calculates the standard boiling point (at 101.325 kPa) using the equationabove1) A. Siwoboloff: Ber. d. chem. Ges. 19, 795 (1886)

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3-14 METTLER TOLEDO FP900 97/04

3.4.2 Preparing the samples

If at all possible use boiling point tubes from Mettler. If these are not available check othertubes with a hole gauge (minimum 2.9 mm to maximum 3.1 mm outer diameter).

Sample tube and capillary must be completely dry and thoroughly clean. Both can be usedagain after washing and thorough drying.

Add liquid under investigation to a height of 15 ... 18 mm in a boiling point tube ensuringfreedom from air bubbles.

Insert boiling capillary in boiling point tube. The boiling capillary must rest on the base ofthe tube. This is best achieved by slightly tapping the capillary with your finger.

Fig. 3-13 Boiling point tube with inserted boiling capillary

3.4.3 Rules for the most important measurement parameters

Heating rate: Usually 1 °C/min, for precision measurements 0.2 and for exploratorymeasurements 5 °C/min.

Changing the heating rate during the measurement influences the measure-ment result! For this reason, the keys F1 T PROG, F2 HOLD and F4 FASTare inactive in locked operation (for routine measurements).

Start temperature: Approx. 5 min before the expected boiling point (5 times the heatingrate).

End temperature: If you need a well resolved measuring curve use an end temperaturewhich is approx. 10°C above the expected effect. Otherwise take the maximum tempera-ture of the relevant measuring cell as the temperature program automatically terminates assoon as the sample boils (stop after event, see section 2.5).

Waiting: As the boiling point sample is relatively large, a wait time of 2 min is advisable.

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3.4.4 Preparing the method

To determine the boiling point, e.g. of Ethanol you need an appropriate method.

You can call up a stored method, if you have prepared one: Either by direct numeric entryor from the method list via F4 METHOD, F3 LIST.

Or you can do a manual method e.g. based on Method 81, by entering the boiling pointmode F1 MODE and F3 BOIL P. As start temperature for the measurement of Ethanol withan expected boiling point of 78 °C and a heating rate of 1 °C/min, take: 78 °C minus (5times 1) equals 73 °C (enter under F2 T PROG).

You can immediately start measuring in this mode without saving the method.

3.4.5 Starting a boiling point determination

After you have defined the method, press the key or F1 RUN respectively, and enterthe prevailing atmospheric pressure (barometer reading) in the SI unit kPa and the sampleidentification and then insert the sample tube into the large opening located in the center ofthe cell..

♣ Without entry of the barometer reading, the resulting boiling temperature is at thepressure at which it was measured, otherwise the boiling point is corrected to thestandard pressure of 101.325 kPa (equation in section 3.4.2). If you are still working withthe semi-official unit mbar, divide the relevant numeric value by 10 to get it inkPa.

75.2T PROG RESETDISPLAY

FP81 Boiling Point 2'36"

Co

ISO FAST

1 Hz

0 15*

Fig. 3-14 Display (bar display) during the boiling point determination. On heating, the bubble frequencyincreases and reaches 0.6 Hz at the boiling temperature.

If the bubble frequency of 0.6 Hz at the boiling temperature rises to 0.85 Hz in less than 2 s,the warning „bumping“ is outputted.

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3.4.6 Results

As soon as the boiling point has been determined or the end temperature reached, the re-sult of the measurement appears:

73.4RESETDISPLAY

FP81 Boiling Range

Co

Finished

Boiling temperature 77.1 °CBoiling point 78.5 °C

Fig. 3-15 When the measurement is finished, the result appears. The status display above „ °C“ showsthat the measuring cell is being cooled to the start temperature.

In this condition, you can always view, e.g. the bubble frequency curve via F3 DISPLAY. Bymeans of F5 RESET you reach the main menu to start a new measurement using the samemethod.

♣ The boiling point (at standard pressure) of Ethanol is 78.3°C.

3.5 Cloud Point

The cloud point is taken to mean that temperature at which a previously clear, single-phase substance becomes cloudy owing to the appearance of a second phase. Thiscloudiness lowers the transmittance. The clear point (FP81C) is the temperature where apreviously cloudy substance becomes clear (usually on cooling).

The cloud point in the cooling of liquids corresponds to the start of crystallization and ismeasured for, e.g. oils or melted fatty acids.

The phase separation when a substance is heated is caused by a miscibility gap of thecomponents of a sample, e.g. an aqueous surfactant solution. The hydrocarbon group of anonionic surfactant is hydrophobic, the polyethylene oxide chain hydrophilic. The hydro-philic part is responsible for the solubility in water, which improves with increasing degreeof ethoxylation. With a rise in temperature, the water-insoluble hydrophobic part of themolecule stretches and results in increased insolubility. On cooling below the cloud point,the solution becomes clear again. As the cloud point depends on the degree of ethoxyla-tion, the latter can be characterized by a simple cloud point measurement.

The cloud point of aqueous solutions is measurable practically only in the range betweenroom temperature and the boiling point (except in sealed capillaries) and can thus not bedetermined with products with a low degree of ethoxylation, which in any case form acloudy solution, or with substances having a very high degree of ethoxylation, where nocloudiness appears below the boiling point. For this reason, substances with a low degreeof ethoxylation require a „better“ solvent than water, those with a high degree of ethoxylationa „poorer“ – in the first case a 25% solution of butyl diglycol, in the second a 10% salt solu-tion.

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3.5.1 Measuring principle

As with the determination of the melting point up to 3 samples can be measured simulta-neously. The transmittance is monitored as in Fig. 3-3.

100 %

0

Transmittance, τ

Temperature

10 s

Start temperature Cloud point

4 %F

Fig. 3-16 The transmittance curve of a previously clear substance that becomes cloudy. The cloud pointis equal to the temperature at which the transmittance drops by F = 4% of the initial value(mean value over the first 10 s of measurement). Above the cloud point, the transmittance curveis shown dashed as the measurement is usually terminated automatically after attainment ofthe expected event.

3.5.2 Preparing the samples− Depending on its solubility, the surfactant is dissolved in butyl diglycol solution, in water

or salt solution. In practice, possible test solutions include the following:• for products with a low degree of ethoxylation:

5 g test substance in 25 ml 25% butyl diglycol solution in distilled water• for products producing a clear solution in water:

1 g test substance made up to 100 ml with distilled water,• for products with a high degree of ethoxylation:

1 g test substance made up to 100 ml with 10% aqueous salt (NaCl) solution.− For the surfactant in this example (Genapol 0-150, Hoechst), 1 g substance is dissolved

in 100 ml 10% salt solution and added to the 3 capillaries (fill height 4 ... 6 mm) using adisposable syringe with a long needle ensuring freedom from air bubbles. Any remain-ing air bubbles must be carefully removed using the syringe.

− If you expect a cloud point in the region of 100 °C, you should seal the capillary by melt-ing.

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3.5.3 Rules for the most important measurement parameters

Heating rate: Usually 1 °C/min, for exploratory measurements 10 °C/min.

Changing the heating rate during the measurement influences the measure-ment results! For this reason, the keys F1 T PROG, F2 HOLD and F4 FASTare inactive in locked operation (for routine measurements).

Start temperature: Approx. 5 min before the expected cloud point (5 times the heatingrate).

End temperature: If you need a well resolved measuring curve use an end temperaturewhich is approx. 10°C above the expected effect. Otherwise take the maximum tempera-ture of the relevant measuring cell as the temperature program automatically stops as soonas all samples are cloudy (stop after event, see section 2.5).

Schmelzpunktkapillaren: If possible use only Mettler-Toledo capillaries (order no. seeaccessories). If you intend to use other capillaries you must check their diameter with theenclosed hole gauge (1.3…1.5 mm).

3.5.4 Preparing the method

Enter cloud point mode by means of F1 MODE and F4 CLOUD P. Define start tempera-ture (65 °C for Genapol) and heating rate (1 °C/min for Genapol) under F2 T PROG. Youcan measure immediately in this mode without saving the method (manual method).

♣ You can only redefine a method if the FP90 that is not locked (section 2.12).

3.5.5 Starting a cloud point determination

After you have defined the method, press or the F1 RUN key to enter the sampleidentification and insert the samples. Ensure that you insert the capillaries right to the bot-tom of the measuring slots. The retaining clips press the capillaries against the furnace andthus produce some resistance.

Do not forget the insert the center stopper in the boiling point opening. It protects the photosensors against disturbing ambient light.

68.2T PROG RESETDISPLAY

FP81 Cloud Point 4'36"

C100%

0

X100%

0

Y100%

0

Z o

ISO FAST

53*

Fig. 3-17 Display (bar display) during the cloud point determination. On heating, the transmittance de-creases owing to the increasing cloudiness.

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3.5.6 Results

As soon as the cloud points of three inserted samples have been determined or the endtemperature reached, the statistics of this measurement appear:

69.4 RESETDISPLAY

FP81 Cloud point

CX Y Z o

Finished

72.2 72.0 72.6

x= 71.3 s=0.31

Fig. 3-18 As soon as the measurement is finished, the mean value (at least 2 measured samplesneeded) and the standard deviation s (of 3 measurements only) appear. The status displayabove „°C“ shows that the measuring cell is now being cooled to the start temperature.

In this condition, you can still display, e.g. the transmittance curve of measuring cell X usingF3 DISPLAY. By pressing F5 RESET you can start a new measurement employing thesame method.

3.6 Temperature Calibration

We make a basic distinction between a check on the temperature carried out as oftenas liked and the calibration of the temperature scale, which has to be performed when acheck has shown that the deviation is no longer tolerable.

The temperature scale is checked by measuring the melting point of calibration stan-dards, which are pure substances melting without decomposition with an exactly knownmelting point and with little tendency to polymorphism. Owing to the poorer reproducibility,cloud and boiling points are deliberately not used for this purpose.

3.6.1 Checking the temperature− For maximum accuracy, the temperature check methods use heating rates of 0.2

°C/min and the thermodynamic result output. Occasionally the rate is set equal to that ofthe subsequent sample measurements.

− Prepare 6 capillaries of each calibration standard to be used (see section 3.3.2), en-suring above all the same fill height and compactness of the samples.

− If the relative humidity in your lab exceeds 70%, you should dry benzophenone overnightin a desiccator after grinding in an agate mortar. It is best to use 2 mortars for the twosubstances (see below).

− For the range from……room temperature to approx. 130 °C take benzophenone (Tf = 48.1 °C, Mettlermethod 91) and benzoic acid (Tf = 122.4 °C, Mettler method 90),

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− For the range from……above approx. 130 °C take benzoic acid (Tf = 122.4 °C, Mettler method 90) andcaffeine (Tf = 236.3 °C, Mettler method 92).

♣ The exact value of the melting point Tf is always printed on the label of the calibrationstandard. It can vary somewhat from batch to batch.

− Check the 6 single results: each result that deviates by more than 0.2 °C from the meanvalue is considered an outlier in the temperature monitoring. 1 outlier in 6 results can beeliminated. If more than 1 outlier appears, or the differences in the remaining results ismore than 0.2 °C, you should prepare and measure 6 more samples.

− We call the mean value of a good measurement series without the outlier Tmeasured andcompare it with the melting point Tf of the calibration standard.

− As long as the absolute value of the temperature deviation Tmeasured - Tf is less than0.3 °C, your measuring cell is in order and you do not need to perform a recalibration.

3.6.2 Recalibrating the temperature

A temperature calibration that is performed wrongly leads to a wrong tem-perature scale and hence this work should be carried out only by especiallyqualified personnel!

− In the main menu (F6 MENU), press F5 SPECIAL, F3 CALIB and F2 TEMP.− Now enter the two value-pairs Tf and Tmeasured (mean value of the 6 measurements). This

redefines the temperature scale.− To make sure, check using benzoic acid!

The calibrated temperature scale is based on the following equation:

Tcalibrated = A + B TMettler

where A is the parallel displacement and B the slope correction of the factory calibration.The absolute value of A should not exceed 2 °C. B should be between 0.990 and 1.010. Ifnot, check the temperature again.

Should you ever falsify the temperature scale so badly owing to a wrong measurement orentry so that another measurement with the Temp. Check methods is impossible as themelting point lies outside the measured temperature range, enter F1 METTLER under F3CALIB. This activates the original factory calibration of the measuring cell (A = 0, B = 1).Then check the temperature once again from the beginning.

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3.7 Maintenance

The FP81 Measuring Cell does not require a great deal of maintenance in normal use evenover a considerable length of time. As far as the user is concerned, this work is limited tochanging defective light bulbs and cleaning the interior of the cell.

− Switch the instrument off and disconnect the power cable. An electricshock could be lethal.

− Never touch the furnace, furnace lid or a sample you have just removedfrom the furnace (capillary tubes, glass tubes, slides, sample cups or cruci-bles)! The temperature of the furnace can reach 375 °C.

− Notice capillary tubes standing out from the module or other glass parts(e.g. capillaries). Broken glass parts can be very sharp. You may hurt your-self.

− When using chemicals, comply with the instructions of the producer and thegeneral lab safety rules.

1

3

21

1

2

3

4

Fig. 3-19a: Removal of the top housing. 1 Turn toleft, 2 Lift off, 3 Undo nuts.

Changing a defective bulb− Switch off control unit.− Unplug connector at control unit.− Remove capillaries.− Remove top housing (1) by push and

turning to the left. This is easier if a sec-ond person holds the measuring cellfirmly.

− Adjust the furnace module in a way thatyou face the label. Remove the threeslotted nuts (2) with a screwdriver.

− Lift the plate with funnel (3), take off thespacers and remove the measuring celljacket (4).

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56

Fig. 3-19b: Arrangement of the bulb holder

− Remove bulb holder (5) by undoingscrew (6)

− Take bulb holder out of the furnace hous-ing

− Change defective bulb.− Insert bulb holder fully in the furnace

housing and secure.− Adjust bulb (see section below).− Mount measuring cell jacket, position the

three spacers and assemble the platewith funnel and secure with the slottednuts. Take care to keep the measuringcell jacket opposite to the connection ca-ble. Adjust the plate with funnel accordingto the capillary inserting in the furnacebody.

− Mount top housing and secure by pushand turning to the right. This is easier if asecond person holds the measuring cellfirmly.

− Attach measuring cell to control unit.− Order a spare bulb.

Adjusting the lamp of the FP81

If the bulb has been changed or if, for any other reason, the lamp is no longer centered, af-ter switching on (not in boiling point operating mode), the message „Not enough light“ ap-pears. This means that you have to readjust the lamp. This must not be done when the in-strument is cold. It is best to leave the FP90 with the FP81 Measuring Cell switched on forat least 20 min before adjustment. After switching off the FP90, remove the measuring cellhousing. Switch the power on. When the prompt „REMOVE CAPILLARIES“ appears, at-tempt to shift the lamp holder, in order to get all 3 displayed signals above the markedminimum. This must also hold when the housing is mounted.

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7

7

Fig. 3-20 Checking the retaining clips(6)

Removing broken capillaries from the cellinterior− Switch off control unit.− Remove top housing by push and turning

to the left (Fig. 3-19). This is easier if asecond person holds the measuring cellfirmly.

− Adjust the furnace module so that it is op-posite to the label. Loose and remove theslotted nuts with a screwdriver.

− Lift the plate with funnel and put beside.− Carefully lift out broken capillaries pro-

jecting above the furnace by hand.− Take out broken sample tubes in the inte-

rior of the furnace with retaining clip (7).− Hold retaining clips with tweezers at bent

arm and take out.− Reinstall retaining clips; any bent retain-

ing clips should be replaced (10 are in-cluded in the standard accessories).

− Assemble the plate with funnel and se-cure with the slotted nuts. Adjust the platewith funnel according to the capillary in-serting in the furnace body.

− Mount top housing and secure by pushand turning to the right. This is easier if asecond person holds the measuring cellfirmly.

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Cleaning the measuring slots in the cell interior

The measuring slots in the furnace interior should be cleaned only if the contamination isexpected to have an adverse effect on the measurement results.

Never touch the furnace, furnace lid or a sample you have just removed fromthe furnace (capillary tubes, glass tubes, slides, sample cups or crucibles)!The temperature of the furnace can reach 375 °C.

Notice capillary tubes standing out from the module or other glass parts (e.g.capillaries). Broken glass parts can be very sharp. You may hurt yourself.

− Switch off control unit.− Remove capillaries.− Remove top housing by push and turning to the left (Fig. 3-19). This is easier if a second

person holds the measuring cell firmly.− Adjust the furnace module so that it is opposite to the label. Loose and remove the slot-

ted nuts with a screwdriver.− Lift the plate with funnel and put beside− Take out retaining clips (7) of all three measuring slots using tweezers.− Check that 1 mm holes for the light beam in the lower half of the retaining clips are clear.− Blow out measuring slots with a rubber puffer (not with compressed air) or carefully push

through with thin pipe cleaners.− Reinsert retaining clips, replace bent retaining clips (10 are included in the standard ac-

cessories) and particularly ensure that the shape of the contact pressure arm is correct.− Mount plate with funnel and secure with slotted nuts. Adjust the plate with funnel accord-

ing to the capillary inserting in the furnace body.− Mount top housing and secure by push and turning to the right. This is easier if a second

person holds the measuring cell firmly.

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3.8 Accessories

Standard Accessories to FP81 Order No.

• 2 Sets of melting point capillaries, each containing 150 pieces− 1 Set, ø 1,3 - 1,5 mm, including 5 tamping wires 18552

• 2 Sets of boiling point tubes, each containing 40 pieces− 1 Set, ø 2,9 - 3,1 mm 18572

• 1 Set of boiling capillaries, each containing 10 pieces 18574

• 1 Sample stand 18399

• 1 Hole gauge 18390

• 1 Set of lamp bulbs, 24 V, 2 W; 3 pieces 18562

• 1 Set of retaining clips, 10 pieces 18567

• 1 Dust cover 18571

• Disposable syringe, 1 mL with long needle see "Optional accessories"

• Tutorial sample, Ethanol 18874

Optional Accessories to FP81 Order No.

• 1 Package of disposable syringes, 1 ml; 100 pieces 71492

• 1 Package of syringe needles, 0.8 x 80 mm; 12 pieces 71484

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3.9 Technical Data

FP81 MBC Cell

Temperature range RT …375 °C− with external cooling (deep freezer) -20 °C…375 °CAbsolute accuracy of the melting point determi-nation with a heating rate of 0.2 °C/min

-20...+30 °C30...200 °C

200...375 °C

± 0.4 °C± 0.2 °C± 0.5 °C

Temperature accuracy after calibration: in thetemperature range covered by the two calibra-tion standards

± 0.2 °C

Absolute accuracy of the melting range with aheating rate of 0.2 °C/min

− Start of melting(typically)− End of melting

Twice tolerance of the melting pointas melting point

Absolute accuracy of the boiling point determi-nation

Three times tolerance of the melting pointException: Water gives values toohigh by approx. 2 °C owing to its highsurface tension.

Reproducibility, (standard deviation of themelting point of the calibration standard:

− Benzoic acid at 0.2 °C/min− Boiling point of Ethanol at 1 °C/min

± 0.1 °C± 0.3 °C

Sample throughput per hour approx. 30 melting pointsor approx. 10 boiling points

Required amount of sample Melting point/range: 1…3 mgBoiling point: 50…80 µl

Melting point capillaries glass, melted shut on one end;external diameter 1,3…1,5 mm,length approx. 90 mm

Boiling point tubes glass, melted shut on end;diameter 2,9…3,1 mm,length approx. 80 mm

Housing Metal, polybutylterephthalateDimensions Diameter of base area 162 mm,

height 219 mmLength of connection cable 75 cmWeight 1,9 kg