i
`
`14
`
`i1
`
`4
`
`Chem. A 62485 4
`261
`.W63
`1960a
`
`LABORATORY TECHNIQUE
`ORGANIC CHEMISTRY
`K.Wiberg
`
`IN
`
`Published on demand by
`UNIVERSITY MICROFILMS
`Xerox University Microfilms, Ann Arbor, Michigan.USA.
`University Microfilms Limited, High Wycombe, England
`
`1
`
`1
`
`I
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`ft
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`amumumgmcmo
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`Emmohmwomgmm.nusxnmeNDm:.Bucmzéufixumqwu;\Eummuma3.5.33:o535?.E33.63::
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`Buooaénummzlmmmuumxugo.umEHEumzészxxauu;\umNflHuHuéESo.5955:33.
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`

`This is an authorized facsimile
`of the original book,
`and was produced in 1975 by microfilm-xerography
`by Xerox University Microfilms,
`Ann Arbor, Michigan, U.S.A.
`
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`amumumgmcmo
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`

`Laboratory Technique
`in Organic Chemistry
`
`KENNETH B. WIBERG
`Professor of Chemistry
`University of Washington
`
`McG RAW-HILL BOOK COMPANY
`New York
`Toronto
`1960
`
`London
`
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`

`Chemistry Library
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`|09(*2.49- no
`
`
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`BuooaénfimmImmmuumxugoEwflfimzésixauu;\umNflHuHuéESo.5955:33.
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`
`Emmohmwomgmm.nusxnmeNDm:.Bucmzéufixumqwu;\Eummuma3.5.33:o535?.E33.63::
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`
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`

`Harold H. Williams
`
`Theory of Molecular Exoitona
`
`W. Conard Fernelius
`
`McGRAW-HILL SERIES IN ADVANCED CHEMISTRY
`Senior Advisory Board
`Louis P. Hammett
`Editorial Board
`David N. Hume
`Gilbert Stork
`Edward L. King .
`Dudley R. Herschbach
`John A. Pople
`BAIR
`Instrumentation
`Introduction to Chemical
`BALLHAUSEN
`Introduction to Ligand Field Theory
`BENSON
`The Foundations of Chemical Kinetics
`BIEMANN
`Mass Spectrometry (Organic Chemical Applications)
`DAVIDSON
`Statistical Mechanics
`DAVYDOV
`(Trant. Kasha and Oppenheimer)
`DEAN
`Flame Photometry
`Optical Rotatory Dispersion
`DJERASSI
`ELIEL
`Stereochemistry of Carbon Compounds
`FITTS
`Nonequilibrium Thermodynamics
`FRISTROM AND WESTENBERG
`Flame Structure
`HELFFERICH
`Ion Exchange
`HILL
`Statistical Mechanics
`HINE
`Physical Organic Chemistry
`KIRKWOOD AND OPPENHEIM
`KOSOWER
`Molecular Biochemistry
`LAITINEN
`Chemical Analysis
`MANDELKERN
`Crystallization of Polymers
`McDOWELL
`Mass Spectrometry
`PITZER AND BREWER
`(Revision of Lewis and Randall)
`POPLE, SCHNEIDER, AND BERNSTEIN
`High-resolution
`Resonance
`PRYOR
`Free Radicals
`PRYOR
`Mechanisms of Sulfur Reactions
`ROBERTS
`Nuclear Magnetic Resonance
`ROSSOTTI AND ROSSOTTI
`The Determination of Stability Constants
`SOMMER
`Stereochemistry Mechanisms, and Silicon
`STREITWIESER
`Solvolytio Displacement Reactions
`SUNDHEIM
`Fused Salts
`WIBERG
`Laboratory Technique in Organic Chemistry
`
`Chemical Thermodynamics
`
`Thermodynamics
`Nuclear Magnetic
`
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`amumumgmcmo
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`Emmohmwomgmm.nusxnmeNDm:.Bucmzéufixumqwu;\Eummuma3.5.33:o535?.E33.63::
`
`
`
`
`
`
`
`\A
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`
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`BuooaénfimmImmmuumxugoEwflfimzésixauu;\umNflHuHuéESo.5955:33.
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`

`LABORATORY TECHNIQUE IN ORGANIC CHEMISTRY
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`

`LMOIATOIY TICHNIQUI
`IN OtOANIC CHIMISTtY
`Copyright © 1960 by the McGraw-Hill Book Company,
`Inc. Printed
`in the United States of America. All
`reserved. This book, or
`rights
`parts thereof, may not be reproduced in any form without permission
`the publishers. Library of Congress - Catalog Card Number 59-11950
`of
`789-MP-9
`70095
`
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`

`PREFACE
`
`Although there are a number of monographs
`available which deal
`with an aspect of the techniques required in dealing with organic com
`there has for some time been no book which gives a brief de
`pounds,
`scription of most of the important techniques. This book is written in
`an effort to fill this need and is directed mainly to the advanced un
`dergraduate or beginning graduate student who is about to undertake
`a program of research work.
`Each of the three types of matter,
`is con
`liquids, solids and gases,
`to both its properties and the methods of puri
`sidered with respect
`It is
`of the properties of the
`fication.
`felt
`that an understanding
`substances adds materially to the appreciation of the methods of puri
`fication. Methods which involve distribution between two phases are
`is examined in relation to
`then considered. Finally,
`the reaction itself
`the apparatus and techniques involved.
`In organic chemical
`the use of the proper
`laboratory technique,
`apparatus is important. A drawing of a commonly used piece of equip
`of
`ment has generally
`been provided to accompany the description
`each method. These drawings are for the most part derived from the
`working drawings used in the shops at the University of Washington,
`important dimensions are given in millimeters.
`and in most cases all
`In writing a book of this type, it is very difficult to give credit to
`r
`
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`vi
`Preface
`for a piece of equipment or to the originator of a
`a specific designer
`The art of
`laboratory work in organic chemistry has
`technique.
`evolved from the experiments and modifications of many technicians,
`and only rarely can the contribution of an individual be specifically
`recognized.
`
`Kenneth B. Wiberg
`
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`

`CONTENTS
`
`Preface
`
`Chapter 1. Liquids .
`Physical Properties of Liquids
`.
`'.
`Density. Refractive Index. Vapor Pressure. Determination of the
`Boiling Point. Determination of Molecular Weights. Effect of
`Structure on the Boiling Point. Effect of Pressure on the Boiling
`Point.
`Purification of Liquids
`Pretreatment prior to Distillation. Simple Distillation. Distillation
`under Reduced Pressure. Fractional Distillation. Types of Frac
`Fractional Distilla
`tionating Columns. Accessory
`Equipment.
`Distillation.
`Pressure. Molecular
`tion under Reduced
`Steam
`Distillation.
`
`Chapter 2. Solids
`Physical Properties of Solids
`Pressure. Melting Points of Solids. Determination of
`Vapor
`and Thermometer Corrections.
`Melting Points. Thermometers
`Melting-point Depression. Melting Points of Mixtures. Solubility.
`Purification of Solids
`Filtration. Drying.
`Inducing Crystallization.
`Reerystallization.
`Fractional Crystallization. Precipitation. Distillation. Sublimation.
`Fractional Freezing. Zone Melting.
`vii
`
`v
`
`1
`
`1
`
`20
`
`75
`
`75
`
`98
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`vllt
`Chapter 3. Oum .
`.
`Physical Properties or Gases
`Pressure. Vapor Density. Heat Capacity
`ductivity.
`Purification op Gases
`Vacuum Lines. Pumping System. Vacuum Manifold. Toepler
`Frac
`Simple Distillation.
`Lubricants.
`and
`Pump. Stopcocks
`tional Distillation. Diffusion. Purification of Gases by Chemical
`Methods.
`
`and Thermal' Con
`
`.
`
`Contents
`120
`120
`
`Chapter 4. Adsorption and Extraction
`Adsorption
`between Liquid and Solid
`Distribution
`Phases. Adsorbents.
`Standardization of Adsorbents. Effect of
`the Structure of the
`Solute on the Degree of Adsorption. Batchwise Adsorption and
`Dccolorization.
`Limitations
`Chromatography.
`Advantages
`and
`of Chromatography.
`Partition
`Paper
`and
`Chromatography
`Ion Exchange.
`Chromatography. Vapor-phase Chromatography.
`Extraction
`Simple Extraction. Continuous Liquid-Liquid Extraction. Con
`tinuous Solid-Liquid Extraction. Multiple-contact
`Pseudocounter-
`current Extraction. Countercurrent
`Extraction.
`
`Chapter
`5. The Reaction
`Apparatus
`and Stirring Motors. Addition of
`Flasks. Condensers. Stirrers
`Liquids. Addition of Solids. Addition of Gases. Heating and
`Cooling Baths. Water Separators. Apparatus for Conducting Re
`actions at High Dilution. Reactions Effected in an Inert Atmos
`phere. Semimicro Scale Preparations. Thermostats. Hydrogena-
`tion Apparatus. Ultraviolet Light Sources.
`Purification of Solvents
`Chapter 6. Literature of Synthetic Organic Chemistry
`
`Index
`
`129
`
`149
`149
`
`179
`
`191
`
`191
`
`240
`
`253
`
`257
`
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`

`Chapter 2
`
`SOLIDS
`
`PHYSICAL PROPERTIES OP SOLIDS
`The properties of a solid which are of greatest interest to an organic
`chemist are vapor pressure, melting point, and solubility. The most
`common methods of purifying and characterizing solids depend on
`these properties. Other properties such as crystal structure, density,
`and refractive index are now rarely used and will not be considered
`here.
`
`Vapor Pressure
`Solids generally have a lower vapor pressure than liquids. This re
`sults from the necessity ot supplying both the heat of vaporization and
`the heat of fusion in vaporizing a solid, whereas only the former need
`be supplied in vaporizing a liquid. Some solids, such as iodine, have a
`a process known as
`relatively high vapor pressure, which facilitates
`is transferred from a heated surface to
`sublimation. Here the material
`first being converted to a liquid. Com
`is cooled, without
`one that
`pounds which exhibit this behavior are, as will be shown in the next
`those with relatively high symmetry. This gives them a rela
`section,
`tively high melting point and high vapor pressure at
`temperatures
`below the melting point.
`The vapor pressure of a solid may be determined in much the same
`way as for a liquid. The solid is placed in a container, which is then
`evacuated. The pressure in the container is measured, and if all foreign
`75
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`76
`Laboratory Technique in Organic Chemistry
`gases have been removed and if sufficient time is allowed for equilib
`rium to be set up, the observed pressure will be the vapor pressure. If
`the vapor pressure is determined as a function of temperature by im
`mersing the container in baths having different temperatures,
`the heat
`of sublimation AH, may be determined from
`AH. r.-T.
`,0g p,
`2.303/?
`The vapor pressure-temperature
`curves for n-pentane and neopen-
`tane are shown in Fig. 2-1. 1 The break in the neopentane curve occurs
`
`BP\^ BPS.
`
`V MP
`
`\
`
`\Neopenton«
`
`MO
`too
`400
`
`aoo
`
`wo
`eo
`■0
`40
`
`to
`
`10
`
`•f
`
`4 I
`
`II
`
`-60
`-40
`Temperoturc,
`*C
`curvos for neopontane and n-pontane.
`Fig. 2-1 . Vapor prossuro-tomporaturo
`It can be seen that the vapor pressure of
`at the melting point (—16°).
`liquid neopentane is higher than that of n-pentane, and correspond
`ingly, neopentane has a lower boiling point than n-pentane. As was
`shown previously, it is generally true that the more symmetrical com
`pound has the lower boiling point.
`The most striking fact is the very high vapor pressure of solid neo
`its vapor pressure is higher than that of liquid n-pentane at
`pentane;
`'American Petroleum Institute Research Project 44, "Selected Values of Prop
`erties of Hydrocarbons," table lk, Vapor Pressures and Boiling Points, Carnegie
`Institute of Technology, Pittsburgh, Pa., Dec. 31, 1952.
`
`\
`
`\
`\ /i-Pentone
`\
`
`-80
`
`-100 -120
`
`MP
`-140
`
`1
`40
`
`1
`20
`
`1
`0
`
`-20
`
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`77
`Solids
`any temperature. This again IS a c&mmon characteristic of symmetrical
`compounds. They have a relatively high melting point and a high
`vapor pressure. Note that the melting point of w-pentane is —130°.
`Melting Points of Solids
`The melting point of a solid is defined as that temperature at which
`the solid and the liquid are at equilibrium with each other at an exter
`nal pressure of one atmosphere. Since the change in volume on melting
`is usually very small, the change in melting point with changing exter
`nal pressure is also very small and may usually be neglected unless very
`high pressure is involved. It may be seen from Fig. 2-1 that the vapor
`pressure of a liquid shows a smaller
`rate of change with temperature
`than does the vapor pressure of a solid. The liquid and the solid will
`be in equilibrium only when their vapor pressures are the same, and
`therefore the melting point corresponds to the intersection of the
`curves. The intersection, and thus the melting point, is then determined
`by the relative slopes of the two curves (which in turn are governed
`respectively) and by
`by the heat of vaporization and of sublimation,
`the intercept if the curves were extrapolated to absolute zero (gov
`erned by the entropy of vaporization and of sublimation,
`respectively).
`These enthalpies and entropies are of course a function of the struc
`ture of the compound. The thermodynamic quantities for vaporization
`were discussed previously (page 14), and it was seen that this factor
`varied in a reasonably predictable way with a change in structure. The
`for sublimation are the sum of those for
`thermodynamic quantities
`vaporization and for fusion, and it is now necessary to consider the
`latter. These are not as simple a function of the structure as is the boil
`ing point, because they depend on the crystal structure which is pos
`attractive forces which
`sible with the compound and on short-range
`operate in the crystal. Certain generalizations may, however, be made.
`In general,
`the more symmetrical compounds have higher melting
`iess symmetrical compounds. This is illus
`points than do the related,
`trated in Table 2-1. The effect of symmetry on these somewhat
`ex
`treme examples is quite profound. It may be noted that in each case the
`compound. This is
`heat of fusion is lowest for the more symmetrical
`easily understood for these cases in which polar effects are not operat
`the solid state is the van der
`ing. The energy factor which favors
`Wnals' attraction between molecules, which is stronger in the solid
`
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`78
`
`Laboratory Technique in Organic Chemistry
`Table M
`Effect of Structure on Melting Point*
`
`Compound
`
`nip
`
`e Pontine
`Neopentane
`
`Methylcyclohexane
`Cyclohexane
`
`Methylcyclopenrane
`Cyclopentane
`
`- 16.6
`-129.7
`-126.6
`+ 6.5
`-142.5
`
`- 93.9
`- 56.8
`
`&H,
`
`2.01
`0.78
`
`1.61
`0.64
`
`1.66
`0.15
`
`AS/
`
`14.0
`3.03
`
`11.0
`2.29
`
`12.67
`0.81
`
`n-Octane
`4.96
`22.91
`2,2,3, 3-Tctramcthyl-butane
`1.80
`4.82
`+ 100.7
`* Data from American Petroleum Institute Research Project
`44, "Selected Values of Properties of Hydrocarbons,"
`table lz,
`Heat and Entropy of Fusion, Carnegie Institute of Technology,
`Pittsburgh, Pa., Dec. 31, 1952.
`state than in the liquid state because the molecules are closer together
`in the former case. The interaction between molecules is less for a sym
`metrical compound, because the individual atoms of
`two adjacent
`are effectively kept farther apart than in the less symmetri
`molecules
`cal case. This may be compared with the relative extents to which
`marbles and sticks may come in contact with each other. The lowered
`interaction results in a lower enthalpy of fusion for the symmetrical
`compounds.
`The fact that increased symmetry decreases the heat of fusion would
`make the melting points of these compounds very low if it were not
`for the fact that the change in the entropy of fusion is even larger. At
`the melting point, the free energy of fusion i
`of course, zero since the
`s at equilibrium. Thus,
`(on the absolute
`the melting point
`system i
`s given by the ratio of the heat of fusion to the entropy of fu
`scale) i
`sion. The remarkably low entropy of fusion for the symmetrical com
`a result of the relatively low gain in the degree of freedom
`
`poundsi
`n going from the solid to the liquid state. The sym
`for the molecules i
`rotational degrees of freedom,
`metrical molecules have few internal
`n going to the liquid state. The un-
`and thus they have little to gain i
`for rotation of
`symmctrical molecules have much greater possibilities
`to the other and also for low
`one part of the molecule with respect
`
`s,
`
`s
`
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`

`79
`Solicis
`frequency bending vibrations involving the whole molecule. These
`motions are, of course, severely restricted in the solid, and therefore
`there is a large gain in entropy in going to the liquid state.
`The melting point of a compound represents a balance between two
`opposing factors, and therefore it will be quite sensitive to changes in
`structure, usually in a rather unpredictable way. The sensitivity to
`changes in structure i
`however, very convenient becausei
`t makes the
`melting point a useful property for characterizing compounds. Herei
`desired that compounds with similar structures should have consider
`ably different melting points.
`The melting points, as commonly determined
`(page 81), are nota
`single temperature but cover a range from the beginning of melting
`until all the material has melted. This range results from the presence
`of
`insufficient heat transfer
`to the sample, and sometimes
`impurities,
`from decomposition of the sample itself. The magnitude of the melting-
`s often used asa criterion of the purity of the compound.
`point range i
`s usually taken as indicative of
`In general, a range of from 0.3 to 0.5°i
`ranges usually indicate that the
`a fairly pure sample, whereas larger
`impure. This i
`s only a rough indication, because
`s somewhat
`sample i
`the range depends on a number of factors which are not easily taken
`into account.
`
`t
`
`s,
`
`is
`
`Determination of Melting Points,
`The most precise method of determining the melting point ofacom
`s to take a sample of the liquid form of the compound and
`pound i
`t to crystallize slowly, noting the temperature of the material as
`allowi
`a function of time. These data may be represented bya curve such as
`that shown in Fig. 2-2. The initial downward slope corresponds
`to the
`rate of cooling of the liquid. The temperature usually drops somewhat
`the first
`below the true melting point
`(supercooling), and then as
`crystals appear, the temperature rises to the melting point and contin
`ues at that temperature until all of the sample has crystallized. The sub
`sequent downward slope represents the rate of cooling of the solid. If
`the plateau will not have a downward slope, and
`the sample i
`s pure,
`s given by the temperature of the plateau. If
`the true melting point i
`the plateau will slope downward, and this will be
`the sample i
`s impure,
`considered under the effects of impurities on melting points.
`The most generally used procedure for the determination of melting
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`Laboratory Technique in Organic Chemistry
`80
`points consists of placing a small amount of the solid in a thin-walled
`capillary tube, which is then heated by an air or liquid bath, and not
`ing the temperature at which the material melts. The capillaries must
`be made so that they will have a high heat conductivity (thin wall)
`in order that only a few crystals be
`and small diameter
`(~1 mm)
`used. If too much material
`is placed in the tube, the melting point will
`less sharp because of the quantity of heat necessary to melt all
`appear
`the material.
`The capillaries are best made by drawing out carefully washed (dis
`tilled water) pyrex tubing- (~ IS to
`20 mm diameter). It is very impor
`the tubes be clean, since
`tant
`that
`inorganic materials on the inside
`surface of the tube may cause de
`composition of
`the
`the sample at
`relatively high temperatures often
`required for melting-point
`deter
`minations.
`If 3 li(luid bath is used' k is im"
`Fig. 2-2. Tlme-ceoling
`portant to maintain a steady, slow
`pure solid.
`rate of heating and to keep the
`liquid well stirred. A small beaker may be conveniently used if a
`stirrer is provided. An inexpensive stirrer may be made by attaching a
`glass rod having a propeller at the end to a small phonograph motor
`(obtainable from radio supply houses). The propeller is made by at
`taching a long (~10 in.) glass rod to the center of a short rod (~1
`in.). The latter is heated and flattened with a pair of pliers. The ends
`may then be twisted to supply the proper pitch for the propeller. The
`bath may be heated with a microburner equipped with a chimney or
`by the use of an electrically heated coil of resistance wire. Mineral oil
`is commonly used as the fluid, although silicone fluid (which may be
`and Carbowax have certain advantages.
`used at higher temperatures)
`Sulfuric acid has enough disadvantages to preclude recommending its
`use as the fluid. The heating rate should be less than 1° per min when
`the melting point is approached, but a much faster rate of heating i
`of course, desirable up to about 10° below the melting point.
`A more satisfactory
`
`apparatus is the Hershberg2
`type (Fig. 2-3).
`Ind. Eng. Chem. Anal. Ed., 8: }12 (1936).
`*E. B. Hershberg.
`
`curve for a
`
`s,
`
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`

`81
`Solids
`This provides a relatively long path between the heater and the sam
`ple, permitting the heated fluid to become completely homogeneous
`with regard to temperature variation before reaching the sample. This
`apparatus also provides a stirrer and an internal electric heater, accepts
`
`Heating wire inlets,
`one in front and
`one in bock
`
`6 feet 032
`nichrome wire
`internal
`[coiled)
`heater
`
`*26Pt wire
`for
`support
`thermometer
`and capillaries
`
`Shield, with 3
`spacers at top
`and at bottom
`
`Shield support
`( 2 indentations)
`
`0710 Silicone
`
`Fig. 2-3. Modified Hershberg melting-point apparatus.
`and gives corrected melting points di
`total-immersion thermometers,
`rectly (see the next section on thermometers). This is probably the
`reliable apparatus now available, and it should be used for the
`most
`in the
`of melting points which are to be recorded
`determination
`literature.
`For determining routine melting points, an electrically heated alumi
`
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`Laboratory Technique in Organic Chemistry
`82
`num. or copper block is very convenient. A typical block (Fig. 2-4)
`has two holes drilled at right angles on its side. One is supplied with a
`standard 1 10-volt radio pilot light with the glass face removed, and the
`second is supplied with a magnifier from a standard thermometer
`is convenient because it is mounted in a tube and
`reader. The latter
`can be moved back and forth in the hole in the block in order to focus
`on the sample. Holes are drilled in the top surface for three capillaries
`
`Holes tor capillaries,
`3mm diom
`
`Thermometer well,
`8mm diom
`
`: support rod
`
`115-v radio pilot
`light with cover
`removed, screwed
`into block
`
`Diometer reduced
`to 60mm, covered
`with asbestos —
`paper a/id wound
`with 15 ft of #26
`nichrome wire
`
`Eyepiece from
`thermometer reader
`block.
`Fig. 2-4. Melting-point
`and a thermometer. The block is electrically heated with 15 ft of 26-
`gaugc nichrome wire which is insulated from the block by asbestos
`paper. The inside of the block is blackened by using a smoky flame
`from a microburner.
`In this apparatus,
`three samples may be observed simultaneously.
`is particularly convenient for mixed melting-point
`This arrangement
`The samples are brightly displayed
`against a black
`determinations.
`background, making observation of the melting point very easy. With
`it is important to heat slowly near the melting point,
`this apparatus,
`than with liquid baths.
`is less efficient
`since heat transfer
`A number of "hot stages" for use with microscopes have been de
`signed, and these arc of considerable value when the melting point of
`
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`

`Solid*
`83
`a very small sample is to be determined.1 They have, however, little
`advantage over the more common apparatus when the usual quantities
`of material are available. Several heated blocks are available commer
`cially in which the sample is placed on a cover glass on the surface of
`the block. These are usually subject
`to considerable errors caused by
`poor attainment of equilibrium between the heated surface and the
`sample. They are sometimes convenient for routine checks on melting
`points, but they should not be used in determining melting points
`which are to be recorded in the literature.
`If one considers the amount of time usually spent in the preparation
`of a new compound, the few minutes required to obtain a good melt
`ing point seems little enough to make it inexcusable to report carelessly
`the use of the Hershberg
`determined melting points. For this purpose,
`is strongly recom
`apparatus and calibrated Anschutz thermometers
`mended.
`
`Thermometers and Thermometer Corrections
`are designed for either total
`im
`Ordinary laboratory thermometers
`immersion. One can readily see that there would be
`mersion or partial
`immersion
`a difference between these two conditions, since with total
`the
`the entire mercury column would be heated, and consequently
`mercury would have a larger volume than obtained with partial
`im
`mersion. The amount which must be added to a total-immersion ther
`mometer when it is only partially immersed is given by
`
`AT = 0.0001 54(/)(f - n)
`where AT is the correction,
`/ is the length in degrees of the mercury
`t is the observed temperature, and n is the
`column above the liquid,
`average temperature of the emergent stem.
`For most purposes,
`to use partial-immersion ther
`it is convenient
`mometers, because the corrections required at any temperature will
`regardless of which type is used, it should
`usually be smaller. However,
`for which the calibrations arc
`be calibrated against some thermometers
`known. The common practice of calibrating thermometers by using
`the melting points of known substances will often lead to considerable
`errors if the capillary method is used.
`type of ther-
`For ordinary laboratory work,
`the most satisfactory
`' VV. C. McCronc,
`Jr., "Fusion Methods in Chemical Microscopy," p. 15, Inter-
`Inc., New York, 1957.
`science Publishers,
`
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`

`M
`
`Laboratory Technique in Organic Chemistry
`
`Fig. 2-5. Antchutz
`
`thermometer.
`
`mometcr both for determining accurate melting points and for calibrat
`ing other laboratory thermometers
`is an Anschutz thermometer
`(Fig.
`2-5). These total-immersion thermometers
`are
`graduated to 0.2° and cover a range of about 50°.
`They usually come in a set of six thermometers
`which cover a range from 0 to 300°. If possible,
`should be obtained with an aux- .
`the thermometers
`iliary scale at 0° on each thermometer,
`so that
`they will meet
`the requirements of the National
`Bureau of Standards
`for certification. One set
`should be calibrated and used to calibrate other
`Anschutz
`and other
`thermometers.
`laboratory
`This operation may best be done by using the
`the
`Hershberg melting-point
`raising
`apparatus,
`temperature very slowly, and noting the tempera
`ture indications on the two thermometers
`simul
`If a partial-immersion thermometer
`is
`taneously.
`it should be calibrated under conditions
`calibrated,
`the length of
`approximating its ultimate use (i.e.,
`the stem which is immersed and the temperature
`of the stem).
`Another type of thermometer
`is the Beckmann
`(Fig. 2-6). Beckmann thermometers
`thermometer
`are graduated in 0.001° intervals and cover a range
`for determining
`from 3
`to 6°. They are useful
`changes in temperature very precisely but do not
`temperatures. They are supplied
`give absolute
`with an auxiliary mercury supply so that the tem
`Fig. 2-6. Beckmann
`range of
`the
`thermometer may
`be
`perature
`thermometer.
`changed by adding or removing some mercury
`from the main reservoir. They are commonly used for determining
`boiling-point elevations and melting-point depressions.
`Thermometers are not the only useful
`temperature-measuring
`
`de-
`
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`

`85
`and the related ther
`
`r--W\,
`
`15 «
`\\
`
`$^A<—Wv
`\—vwyvwvwwv —
`
`Solids
`resistance thermometers,
`vices. Thermocouples,
`mistors are becoming increasingly widely used.
`are constructed by joining two dissimilar metals.
`Thermocouples
`Such a junction will produce a potential which is proportional to the
`temperature. These couples are almost always used in pairs and con
`nected to oppose each other's potential. One is kept at a fixed tem
`perature (usually the melting point of ice), and the other is used as
`the two
`element. The potential
`the temperature-sensing
`between
`couples, measured with a potentiometer, will be proportional
`to the
`temperature difference between the reference
`and the
`temperature
`observed temperature.
`Thermocouples have several ad
`Be-
`vantages over
`thermometers.
`' cause they are small in size and flex
`into
`be inserted
`they can
`ible,
`for
`places which are
`impractical
`Also,
`thermometers.
`the temper
`ature may be observed at a point
`remote from the sensing element,
`and this is particularly convenient
`for measuring the temperature
`at
`the head of a large fractionating
`column or in the jacket of such a
`column.
`One of the main deterrents
`to
`in organic laboratory work
`the wider application of thermocouples
`has been the cost of commercial potentiometers. This may in part be
`circumvented in two ways. First, one may measure the current sup
`having a given resist
`plied by the thermocouple to a microammeter
`ance. This is not very satisfactory since the current for any temper
`ature is a function of the resistance of the thermocouple and that of
`In addition, the meters commonly used are not sufficiently
`the meter.
`sensitive. This method is often used for rough work such as determin
`ing the temperature in a heating mantle.
`A better arrangement utilizes a Hclipot (a 10-turn potentiometer),
`It may
`which is both comparatively inexpensive and quite accurate.
`be read to one part per thousand. A typical circuit is shown in Fig.
`2-7. The reference thermocouple is inserted into a small glass
`tube
`
`ThorstocoMpJo
`Fig.
`2-7.
`fa, 2SO
`polontlomotor.
`fa. SO
`10-turn Holleotr
`variobto mlihv
`(fin*
`fa. 1000 var
`control)}
`iable
`resistor
`(errors*
`fa,
`400 O
`control))
`tol-
`wlro woundj M,
`vanomotor.
`
`To thermocouple
`
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`

`86
`Laboratory Technique In Organic Chemistry
`which is then placed in an ice-water mixture kept in a small Dewar,
`and the measuring couple is heated to the boiling point of water. For
`a range of 0 to 200°, the Helipot is set at the number given in Table
`2-2, which depends on the atmospheric
`pressure. The potentiometer
`circuit is balanced by using the resistors R2 and Rs and observing the
`null with the galvanometer. The potentiometer will
`then read five
`times the observed temperature and will be accurate to about ±0.5°,
`from 0 to 150°, depending on the sensitivity of the galvanometer.
`
`Table 2-2
`Settings for Thermocouple
`Potentiometer
`Atmospheric
`pressure, mm
`740
`750
`760
`770
`780
`
`Setting
`496
`498
`500
`502
`504
`
`Table 2-3
`Potential of Chromel-Alumel
`(In millivolts)
`
`Thermocouples*
`
`•c
`
`0
`100
`200
`300
`400
`
`0
`
`0.00
`4.10
`B.I)
`12.21
`16.39
`
`10
`
`20
`
`30
`
`40
`
`fO
`
`60
`
`70
`
`0.40
`4.51
`8.53
`12.62
`16.82
`
`0.80
`4.92
`8.93
`13.04
`17.24
`
`1.20
`5.33
`9.34
`13.45
`17.66
`
`1.61
`5.73
`9.74
`13.87
`18.08
`
`2.02
`6.13
`10.15
`14.29
`18.50
`
`2.43
`6.53
`10.56
`14.71
`18.93
`
`2.85
`6.93
`10.97
`15.13
`19.36
`
`N
`
`3.26
`7.33
`11.38
`15.55
`19.78
`
`90
`
`3.68
`7.73
`11.80
`15.97
`20.21
`
`* Data from the Natl. Bur. Standards Research Papers 767, 768, and 1278.
`
`The most common thermocouple uses a chromel-alumel
`junction,
`change per unit
`tem
`since this gives a
`reasonably
`large potential
`perature change (Table 2-3) and is quite stable and quite reproducible.
`The junction may be made by tightly winding together about % in.
`of the ends of the two wires (well cleaned with sandpaper) and dip
`ping them in some slightly moistened borax. The junction is then
`heated in a hot bunsen flame until the wires are fused, with the borax
`acting as a flux.
`In order to obtain consistent
`
`it is advisable to have three
`
`results,
`
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`

`87
`Solicit
`junctions, all made in the same manner. The reference junction and
`the measuring junction would be between the two different metals,
`and the third junction would be between two similar metals. The
`reference junction and' the last junction would be maintained at the
`same temperature (Fig. 2-8).
`The potential
`from the thermocouple may also be used to supply
`a signal to an electronic recorder. This is particularly convenient with
`for one can then follow the course of the dis
`distillation equipment,
`tillation without being present constantly.
`In some applications,
`it is desired to determine the difference be
`tween two temperatures. A typical case would be the adiabatic opera
`tion of the jacket of a fractionating column. Here, it is desired to main-
`
`"V Hot junction
`
`Metal A
`
`Metal B
`
`Metal B
`
`To potentiometer
`
`C
`
`^ }
`Metal A
`Fig. 2-8. Connection of thermocouple wires.
`
`Metal A
`
`Cold junction
`
`tain the temperature of the heating jacket equal to that of the material
`in the column. If one couple is suspended in the jacket and the other
`is attached to the wall of the inside tube of the column and insulated
`from the heat supply, then the potential between the couples, as de
`tected with a galvanometer, would give the temperature difference.
`If this were maintained at zero,
`the column would then be operating
`adi