The Characterization of Chemical Purity. Organic Compounds

170 Pages·1971·6.956 MB·English

by  L. A. K. Staveley (Auth.)

COMMISSION ON PHYSICOCHEMICAL
MEASUREMENTS AND STANDARDS
Chairman: D. R. Stull
Vice-Chairman and Secretary: E. F. G. Herington
Titular Members
I. Brown Y. Mashiko
J. Franc W. W. Meinke
H.Kienitz I. I. Novikov
Associate Members
J. P. Cali L. A. K. Staveley
A. Juhasz S. Sunner
W. Simon J. Terrien
National Representatives
R. P. Graham (Canada) M. Milone (Italy)
H. Feuerberg (Germany) W. M. Smit (Netherlands)
J. N. Mukherjee (India) T. Plebanski (Poland)
Observer
G. Waddington
INTERNATIONAL UNION OF
PURE AND APPLIED CHEMISTRY
PHYSICAL CHEMISTRY DIVISION
COMMISSION ON PHYSICOCHEMICAL MEASUREMENTS
AND STANDARDS
THE CHARACTERIZATION OF
CHEMICAL PURITY
ORGANIC COMPOUNDS
L. A. K. STAVELEY
The Inorganic Chemistry Laboratory
University of Oxford
LONDON
BUTTERWORTHS
1971
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Published as a supplement to
Pure and Applied Chemistry
Suggested U.D.C, numbers: 543*5 : 547
©
International Union of Pure and Applied Chemistry
1971
ISBN 0 408 70145 5
Printed in Great Britain by Page Bros. (Norwich) Ltd., Norwich
PREFACE
The suggestion that the IUPAC Commission on Physicochemical Measure­
ments and Standards should organize the production of a monograph on
chemical purity was made by Professor Stig Sunner and Dr Guy Waddington
at a meeting of the Commission (then known as the Commission on Data and
Standards) held in London in 1963 under the Chairmanship of Dr Edward
Wichers. At the same meeting I was elected to succeed Dr Wichers as
Chairman of the Commission, and I agreed to edit the monograph. Detailed
plans were drawn up which were discussed when the Commission met again
in 1965 in Paris, and by the time of the next meeting in Prague in 1967 a
substantial part of the book had been written. A small Committee was
appointed to deal with the final stages of its preparation, which consisted of
the present Chairman (Dr D. R. Stull), Dr E. F. G. Herington, Dr W. M.
Smit, and myself. This Committee met at Oxford in July 1968.
I would like to thank all those members of the Commission on Physico-
chemical Measurements and Standards who in one way or another have
helped in this undertaking, as well as those authors who are not members of
the Commission. The contributors wish to express their gratitude to Dr I. J.
Lawrenson, Dr C. S. G. Phillips, Dr J. R. Rands and Dr Guy Waddington
for their helpful comments, criticism and advice.
L. A. K. STAVELEY
The Inorganic Chemistry Laboratory
Oxford, 1971
v
INTRODUCTION
L. A. K. STAVELEY
Inorganic Chemistry Laboratory, Oxford University, Oxford, England
There can be few chemists who at some stage or other of their working
lives do not have to ask themselves what degree of purity of a particular
compound is adequate for their purpose, and what are the most suitable
criteria for deciding whether the required standard of purity has in fact been
reached. To some chemists, the determination of purity may be an end in
itself, as for example in laboratories where standard reference substances are
prepared and their physical properties determined. To many others, the
characterization of the purity of the substances they are working with may
be a tiresome operation of little intrinsic interest, but they recognize that it
is necessary if the value of their work is to be properly appreciated and
assessed. It is hoped that this publication will be of some value to all chemists
who, for whatever reasons, have to concern themselves with the problem of
chemical purity.
It seemed to the members of the Commission on Physico-chemical Data
and Standards of IUPAC that for several reasons their Commission was a
particularly suitable body to organize the preparation of a Monograph on
chemical purity. One of its first members, Jean Timmermanns, was a pioneer
in placing the determination of the purity of substances on a sound physico-
chemical basis and in raising the standards of chemical purity, partly by his
own experimental researches and partly by his books Chemical Species1
and Physico-chemical Constants of Pure Organic Compounds2. Furthermore,
what are in effect national standardizing laboratories have a particular
interest in problems of purification and purity. It was clearly desirable to
draw on the experiences of as many such institutions as possible, and the
Commission is well placed to do this since its members include representatives
of several such laboratories. The Commission has also been able to seek the
advice of members of allied Commissions of the Division of Physical Chemi­
stry, notably those on Molecular Structure and Spectroscopy and on Thermo­
dynamics and Thermochemistry, as well as of the various Commissions of the
Division of Analytical Chemistry. Moreover, the Commission has already
been involved in two successful enterprises concerned with the characteriza­
tion of purity. It organized the International Symposium on Purity Control
by Thermal Analysis held at Amsterdam in 19573, and then appointed a
Committee to implement the recommendation made at the Symposium
that there should be a co-operative determination by thermal methods of the
purity of controlled samples. This led to co-ordinated experiments on samples
of benzene of varying degrees of purity, involving twenty groups of workers
1
C.C.P.—B
L. A. K. STAVELEY
in six different countries. The results of these experiments were discussed at a
Symposium in Ottawa in 1961.
The Commission has had to consider whether to produce a Monograph
dealing primarily with the characterization of purity, or whether the under­
taking should be a more comprehensive one, leading to the publication of a
work treating both the characterization of purity and methods of purification.
While these two subjects are so related that it is almost impossible to write
sensibly about one of them without making at least some reference to the
other, it was decided to limit this Monograph essentially to the characteriza­
tion of purity, primarily because of the greater chance of completing this less
ambitious project within a reasonable time. (If it later appears that the
publication of this Monograph has served a useful purpose, the members of
the Commission will feel encouraged to consider the preparation of a similar
work on purification methods.) It was then necessary to consider whether
the Monograph should deal with all known methods of purity determination
or concentrate on techniques suitable for certain types of substance only.
Great importance now attaches to the preparation in an extremely pure
condition of certain solid elements and of relatively simple inorganic solids.
The methods used in these preparations are such that they tend to be the
concern of specialist laboratories and manufacturers, and the chemist who
needs, for example, germanium containing more than 99-999 per cent Ge
will almost certainly buy his sample and rely on the supplier's statement about
its purity. The object of this Monograph is rather to help those who have
either to prepare for themselves the substances on which they intend to work
or at least have to purify the best materials commercially available, and who
then have to carry out further experiments to assess the purity of their final
products. Generally such materials are organic compounds or comparatively
volatile inorganic compounds, and the Commission decided to restrict this
Monograph to techniques applicable to such substances, and for the time
being at any rate not to deal with methods better suited to involatile inor­
ganic solids.
Even with these limited objectives it would still be possible to treat the
relevant experimental methods in great detail and so produce a book of
considerable size. We have refrained from doing this chiefly because we hope
that this Monograph will be read especially by those who feel they should
know more about the determination of purity, but who can only devote a
limited amount of time and mental effort to the subject since their main
activities lie in other fields. The Commission considered that the interests of
such chemists would be better served by presenting them with summarized
accounts of the various methods for characterizing purity, and in particular
with accounts which consider objectively the advantages and disadvantages of
each method. Reference to the original literature shows that the choice of
criteria of purity is not always made on the most scientific grounds. Some­
times it appears to depend on what apparatus happens to be available at the
time, and sometimes on a parochial devotion to a favourite method. Thus,
one chemist or group of chemists in a series of papers can show an apparent
preference for characterizing purity by measurement of just one or two
physical properties, for example, density and refractive index. The Commis­
sion has neither the wish nor the mandate to attempt to arbitrate between the
2
INTRODUCTION
claims of different ways of estimating purity, but it considered that it would
be useful to present a dispassionate analysis of the merits and limitations of
the various methods which will enable chemists to decide for themselves for
any particular substance what are the best criteria to apply. Where it has
been thought desirable in the interests of brevity to omit experimental detail,
references have been supplied which should lead the reader to the information
he needs for the practical application of the method under discussion.
It used to be said that physicists make accurate measurements on impure
substances, while chemists make inaccurate measurements on pure sub­
stances. If this was ever true, it has become less so with the passage of time
and with the gradual disappearance of the dividing line between physics and
chemistry. Unfortunately, it is still often difficult to decide if measurements
(whether made by physicists or chemists) have in fact been carried out on
samples of an acceptable degree of purity, simply because the information
supplied is insufficient or incomplete, 01 because unsuitable or insensitive
criteria were applied. Sometimes the evidence for purity is so brief as to be
virtually worthless, as in a recent paper in a reputable journal, on the semi­
conducting properties of some organometallic compounds, where the only
statement made about the purity of the materials was simply that they were
'analytically pure'. It is also commonly found that the values of properties
like melting-point, boiling-point and refractive index are quoted as evidence
of purity with no information about the precision with which they have been
made, or about the apparatus or equipment used to measure them. Moreover,
properties like boiling-point and refractive index are frequently measured
under non-standard conditions which makes comparison with other literature
values unnecessarily laborious. Thus, it is deplorable that when the boiling-
point of an unassociated liquid has been determined at a pressure close to,
but not actually at, the standard atmosphere, such a boiling-point is still
usually recorded as that at say 770 mm Hg, when the small correction needed
to give the value at the standard atmosphere could usually have been evalua­
ted with sufficient accuracy. Similarly, the value of much density and refrac­
tive index data would be enhanced if those who made the determinations
would take the trouble to control the temperature of the sample. In a recent
paper which dealt with certain properties of five organic liquids, the refrac­
tive indices of four of these had been measured as an indication of their
purity, but at a different temperature for each liquid, which in no instance was
a rounded temperature such as 25°C or 20°C.
It is hoped that those who referee original papers will pay more attention
to points such as these, and that at the same time some authors will be helped
by this Monograph to avoid the friction which might otherwise arise by
referees questioning the purity of their materials.
In this introduction, the word purity has been used repeatedly, but no
attempt has yet been made to say precisely what it means. For the molecular
solids and liquids with which this Monograph is concerned, one might postu­
late that an absolutely pure sample of an element or compound is one all the
molecules of which are identical. This, for many substances at least, must
represent an unattainable ideal, and leave quite open the question of what
steps should be taken for any particular specimen to find how nearly the ideal
has been approached. Moreover, even if every single molecule of the specimen
3
L. A. K. STAVELEY
is of the same chemical species, it would still be possible for the properties to
vary from sample to sample, owing to variability in isotopie composition.
This is not a purely academic point, since some modern purification methods
are efficient enough to effect some degree of isotopie separation, and in any
case the isotopie composition of elements can vary with their source. Presum­
ably, therefore, the ideal standard sample would not only be a specimen for
which every single molecule belongs to the same chemical species, but also
one which has some definite, if arbitrarily prescribed, isotopie composition.
For nearly all practical purposes, however, variations in isotopie abundances
can be ignored. The conventional way of expressing purity is then in fact
based on the above definition by stating as a percentage the number of moles
of the specimen which consist of the major component. Thus, if a compound is
stated to be 99-992 moles per cent pure, it means that 8 out of every 105
molecules are molecules of some impurity.
An apparently more practical definition of a pure substance is one the
properties of which are no longer changed by subjecting it yet again to the
procedures used to purify it. The quality of the final sample will depend
on the precision with which the relevant properties can be measured, and
with the effect that the likely impurities will have on these properties. Thus,
since the lower pyridine bases all have refractive indices between 1*50 and
1-52, measurement of this property cannot be a very sensitive criterion of the
extent to which, say, one of the picolines has been freed from its isomers.
(On the other hand, it might be expected to provide a much better indication
of freedom from water, with a refractive index of 1-33. In fact, careful studies
of the effect of water on the refractive index of pyridine and its methyl homo­
logues have been made. While small amounts of water decrease the refractive
indices of pyridine and ß- and y-picoline, they have no effect on that of a-
picoline, and up to concentrations of approximately one per cent actually
increase that of 2:6-lutidine. So in spite of the large difference in refractive
index between water and the pyridine base, measurement of this property
for slightly moist a-picoline or 2:6-lutidine would tell one nothing about the
water content4.) The question of the choice of properties to be monitored as
the purification proceeds is a very important one, and ought to be considered
in relation to the experiments for which the final specimen is to be used. Some
chemists, for example, seem to attach particular value to the description
'spectroscopically pure'. But as applied to, say, cyclohexane, it is well known
that this term simply means that the sample is free from the olefines and
aromatics which absorb in the ultra-violet, and such cyclohexane will usually
contain paraffins and alicyclic hydrocarbons such as methylcyclopentane.
Some samples of 'spectroscopically pure' cyclohexane can in fact freeze as
much as two degrees below the freezing-point of the pure compound, and
would clearly be quite unsuitable for many purposes without further purifica­
tion. It is high time that the expression 'spectroscopically pure' vanished from
the chemical literature, as being as valueless as 'chemically pure' and 'analyti­
cally pure'. As a rather different example of the need to bear in mind the
ultimate purpose for which the substance has been purified, it has been
pointed out that a specimen of water may have been de-ionized to a degree
which makes it completely acceptable for conductivity work, and yet the
sample can be contaminated with grease to an extent which makes it quite
4
INTRODUCTION
unsuitable for surface tension measurements5. Many illustrations of this
kind could be given, and it is evident that if apparent constancy o fa physical
property is to be the criterion of purity, the property should if possible be
related to the purpose for which the specimen is to be used. It is also much
more convincing to demonstrate the constancy of two or even three properties
rather than just one. Sometimes the only evidence quoted for the purity of an
organic compound is the analysis for the component elements other than
oxygen, (perhaps because one can pay to have such an analysis done by
someone else). It is therefore worth pointing out that appreciable amounts
of impurities which are chemically similar to the main constituent may only
change the analytical figures slightly. Thus, the presence of one mole per cent
of a toluic acid in benzoic acid would only alter the carbon percentage by
002 and the hydrogen percentage by 0Ό1. Discrepancies between the observed
and theoretical percentages ten times and more larger than these are usually
passed over by authors without comment.
More than one title might be given to this Monograph, but one which
associated the words 'purity' and 'determination' would perhaps not be
particularly apposite since the purity of a chemical substance of the sort we
are considering is seldom examined in an experiment which gives directly the
moles per cent of the major constituent. Such an experiment may occasionally
be useful and feasible for a grossly impure sample, but it is clearly much less
so for a specimen of purity of say 99 moles per cent or better, and it is rather
this latter situation which we have in mind. The techniques described in this
Monograph are in fact much more directed to the determination of impurities
in the specimen. The method may give the total amount of the impurities
with no indication of their identity, or it may give information about the
amount and nature of the individual contaminants. An example of the first
type of technique is thermal analysis, the potentialities of which, as already
mentioned, have been thoroughly explored in an extensive series of experi­
ments on samples of benzene of controlled degrees of purity. On the other
hand, methods which in effect separate the impurities from the main constitu­
ent and also from each other, such as Chromatographie and mass-spectro-
metric techniques, are examples of the second type of method which can
furnish more detailed information.
Another way of looking at the various methods of characterizing chemical
purity is that some of them are absolute in the sense that in principle they
only require experiments on the sample itself, while others are comparative
and demand that there should be available standard samples of known purity.
There is increasing interest in the U.S.A., in Europe, and in Japan in standard
and reference substances, and it is important that all chemists interested in
considerations of purity should know of these activities, of the steps being
taken to co-ordinate them, and of the availability of standard samples at the
present time. These important topics are dealt with in this Monograph in
the chapter on the use and availability of standard samples.
It is hoped that this publication will be of some use not only to those
engaged in pure research but also to those concerned with the manufacture of
chemical substances. The latter have their own reasons for being interested
in purity, since the chemical industry itself now frequently needs starting
materials which are almost free from contaminants. But there is also the point
5
L. A. K. STAVELEY
that those engaged in pure research regret the time spent in purifying the
materials they need, and so they are usually willing to pay more for samples
which have already been adequately purified. It has of course been possible for
many years to pay higher prices for superior samples and to be given some
information about the impurities in the specimen purchased. Unfortunately,
this information is often not as helpful as it might be. For example, for an
organic compound it may take the form of figures for the very small amounts
of inorganic impurities, while being so imprecise about the melting range that
nothing quantitative can be inferred about the probable concentration of
organic contaminants. There would seem to be room for improvement here.
Finally, commercial laboratories which undertake analyses, and industrial
laboratories which serve manufacturing plants, should generally be well
equipped with the apparatus to carry out the techniques dealt with in this
Monograph. These laboratories could provide their customers with a thorough
characterization-of-purity service.
References
U. Timmermanns, Chemical Species, (Trs. R. E. Oesper), MacMillan, London, 1941.
2J. Timmermanns, Physico-Chemical Constants of Pure Organic Compounds, Elsevier,
New York, 1950.
^Purity Control by Thermal Analysis, (W. M. Smit, Ed.,) Elsevier, Amsterdam, 1957.
4D. P. Biddiscombe, E. A. Coulson, R. Handley and E. F. G. Herington, J. Chem. Soc.
1954 (1957).
5E. F. G. Herington, Mass Spectrometry, p. 157. The Institute of Petroleum, London, 1952.
6