J. J. BERZELIUS
The Perfection of Chemical Measurement
Jons
Jacob Berzelius was born at Vaversande, Ostergotland, in Sweden, in 1779. His father
was a teacher, but died while Berzelius was still an infant. His mother married
again, but she too died very shortly, and he was brought up by his mother's
sister, `Auntie Flora'. When she married a widower with a young family the boy
was not welcome and was sent to an uncle. At twelve he was sent to school at
Linkoping, where he largely supported himself by private tutoring. At this time
he had a great interest in natural history. But there were troubles at the
school. He was not as diligent as he should have been, and left, perhaps at the
suggestion of the school authorities. In 1796 he began medical studies at
Uppsala. He was very fortunate to be able, at least for a time, to learn
chemistry from A. G. Ekeburg, an excellent teacher and a chemist of repute, who
had discovered titanium. However, he was forced to withdraw from these studies
since he could not afford the course.
The
financial crisis was solved by his uncle who apprenticed him to a pharmacist,
and then to a physician at a health spa. During this time he learned the
techniques of quantitative analysis. Part of the mystique of the spa cures was
to advertise the composition of the minerals in the spa water. At this time his
interests were exclusively medical, and his doctoral dissertation, which he
worked up at this time, was a study of galvano‑therapy, on the uses of
electricity in medicine. In 1800 he became assistant to the Professor of
Surgery at Stockholm, but at about the same time began a series of chemical
studies in collaboration with a young mine owner, Wittisinger. In 1805 he was
appointed `physician to the poor' in East Stockholm. He evidently continued his
chemical studies during this time, since in 1807 he became Professor of
Chemistry at the Karolinska Medical Institute. His first work in this post
concerned the composition of animal products, but he soon turned to inorganic
analysis. He brought in quite new standards of rigour that transformed the
chemistry of the day. In 1832 he resigned his professorship over the refusal of
the National Education Commission of Sweden to grant the Institute full
university status. Late in life, in 1835, he married Elisabeth Poppins. By this
time Berzelius had reached international eminence, and he was created a Baron
on his marriage. Despite his great fame and the honours heaped upon him he
became very depressed in his old age. `God knows', he said, `what happens to
your time once you have begun to get old. You are busy all the time, you do
important things, you work, and yet when you sum it all up the result is
nothing.' He died in 1848.
Analytical chemistry before Berzelius
In
1810 the study of chemistry had run up against a serious inadequacy in its
empirical methods. Dalton had proposed, generalizing both brilliantly and
wildly from very rough data, that when elements combined to form compounds they
did so atom to atom, so to speak. Allowing for the differences in weight
between the atoms of distinct elements, this combining principle leads to the
hypothesis that there should be simple and fixed ratios between the amounts of
constituent elements that go to form a particular compound. The basic structure
of the reasoning behind all the analytical work of the period can be
illustrated as follows: if sodium hydroxide is formed by the combination of
clusters of atoms in which one atom of sodium combines with one of oxygen and
one of hydrogen, and sodium atoms are 23 times as heavy as hydrogen atoms,
while oxygen atoms are 16 times as heavy as hydrogen atoms, then in any sample
of the compound the weights of sodium, oxygen and hydrogen ought to be in the
ratios 23:16:1. Working backwards one ought to be able to compare a great many
compounds to guess the unit weight of the atoms of elements. Then, by dividing
the weights of each element found in an analysis of a compound by the relative
unit weight of atoms, one can find the atomic constitution of the most
elementary units of a compound. We have come to call these compound
constituents `molecules'. For instance, if one supposed that the weight of an
atom of sulphur was thirty‑four times that of an atom of hydrogen, and
found that in a sample of hydrogen sulphide 0.04 grams of hydrogen had combined
with 0.68 grams of sulphur, one could conclude by simple arithmetic that the
proportion of hydrogen and sulphur atoms in hydrogen sulphide was 2:1.
Berzelius
was greatly disenchanted with the inaccuracy and inadequacy of the methods of
analysis in use in his day. He had started to write a textbook of chemistry for
the cadets at the Military Academy and for medical students. When he tried to
bring some order and system into the existing quantitative data he found not
only confusion but downright contradiction. When results were coordinated
across a variety of compounds, inconsistencies appeared. The atomic theory, as
elaborated by Dalton, placed strict requirements on the relationships between
elements. If a given weight of an element A combines with a certain weight of
element B, and the same weight of A combines with so much by weight of element
C, then there should be a definite relationship between the weights of B and C
when they combine. They should either be in the same ratio as they each bear to
A, or some integral multiples of those weights. This allows for the possibility
of there being different numbers of atoms of B and C in combination when they
combine with each other, from when each combines with A. But Berzelius found it
impossible to make existing results of measurements of relative weight fit in
with these requirements. So began his obsession with precise measurement. He
realized by about 1810 that progress in chemistry needed a new kind of
experiment, one of meticulous, painstaking accuracy. Only then could reliable
hypotheses about the atomic constitution of compounds be arrived at. He set out
on ten years of devoted work to just this end.
Both
Dalton (the originator of the chemical version of the atomic theory) and
Wollaston (an English chemist who had pioneered quantitative chemistry) were
convinced that the proportions by weight of combining substances must be
integral ratios, such as 1:1 or 1:2 or 3:2 and so on. This followed directly
from the atomic theory, along with the assumption that the atoms of different
elements had different but constant weights. Berzelius was familiar with the
work of
these
English chemists, and he knew also of Gay‑Lussac's successful
demonstration that when gases combined chemically, they did so in integral
ratios of volumes, so that water was formed by the combination of two volumes
of hydrogen to one of oxygen. At that time, it must be remembered, the familiar
distinction between atoms and molecules had not been formulated. Contemplation
of all these matters led Berzelius to the conviction that equal volumes of
permanent gases (those which could not then be liquified) must, at the same
temperature and pressure, contain equal numbers of atoms.. There must then be a
relation between the integral ratios of volumes and the integral ratios of
weights, revealed in studies of chemical combination. This notion was later to
be incorporated in more refined form into chemistry as Avogadro's Hypothesis.
Incomplete though these ideas proved to be they were sufficient to give
Berzelius a powerful enough theoretical basis for his purposes, a theory which
foretold that combining weights must be in integral proportions. This enabled
him to formulate the idea of a 'correct' measurement.
A
measurement was correct when it gave integral proportions, for that was
required by the atomic theory. In his autobiography Berzelius notes, many times
I had to repeat my analysis with different methods to find that method which
was most certain to give the correct result', that is the result in accordance
with the atomic theory. Berzelius did not discover that the elements combined
in integral proportions. By assuming that that was indeed the way they must
combine he corrected and improved and adjusted his experimental technique until
his results were in accordance with this principle.
The analytical programme
The
secret of his success was a kind of perfectionism, an obsession with accuracy.
'My first attempts in this were not successful,' he says. 'I still had no
experience regarding the great accuracy that was needed, nor how a greater
accuracy could be obtained in the final results.' The answer to these troubles
lay in attention to detail. Equipment had to be designed so that there was as
little loss of material as possible. In reactions which required pouring the
vessels had to have lips that discharged the very last drop. Filter papers not
only had to have a standard residue of ash, but it was advisable to wet them
before they were to be used, to prevent some of the substances dissolved in the
solute being absorbed by the fibres of the paper. But above all the
manipulative technique had to be precise. It consisted in 'observing a large
number of small details which, if overlooked, often spoil several weeks of
careful work'.
Atomic
weight determinations depended on two things. It was necessary to know the
relative numbers of atoms of different elements in compounds, for instance,
whether an oxide was ZnO or Zn02 or Zn20 and so on. It was also necessary to
know the equivalent weights of the elements so combined. Knowing the relative
weights of zinc and oxygen in zinc oxide, and knowing that the atomic
composition of the oxide is one atom of zinc to one of oxygen, the relative
weights are the relative weights of the atoms of each element. Standardization
was achieved by referring all weights to that of oxygen.
The
basic method perfected by Berzelius involved oxygen compounds. These were more
common and much more easily handled than the hydrogen compounds that had been
favoured
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in England.
But the use of hydrides was fairly common, and Berzelius gives his results in
terms of both the hydrogen and the oxygen standard. One could either start with
a given amount of a metal and form the oxide, or start with the oxide and
reduce it to the metal with hydrogen. The particular technique chosen by
Berzelius depended on the ease of manipulation and the possibility of error.
The
reasoning is very simple. The combining ratio is
Weight of oxide ‑ weight of metal
weight of metal
If
the atomic proportions are known by comparison with other analyses it is a
simple matter to calculate the ratios of the atomic weights. For instance, if
the oxide consists of two atoms of oxygen to one of the metal then the above
ratio must be divided by 2.
Here
is Berzelius's description of the steps involved in finding the atomic weight
of chlorine relative to oxygen and to hydrogen. In his Treatise on Chemistry,
volume V, he says, `I established its [chlorine's] atomic weight by the
following experiments: (1) From the dry distillation of 100 parts of anhydrous
potassium chlorate, 38.15 parts of oxygen are given off and 60.85 parts of
potassium chloride remain behind. (Good agreement between the results of four
measurements.) (2) From 100 parts of potassium chloride 192.4 parts of silver
chloride can be obtained. (3) From 100 parts of silver 132.175 parts of silver
chloride can be obtained. If we assume that chloric acid is composed of 2 Cl
and 5 O, then according to these data 1 atom of chlorine is 221.36. If we calculate
from the density obtained by Lussac, the chlorine atom is 220 [relative to the
atomic weight of oxygen]. If it is calculated on the basis of hydrogen then it
is 17.735.'
The
simplicity of the reasoning and the need for careful manipulation are vividly
illustrated in this passage. To get to the final ratio between the element in
question (chlorine) and the standard (oxygen), several different ratio
determinations have to be gone through, each of which must be as accurate as
possible. Berzelius's result is in good agreement with modern determinations,
but for one thing. It is only half the modern Value. The reason lies in the way
the hydrogen standard was computed. Without the distinction between atoms and
molecules it was natural to think of hydrogen as a monoatomic gas. If one
thinks of the ultimate particles of hydrogen as atoms, single Hs, when as we
now think they are really molecules, HZS, pairs of atoms, one will be inclined
to take 2H = 1 as the standard, and this is just what Berzelius did. Correcting
the value gives us an atomic weight for chlorine of 35.47, relative to
hydrogen.
From
the point of view of scientific method it is worth noticing Berzelius's
devotion to the `intensive design'. There are two ways of gaining general
knowledge by experiments. One can study a great many samples and then find
their typical properties by some sort of averaging. This is called the
`extensive design'. Or one can take one, or at most a very few cases, and
assume that they are typical. Their properties will then be the defining
properties of all samples similar to them. This is called the `intensive
design'. As MacNevin says of Berzelius, `The selection of the proper method of
analysis seemed far more important to him than the frequent repetition of the
measurement common today . . . he seldom repeated any of it once completed and
was ready to defend its reliability.'
By
1818 Berzelius was ready to announce the atomic weights of 45 of the 49 known
elements. Throughout his life he continued to improve and extend these results.
Berzelius
was not just a superb experimenter. He developed, in much the same way as had
Davy, an electrical theory of chemical combination, but with a more detailed
and precise form. Soderbaum, quoted in Jorpes, gives Berzelius as saying,
`Atoms contain both types of electricity, these being placed at different poles
in them, but one type is dominant. Affinity is due to the effect of the
electrical polarities of the particles. Thus, all compounds are composed of two
parts, these parts differing in the nature of their electricity, and are bound
together by attraction. All compounds can therefore be divided into two
oppositely charged parts irrespective of the number of elements from which the
compound is composed.'
This
was a powerful theory. It did very well for inorganic compounds, but the
discovery that chlorine could be substituted for hydrogen, atom by atom, in
organic compounds, brought it into temporary disfavour (and brought the discoverer
of substitution, Liebig, into permanent disfavour with Berzelius!). If hydrogen
is electropositive, any atom which