[Footnote 1: An abstract of thesis by E.A. Partridge, class of
'89, Univ. of Pa. Read before the Chemical Section of the
Franklin Institute by Prof. S.P. Sadtler.]
It is well known that the paraffine obtained by the distillation of
petroleum residues is crystalline, while that obtained directly (as in
the filtration of residuum) is amorphous. Ozokerite or ceresine
differs but slightly from paraffine, the principal distinction being
want of crystalline structure in it as found. Other characteristics,
such as the melting point, specific gravity, etc., vary in both, and
so are not of importance in a comparison. Hence it has been asked, Is
the paraffine occurring in petroleum and ozokerite identical with that
which is produced by their distillation? As crystalline paraffine
could be obtained from ozokerite by distillation alone, many persons
have supposed that it was engendered in the process. Recently,
however, crystalline paraffine has been obtained from ozokerite by
dissolving the latter in warm amyl alcohol; on cooling the greater
part separates out in crystals having the luster of mother-of-pearl.
By repetition of this process, a substance is obtained that is
scarcely to be distinguished from the paraffine obtained by
distillation. Apparently there exists then in ozokerite, together with
paraffine, other substances not capable of crystallization which keep
the paraffine from crystallizing. These colloids appear to be
separated by amyl alcohol in virtue of their greater solubility in
that menstruum. It is also reasonable to suppose that they undergo
change or decomposition by distillation.
So as petroleum residues are amorphous, and the crystalline paraffine
is first produced by distillation, it has been argued that the
paraffine present in crude petroleum is approximately the same thing
as ozokerite.
This, however, is not sufficient to establish the pyrogenic origin of
all crystallized paraffine, as crystals can be obtained from the
amorphous residues by distillation at normal or reduced pressure or in
a current of steam. To explain these facts two assumptions are
possible. Either the chemical and physical properties of all or some
of the solid constituents are changed by the distillation, and the
paraffine is changed from the amorphous into the crystalline variety,
or the change produced by the distillation takes place in the medium
(i.e., the mother liquid) in which the paraffine exists. The change
effected in ozokerite and in petroleum residues when crystalline
paraffine is obtained by distillation is to be regarded as a
purification, and can be effected partially by treatment with amyl
alcohol. In the same way, by repeated treatment of petroleum residuum
with amyl alcohol, a substance of melting point 59° C. can be
obtained, which cannot be distinguished from ordinary paraffine.
The treatment with amyl alcohol has therefore accomplished the same
results as was obtained by distillation, and the action is probably
the same, i.e., a partial separation of colloid substance. These
facts point to the conclusion that crystallizable paraffine exists
ready formed in both petroleum and in ozokerite, but in both cases
other colloidal substances prevent its crystallization. By
distillation, these colloids appear to be destroyed or changed so as
to allow the paraffine to crystallize.
It is a generally known fact that liquids always appear among the
products of the distillation of paraffine, no matter in what way the
distillation be conducted. This shows that some paraffine is
decomposed in the operation.
The name _proto-paraffine_ has been given to ozokerite and to the
paraffine of petroleum in contradistinction to _pyro-paraffine_, the
name that has been applied to the paraffine obtained by distillation
from any source.
According to Reichenbach, paraffine may crystallize in three forms:
needles, angular grains, and leaflets having the luster of
mother-of-pearl. Hofstadter, in an article on the identity of
paraffine from different sources, confirmed this statement, and added
further that at first needles, then the angular forms, and then the
leaflets are formed. Fritsche found, by means of the microscope, in
the ethereal solution of ozokerite, very fine and thin crystal
leaflets concentrically grouped, and in the alcoholic solution fine
irregular leaflets. Zaloziecki has recently developed these
microscopic investigations to a much greater extent. According to this
observer, the principal part of paraffine, as seen under the
microscope, consists of shining stratified leaflets with a darker
edge. The most characteristic and well developed crystals are formed
by dissolving paraffine in a mixture of ethyl and amyl alcohols and
chilling. The crystals are rhombic or hexagonal tablets or leaves, and
are quite regularly formed. They are unequally developed in different
varieties of paraffine. The best developed are those obtained from
ceresine. Their relative size and appearance give an indication as to
the purity of the paraffine, and, as they are always present, they are
to be counted among the characteristic tests for paraffine.
Reichenbach observed that mere traces of empyreumatic oil prevented
their formation.
The old method of determining the amount of paraffine in petroleum was
to carry out the refining process on a small scale; that is, to
distill the residue from the kerosene oils to coking, chill out the
paraffine, press it thoroughly between filter paper, and weigh the
residue. The sources of error in this procedure are manifold; the
principal one is the solubility of paraffine in oils, which depends
upon the character of both the paraffine and the oil, and also upon
the temperature. The next greatest source of error is variation in the
process of distillation and the difference between working on the
small scale and on the large scale.
In most cases, where a paraffine determination is to be carried out,
one has to deal with a mixture of paraffine with liquid oils. Now,
paraffine is not a substance defined by characteristic physical
properties which distinguish it from the liquid portions of petroleum.
It consists of a mixture of homologous hydrocarbons, which form a
solid under ordinary conditions. The hydrocarbons of this mixture show
a gradation in their properties, and gradually approximate to those
which are liquid at ordinary temperatures. It is a well known fact
that a separation of these homologues is entirely impossible by
distillation. It has also been ascertained that the liquid
constituents of petroleum do not always possess boiling points that
are lower than those of the solid constituents. This shows that we
have to deal not merely with hydrocarbons of one, but of several
series.
When determinations of the amount of paraffine are to be made, then it
becomes necessary to specify with exactness what is to be called
paraffine. The most definite property that can be made use of for this
purpose is the melting point. For several reasons it is convenient to
include under this name hydrocarbons of melting point as low as
35°-40° C.
The method proposed by Zaloziecki for the determination of paraffine
is the following: The most volatile portions of the petroleum are
separated by distillation, until the thermometer shows 200° C. These
portions are separated, as they exert great solvent action upon
paraffine. At the same time he finds that no pyro-paraffine is formed
under this temperature. A weighed portion of the residue is taken and
mixed with ten parts by weight of amyl alcohol and ten parts of
seventy-five per cent. ethyl alcohol: the mixture is then chilled for
twelve hours to 0° C. It is then filtered cold, washed first with a
mixture of amyl and ethyl alcohols, and then with ethyl alcohol alone.
The paraffine is transferred to a small porcelain evaporating dish and
dried at 110° C. It is then heated with concentrated sulphuric acid to
150°-160° C. for fifteen to thirty minutes with constant stirring. The
acid is then neutralized and the paraffine extracted by petroleum
ether. On evaporation of the solvent, the paraffine is dried at 100°
C. and weighed. Zaloziecki found, according to this method, in three
samples of Galician petroleums, 4.6, 5.8 and 6.5 per cent.,
respectively, of proto-paraffine. The method was carried out as above
with four samples of American petroleums, Colorado oil from Florence,
Col.; Warren County oil from Wing Well, Warren, Pa.; Washington oil
from Washington County, Pa.; Middle District oil from Butler County,
Pa., all furnished by Professor Sadtler.
They were very different in physical properties and in appearance, the
Colorado oil being a much heavier oil than the others and the
Washington oil being an amber oil, while the other two were of the
ordinary dark green color and consistence. The losses on distillation
to 200° C. were very different, being about one-tenth in the case of
the Colorado oil and nearly one-half in the case of the others. The
percentages of partially refined proto-paraffine in the four reduced
oils (all below 200° C. off) were as follows: for the Colorado oil,
23.9 per cent.; for the Warren oil, 26.5 per cent.; for the Washington
oil, 26.6 per cent.; and for the Middle District oil, 28.2 per cent.
The question now arises, What value has this determination of the
proto-paraffine which may exist in an oil? As before said, a portion
of the paraffine is always decomposed in distillation at temperatures
sufficiently high to drive over the paraffine oils, so the yield of
pyro-paraffine is always less than the proto-paraffine shown to be
present originally. Zaloziecki found this in the case of the several
Galician oils he examined. Corresponding to the 4.6, 5.8 and 6.5 per
cent. of proto-paraffine in the several oils he obtained 2.18, 2.65
and 2.35 per cent., respectively, of pyro-paraffine.
For the present, however, the extraction of proto-paraffine on a large
scale by means of such solvents as amyl and ethyl alcohols is out of
the question on account of their cost. A distillation, under reduced
pressure and with superheated steam, would, however, prevent much of
the decomposition of the original proto-paraffine and increase the
yield of pyro-paraffine.
This study of Zaloziecki's method and the examination of American oils
was suggested by Professor Sadtler and carried out in his laboratory.
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