[Footnote 1: This substance is also known by the name Schweinfurt green.]
By ROBERT GALLOWAY, M.R.I.A.
The poisonous effects of wall-paper stained with emerald-green
(aceto-arsenite of copper) appears to be a very favorite topic in many
journals; it is continually reappearing in one form or another in
different publications, especially medical ones; there has recently
appeared a short reference to it under the title, "The Poisonous Effect of
Wall-paper." As some years ago I became practically acquainted with its
properties and manufacture, a few observations on these subjects may not
be without interest.
In the paragraph referred to, it is stated that the poisonous effect of
this pigment cannot be _entirely_ due to its mere mechanical detachment
from the paper. This writer therefore attributes the poisonous effects to
the formation of the hydrogen compound of arsenic, viz., arseniureted
hydrogen (AsH_{3}); the hydrogen, for the formation of this compound,
being generated, the writer thinks probable, "by the joint action of
moisture and organic matters, viz., of substances used in fixing to walls
papers impregnated with arsenic." In some of our chemical manuals, Dr.
Kolbe's "Inorganic Chemistry," for example, it is also stated that
arseniureted hydrogen is formed by the _fermentation_ of the starch-paste
employed for fastening the paper to the walls. It is perfectly obvious
that the fermentation of the starch-paste must cease after a time, and
therefore the poisonous effects of the paper must likewise cease if its
injurious effects are caused by the fermentation. I do not think that
arseniureted hydrogen could be formed under the _conditions_, for the
oxygen compound of arsenic is in a state of combination, and the compound
is in a dry solid state and not in solution and the affinities of the two
elements--arsenic and hydrogen--for each other are so exceedingly weak
that they cannot be made to unite directly except they are both set free
at the same moment in presence of each other. Further, for the formation
of this hydrogen compound by the fermentation of the starch, or by the
growth of minute fungi, the _entire_ compound must be broken up, and
therefore the pigment would become discolored; but aceto-arsenite of
copper
(3CuAs_{2}O_{4}+Cu(C_{2}H_{3}O_{2})_{2})
is a very stable compound, not readily undergoing decomposition, and is
consequently a very permanent color. It has also been not unfrequently
stated that the injurious effects of this pigment are due to the arsenious
oxide volatilizing from the other constituents of the compound. This
volatilization would likewise cause a breaking up of the entire compound,
and would consequently cause a discoloration of the paper; but the
volatilization of this arsenic compound is in every respect most
improbable.
The injurious effects, if any, of this pigment must therefore be due to
its mechanical detachment from the paper; but has it ever been
conclusively proved that persons who inhabit rooms the wall-paper of which
is stained with emerald-green suffer from arsenical poisoning? If it does
occur, then the effects of what may be termed homoeopathic doses of this
substance are totally different from the effects which arise from larger
doses. During the packing of this substance in its dry state in the
factory, clouds of its dust ascend in the air, and during the time I had
to do with its manufacture I never heard that any of the factory hands
suffered, nor did I suffer, from arsenical poisoning. If there is any
abrasion of the skin the dust produces a sore, and also the delicate
lining of the nostrils is apt to be affected. It is in this way it acts in
large doses; I am therefore very skeptical as to its supposed poisonous
effects when wall-paper is stained with it.
Different methods are given in works on chemistry for the manufacture of
this pigment, but as they do not agree in every respect with the method
which was followed in English color factories some years ago, it will be
as well, for the full elucidation of the manufacture of this substance, to
briefly recite some of these methods before describing the one that was,
and probably is still, in use; and I will afterward describe a method
which I invented, and which is practically superior to any other, both in
the rapidity with which the color can be formed, and for producing it at a
less cost.
It is stated in Watts' "Dictionary of Chemistry" that it is "prepared on a
large scale by mixing arsenious acid with cupric acetate and water. Five
parts of verdigris are made up to a thin paste, and added to a boiling
solution of 4 parts or rather more of arsenious acid in 50 parts of water.
The boiling must be well kept up, otherwise the precipitate assumes a
yellow-green color, from the formation of copper arsenite; in that case
acetic acid must be added, and the boiling continued a few minutes longer.
The precipitate then becomes crystalline, and acquires the fine green
color peculiar to the aceto-arsenite." I do not know from personal
knowledge, but I have always understood that the copper salt employed in
its manufacture in France is the acetate. This would account, in my
opinion, for the larger crystalline flakes in which it is obtained in
France than can be produced by the English method of manufacturing it.
Cupric acetate is never employed, I believe, in England--the much cheaper
copper salt, the sulphate, being always employed.
In "Miller's Chemistry" it is stated it "may be obtained by _boiling_
solutions of arsenious anhydride and cupric acetate, and adding to the
mixture an equal bulk of _cold_ water." Why it should be recommended to
add _cold water_, I am at a loss to understand.
In Drs. Roscoe and Schorlemmer's large work on "Chemistry," and in the
English edition of "Wagner's Handbook of Chemical Technology," edited by
Mr. Crookes, the process as described by Dr. Ehrmann in the "Ann. Pharm.,"
xii., 92, is given. It is thus stated in Wagner's work: "This pigment is
prepared by first separately dissolving equal parts by weight of arsenious
acid and neutral acetate of copper in boiling water, and next mixing these
solutions while boiling. There is immediately formed a flocculent
olive-green colored precipitate of arsenite of copper, while the
supernatant liquid contains free acetic acid. After a while the
precipitate becomes gradually crystalline, at the same time forming a
beautiful green pigment, which is separated from the liquid by filtration,
and after washing and carefully drying is ready for use. The mode of
preparing this pigment on a large scale was originally devised by M.
Braconnot, as follows: 15 kilos. of sulphate of copper are dissolved in
the smallest quantity of boiling water, and mixed with a boiling and
concentrated solution of arsenite of soda or potassa, so prepared as to
contain 20 kilos. of arsenious acid. There is immediately formed a dirty
greenish-colored precipitate which is converted into Schweinfurt green by
the addition of some 15 liters of concentrated wood-vinegar. This having
been done, the precipitate is immediately filtered off and washed."
As I have already stated, the copper salt used in the manufacture of this
pigment in England is the sulphate, and it is carried out pretty much
according to Braconnot's method as described by Dr Ehrmann; but any one
would infer, from reading his description of the manufacturing process,
that the compound, aceto-arsenite of copper, was formed almost immediately
after the addition of the acetic acid, a higher or lower atmospheric
temperature having no effect in hastening or retarding the formation.
Furthermore, it is not stated whether the compound forms more readily in
an acid or neutral solution, or whether it can or cannot be formed in a
neutral one; now both these points are important to notice in describing
its manufacture. As regards the former I shall notice it presently, and,
as far as my knowledge extends, the pigment will not form when the
solution is neutral.
The operation is conducted in the following manner in the factory: The
requisite quantity of sulphate of copper is placed in a large wooden vat,
and hot water added to dissolve it; the requisite quantity of arsenic
(arsenious anhydride) and carbonate of soda, the latter not in quantity
quite sufficient to neutralize the whole of the sulphuric acid set free
from the sulphate of copper on the precipitation of the copper as
arsenite, are placed in another wooden vessel; water is then added, and
the formation of the arsenite of soda and its solution are aided by the
introduction of steam into the liquid. When complete solution has been
effected the arsenic solution is run off into the vat containing the
solution of the sulphate of copper, arsenite of copper being at once
precipitated. The necessary quantity of acetic acid is afterward added. In
_warm_ weather the formation of the aceto-arsenite soon commences after
the addition of the vinegar; but, even in that case, it takes a week or
more to have the whole of a big batch of arsenite converted into the
aceto-arsenite; and perfect conversion is necessary, as the presence of a
very minute quantity of unchanged arsenite lowers very much the price of
the emerald pigment, and a by no means large quantity renders the pigment
unsalable, owing to its dirty yellowish-green color. In cold weather a
much longer time is required for its complete conversion; even at the end
of a fortnight or three weeks there frequently remains sufficient
unconverted arsenite to affect seriously the selling price of the color;
when this occurs the manufacturer generally removes these last traces by a
most wasteful method viz, by adding a quantity of free sulphuric acid. The
acid of course dissolves the arsenite, but it dissolves in very much
larger quantities the aceto-arsenite; and this costly solution is not
utilized, but is run into the factory sewer.
By my method of manufacturing it, it can be produced in winter as well as
in summer in one or two hours, and the quantity of free acid required for
its formation is reduced to the lowest amount. I proceed as follows: After
having dissolved in hot water the requisite quantity of cupric sulphate, I
decompose one-fourth of this salt by adding just sufficient of a solution
of carbonate of soda to precipitate the copper, in that quantity of the
sulphate, as carbonate. I then add just sufficient acetic acid to convert
the carbonate into acetate. I have now got in solution--
3CuSO_{4} + Cu(C_{2}H_{3}O_{2})_{2},
and I have to transform it into--
3CuAs_{2}O_{4} + Cu(C_{2}H_{3}O_{2})_{2}.
It is at once seen that I have got the requisite quantity of acetate
formed. I next dissolve the requisite quantity of arsenious anhydride in
an amount of carbonate of soda _rather less_ than is sufficient to
neutralize the acid in the remaining cupric sulphate, and I then bring the
solution to or near the boiling-point by introducing steam into it; the
arsenic is dissolved not in the same vessel as the copper salt, but in a
separate one. When the arsenic solution is fully heated, a small current
of it is allowed to flow into the vat containing the copper salts, and
brisk stirring is kept up in the vat. The emerald green is at once formed;
but if there should be the slightest formation of any arsenite, the flow
of the arsenic solution is at once stopped until every trace of the
arsenite has been converted; the arsenic solution is then allowed to flow
in again, with the same precautions as before; in this way a large batch
of emerald-green can he formed in one or two hours, without containing the
slightest trace of the arsenite. I keep the arsenic solution near the
boiling-point during the whole of the time it is flowing into the other
vessel. By varying the proportions of water I could either make it coarse
or fine, as I wished, which is an important matter to have complete
control over in its manufacture.
Two points of interest occurred to me during the time I was occupied with
the research, which I had not time to complete; one was whether the
aceto-arsenite can be formed, adopting the old method for its formation,
if there is more than a certain quantity of water; from some experiments I
made in this direction I was inclined to the opinion it could not. I have
already stated that emerald-green is soluble to a certain extent in acids,
and that it is formed in a more or less acid solution; consequently a
varying amount of the pigment is always lost by being dissolved in the
supernatant liquid. To prevent to a certain extent this loss I
precipitated the copper from it as arsenite; but I was not successful in
the few experiments I had time to make on this part of the subject of
reconverting the copper arsenite thus obtained into the aceto-arsenite by
the addition of acetic acid.--_Jour. of Science._
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