§ 688. Oxalic acid is widely distributed both in the free state and in
combination with bases throughout the vegetable kingdom, and it also
occurs in the animal kingdom. In combination with potash it is found in
the _Geranium acetosum_ (L.), _Spinacia oleracea_ (L.), _Phytolacca
decandra_ (L.), _Rheum palmatum_ (L.), _Rumex acetosa_, _Atropa
belladonna_, and several others; in combination with soda in different
species of _Salsola_ and _Salicornia_; and in combination with lime in
most plants, especially in the roots and bark. Many lichens contain half
their weight of calcic oxalate, and oxalic acid, either free or
combined, is (according to the observations of Hamlet and
Plowright[679]) present in all mature non-microscopic fungi. Crystals of
oxalate of lime may be frequently seen by the aid of the microscope in
the cells of plants. According to Schmidt,[680] this crystallisation
only takes place in the fully mature cell, for in actively growing cells
the oxalate of lime is entirely dissolved by the albumen of the plant.
[679] _Chem. News_, vol. xxxvi. p. 93.
[680] _Ann. Chem. Pharm._, vol. lxi. p. 297.
In the animal kingdom oxalic acid is always present in the intestinal
contents of the caterpillar. In combination with lime, it is constantly
found in the allantois liquor of the cow, the urine of man, swine,
horses, and cats. With regard to human urine, the presence or absence of
oxalate of lime greatly depends upon the diet, and also upon the
individual, some persons almost invariably secreting oxalates, whatever
their food may be.
§ 689. =Oxalic Acid=, H₂C₂O₄2H₂O (90 + 36), specific gravity 1·64,
occurs in commerce in prismatic crystals, very similar to, and liable to
be mistaken for, either magnesic or zincic sulphates. The crystals are
intensely acid, easily soluble in water (1 part requiring at 14·5° 10·46
parts of water); they are also soluble in parts of cold, and readily in
boiling, alcohol. Oxalic acid is slightly soluble in cold absolute
ether; but ether, although extracting most organic acids from an aqueous
solution, will not extract oxalic acid.
Oxalic acid sublimes slowly at 100°, but rapidly and completely at
150°; the best means of obtaining the pure anhydride is to put a
sufficient quantity of the acid into a strong flask, clamp it by
suitable connections to a mercury pump, and sublime in a vacuum; in this
way a sufficient quantity may be sublimed a little above 100°. It is
well to remember, not only its low subliming temperature, but also that
an aqueous solution, if kept at 100°, loses acid; hence all evaporating
or heating operations must not exceed 98°, or there will be some loss.
The effect of heat is first to drive off water, then, if continued up to
about 190°, there is decomposition into carbon monoxide, carbon dioxide,
water, and formic acid; the two reactions occurring simultaneously--
C₂H₂O₄ = CO₂ + CO + H₂O.
C₂H₂O₄ = CO₂ + CH₂O₂.
Heated with sulphuric acid to 110°, the following decomposition takes
place:--
H₂C₂O₄ = H₂O + CO₂ + CO.
Oxalic acid decomposes fluor spar, the phosphates of iron, silver, zinc,
copper, and the arseniates of iron, silver, and copper. It may be used
to separate the sulphides of iron and manganese from the sulphides of
zinc, cadmium, uranium, cobalt, mercury, and copper--dissolving the
former, not the latter. Many minerals and other substances are also
attacked by this acid.
If a solution of oxalic acid in water is boiled with ammonio or sodio
terchloride of gold (avoiding direct exposure to light) the gold is
precipitated--
2AuCl₃ + 3H₂C₂O₄ = 6CO₂ + 6HCl + Au₂.
When black oxide of manganese (free from carbonate) is mixed with an
oxalate, and treated with dilute sulphuric acid, the oxalic acid is
decomposed, and carbon dioxide evolved--
MnO₂ + H₂C₂O₄ + H₂SO₄ = MnSO₄ + 2H₂O + 2CO₂.
A similar reaction occurs with permanganate of potash.
If to a solution of oxalic acid, which may be neutralised with an
alkali, or may contain free acetic acid, a solution of acetate of lime
be added, oxalate of lime is thrown down. This salt, important in an
analytical point of view, it will be well to describe.
§ 690. =Oxalate of Lime= (CaC₂O₄H₂O), 1 part ·863 crystallised oxalic
acid. This is the salt which the analyst obtains for the quantitative
estimation of lime or oxalic acid; it is not identical with that
occurring in the vegetable kingdom, the latter containing 3H₂O. Oxalate
of lime cannot be precipitated for quantitative purposes from solutions
containing chromium, aluminium, or ferric iron, since somewhat soluble
salts are formed. It dissolves in solutions of magnesium and
manganese,[681] and citrate of soda, and is also decomposed by boiling
with solutions of copper, silver, lead, cadmium, zinc, nickel, cobalt,
strontium, or barium. It is insoluble in solutions of chlorides of the
alkalies and alkaline earths, and in water, in alkaline solutions, or in
acetic acid; and is soluble in mineral acid only when the acid is strong
and in considerable excess. It is unalterable in the air, and at 100°.
When carefully and slowly ignited it may be wholly converted into
carbonate of lime; if the heat is not properly managed (that is, if
excessive), caustic lime may be formed in greater or smaller quantity.
[681] But it is reprecipitated unaltered by excess of alkaline oxalate.
§ 691. =Use in the Arts.=--Oxalic acid is chiefly used by dyers and
calico-printers, but also by curriers and harness-makers for cleaning
leather, by marble masons for removing iron stains, by workers in straw
for bleaching, and it is applied to various household purposes,[682]
such as the whitening of boards, the removing of iron-mould from linen,
&c. The hydropotassic oxalate (binoxalate of potash), under the popular
names of “_essential salt of lemons_” and salts of sorrel, is used for
scouring metals and for removing ink-stains from linen.
[682] A “Liquid Blue,” used for laundry purposes, contains much free
oxalic acid.
§ 692. =Hydropotassic Oxalate, Binoxalate of Potash=, KHC₂O₄(H₂O), is a
white salt, acid in reaction, soluble in water, and insoluble in
alcohol. Heated on platinum foil it leaves potassic carbonate, which may
be recognised by the usual tests. Its aqueous solution gives, with a
solution of acetate or sulphate of lime, a precipitate of calcic oxalate
insoluble in acetic acid.
§ 693. =Statistics.=--Poisoning by oxalic acid is more frequent in
England than in any other European country. In the ten years 1883-92,
there were registered in England and Wales 222 deaths from oxalic
acid--of these 199, or 89·6 per cent., were suicidal, the remainder
accidental. The age and sex distribution of these cases is set out in
the following table:--
POISONING BY OXALIC ACID IN ENGLAND AND WALES DURING THE TEN YEARS
1883-1892.
ACCIDENT OR NEGLIGENCE.
Ages, 0-1 1-5 5-15 15-25 25-65 65 and Total
above
Males, 1 ... ... 2 ... 14 17
Females, ... ... ... 1 5 ... 6
---------------------------------------------
Total, 1 ... ... 3 5 14 23
---------------------------------------------
SUICIDE.
Ages, 15-25 25-65 65 and Total
above
Males, 9 102 3 114
Females, 21 62 2 85
----------------------------
Total, 30 164 5 199
----------------------------
§ 694. =Fatal Dose.=--The smallest dose of oxalic acid known to have
destroyed life is, according to Dr. Taylor, 3·88 grms. (60 grains); but
recovery has taken place, on prompt administration of remedies, after
eight times this quantity has been swallowed.
With regard to oxalate of soda, or binoxalate of potash, 14·2 grms.
(half an ounce) have been taken without fatal result, although the
symptoms were very serious; and it may be held that about that quantity
would usually cause death. Oxalic acid is not used in medicine, save as
a salt, _e.g._, oxalate of cerium.
§ 695. =Effects of Oxalic Acid and Oxalates on Animals.=--The first
cases of poisoning by oxalic acid occurred early in the nineteenth
century, a little more than fifty years after its discovery.
Thompson[683] was the first who attempted, by experiment on animal life,
to elucidate the action of the poison; he noted the caustic action on
the stomach, and the effects on the heart and nervous system, which he
attributed simply to the local injury through the sympathetic nerves.
Orfila[684] was the next who took the matter up, and he made several
experiments; but it was Robert Christison[685] who distinctly recognised
the important fact that oxalic acid was toxic, quite apart from any
local effects, and that the soluble oxalates, such as sodic and potassic
oxalates, were violent poisons.
[683] _Lond. Med. Rep._, vol. iii. p. 382.
[684] _Traité de Toxicologie._
[685] _Edin. Med. and Surg. Journ._, 1823.
§ 696. Kobert and Küssner[686] have made some extended researches on the
effects of sodic oxalate on rabbits, cats, dogs, guinea-pigs, hedgehogs,
frogs, &c.--the chief results of which are as follows:--On injection of
sodic oxalate solution in moderate doses into the circulation, the
heart’s action, and, therefore, the pulse, become arhythmic; and a
dicrotic or tricrotic condition of the pulse may last even half a day,
while at the same time the frequency may be uninfluenced. The
blood-pressure also with moderate doses is normal, and with small atoxic
doses there is no slowing of the respiration. On the other hand, toxic
doses paralyse the respiratory apparatus, and the animal dies
asphyxiated. With chronic and subacute poisoning the respiration becomes
slower and slower, and then ceases from paralysis of the respiratory
muscles. The first sign of poisoning, whether acute or chronic, is a
sleepy condition; dogs lie quiet, making now and then a noise as if
dreaming, mechanical irritations are responded to with dulness. The hind
extremities become weak, and then the fore. This paresis of the hind
extremities, deepening into complete paralysis, was very constant and
striking. Take, for example, from the paper (_op. cit._) the experiment
in which a large cat received in six days five subcutaneous injections
of 5 c.c. of a solution of sodic oxalate (strength 1 : 30), equalling
·16 grm.; the cat died, as it were, gradually from behind forwards, so
that on the sixth day the hinder extremities were fully motionless and
without feeling. The heart beat strongly. The temperature of the
poisoned animal always sinks below the normal condition. Convulsions in
acute poisoning are common, in chronic quite absent; when present in
acute poisoning, they are tetanic or strychnic-like. In all the
experiments of Kobert and Küssner, lethal doses of soluble oxalates
caused the appearance of sugar in the urine.
[686] _Exper. Wirkungen der Oxalsäure, Virch. Archiv_, Bd. lxxvii. S.
209.
J. Uppmann[687] made forty-nine experiments on dogs, in which he
administered relatively large doses by the stomach; no poisonous effect
followed. Emil Pfeiffer[688] gave a dog in three successive days ·2, ·5,
and lastly 1 grm. oxalic acid with meat, but no symptoms resulted. Yet
that oxalic acid, as sodic oxalate, is poisonous to dogs, if it once
gets into the circulation, cannot be disputed. The accepted explanation
is that the large amount of lime phosphates in the digestive canal of
dogs is decomposed by oxalic acid, and the harmless lime oxalate formed.
[687] _Allg. Med. central Ztg._, 1877.
[688] _Archiv der Pharm._ (3 R.), Bd. xiii. S. 544, 1878.
Oxalic acid is absorbed into the blood, and leeches have been known to
die after their application to a person who had taken a large dose. Thus
Christison[689] quotes a case related by Dr. Arrowsmith, in which this
occurred:--“They were healthy, and fastened immediately; on looking at
them a few minutes after, I remarked that they did not seem to fill, and
on touching one it felt hard, and instantly fell off motionless and
dead; the others were in the same state. They had all bitten, and the
marks were conspicuous, but they had drawn scarcely any blood. They were
applied about six hours after the acid had been taken.”
[689] _Treatise on Poisons._
§ 697. =Effects of Vaporised Oxalic Acid.=--Eulenberg has experimented
on pigeons on the action of oxalic acid when breathed. In one of his
experiments, ·75 grm. of the acid was volatilised into a glass shade, in
which a pigeon had been placed; after this had been done five times in
two minutes, there was uneasiness, shaking of the head, and cough, with
increased mucous secretion of the nasal membrane. On continuing the
transmission of the vapour, after eight minutes there was again
restlessness, shaking of the head, and cough; after eleven minutes the
bird fell and was convulsed. On discontinuing the sublimation, it got up
and moved freely, but showed respiratory irritation. On the second day
after the experiment, it was observed that the bird’s note was hoarse,
on the fourth day there was slowness of the heart’s action and refusal
of food, and on the sixth day the bird was found dead. Examination after
death showed slight injection of the cerebral membranes; the cellular
tissue in the neighbourhood of the trachea contained in certain places
extravasations of blood, varying from the size of a pea to that of a
penny; the mucous membrane of the larynx and trachea was swollen and
covered with a thick croupous layer; the lungs were partially hepatised,
and the pleura thickened; the crop as well as the true intestines still
contained some food.[690]
[690] _Gewerbe Hygiene_, p. 423.
§ 698. =The Effects of Oxalic Acid and Hydropotassic Oxalate on
Man.=--The cases of oxalic poisoning have been invariably due to either
oxalic acid or hydropotassic oxalate, the neutral sodic or potassic
oxalates having hitherto in no instance been taken. The symptoms, and
even the locally destructive action of oxalic acid and the acid oxalate,
are so similar that neither from clinical nor _post-mortem_ signs could
they be differentiated by anyone not having a previous knowledge of the
case.
The external application of oxalic acid does not appear to cause
illness; workmen engaged in trades requiring the constant use of the
acid often have the nails white, opaque, and brittle; but no direct
injury to health is on record.
A large dose of either causes a local and a remote effect; the local is
very similar to that already described as belonging to the mineral
acids, i.e., more or less destructive of the mucous membranes with which
the acid comes in contact. The remote effects may only be developed
after a little; they consist essentially of a profound influence on the
nervous system. Though more than 120 cases of oxalic acid poisoning have
occurred since Christison wrote his treatise, his graphic description
still holds good. “If,” says he, “a person immediately after swallowing
a solution of a crystalline salt, which tasted purely and strongly acid,
is attacked with burning in the throat, then with burning in the
stomach, vomiting, particularly of bloody matter, imperceptible pulse,
and excessive languor, and dies in half an hour, or still more, in
twenty, fifteen, or ten minutes, I do not know any fallacy which can
interfere with the conclusion that oxalic acid was the cause of death.
No parallel disease begins so abruptly, and terminates so soon; and no
other crystalline poison has the same effect.” The local action is that
of a solvent on the mucous tissues. If from 10 to 30 grms. are
swallowed, dissolved in water, there is an immediate sour taste, pain,
burning in the stomach, and vomiting. The vomit may be colourless,
greenish, or black, and very acid; but there is a considerable variety
in the symptoms. The variations may be partly explained by saying that,
in one class of cases, the remote or true toxic effects of the poison
predominate; in a second, the local and the nervous are equally divided;
while in a third, the local effects seem alone to give rise to symptoms.
In a case at Guy’s Hospital, in 1842, there was no pain, but vomiting
and collapse. In another case which occurred in 1870, a male (aged 48)
took 10·4 grms. (162 grains); he had threatening collapse, cold sweats,
white and red patches on the tongue and pharynx, difficulty in
swallowing, and contracted pupils. Blood was effused from the mouth and
anus; on the following day there were convulsions, coma, and death
thirty-six hours after taking the poison. In another case, there was
rapid loss of consciousness and coma, followed by death in five hours.
Death may be very rapid, _e.g._, in one case (_Med. Times and Gaz._,
1868) it took place in ten minutes; there was bleeding from the stomach,
which doubtless accelerated the fatal result. Orfila has recorded a
death almost as rapid from the acid oxalate of potash; a woman took 15
grms.; there was no vomiting, but she suffered from fearful cramps, and
death ensued in fifteen minutes. In another case, also recorded by
Orfila, there was marked slowing of the pulse, and soporific tendencies.
With both oxalic acid and the acid oxalate of potash, certain nervous
and other sequelæ are more or less constant, always provided time is
given for their development. From the experiments already detailed on
animals, one would expect some paresis of the lower extremities, but
this has not been observed in man. There is more or less inflammation of
the stomach, and often peritonitis; in one case (_Brit. Med. Journal_,
1873) there were cystitis and acute congestion of the kidneys with
albuminuria.
In two cases quoted by Taylor, there was a temporary loss or
enfeeblement of voice; in one of the two, the aphonia lasted for eight
days. In the other, that of a man who had swallowed about 7 grms. (¼
oz.) of oxalic acid, his voice, naturally deep, became in nine hours low
and feeble, and continued so for more than a month, during the whole of
which time he suffered in addition from numbness and tingling of the
legs. As a case of extreme rarity may be mentioned that of a young
woman,[691] who took 12 grms. (185 grains) of the acid oxalate of
potash, and on the third day died; before death exhibiting delirium so
active and intense that it was described as “madness.”
[691] _Journ. de Chim. Méd._, 1839, p. 564.
§ 699. =Physiological Action.=--Putting on one side the _local_ effects
of oxalic acid, and regarding only its true toxic effects, there is some
difference of opinion as to its action. L. Hermann considers it one of
the heart poisons, having seen the frog’s heart arrested by subcutaneous
doses of sodic oxalate, an observation which is borne out by the
experiments of Cyon,[692] and not negatived by those of Kobert and
Küssner. The poison is believed to act on the extracardial ganglia.
Onsum[693] held at one time a peculiar theory of the action of oxalic
acid, believing that it precipitated as oxalate of lime in the lung
capillaries, causing embolic obstruction; but this view is not now
accepted--there are too many obvious objections to it. Kobert and
Küssner do not consider oxalic acid a heart poison, but believe that its
action is directed to the central nervous system, as attested by sinking
of the blood-pressure, the arhythm and retardation of the pulse, the
slow breathing, the paralytic symptoms, and the fibrillary muscular
contraction; but, with regard to the latter, Locke[694] has observed
that a frog’s sartorius, immersed in 0·75 sodium oxalate solution,
becomes in a few seconds violently active, much more so than in
Biederman’s normal saline solution. After thirty to forty-five minutes
it loses its irritability, which, however, it partially recovers by
immersion in 0·6 sodium chloride solution. He thinks this may explain
the symptoms of fibrillary muscular contraction observed by Kobert and
Küssner, which they ascribe to an action on the central nervous system.
[692] _Virch. Archiv_, Bd. xx. S. 233.
[693] Almen afterwards supported Onsum’s view; he made a number of
microscopical observations, and appears to have been the first who
identified oxalate of lime in the kidneys (Upsala, _Läkareförenings
förhandl._, Bd. ii. Hft. iv. S. 265).
[694] F. S. Locke, _J. Phys._, xv. 119; _Journ. Chem. Soc._, 1893, 480.
§ 700. =Pathological Changes.=--Kobert and Küssner observed that when
oxalate of soda was subcutaneously injected into animals, there was
often abscess, and even gangrene, at the seat of the injection. If the
poison were injected into the peritoneal cavity, death was so rapid as
to leave little time for any coarse lesions to manifest themselves. They
were not able to observe a cherry-red colour of the blood, nor did they
find oxalate of lime crystals in the lung capillaries; there were often
embolic processes in the lung, but nothing typical. They came,
therefore, to the conclusion that the state of the kidneys and the urine
was the only typical sign. The kidneys were dark, full of blood, but did
not show any microscopic hæmorrhages. Twelve hours after taking the
poison there is observed in the cortical substance a fine striping
corresponding to the canaliculi; in certain cases the whole boundary
layer is coloured white. If the poisoning lasts a longer time, the
kidneys become less blood-rich, and show the described white striping
very beautifully; this change persists several weeks. The cause of this
strange appearance is at once revealed by a microscopical examination;
it is due to a deposition of oxalate of lime; no crystals are met with
in the glomerules. Both by the microscope and by chemical means it may
be shown that the content of the kidney in oxalates is large.[695] So
far as the tissues generally are concerned, free oxalic acid is not
likely to be met with; there is always present sufficient lime to form
lime oxalate. The urine was always albuminous and contained a reducing
substance, which vanished about the second day after the dose. Hyaline
casts and deposits of oxalates in the urine never failed.[696]
[695] The important fact of the oxalate-content of kidneys and urine,
and the expulsion of casts, was first observed by Mitscherlich in 1854.
He noticed in a rabbit, to which had been given 7·5 grms. of oxalic
acid, and which had died in thirteen minutes, “_renes paululum magis
sanguine replete videbantur, in urina multa corpora inveniebantur, quæ
tubulos Bellenianos explese videntur_” (_De acidi acetici, oxalici,
tartarici, citrici, formici, et boracici, &c., Berlin_).
[696] Rabuteau has discovered by experiment that even the oxalates of
iron and copper are decomposed and separated by the kidneys. _Gaz. Méd.
de Paris_, 1874.
§ 701. Observations of the pathological effects of the oxalates on man
have been confined to cases of death from the corrosive substances
mentioned, and hence the intestinal tract has been profoundly affected.
In the museum of St. Thomas’ Hospital is a good example of the effects
produced. The case was that of a woman who had taken a large, unknown
quantity of oxalic acid, and was brought to the hospital dead. The
mucous membrane of the gullet is much corrugated and divided into
numerous parallel grooves, these again by little transverse grooves, so
that the intersection of the two systems makes a sort of raised pattern.
It is noted that in the recent state the mucous membrane could be
removed in flakes; in the upper part it was whitish, in the lower
slate-coloured. The stomach has a large perforation, but placing the
specimen beside another in the same museum which illustrates the effect
of the gastric juice, in causing an after-death solution of a portion of
the stomach, I was unable to differentiate between the two. The mucous
membrane had the same shreddy flocculent appearance, and is soft and
pale. The pyloric end is said to have been of a blackish colour, and no
lymph was exuded.
§ 702. The pathological changes by the acid oxalate of potash are
identical with those of oxalic acid, in both the gullet and stomach
being nearly always more or less inflamed or corroded; the inflammation
in a few cases has extended right through into the intestinal canal;
there are venous hyperæmia, hæmorrhages, and swelling of the mucous
membrane of the stomach. The hæmorrhages are often punctiform, but
occasionally larger, arranged in rows on the summits of the rugæ;
sometimes there is considerable bleeding. In the greater number of cases
there is no actual erosion of the stomach, but the inner layer appears
abnormally transparent. On examining the mucous membrane under the
microscope, Lesser[697] has described it as covered with a layer which
strongly reflects light, and is to be considered as caused by a fine
precipitate of calcic oxalate. Lesser was unable to find in any case
oxalic acid crystals, or those of the acid oxalate of potash. There are
many cases of perforation on record, but it is questionable whether they
are not all to be regarded as _post-mortem_ effects, and not
life-changes; at all events, there is little clinical evidence to
support the view that these perforations occur during life. In the case
(mentioned _ante_) in which death took place by coma, the brain was
hyperæmic. The kidneys, as in the case of animals, show the white zone,
and are congested, and can be proved by microscopical and chemical
means to be rich in oxalates.
[697] Virchow’s _Archiv_, Bd. lxxxiii. S. 218, 1881.
§ 703. =Separation of Oxalic Acid from Organic Substances, the Tissues
of the Body, &c.=--From what has been stated, no investigation as to the
cause of poison, when oxalic acid is suspected, can be considered
complete unless the analyst has an opportunity of examining both the
urine and the kidneys; for although, in most cases--when the acid
itself, or the acid potassic salt has been taken--there may be ample
evidence, both chemical and pathological, it is entirely different if a
case of poisoning with the neutral sodic salt should occur. In this
event, there may be no congested appearance of any portion of the
intestinal canal, and the evidence must mainly rest on the urine and
kidneys.
Oxalic acid being so widely distributed in the vegetable kingdom, the
expert must expect, in any criminal case, to be cross-examined by
ingenious counsel as to whether or not it was possible that the acid
could have entered the body in a rhubarb-pie, or accidentally through
sorrel mixed with greens, &c. To meet these and similar questions it is
important to identify, if possible, any green matters found in the
stomach. In any case, it must be remembered, that although rhubarb has
been eaten for centuries, and every schoolboy has occasionally chewed
small portions of sorrel, no poisoning has resulted from these
practices. When oxalic acid has been taken into the stomach, it will
invariably be found partly in combination with lime, soda, ammonia, &c.,
and partly free; or if such antidotes as chalk has been administered, it
may be wholly combined. Vomiting is nearly always present, and valuable
evidence of oxalic acid may be obtained from stains on sheets, carpets,
&c. In a recent case of probably suicidal poisoning, the writer found no
oxalic acid in the contents of the stomach, but some was detected in the
copious vomit which had stained the bed-clothes. The urine also
contained a great excess of oxalate of lime--a circumstance of little
value taken by itself, but confirmatory with other evidence. If a liquid
is strongly acid, oxalic acid may be separated by dialysis from organic
matters, and the clear fluid thus obtained precipitated by sulphate of
lime, the oxalate of lime being identified by its microscopic form and
other characters.
The usual general method for the separation of oxalic acid from organic
substances or mixtures is the following:--Extract with boiling water,
filter (which in some cases must be difficult or even impossible), and
then precipitate with acetate of lead. The lead precipitate may contain,
besides oxalate of lead, phosphate, chloride, sulphate, and various
organic substances and acids. This is to be decomposed by sulphuretted
hydrogen, and on filtering off the sulphide of lead, oxalic acid is to
be tested for in the filtrate. This process can only be adopted with
advantage in a few cases, and is by no means to be recommended as
generally applicable. The best general method, and one which insures
the separation of oxalic acid, whether present as a free acid, as an
alkaline, or a calcic oxalate, is perhaps the following:--The substance
or fluid under examination is digested with hydrochloric acid until a
fluid capable of filtration is obtained; the free acid is neutralised by
ammonia in very slight excess, and permitted to deposit, and the fluid
is then carefully decanted, and the deposit thrown on a filter. The
filtrate is added to the decanted fluid, and precipitated with a slight
excess of acetate of lime--this precipitate, like the first, being
collected on a filter. The first precipitate contains all the oxalic
acid which was in combination with lime; the second, all that which was
in the free condition. Both precipitates should be washed with acetic
acid. The next step is to identify the precipitate which is supposed to
be oxalate of lime. The precipitate is washed into a beaker, and
dissolved with the aid of heat by adding, drop by drop, pure
hydrochloric acid; it is then reprecipitated by ammonia, and allowed to
subside completely, which may take some time. The supernatant fluid is
decanted, and the precipitate washed by subsidence; it is lastly dried
over the water-bath in a tared porcelain dish, and its weight taken. The
substance is then identified by testing the dried powder as follows:--
(_a_) It is whitish in colour, and on ignition in a platinum dish leaves
a grey carbonate of lime. All other organic salts of lime--viz.,
citrate, tartrate, &c.--on ignition become coal-black.
(_b_) A portion suspended in water, to which is added some sulphuric
acid, destroys the colour of permanganate of potash--the reaction being
similar to that on p. 511--a reaction by which, as is well known, oxalic
acid or an oxalate may be conveniently titrated. This reaction is so
peculiar to oxalic acid, that there is no substance with which it can be
confounded. It is true that uric acid in an acid solution equally
decolorises permanganate, but it does so in a different way; the
reaction between oxalic acid and permanganate being at first slow, and
afterwards rapid, while the reaction with uric acid is just the
reverse--at first quick, and towards the end of the process extremely
slow.
(_c_) A portion placed in a test-tube, and warmed with concentrated
sulphuric acid, develops on warming carbon oxide and carbon dioxide; the
presence of the latter is easily shown by adapting a cork and bent tube
to the test-tube, and leading the evolved gases through baryta water.
Alexander Gunn[698] has described a new method of both detecting and
estimating oxalic acid; it is based on the fact that a small trace of
oxalic acid, added to an acid solution of ferrous phosphate, strikes a
persistent lemon-yellow colour; the depth of colour being proportionate
to the amount of oxalic acid.
[698] _Pharm. Journal_, 1893, 408.
The reagents necessary for both quantitative and qualitative testing are
as follows:--A standard solution of oxalic acid, of which 100 c.c. equal
1 grm., and a solution of ferrous phosphate, containing about 12·5 per
cent. of Fe₃2PO₄, with excess of phosphoric acid.
Into each of two Nessler graduated glasses 7·5 c.c. of the ferrous
phosphate solution are run and made up to 50 c.c. with distilled water;
both solutions should be colourless; 1, 2, or more c.c. of the solution
to be tested are then run into one of the Nessler glasses; if oxalic
acid be present, a more or less deep tint is produced; this must be
imitated by running the standard solution of oxalic acid into the second
Nessler cylinder--the calculation is the same as in other colorimetric
estimations. It does not appear to be reliable quantitatively, if alum
is present; and it is self-evident that the solution to be tested must
be fairly free from colour.
§ 704. =Oxalate of Lime in the Urine.=--This well-known urinary sediment
occurs chiefly as octahedra, but hour-glass, contracted or dumbbell-like
bodies, compound octahedra, and small, flattened, bright discs, not
unlike blood discs, are frequently seen. It may be usually identified
under the field of the microscope by its insolubility in acetic acid,
whilst the ammonio mag. phosphate, as well as the carbonate of lime, are
both soluble in that acid. From urates it is distinguished by its
insolubility in warm water. A chemical method of separation is as
follows:--The deposit is freed by subsidence as much as possible from
urine, washed with hot water, and then dissolved in hydrochloric acid
and filtered; to the filtrate ammonia is added in excess. The
precipitate may contain phosphates of iron, magnesia, lime, and oxalate
of lime. On treatment of the precipitate by acetic acid, the phosphates
of the alkaline earths (if present) dissolve; the insoluble portion will
be either phosphate of iron, or oxalate of lime, or both. On igniting
the residue in a platinum dish, any oxalate will be changed to
carbonate, and the carbonate of lime may be titrated with d. n. HCl acid
and cochineal solution, and from the data thus obtained the oxalate
estimated. The iron can be tested qualitatively in the acid solution by
ferrocyanide of potassium, or it can be determined by the ordinary
methods. If the qualitative detection of oxalate of lime in the deposit
is alone required, it is quite sufficient evidence should the portion
insoluble in acetic acid, on ignition in a platinum dish, give a residue
effervescing on the addition of an acid.
§ 705. =Estimation of Oxalic Acid.=--Oxalic acid is estimated in the
free state by direct weighing, or by titration either with alkali or by
potassic permanganate, the latter being standardised by oxalic acid. If
(as is commonly the case) oxalic acid is precipitated as oxalate of
lime, the oxalate may be--
(_a_) Dried at 100° and weighed directly, having the properties already
described.
(_b_) Titrated with dilate sulphuric acid and permanganate.
(_c_) Ignited, and the resulting carbonate of lime weighed; or dissolved
in standard acid and titrated back--one part of calcic carbonate
corresponds to 1·26 part of crystallised oxalic acid, or 0·90 part of
H₂C₂O₄; similarly, 1 c.c. of standard acid equals ·05 of calcic
carbonate (or ·063 of crystallised oxalic acid).
(_d_) The oxalate may be dissolved in the smallest possible amount of
hydrochloric acid, and boiled with ammonio chloride of gold, avoiding
exposure to light; every part of gold precipitated corresponds to ·961
part of crystallised oxalic acid.
(_e_) The oxalate may be placed in Geissler’s carbonic acid apparatus,
with peroxide of manganese and diluted sulphuric acid. The weight of the
gas which at the end of the operation has escaped, will have a definite
relation to that of the oxalate, and if multiplied by 1·4318 will give
the amount of crystallised oxalic acid.
CERTAIN OXALIC BASES--OXALMETHYLINE--OXALPROPYLINE.
§ 706. Hugh Schulz[699] and Mayer have contributed the results of
some important researches bearing upon a more exact knowledge of the
effects of the oxalic group of poisons, and upon the relation
between chemical constitution and physiological effects. They
experimented upon _oxalmethyline_, _chloroxalmethyline_, and
_oxalpropyline_.
[699] _Beitrag zur Kenntniss der Wirkung der Oxalbasen auf den
Thierkörper. Arch. f. exper. Path. u Pharm._, 1882.
=Chloroxalmethyline= (C₆H₅ClN₂) is a liquid, boiling at 205°, with a
weakly narcotic smell. A solution of the hydrochlorate of the base
was employed. Subcutaneous injections of ·05 grm. into frogs caused
narcosis, and both this and the ethylic compound deranged the
heart’s action, decreasing the number of beats. Thus ·05 grm.
decreased the number of the beats of the heart of a frog in the
course of one and three-quarter hours as follows: 72, 60, 56, 50,
44, 40, 35, 0.
=Oxalmethyline= produces somewhat similar symptoms, but the nervous
system is more affected than in that which contains chlorine.
=Oxalpropyline= also causes narcosis, and afterwards paralysis of
the hinder extremities and slowing of the heart.
The difference between the chlorine-free and the chlorine-containing
oxalic bases are summarised as follows:--
FROGS.
CHLORINE-HOLDING BASES. CHLORINE-FREE BASES.
Notable narcosis; no heightened Narcosis occurs late, and is
reflex action, muscular cramps, little pronounced; a notable in-
nor spontaneous convulsions. crease of reflex excitability;
more and more muscular paralysis;
between times, muscular cramps.
CATS.
Notable narcosis and salivation; Great excitement; general
no mydriasis; convulsions and shivering, rising to pure clonic
paralysis; no change in the convulsions; paralysis of the
respirations. hind legs; notable mydriasis,
jerking, and superficial res-
piration; weak narcosis.
DOGS.
Notable narcosis; occasional Narcosis evident; the rest as in
vomiting; the rest as in cats. cats.