Arsenic acid

“ Behavior of Arsenic Acid toward Hydrogen Sulphide.—In conclusion, special consideration must still be given to the behavior of arsenic acid, H3AsO4. As its name indicates, it is an acid, and, in fact, a rather strong acid, of the order of strength503 of phosphoric acid, H3PO4, which it resembles in composition and in many of its properties. As a strong acid, arsenic acid, when it is ionized, yields chiefly negative arseniate ions, H2AsO4−, HAsO42− and AsO43−. Any basic properties, which it may and, most likely, does possess, must be extremely weak. ”

“ Another oxide likewise exists, which contains two equivalents of metal and five of oxygen, and, possessing strong acid properties, is denominated arsenic acid. The sesquioxide and arsenic acid unite with bases, and produce compounds which, with the exception of those they form with the alkalis, are mostly insoluble. Metallic arsenic unites with sulphur in two proportions, forming an orange-red and a sulphur-yellow compound. The compounds of arsenic have very little chemical action with vegetable and animal principles. ”

“ This substance dissolves slowly in water, forming arsenic acid; by heating to redness it decomposes into arsenic and oxygen. It deliquesces in moist air, and is easily reduced to arsenic by heating with carbon.
Arsenic acid, H3AsO4, is prepared as shown above, the compound 2H3AsO4·H2O on being heated to 100° C. parting with its water of crystallization and leaving a residue of the acid, which crystallizes in needles. On heating to 180° C. it loses water and yields pyroarsenic acid, H4As2O7, which at 200° C. loses more water and leaves 653 a crystalline mass of meta-arsenic acid, HAsO3. ”

“ The oxides of nitrogen may be removed by heating the acid with charcoal, which converts them into volatile gases. Sulphuric acid may be freed from arsenic by heating it with manganese dioxide and then distilling. This oxidises all the arsenic into non-volatile arsenic acid. Without a preliminary oxidation it would partially remain as volatile arsenious acid, and might pass over into the distillate. The arsenic may also be driven off by first reducing it to arsenious acid, and then passing hydrochloric acid gas through the heated acid. ”

“ The hydrogen comes off regularly even in the cold, but it is best to apply a little heat. This test will evolve arsine from arsenious acid, and also from arsenic trisulphide; but it is not available for the detection of arsenic, when the arsenic is in the form of arsenic acid. ”

“ Pure lump-sugar dissolved in an aqueous solution of this acid becomes, in a few hours, of a reddish colour, and afterwards of a magnificent purple. Heated with charcoal it evolves a garlic-like odour, and is reduced to the metallic state. The suspected liquid being treated with sulphurous acid and boiled for a short time, the arsenic acid loses oxygen and is converted into arsenious acid, which may be tested for as such. Sulphuretted hydrogen does not precipitate a solution of arsenic acid, or an acidified arseniate, until after the lapse of several hours ”

“ The vessel is again heated to 180°, and while hot 8 c.c. of pure concentrated nitric acid are added, drop by drop, with continual stirring. After this addition, it is heated to 200° for fifteen minutes, and the result on cooling is a hard carbonaceous residue wholly free from nitric acid. The arsenic is in this way oxidised into arsenic acid, which is easily soluble in water. The contents of the dish are, therefore, perfectly extracted by boiling water, the aqueous extract filtered, and evaporated to dryness. ”

“ No definite evidence of stimulation has yet been obtained with any arsenic compound, however great the dilution at which it is applied. With certain algae a stimulus may occur, and it is possible that arsenic acid is capable of replacing phosphoric acid to some extent under certain conditions. With fungi the toxic effect of great concentrations is marked with certain species, but there are others which are capable of living happily on arsenical compounds and of liberating highly poisonous arsenic gas. ”

“ With arsenic acid on barley a possible stimulus is sometimes indicated to the eye, the plants being fine and of a particularly healthy dark colour, but this is not corroborated by the dry weights. Additional tests were made with peas and barley, treated with sodium arsenite and arsenate, the dilutions being carried down to 1/250,000,000, but no evidence of stimulus was obtained, so that it hardly seems possible that arsenic can act as a stimulative agent for these two plants when grown in water cultures. ”

“ Arsenious oxide is very poisonous. It acts as a reducing agent; it is not convertible into the pentoxide by the direct action of oxygen; and its solution is reduced by many metals (e.g. zinc, tin and cadmium) with precipitation of arsenic and formation of arseniuretted hydrogen. The solution of arsenious oxide in water reacts acid towards litmus and contains tribasic arsenious acid, although on evaporation of the solution the trioxide is obtained and not the free acid. The salts of the acid are, however, very stable, and are known as arsenites. ”

“ Arsenic may be detected by dissolving the substance in hydrochloric acid, or in aqua regia (avoiding an excess of nitric acid) , and adding a little of this solution to the contents of a small flask in which hydrogen is being made by the action of zinc and hydrochloric acid. The ignited jet of hydrogen assumes a blue colour if arsenic is present, and a cold porcelain dish held in the flame (fig. 67) becomes coated with a dark deposit of metallic arsenic. Antimony produces a similar effect, but is distinguished by the insolubility of its deposit in a cold solution of bleaching-powder. ”

“ Fleitmann‘s Test.—Detects arsenic in the presence of antimony, but does not detect arsenic as arsenic acid. When zinc or aluminium is heated with excess of potassium or sodium hydroxide in a mixture containing arsenious anhydride, arseniuretted hydrogen is evolved. The gas may be led into 4 per cent. nitrate of silver solution, or a test tube the top of which is covered with filter paper wet with nitrate of silver. The gas reduces the silver salt, and a black precipitate is produced in the solution, or a black spot on the paper. ”

“ With regard to arsenic acid, and the alkaline arseniates and arsenites, it is probable, from their effects in medicinal doses, that they are as active as the white oxide, if not more so. But they have not been particularly examined, as they are not objects of great interest to the medical jurist.
The fly-powder or black oxide is very active. Renault found that four grains killed a middle-sized dog in ten hours. [583] It has been likewise known to prove quickly fatal to man. ”

“ A millionfold increase in concentration, even when we are dealing with very small numbers, would imply a millionfold increase in the activity of the solution. If, then, the oxidizing component of arsenic acid is the quinquivalent ion, As5+, which would tend to discharge two of its positive (oxidizing) charges, arsenic acid should be a much more powerful oxidizing agent in strong acid solution than in alkaline or neutral solutions. ”

“ Uses of As2O3.-Arsenic is used in shot-manufacture, for hardening the metal. Its most important compound is As2O3, arsenic trioxide, called also arsenious anhydride, arsenious acid, white arsenic, etc. So poisonous is this that enough could be piled on a one-cent piece to kill a dozen persons. Taken in too large quantities it acts as an emetic. The antidote is ferric hydrate Fe2 (OH) 6 and a mustard emetic, followed by oil or milk.
The vapor density of this compound shows that its symbol should be As4O6, but the improper one, As2O3, is likely to remain in use. ”

“ According to the theory, that arsenic acid is an oxidizing agent because it gives up oxygen of a definite pressure, this pressure would be the more effective, the more completely the opposing hydroxide-ion is suppressed by added acid (p. 280; see also p. 272, on the action of ferro-ion on silver-ion in the presence and in the absence of fluorides) . ”

“ The arsenic-eaters begin with a very small dose, which is continued for several weeks or months, until the system gets accustomed to it. The amount is then slightly augmented until relatively large doses are taken with impunity. In one case [720] it appears that a countryman, in good health, and sixty years of age, took daily 4 grains of arsenious acid, a habit which he had inherited from his father, and which he in turn bequeathed to his son. ”

“ Arsenic acid has a very marked action on animal matters; I shall make it known without delay in my second memoir upon gelatine. It preserves bodies well, but appears to favour their desiccation. In the details of experiments made under the surveillance of the commissioners of the two academies, I shall cite the effects produced by the employment of this substance. Acetic acid preserves flesh only by drying it. This acid weakened, or vinegar, retards putrefaction, softens the bones, as well as the muscles, which are discolored by its action. ”

“ The freshly precipitated oxide of iron, taken immediately after poisoning by arsenic, converts the arsenious acid into an insoluble state, by forming a compound on which the acids of the stomach have no action, so that the poisoning cannot proceed. It is remarkable that the inhabitants of certain mountainous countries accustom themselves to taking arsenic, as a means which, according to their experience, helps to overcome the fatigue of mountain ascents. Arsenious anhydride and certain of its salts are also used in medicine, naturally only in small quantities. ”

“ White arsenic is an oxide of arsenic, and is obtained by roasting arsenical ores, and refining the material (crude arsenic) , which condenses in the flues. Arsenic itself is volatile, and many of its compounds have the same property. It forms two well-defined series of salts, corresponding to the oxides: arsenious oxide (As2O3) , and arsenic oxide (As2O5) . These combine with bases to form arsenites and arsenates respectively. Boiling with nitric acid converts the lower into the higher oxide; and powerful reducing-agents, such as cuprous chloride, have the opposite effect. ”

“ Solution.—Where, as in crude arsenic, the substance is arsenious oxide (As2O3) mixed with impurities, the arsenic is best got into solution by warming with caustic soda, and neutralising the excess with hydrochloric acid; it will be present as sodium arsenite. Metals and alloys are acted on by means of nitric acid; or the arsenic may be at the same time dissolved and separated by distilling with a strongly-acid solution of ferric chloride, in the way described under Volumetric Methods. ”

“ If ignited gently in a stream of carbonic acid gas, the weight remains constant. To render it anhydrous, a heat of 200° is required.
The recognition of arsenic in the commercial sulphide is most easily effected by placing 2 grms. or more in a suitable retort (with condenser) , adding hydrochloric acid, and distilling. The chloride of arsenic passes over before the chloride of antimony; and by not raising the heat too high, very little antimony will come over, even if the distillation be carried almost to dryness. The arsenic is detected in the distillate by the ordinary methods. ”

“ Effects of Arsenious Acid on Man.—The symptoms produced by arsenious acid vary according to the form of the poison—whether solid, vaporous, or soluble—according to the condition of bodily health of the person taking it, and according to the manner in which it is introduced into the animal economy, while they are also in no small degree modified by individual peculiarities of organisation and by habit, as, for instance, in the arsenic-eaters. ”

“ They also possess the richest colour; and are, therefore, preferred by artists and dyers. In former times, orpiment, like realgar, was employed in medicine. See Arsenic.
Arsenic, Pentasul′phide of. As2S5. Syn. Sulpharsen′ic acid, &c.; Arsen′ici pentasulphure′tum, &c., L. When a stream of sulphuretted hydrogen is transmitted for some time through a solution of arsenic acid, a precipitate of the PENTASULPHIDE is deposited after some hours’ repose. Its formation is accelerated by boiling the liquid. ”

“ Arsenic is found associated with many other substances in nature, particularly copper and pyrites. Arsenic is commonly present in commercial sulphuric acid manufactured from pyrites containing the metal, and when such acid is used with common salt for the production of hydrochloric acid, the latter also becomes contaminated. It may be safely stated that commercial sulphuric acid, hydrochloric acid, copper and zinc, free from arsenic, do not exist in the market. Hence in the detection of arsenic by the toxicologist the absolute purity of these reagents, which he uses, must be established. ”

“ There is no poison so liable and certain to be found as arsenic, and in almost every case of poisoning with mineral poisons, science is enabled to detect the substance, even when life has been extinct for years, and the body nearly decomposed.
What is arsenic?
Metallic arsenic is an exceedingly brittle metal, of a steel-gray color. It vaporizes, when heated, with a strong odor of garlic, a property not possessed by any other metal.
The substance used as poison, and sometimes known as ratsbane, is arsenious acid, a compound of arsenic and oxygen. ”