Arsenic

Properties

It is a Métalloïde which has three allotropic forms : yellow, black and gray.

Chemical properties

Arsenic is chemically very similar to the Phosphore which precedes it in the same group.

The arsenous Oxide, or white arsenic (improperly called arsenic), As₂O₃ is a violent poison.

The arseniuretted Hydrogen or Arsine , of AsH₃ formula, is a substance vaporizing in a colorless gas, of nauseous alliacée odor, very toxic which was used like Poison gas, in particular at the time of the First World War.

The trioxyde of arsenic is a poison, nevertheless used in injection with very low dose as anti-cancer drug for a form of Leucémie when she does not answer the other products used for chemotherapy, with cardiac side effects including the arrhythmia, even the cardiac arrest resulting in death.
Il is used for the manufacture of glass or crystal, when it was not replaced by antimony trioxyde, also toxic, but not subjected to the Seveso directive.

Toxicology

The toxicity of arsenic depends of its chemical nature: inorganic arsenic is more toxic than organic arsenic. It depends on its oxidation step: Ace (O) > Ace (III) > Ace (V). It increases with the degree of methylation of arsenic.

Arsenic has 3 major effects at the biochemical level:

It cuts the chain of breathing in substituent to phosphorus in the reaction of formation of ATP.
the proteins coagulate when the inorganic arsenic concentration is strong: reaction arsenic/bonds sulfides or reaction arsenic/active site.
arsenic is complexed with the sulfhydryles groups of the enzymes.

Its reputation is not usurped, arsenic in pure form or of mineral compound is dangerous even with low dose, especially in the event of repeated exposure.

In the case of an acute intoxication, the symptoms are immediate. One can quote like characteristic the vomiting, pains oesophagiennes and abdominal signs and the diarrheas sanguinolentes.

The chronic exposure to arsenic is a risk factor of Lung cancer, registers for this reason in the tables of occupational diseases.

The exposure prolonged to arsenic is also at the origin of Cancer of the skin, Cancer of the bladder and Cancer of the kidney.

After an exposure prolonged to arsenic, the first modifications generally relate to the skin from a change of the pigmentation. Cancer occurs more tardily and can put more than 10 years to appear.

The absorption of arsenic by the skin does not present a health risk.

Arsenic is often employed like Poison, from where the title Arsenic and old laces ( Arsenic and old lace ).

Certain researchers suppose that Napoleon i would have been poisoned with arsenic.

Arsenic is known as organics when it is chemically related on carbon or hydrogen, inorganic when it is related on oxygen, chlorine or sulfur. Good example of the proverb of Paracelse on the Poison S, arsenic (only organic arsenic) is also an ultra essential Oligo-élément for the human being, chicken, the goat, the pig and some other species. The needs for the man were evaluated between 10 and 20 µg per day.

Some figures

WHO, the EPA (Environmental Protection Agency), Health Canada, France or the EU fixed the maximum limit of concentration of arsenic in water at 0,01mg/L while the payment on the drinkable water quality of Quebec going back to 2001 fixes the limit at 0,025 mg/L and Bangladesh and India fix the limit at 0,05mg/L.

In spite of these standards, certain countries are still today above the limits of exposure according to WHO. Among them, Argentina, Australia, Bangladesh, Chile, China, the United States, Hungary, India, Mexico, Peru and Thailand. Negative effects on health were observed in Bangladesh, in China, in the United States (see chart of the USGS) and in India.

Usually, arsenic is dispersed and is found in small quantity in the environment. The usual concentrations are:

In the grounds: between 2 and 15 mg/L
In water: less 10µg/L
In food: less than 0,1 mg/kg
In the air: between 0,005 and 0,1 mg/m ³

Layers

Arsenic is a native element; often rather pure, but being able to contain traces of Antimony.

The principal arsenic ores are the Réalgar AsS ( red arsenic ) and the Orpiment As₂S₃ ( yellow arsenic ); arsenic is also present in many other minerals, particularly the Arséniure S, the Arsénate S, some Sulfosel S (Alloclasite, Cobaltite, Enargite, Lautite, Luzonite, Pearcéite, Proustite).

Natural flows

The cycle of arsenic is still known little about, but it is thought that the terrestrial Bactérie S produce approximately 26000 t/an of volatile methylated arsenic leading to the ocean. 17000 tonnes/an are added to it emitted by the volcanos and 2000 tonnes/an approximately resulting from the wind erosion. The marine sediments trap of it a part, which during a certain time will remain however biodisponible. These flows are to be compared with the production by the human activities, which were estimated in the years 1990 at approximately 30000 tonnes/an in the world.

Use

arsenic was tested or used in certain drugs since antiquity (see for example Solution of Fowler, containing Arsénite).
It was used to increase the toxicity of certain chemical weapons, in the form of Arsine in particular, as of the First World War.
In the form of lead arsenate in particular, it was used like Pesticide, which was a frequent source of poisoning of the users or consumers of treated products. It continues has to pollute the environment a long time after its use, the Plomb neither arsenic not being Biodégradable S nor dégradables on human scale of time.
arsenic is used as additive with the mixture Plomb - Antimoine of the electrode S of the accumulating S
It is used as additive (hardener) with the lead of the cartridges of hunting or the ammunition of war (it slows down also the formation of lead oxide).
Mixed with Copper and Chrome (CCA) it is a pesticide of the wood (which gives him a greenish color). Although discussed because of its toxicity, its remanence and owing to the fact that it is partially water soluble and the rains, this treatment remains authorized in the majority of the countries.
It is recently used on the “drums” of the Imprimante S known as “lasers” and of the Photocopieuse S, Fax. Pure or in the form of Séléniure, its sensitivity to the light makes it possible to discharge the electrostatic tension which will retain it “to thunder”.
the alloys made up of arsenic and Gallium (GaAs) or of Indium (InAs) give materials Semi-conducteur S (known as III-V by reference to the columns of the table of the elements), used for the manufacture of photovoltaic cells, electroluminescent diodes (LED) and of Transistor S with very high frequency.

more expensive and of implementation more complex than the Silicon, their market remains marginal, but their role is essential in Opto-électronique, where the performances of silicon are less good.

Anecdote

The green, color of the Devil, were not very good omen for the actors of the Middle Ages which died sometimes because of their costumes, dyes with arsenic. It is only very recently that the green became symbol of hope.

Methods of analysis

Several methods exist

quantification of the total arsenic of a sample
study of the speciation of the sample i.e. the various chemical or organic shapes of arsenic present and quantification.
analyzes arsenic among other metals, via for example the analysis X-Ray fluorescence…

Analyzes water samples

A good method of analysis must above all minimize the interferences so as to be sufficiently sensitive to obtain good limits of detection and quantification. These limits must be lower than the national standards in force. Moreover the method must be validated in term of field of linearity, of replicability, repeatability and accuracy. It is also important to know the rate of recovery to adjust the results.

Taking away and conservation of the sample

The analysis of arsenic being generally an analysis of trace it is essential to take the samples in containers washed beforehand with nitric Acid or Hydrochloric acid and rinsed with the demineralized Eau. To guarantee the conservation of the sample, this one will have to be acidified and not to be in contact with the air.

Standard techniques in laboratory

There exists a broad range of methods to analyze the liquid samples containing arsenic. The choice of the method is done according to the limits of desired detections and the awaited concentration. Among them:

the Colorimetry: little specifies,
the Atomic absorption spectrometry of flame (FAAS): very interfered and misses sensitivity,
the atomic absorption spectrometry with thermoelectric Atomization (GF- AAS). Measurement is based on the direct injection of the sample in a tube graphite, heated electrically with thermoelectric atomization.
the generation of hydride followed by a detection by spectroscopy of atomic absorption (HG-AAS) or by atomic fluorescence (HG-AFS). Initially the Ace (V) is tiny room of Ace (III) with sodium iodide. Arsenic is then transformed into a arsine volatile under the action of the sodium borohydride (NaBH4) in acid medium. The formed arsine is then oxidized out of elementary arsenic in a cell heated and proportioned by spectrophotometry of atomic absorption.
the atomic Spectrometry of emission in an argon plasma (ICP-AES) (see Gun plasma),
the mass spectrometry in an argon plasma (ICP-MS) (see Gun plasma). The sample is introduced into an argon plasma, that allows its ionization, the mass spectrometer separates then the ions according to the m/z report/ratio (mass/load). This method comprises many advantages: since there is one stable isotope for m/z = 75, the determination of arsenic is simple. Moreover limits of detection are very weak. On the other hand, there is a problem of isobaric interferences (see Gun plasma) in the samples with concentration raised out of chloride (presence of ArCl). Moreover the capital cost is high and the user must be qualified.

Methods of analysis on site

The EPA validated some methods of analysis on site. Arsenic Test Kits

The three first are based on the generation of arsine. Measurement is done by comparison of the change of color of the solution with the scales of color provided with the kit

Peters Engineering Aces 75 PeCo test kit,
Envitop Ltd. Have-signal Toilets arsenic test kit,
Industrial Systems Test, Inc., QuickT test kit arsenic analysis systems.

The fourth kit uses a voltametric method.

NANO-BAND™ TO EXPLORE Portable Analyzer Toilets.
Of the portable analyzers (X Ray fluorescence exist also, more expensive)

Analyzes samples of ground

Extraction of the sample

The samples are generally beforehand dried and filtered. One proceeds then to their solubilization either by acid attack on plate or to backward flow, or by acid attack in microwave closed nothing to lose. There does not exist however method standardized for arsenic in the grounds and sediments. The sample is then in liquid and acidified form.

One can in the same way carry out a direct analysis of the sample without preliminary mineralization.

Current methods in laboratory

Mêmes methods that for the analysis of water after the setting in solution of the samples.
the neutron Activation instrumental (INAA): Used as technique of reference but requiring the access to a nuclear reactor.
the X-ray fluorescence and its constraints: preparation of the samples, calibration and taken into account of the effects of matrix.

Speciation of arsenic

The knowledge of the speciation of arsenic makes it possible to better measure the impact and the environmental risk, because the nature of the species affects the Biodisponibilité, the Toxicologie and the mobility and Bioturbation of arsenic. There are many species to identify and quantify, which can rather quickly evolve/move (=> the stability of the samples is important, of sampling to the analysis). The extraction of arsenic should not modify its chemical forms. The techniques of analysis must be sensitive, selective and rapids to avoid the conversion of the species present.

Stability of the sample

It can be obtained by adding a chelating agent like EDTA, and by cooling the samples (with room temperature, only the very concentrated solutions remain stable).

The nature of the analyte also exploits stability: organic arsenic (methylated) is more stable than inorganic arsenic.

Extraction of arsenic

To extract arsenic without modifying its chemical forms, one needs a solvent not interfering with detection.

The scrubbing of the grounds makes it possible to realize of the mobility of the arsenic (which depends on the chemical composition of the ground, the pH, the micro-organisms…). But to extract arsenic quickly, of strong solvents are used and they interfere then with separation HPLC.

Techniques of separation

HPLC is the technique most often used. Thus one will use the chromatography of pair of ion to separate the neutral species from the ionic species (cations or anions), the chromatography of exchange of ions (exchange of anions to separate As (III), As (V), MY, DMA, of cations to separate the arsénobétaïne, the oxide Triméthylarsine and Me₄As⁺).

The chromatography of exclusion can also be used as a préparative technique.

The capillary electrophoresis is used little because of the interferences due to the matrix of the sample. This technique is used for the analysis of standard or samples whose matrix is simple.

Techniques of detection

The principal techniques used are:

the spectroscopy of atomic absorption and the ICP-AES.

These techniques are effective for samples very concentrated but not sensitive enough for the analysis of trace. They should then be combined with the generation of hydride. The generation of hydride is a fast reaction which increases by 10 to 100 times the sensitivity. That also makes it possible to remove the effect ED stamps sample.

ICP-MS.

This technique is supersensitive and makes it possible to analyze various elements at the same time. Moreover the coupling with the HPLC is easy and it is possible to measure the isotopic ratios what makes the determination of the species more precise.

the Spectrometry mass with a électrospray source.

That allows an analysis direct or coupled with HPLC and adds the parameter identification of the arseniuretted species in addition to quantifying them. That is particularly interesting for the complex organic molecules.

Research and development (R & D)

2007, France. A group of researchers highlighted that the bacterium Herminiimonas arsenicoxydans was able, not only to transform arsenic into form less toxic, but such an able to isolate this arsenic in a matrix from sugars.

Pour recall, In Bangladesh and in China, the concentration out of arsenic in water exceeds the rates recommended by WHO fixed at 0,01 mg/L.

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