Abstract

Arsenic acid, systematically named arsoric acid with molecular formula H₃AsO₄, represents the arsenic analogue of phosphoric acid in the pnictogen oxoacid series. This inorganic compound exists primarily in aqueous solution where it behaves as a triprotic acid with dissociation constants pK_a1 = 2.19, pK_a2 = 6.94, and pK_a3 = 11.5. The compound forms white translucent or colorless hygroscopic crystals in its hemihydrate form (2H₃AsO₄·H₂O) with density 2.5 g/cm³. Arsenic acid demonstrates significant oxidizing properties distinct from its phosphorus counterpart and finds limited commercial application due to extreme toxicity. The molecular structure exhibits tetrahedral geometry around the central arsenic atom with As–O bond lengths ranging from 1.66 to 1.71 Å.

Introduction

Arsenic acid occupies a significant position in inorganic chemistry as the highest oxidation state arsenic oxoacid, formally containing arsenic in the +5 oxidation state. This compound belongs to the broader class of pnictogen oxoacids and demonstrates both similarities to and important differences from its phosphorus analogue. The compound's commercial importance has diminished considerably due to its extreme toxicity, though it maintains relevance in specialized industrial processes and as a chemical precursor. Arsenic acid exists in equilibrium with its various conjugate bases—dihydrogen arsenate (H₂AsO₄^-), hydrogen arsenate (HAsO₄^2-), and arsenate (AsO₄^3-)—across the pH range, with the relative concentrations of these species governed by its three acid dissociation constants.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of arsenic acid features tetrahedral coordination around the central arsenic atom, consistent with VSEPR theory predictions for an AX₄E₀ system. The arsenic atom employs sp³ hybrid orbitals to form four σ bonds with oxygen atoms. Experimental measurements indicate As–O bond lengths ranging from 1.66 to 1.71 Å, slightly longer than the corresponding P–O bonds in phosphoric acid due to the larger covalent radius of arsenic. The electronic configuration of arsenic in H₃AsO₄ is [Ar]3d¹⁰4s²4p⁰ with the atom in the +5 oxidation state. The molecule belongs to the C_3v point group when considering the idealized symmetric structure, though hydrogen bonding interactions in the solid state may reduce the effective symmetry.

Chemical Bonding and Intermolecular Forces

The bonding in arsenic acid consists of polar covalent As–O bonds with approximately 50% ionic character based on electronegativity differences. The oxygen atoms attached to hydrogen exhibit significant hydrogen bonding capability. In the crystalline hemihydrate form (2H₃AsO₄·H₂O), extensive hydrogen bonding networks form between arsenic acid molecules and water molecules, creating a stable crystal structure. The molecular dipole moment measures approximately 2.6 D, comparable to phosphoric acid. Intermolecular forces include strong hydrogen bonding (20-40 kJ/mol), dipole-dipole interactions (5-10 kJ/mol), and London dispersion forces. The compound's hygroscopic nature arises from these strong intermolecular interactions with water molecules.

Physical Properties

Phase Behavior and Thermodynamic Properties

Arsenic acid typically crystallizes as a hemihydrate with composition 2H₃AsO₄·H₂O, forming white translucent or colorless hygroscopic crystals. The pure compound melts at 35.5 °C, though it begins to decompose upon further heating with complete decomposition occurring at approximately 120 °C. The density of crystalline arsenic acid hemihydrate measures 2.5 g/cm³ at 20 °C. The compound exhibits high solubility in water—16.7 g per 100 mL at 20 °C—and is also soluble in ethanol. The vapor pressure at 50 °C measures 55 hPa. The hemihydrate form dehydrates with condensation when heated to 100 °C, losing water to form the anhydrous acid which subsequently decomposes. The dihydrate (H₃AsO₄·2H₂O) crystallizes at lower temperatures.

Spectroscopic Characteristics

Infrared spectroscopy of arsenic acid reveals characteristic absorption bands corresponding to O–H stretching vibrations between 3200-2800 cm^-1, As–O stretching vibrations between 850-750 cm^-1, and O–H bending modes around 1400 cm^-1. Raman spectroscopy shows strong bands attributable to symmetric As–O stretching at approximately 810 cm^-1. Nuclear magnetic resonance spectroscopy of arsenic acid in solution displays a ⁷⁵As NMR signal at approximately -450 ppm relative to aqueous AsCl₃ reference, though the quadrupolar nature of the ⁷⁵As nucleus (I = 3/2) results in broad signals. The ¹H NMR spectrum exhibits a singlet at approximately 10.5 ppm in D₂O, consistent with acidic protons undergoing rapid exchange.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Arsenic acid demonstrates complex reactivity patterns dominated by its acidic and oxidizing properties. The compound decomposes thermally above 120 °C through condensation reactions initially forming pyroarsenic acid (H₄As₂O₇) and ultimately arsenic pentoxide (As₂O₅) with liberation of water. In aqueous solution, arsenic acid participates in nucleophilic substitution reactions at the arsenic center, particularly with alcohols to form organoarsenic compounds. The hydrolysis of arsenic acid esters proceeds with rate constants on the order of 10^-3 to 10^-5 s^-1 at neutral pH and room temperature. Unlike phosphoric acid, arsenic acid functions as an oxidizing agent, capable of converting iodide to iodine with a standard reduction potential of approximately +0.56 V for the H₃AsO₄/H₃AsO₃ couple.

Acid-Base and Redox Properties

Arsenic acid behaves as a triprotic acid with successive dissociation constants pK_a1 = 2.19 ± 0.02, pK_a2 = 6.94 ± 0.02, and pK_a3 = 11.5 ± 0.1 at 25 °C. These values are remarkably similar to those of phosphoric acid (pK_a1 = 2.15, pK_a2 = 7.20, pK_a3 = 12.35), reflecting the similar sizes and electronegativities of arsenic and phosphorus. The acid demonstrates maximum buffering capacity near pH 2.2, 7.0, and 11.5 corresponding to the pK_a values. The redox behavior distinguishes arsenic acid from phosphoric acid, with the former readily reducing to arsenous acid (H₃AsO₃) in the presence of suitable reducing agents. The standard reduction potential for the H₃AsO₄/H₃AsO₃ couple is pH-dependent, measuring +0.56 V at pH 0 and decreasing with increasing pH.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most common laboratory synthesis of arsenic acid involves oxidation of arsenic trioxide with concentrated nitric acid. The reaction proceeds according to the equation: As₂O₃ + 2HNO₃ + 2H₂O → 2H₃AsO₄ + N₂O₃. This exothermic reaction requires careful temperature control between 50-70 °C to prevent decomposition of the product. The resulting solution upon cooling yields colorless crystals of the hemihydrate 2H₃AsO₄·H₂O. Alternative synthetic routes include hydrolysis of arsenic pentoxide (As₂O₅) with water or treatment of pyroarsenic acid (H₄As₂O₇) with cold water. A more modern approach employs ozone oxidation of elemental arsenic according to: 2As + 3H₂O + 5O₃ → 2H₃AsO₄ + 5O₂, which proceeds at room temperature with high yield.

Analytical Methods and Characterization

Identification and Quantification

Arsenic acid and arsenate species are routinely analyzed using inductively coupled plasma mass spectrometry (ICP-MS) with detection limits below 0.1 μg/L. Ion chromatography with conductivity detection provides quantitative analysis of arsenate anions with precision of ±2% and accuracy of ±5% in the concentration range 0.1-10 mg/L. Gravimetric methods employing silver nitrate precipitation as silver arsenate (Ag₃AsO₄) offer classical quantification with uncertainty of approximately ±0.5%. Spectrophotometric methods based on the molybdenum blue complex, analogous to phosphate determination, achieve detection limits of 10 μg/L though with potential interference from phosphate. X-ray diffraction analysis of crystalline samples provides definitive identification through comparison with reference patterns for arsenic acid hemihydrate.

Purity Assessment and Quality Control

Purity assessment of arsenic acid typically employs potentiometric titration with standardized sodium hydroxide solution to determine total acid content, with purity expressed as percentage of H₃AsO₄ equivalent. Impurity profiling commonly includes determination of arsenic(III) content iodometrically, with commercial specifications typically requiring less than 0.5% arsenous acid contamination. Heavy metal impurities are quantified using atomic absorption spectroscopy, with lead and cadmium limits typically below 10 mg/kg. Moisture content determination by Karl Fischer titration is essential due to the compound's hygroscopic nature, with specifications generally requiring less than 1.0% water in analytical grade material. Stability testing indicates that arsenic acid solutions are stable for extended periods when stored in glass or polyethylene containers at room temperature.

Applications and Uses

Industrial and Commercial Applications

Commercial applications of arsenic acid are severely limited by toxicity concerns but include specialized uses as a precursor to arsenic-based pesticides, particularly lead hydrogen arsenate formerly used as an insecticide. The compound functions as a wood preservative through formation of insoluble arsenates with copper and chromium in commercial preservative formulations. Arsenic acid serves as a finishing agent for glass and metal surfaces, providing corrosion resistance and modified surface properties. The compound finds application in the synthesis of various dyestuffs and organic arsenic compounds, including pharmaceuticals and veterinary products. Industrial production remains limited to specialized manufacturers with global production estimated at less than 1000 metric tons annually. Economic significance has declined steadily since the mid-20th century due to environmental and health concerns.

Historical Development and Discovery

The development of arsenic acid chemistry parallels the broader understanding of arsenic compounds throughout the 19th century. Early investigations by Swedish chemist Carl Wilhelm Scheele in the 1770s established the conversion of arsenic trioxide to arsenic acid using nitric acid. The compound's molecular formula was definitively established following the adoption of atomic theory in the early 1800s. The structural relationship to phosphoric acid became apparent with the development of structural chemistry concepts in the mid-19th century. Industrial applications expanded during the late 19th and early 20th centuries, particularly in agricultural chemicals, before declining with increased awareness of arsenic toxicity. The precise determination of acid dissociation constants occurred through potentiometric studies in the 1920s-1940s, while modern spectroscopic characterization developed throughout the latter half of the 20th century.

Conclusion

Arsenic acid represents a chemically significant compound that illustrates both the periodic trends within the pnictogen group and the important differences that emerge with increasing atomic number. The compound's tetrahedral molecular structure and stepwise acid dissociation behavior closely mirror those of phosphoric acid, while its pronounced oxidizing character distinguishes it from its lighter congener. The extreme toxicity of arsenic compounds has severely limited practical applications of arsenic acid, though it maintains importance as a chemical precursor and reference compound in inorganic chemistry. Future research directions may include further exploration of arsenic acid's coordination chemistry, development of improved analytical methods for arsenic speciation, and investigation of its behavior in environmental systems where arsenic contamination remains a significant concern.