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The sulfate ion is a polyatomic anion with the empirical formula SO2−
4
and a molecular mass of 96.06 daltons (96.06 g/mol); it consists of a central sulfur atom surrounded by four equivalent oxygen atoms in a tetrahedral arrangement. The symmetry is very similar to that of methane, CH4, The sulfur atom is in the +6 oxidation state while the four oxygen atoms are each in the −2 state. The sulfate ion carries a negative two charge and is the conjugate base of the bisulfate (or hydrogen sulfate) ion, HSO
4
, which is the conjugate base of H2SO4, sulfuric acid. Organic sulfates, such as dimethyl sulfate, are covalent compounds and esters of sulfuric acid.

PbS was once used as a black pigment, but current applications exploit its semiconductor properties, which have long been recognized.[9] PbS is one of the oldest and most common detection element materials in various infrared detectors. As an infrared detector, PbS functions as a photon detector, responding directly to the photons of radiation, as opposed to thermal detectors, which respond to a change in detector element temperature caused by the radiation.

Applications:
A PbS element can be used to measure radiation in either of two ways: by measuring the tiny photocurrent the photons cause when they hit the PbS material, or by measuring the change in the material's electrical resistance that the photons cause. Measuring the resistance change is the more commonly used method.

At room temperature, PbS is sensitive to radiation at wavelengths between approximately 1 and 2.5 μm. This range corresponds to the shorter wavelengths in the infra-red portion of the spectrum, the so-called short-wavelength infrared (SWIR). Only very hot objects emit radiation in these wavelengths.

Cooling the PbS elements, for example using liquid nitrogen or a Peltier element system, shifts its sensitivity range to between approximately 2 and 4 μm. Objects that emit radiation in these wavelengths still have to be quite hot—several hundred degrees Celsius—but not as hot as those detectable by uncooled sensors. Other compounds used for this purpose include indium antimonide (InSb) and mercury-cadmium telluride (HgCdTe), which have somewhat better properties for detecting the longer IR wavelengths. The high dielectric constant of PbS leads to relatively slow detectors (compared to silicon, germanium, InSb, or HgCdTe).

General Properties:
Crystal structure: Halite (cubic), cF8
Molar mass:   239.30 g/mol
Density:         7.60 g/cm3
Melting point:    1118 °C, 1391 K, 2044 °F
Boilling point:    1281 °C, 1554 K, 2338 °F
Solubility in water: 2.6×10−11 kg/kg (calculated, at pH=7, 8.6×10−7 kg/kg
Lattice constant: a = 5.936 Angstroms
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