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Products » Silica Gel
Silica gel is most commonly encountered in everyday life as beads packed in a semi-permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage of some goods. Because of poisonous dopants (see below) and their very high absorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents, but to throw them away instead. If consumed, the pure silica gel is unlikely to cause acute or chronic illness, but would be problematic nonetheless.
Silica
However, some packaged desiccants may include fungicide and/or pesticide poisons. It is not known whether these would be labelled specifically. Food-grade desiccant should not include any poisons which would cause long-term harm to humans if consumed in the quantities normally included with the items of food.
Amorphous Silica
Silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University, Baltimore, Maryland in 1919. Prior to that, it was used in World War I for the absorption of vapors and gases in gas mask canisters. The substance was in existence as early as the 1640s as a scientific curiosity.
In World War II, silica gel was indispensable in the war effort for keeping penicillin dry, protecting military equipment from moisture damage, as a fluid cracking catalyst for the production of high octane gasoline, and as a catalyst support for the manufacture of butadiene from ethanol, feedstock for the synthetic rubber program.
Amorphous Silica Gel
Silica gel's high surface area (around 800 m²/g) allows it to adsorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating to 150 °C (300 °F) for 1.5 hours per litre of gel. Some types of silica gel will "pop" when exposed to enough water.
Ferrous Ammonium Sulphate
Mohr's Salt, ammonium iron sulfate, is a double salt of iron sulfate and ammonium sulfate, with the formula [NH4]2[Fe][SO4]2·6H2O. Mohr's salt is preferred over iron(II) sulfate for titration purposes as it is much less affected by oxygen in the air than iron(II) sulfate, solutions of which tend to oxidise to iron(III). The oxidation of solutions of iron(II) is very pH dependent, occurring much more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which prevents this oxidation occurring. The relevant equation for this is:
4 Fe2+ + O2 + (4+2x) H2O, 2 Fe2O3.xH2O + 8 H+
The presence of protons keeps this equilibrium to the left, the Fe(II) side.
Ferric Ammonium Sulphate
Inhalation : Causes irritation to the respiratory tract. Symptoms may include coughing, shortness of breath.
Ingestion : Low toxicity in small quantities but larger dosages may cause nausea, vomiting, diarrhea, and black stool. Pink urine discoloration is a strong indicator of iron poisoning. Liver damage, coma, and death from iron poisoning has been recorded.
Skin Contact : Causes irritation to skin. Symptoms include redness, itching, and pain. May cause skin discoloration with irritation.
Eye Contact : Causes irritation, redness, and pain.
Sodium Chlorite
The free acid, chlorous acid, HClO2, is only stable at low concentrations. Since it cannot be concentrated, it is not a commercial product. However, the corresponding sodium salt, sodium chlorite, NaClO2 is stable and inexpensive enough to be commercially available. The corresponding salts of heavy metals (Ag+, Hg+, Tl+, Pb2+, and also Cu2+ and NH4+) decompose explosively with heat or shock.
Sodium chlorite is derived indirectly from sodium chlorate, NaClO3. First, the explosively unstable gas chlorine dioxide, ClO2 is produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, sodium chloride, sulfur dioxide, or hydrochloric acid). The chlorine dioxide is then absorbed into an alkaline solution and reduced with hydrogen peroxide, H2O2 yielding sodium chlorite.
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