Perchloric acid hoods

No organic materials should be stored in the perchloric acid hood. 

Perchloric acid is effective in dissolving certain solids but is not used here because it poses an explosion hazard, notably with carbon. The acid should only be used to dissolve carbon-containing materials where perchloric acid hoods are available and the chemist is fully trained and certified to work with perchloric acid for this purpose. 

Arsenic can be determined in urine and air, P CAM 139, by filter collection and hydride flame AAS. Samples are analyzed in 5 ml of 3 1 1 nitric sulfuric perchloric acids on a hot plate at 130-150°C. The sample may be wet ashed with 5 ml of 3 2 nitric sulfuric acids when no perchloric acid hood is available. The sodium borohydride is added to the sample and acidified to generate arsine. This operation should be performed cautiously in a well-ventilated area because arsine gas is extremely toxic. 

Alternatively, the following perchloric acid procedure can be used. Perchloric acid can be used on inert (glass or quartz fiber) filters or with filters where a pretreatment has oxidized most of the organic material. In no event should reactive organic compounds be in contact with fuming (hot, concentrated) perchloric acid, nor should perchloric acid be fumed without an adequate fume trapping system or the use of a perchloric acid hood. This procedure is not suitable for preparation of samples to be analyzed by ETA. For proper safety precautions refer to Perchloric Acid and Perchlorates by Schilt [15]. 

The method cited here has been in routine use since 1972 without incident. The only major precaution has been extreme care in preventing any sample from evaporating to dryness. All digestions are done in a perchloric acid hood. 

The 72% acid is perfectly stable when cold and cannot be made to detonate. The 85% acid is a white solid, m.p. 50 C, and is so stable it can be shipped by common carrier. However, the 100 % (anhydrous) is not stable. It is a heavy oily liquid and eventually will detonate. It should not come in contact with organic matter or easily reduced inorganic compounds. It must be kept cool and should not be made with the intentions of storing it. Perchloric acid hoods are available for using this acid when fumes might escape. These hoods are equipped with a wash down system to prevent any formation of anhydrous acid in the exhaust duct. 

You can filter cold HCIO4 solutions, but wash the paper thoroughly or it may explode if placed in a drying oven. Wash with NH4CIO4, if possible. Wash down the fumes. Use an air eondenser like that in Figure 41-1, p. 475 (inner part) as a minimum, if a perchloric acid hood is not used. Place lead shot in the flask to keep it from tipping and cover the shot with NaOH solution. Use a gentle suction. 

What is the difference between a perchloric acid hood and a regular hood and why ... 

A perchloric acid hood has a water wash down feature, because HCIO4 is not an oxidizing agent if it is dilute and cold. 

Hoods for Compliance with the Americans with Disabilities Act (ADA) Other Modes of Exhaust Perchloric Acid Hood Radioisotope Fume Hood Carcinogen Fume Hood Exhaust Ducts... 

Stainless steel Stainless steel may be attacked by some chemicals but type 316 stainless or equivalent is commonly used for the lining of perchloric acid hoods. Type 304 stainless may be used for radioisotope hoods which need to be easily decontaminated. Because of its vulnerability to some chemicals and its relatively high cost, it is not recommended for general purpose fume hood use. Among the problem chemicals for stainless steel are acids and compounds containing haUdes. 

Polyvinyl chloride (PVC) PVC offers good protection for a wide range of chemicals but is affected by some, including liquid ammonia, amyl acetate, aniline, benzene, benzaldehyde, bromine, carbon disulfide, carbon tetrachloride, chloroform, ether, fluorine, nitric acid, and firming sulfuric acid. Some perchloric acid hoods use an unplasticized version of this material. 

It is recommended that the interior fittings of a perchloric acid hood should be non-sparking and the lights should be explosion-proof This concept should be extended to any apparatus placed in the hood. With the dangers already represented by perchloric acid, there should be no contributory factors that could initiate an explosion. PVC ductwork can be employed instead of stainless steel, but it would be much less likely to remain intact in the event of a significant fire exposure. However, if the duct work is enclosed within a 2-hour fire-rated chase, as it usually should be, this would not be a serious drawback. 

Perchloric acid digestions or any procedure involving heating of perchloric acid must always be done in a fume hood designed as a perchloric acid hood. This type of hood... 

If perchloric acid has been used in a modem perchloric acid hood system, consisting of a stainless steel hood and duct system, welded seams, an automatic wash down system, which... 

Perchloric acid quantities maintainedat minimumlevels perchloric acid hoods available for hot perchloric acid applications... 

Specific systems serve isotope hoods, perchloric acid hoods, or other high-hazard sources that require isolation... 

Certain types of fume hoods and exhaust sources, such as perchloric acid hoods, should not be manifolded with other types of fume hood exhausts. In large buildings where the designer wishes to take advantage of the benefits of manifolded exhaust systems but wishes to isolate a few exhaust streams, a combination, or hybrid, of these two types of systems is usually the most prudent and cost-effective alternative. 

There are two special types of chemical hoods that we will mention only briefly—perchloric acid hoods and radioactive materials hoods." ... 

Operations using perchloric acid for digestions or heated dissolutions should be performed in perchloric acid hoods that are specifically constructed for this purpose. Perchloric acid, being extremely reactive, can produce explosive perchlorates with metals that are touch- or shock-sensitive. Thus, the interior lining of these hoods must be constructed of nonreactive materials (such as stainless steel) with sealed seams to facilitate decontamination. These hoods are normally fitted with equipment for washing down the interior of the hood exhaust system after perchloric acid use to prevent the buildup of explosive perchlorate residues. The entire exhaust system must be independent of other exhaust systems connected to other chemical hoods. These hoods must be clearly labeled as perchloric acid work only. Anyone using perchloric acid should seek special training in these operations. 

Operations using perchloric acid for digestions or heated dissolutions should be performed in perchloric acid hoods that are specifically constmcted for this purpose. Perchloric acid, being extremely... 

The perchloric acid hood is designed to minimize the buildup of recrystallized material, as well as the number of places available for perchloric acid condensation. The chamber of the hood is fabricated from seamless, coved type-316 stainless steel, which eliminates any joints or comers for crystals to collect. The ductwork must also be constructed from type-316 stainless steel or polyvinylchloride, and must be fabricated so that it extends vertically from the hood with no bends. Spray heads activated from the hood face are incorporated in the ductwork, permitting washing down of the duct system and air plenums of the hood. The water dissolves any residual perchloric acid crystals and the waste rinse water is drained into a trough located behind the hood baffle. 

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