Perchloric Acid: Common Questions

1. We just spilled some perchloric acid on the floor, what do we do?

A. Dilute the acid immediately with some water and neutralize it with soda ash, sodium bicarbonate or lime. Take up the solution with an inorganic based absorbent or with a mop, preferably a plastic foam type, or flush the neutralized spill to a floor drain with water. The absorbent material containing the perchlorate may be flushed to the sanitary sewer or placed in a landfill or leach bed. Any mops must be thoroughly washed with water. Do not mop up the spill with rags, paper towels, or sawdust and then put it aside to dry out. Such materials must be considered fire hazards. If the floor surface is bare concrete, painted concrete, asphalt, vinyl tiles, or ceramic, little hazard is posed by the spill, but the floor should be washed thoroughly with water. If the flooring is wood, such as is often encountered in trucks, the wood should be soaked thoroughly with water and then the contact area should be removed and replaced with a new flooring. Wood which has contacted perchloric acid or perchlorate salts may become quite flammable when it dries out and can catch fire with little provocation.

2. What will happen if hot perchloric acid gets spilled?

A. The result of such a spill depends greatly upon the answers to other questions: How hot? How concentrated? What materials are contacted? Under the worst set of circumstances, boiling concentrated perchloric acid spilled onto a combustible organic material, the result will be a fire or an explosion. Boiling concentrated perchloric acid spilled on an inorganic material or metal object will not give nearly so drastic a result. Hot, diluted perchloric acid (50%) or concentrated acid at temperatures under 100-120oC will not likely cause fires, but it must be remembered that it is still very corrosive. Hot, concentrated perchloric acid will cause fire with many things not really thought of as combustible, such as rubber gloves, polyethylene tags, and human skin, so caution is an absolute requirement when using it.

3. If we have a fire and perchlorates are present, what type of extinguisher should we use?

A. The only extinguisher for fires involving perchlorates is water. Dry chemicals and CO2 may help to prevent the spread of the fire, but water will also do this. Water is also the best preventive measure against the occurrence of such fires.

4. I got some perchloric acid on my hands accidentally but I didn't get burned. Is there something wrong with this bottle?

A. While perchloric acid is the strongest of the mineral acids, it is surprisingly mild on skin when cool. In fact, for a demonstration only, the author has washed his hands with cold concentrated perchloric acid. We do not recommend this as there are generally better cleansers available. The immediate effect on contact with perchloric acid is a warming of the area due to the acid reacting with moisture on and in the skin. Washing off with water prevents further reaction. Prolonged or repeated contact will result in the loss of some skin or even deeper effects. Any cuts or scratches in the skin will be immediately felt. The eyes and mucous membranes are much more sensitive, and eye contact is quite dangerous, still not as bad as sulfuric acid or nitric acid, but must be avoided. Therefore, eye protection is a must at all times while handling any mineral acid.

5. How are perchloric acid wastes handled?

A. Generally speaking, the neutralized (pH 6-10) waste from small scale perchloric acid uses can be flushed to the sanitary system. Perchlorate does not constitute any hazard in sanitary plumbing, nor is the concentration likely to build up in the surface waters. Much of the perchlorate may be degraded to chloride through the action of bacteria in sewage treatment systems. Disposal techniques that call for adding a reducing agent such as ferrous sulfate or sodium bisulfite are only adding extra pollutants to the effluent. Such agents do not lessen any risks and do not cause the reduction of any perchlorate under the conditions present.

Suitable treatment of waste perchlorates on a large scale is being studied, but will probably come down to two choices: incineration of the wastes (thermal decomposition) yields primarily chloride in the form of a salt or hydrochloric acid; biological reduction with suitable bacteria may prove to be useful for certain waste streams, particularly if there is a lot of water and organic matter to treat.

6. Perchloric acid is listed by the U.S. Department of Transportation as an oxidizer. Doesn't it also qualify as a corrosive?

A. Yes, perchloric acid is corrosive and we recommend using both oxidizer and corrosive labels on the packages. Actually, perchloric acid as it is shipped is not an oxidizing material, only a potential one. At 70% concentration and normal outdoor temperatures (less than 40oC) perchloric acid will oxidize nothing, so the DOT oxidizer category is actually incorrect. However, DOT does not show any interest in correcting the situation, so the shippers will just have to go along. Of course, the initial danger of a spill is the corrosion of nearly anything it contacts, including skin and eyes. Long-term dangers include the formation of perchlorated cellulose fibers in wood, which are easily ignited. Perchloric acid is oxidizing only at elevated temperatures (more than 150oC).

7. We have heard about some environmental concerns involving perchlorates. What is happening?

A. Improper disposal or spillage of large amounts of perchlorate has resulted in the contamination of groundwater in several areas. The U.S. EPA and several state agencies have initiated studies to identify the sources of the contamination, treat the water, and to determine the health effects of perchlorate when ingested at low levels. Perchlorate in higher amounts (about 200 mg/day or more) has been used to treat hyperthyroidism by displacing some of the iodine used in the thyroid gland. At levels encountered environmentally, and at levels commonly encountered by workers who were exposed, the risk of health effects such as hyrothyroidism or enlarged thyroid glands are reported to be the same as for unexposed groups. We can be assured that there will be many more studies to come. A government report on the presence of traces of perchlorate in fertilizer was issued in 2001, and concluded that while there are traces of perchlorate in Chilean nitrate deposits, there is no evidence for

8. A bottle of perchloric acid has been in our storage area for many years. Is it safe to use or even move it?

A. A bottle of old perchloric acid is perfectly safe to move and use and requires no more precautions than any new bottle. The acid is completely stable and no degradation or change in the product will be observed in a hundred years on the shelf if one assumes that the integrity of the container has been maintained. However, the plastic caps used on the bottles probably will not last that long, so it is a good idea to inspect the caps of acid which has spent a long time in storage and replace any which appear to be deteriorating.

9. Crystals have formed on the outside (around the neck and cap) of a bottle of perchloric acid in our corrosives storage cabinet, what do we do?

A. The crystals that form around the neck of the bottles of perchloric acid (as well as nitric acid and hydrochloric acid) are due to escape of ammonia or another amine into normal lab atmospheres, particularly the storage cabinet. Such a deposit of ammonium perchlorate can be washed off with water. The bottle should then be dried and placed back in the storage area.

10. The acid in an old bottle labeled "e;perchloric acid"e; has turned dark and has crystals forming in the bottom of the bottle. What should we do?

A. This is probably the one case where one calls for help. he perchloric acid (if present at all) has quite obviously been grossly contaminated and a potentially dangerous situation exists. Under most circumstances, the cap may be removed, water added gently and the acid then neutralized by pouring into (or submersing in) water containing a little soda ash. However, it is probably best to call the bomb squad and have it removed for demolition. The chances are that a bottle found in this condition will not contain perchloric acid; that someone has used the bottle for storage of something else or has perpetrated a deliberate hoax.

11. What materials of construction are suitable to handling perchloric acid?

A. The recommending of materials may depend on the end use, but several materials are good for any use. These are glass, TeflonJ, and DurironJ. Hot perchloric acid can be handled in any of the three. For cold acid, PVC can be used and is also good for hood and ductwork. For dilute solutions polyethylene and polypropylene will hold up well and can be used for handling of cool concentrated acid. "e;O"e; rings and seals made of one of the fluorocarbons such as "e;VitonJ"e; have been found to be acceptable. For heat transfer and/or lubrication PCB and "e;FluorolubeJ"e; have proved to be satisfactory. Fume hood construction of stone, PVC, or transite is preferred, and stainless steel will perform well if it is washed down thoroughly on a regular basis.

12 What safety equipment is recommended while handling and using perchloric acid?

A. Due to the corrosive nature of the acid toward the eyes, it is absolutely necessary to wear safety glasses, goggles, or a full-face shield at all times. Rubber gloves and a rubber or plastic apron may save some discomfort or clothing as even a small drop of acid can become irritating or show up as a large hole the next time a garment is washed. If the acid is to be heated and some will be allowed to volatilize, it is necessary to provide for vapor control such as using a fume hood (more on these later). If digestions of organic matter or other unfamiliar reactions are being done, it is wise to experiment on small portions and use a safety shield or close the sash on the fume hood. If one is handling or transferring large volumes, it is recommended that a water hose be kept handy to dilute any spills.

13. Our safety officer limits quantities of perchloric acid in our laboratory. Is this necessary?

A. Obviously not. Perchloric acid in the lab or the acid storage area in the lab is no more hazardous than the other acids there. Anyone who wants to do something foolish or careless with perchloric acid can make an utter mess of the place with a pint as easily as with a gallon. In the best instance, a safety officer who acts in this matter knows something about perchloric acid, but wants to call attention to potential hazards. Normally, though, it is a reaction to fear brought about by ignorance. Safety personnel who would deny access to the use of this product by rational, technically competent, scientific and engineering people need additional education.

14. I was told to store perchloric acid in the refrigerator. Why?

A. The reasoning behind this is a total mystery to us. There is no reason whatever to store perchloric acid anywhere except with the other mineral acids, such as nitric acid or sulfuric acid. Refrigerated storage is normally reserved for things that decompose readily at room temperature or are potentially unstable. These compounds are exactly the wrong type of materials to keep with perchloric acid. Thus, the refrigerator is the second worst place to keep it. The "e;flammables"e; cabinet is the worst.

15. What is the shelf life of perchloric acid?

A. 70% perchloric acid will keep on the shelf forever if package integrity is maintained. It is not subject to any form of decomposition, even if the storage area is an unusually warm or cold room.

16. Is a special perchloric acid fume hood necessary for all uses of perchloric acid?

A. No, definitely not. The purpose of a fume hood is to remove the hazardous vapors or gases generated in the work being done. Use of cold or dilute perchloric acid does not result in any hazardous vapors or gases and does not require a hood at all. If other vapors or gases are generated such as hydrogen or hydrogen sulfide, any fume hood can be used. The perchloric acid fume hood is intended for removal of acid vapors that result when perchloric acid is evaporated and vented to the atmosphere. This only happens when perchloric acid is concentrated and heated to a point where the vapors can escape the container. At room temperatures, concentrated perchloric acid evaporates very slowly. In fact, it will absorb moisture from the atmosphere to which it is exposed until a balance is achieved in concentration and relative humidity.

17. We don't have a perchloric acid fume hood and only do an occasional procedure where we have to evaporate some perchloric acid. What should we do?

A. Several miniature fume hoods (or similar equipment) are available. GFS Chemicals has a fume eradicator which can be used as a hood over a single digestion. The Bethge apparatus is a device for condensing the acid fumes and refluxing or removing the condensate and is also available from GFS. Kjeldahl racks with a fume collection system have also been adapted for this purpose. The author does not recommend Kjeldahl flasks for perchlorate oxidations, however.

18. Our old fume hood has been used for all kinds of general lab uses including a lot of perchloric acid digestions. It may never have been washed out. Is it a hazard?

A. Definitely, depending on the construction of the hood and a lot of other factors, it could be a potential bomb.

19. How can we clean up a hood that has been misused, i.e. Question 18?

A. Perchlorated dusts and residues will find the natural collection points in the system and must be removed. This is best accomplished by use of water under some pressure to help dislodge any stubborn particles as well as dissolve soluble salts. All cracks and joints in the system should be thoroughly flushed out, including the full length of the ductwork. The fan, if it is in contact with the air from the hood, must also be cleaned and bearings must get particular attention. No hood fan that has exposed greased bearings should be used on a perchloric acid fume hood. A fan with external bearings or a venturi system where there is no contact between fan and fumes are the acceptable choices.

20. How can we remove a hood that has been used for perchloric acid digestions?

A. If the hood has been regularly maintained and washed down, the routine wash down procedure is followed and the hood is dismantled. If the hood has not been regularly washed down, it must be thoroughly cleaned (see Question 19) and then dismantled while wet. In fact, it is best to use a small stream of water during the operation on any contact area to be cut, struck, or otherwise taken apart.

The issue of fume hood safety and disassembly has been addressed in various forums. We recommend an excellent article by Richard J. Kelley published in the May/June 2000 issue of Chemical Health and Safety (pp 5-10; an ACS publication). It also cites extensive work done by Oak Ridge National Lab Industrial hygienists on the problem of decontaminating and disposing of perchlorate fume hoods. Additional information can be obtained from Mr. Marwan Bader at ORNL (865-576-1231; e-mail: b6v@ornl.gov.

21. How can perchlorates be identified in residues and wastes or in the lab environment (hood or sink)?

A. There are several ways to identify perchlorates or detect them. A perchlorate-sensitive electrode has been developed for use with a specific-ion meter which will detect perchlorate as low as about 1 ppm in aqueous solutions. It is ideal for looking at wastes or a wash solution from a fume hood. Several other tests can be used for generally high concentrations. Perchlorate can be precipitated with tetraphenylarsonium chloride, but the precipitate must be identified by infrared examination. It will precipitate with zinc-free methylene blue, or with potassium ion in larger quantities. For a review of these methods, see the GFS publication Perchloric Acid and Perchlorates, by A.A. Schilt and the Second Edition of Perchloric Acid and Perchlorates, published in 2003. (See list of publications on page facing inside back cover)

22. The handbooks say that anhydrous perchloric acid is explosive. Under what conditions is anhydrous perchloric acid formed?

A. Anhydrous perchloric acid is formed when concentrated perchloric acid is mixed with a strong dehydrating agent such as fuming sulfuric acid (SO3), anhydrous magnesium perchlorate, or phosphorus pentoxide. It can also be formed by mixing a strong mineral acid, such as sulfuric acid, with an anhydrous perchlorate salt, such as potassium perchlorate. Anhydrous perchloric acid is not stable and will undergo a spontaneous detonation, usually within 2-4 weeks at room temperature, due to the accumulation of decomposition products.

23. Why isn't there danger of anhydrous perchlorate acid being formed by evaporation of water from a solution?

A. Perchloric acid forms a constant-boiling azeotrope with a composition of about 72.5% at atmospheric pressure when heated or allowed to evaporate in a dry room. Under normal temperature and humidity conditions, this azeotrope will actually pick up water and the concentration will change to somewhere between 50 and 70%. Leaving the cap off a full bottle of 70% perchloric acid will result in the bottle overflowing due to the absorption of water.

24. We use perchloric acid mixed with other acids. Can we premix these acids and store them?

A. Generally speaking, it is all right to premix perchloric acid with the other acids with which it is routinely used, i.e. nitric acid, sulfuric acid, phosphoric acid and acetic acid. Solutions of perchloric acid and acetic acid do not always store well due to organic impurities in the acetic acid. Such solutions may discolor considerably on standing, maybe sooner if allowed to heat up while mixing. Mixing of all the acids mentioned should be done by adding, slowly and with sufficient cooling, the acid with the higher specific gravity to the lower.

25. I use perchloric acid titrant in non-aqueous work. Isn't the perchloric acid actually anhydrous in this work?

A. No, the perchloric acid dihydrate (72%) that you start with ionizes completely in a solution of acetic acid to hydronium (oxonium) ion and perchlorate ion (H3O+and ClO4-). The acetic anhydride added to such a titrant absorbs only the excess water from the perchloric acid and other incidental water. This ionization is the same in many other solvents and, in fact, perchloric acid dihydrate may be a misnomer, perhaps oxonium perchlorate monohydrate would be a better name.

26. Can other organic solvents be used with perchloric acid and perchlorate salts?

A. Perchloric acid and many perchlorate salts are soluble in a variety of organic solvents and are often used with them. Generally, several things should be avoided and nothing should be taken for granted. High concentrations and high temperatures should be avoided. Solvents, which would readily form esters (alcohols, for instance), should not be heated with or evaporated from perchloric acid or perchlorate salts as shock-sensitive compounds may be formed. Generally, it is inadvisable to evaporate any solvent containing any perchlorate unless a large amount of water has been added and the solvent can be removed before the water will be volatilized. Inadvertent synthesis of unstable organic perchlorate has been the reason for many accidents. Use of a safety shield and proper personal protection is mandatory in experimentation with such systems.

27. Has safety data been generated for perchlorates in organic systems?

A. The behavior of perchlorate salts in organic media has been studied for decades. Much of the data that have been published has been derived from research on battery electrolytes containing up to 25 wt.% of anhydrous lithium perchlorate. Most chemists who have taken the time to become truly familiar with these systems consider them to be non-hazardous under anticipated operating conditions due to the inertness of the perchlorate.

GFS is sponsoring an ongoing program to clarify safety issues regarding the behavior of perchlorate salts In organic media and synthetic processes. Adiabatic Calorimetry performed on a saturated solution (ca. 50% w/w) of anhydrous lithium perchlorate in diethyl ether provides one clue to perchlorate stability in this volatile environment. As the system temperature was increased in ten-degree increments to 150°C, the mixture was not able to demonstrate a self-sustaining exotherm. Upon reaching about 160oC, a controlled exotherm was seen to occur in the system. This behavior continued to approximately 220o C, at which point the exotherm proceeded in an uncontrolled fashion.

Although data gathering will continue for some time, this behavior appears to be consistent with observations derived from perchloric acid oxidations, which typically proceed most rapidly when a temperature around or above 150o C is applied. Other factors (e.g. catalytic agents) may affect these properties, so one is strongly cautioned not to make assumptions in this regard without elaborate, small scale sample testing under a wide range of conditions of temperature, concentration, and reaction substrate composition.

28. Perchloric acid has been used in electropolishing of metal parts. Is this effective?

A. Electrolytic bright polishing using perchloric acid solutions was certainly one of the best electrolytic polishing techniques ever devised. However, in 1947 a polishing operation in Los Angeles was destroyed in an explosion, which was subsequently blamed on the faulty operation of the perchloric acid process, and it has largely been discontinued in this country. It has since been used in other parts of the world without reported incident. Due to the widespread destruction in the Los Angeles explosion, it seems apparent that a gas phase explosion had occurred, but it was demonstrated that there was a possible combination of the ingredients used in the process that could explode if sufficiently stimulated.

29. Hasn't perchloric acid been responsible for a number of accidents?

A. Responsibility for accidents rightly belongs on the operator. Perchloric acid can only act as perchloric acid. There have been some incidents where damage has been done (see Question 28 above), but in all cases it could be shown that a person is responsible. Unfortunately, the person who gets hurt is not always the one who is responsible. There have been workmen hurt removing hoods and ductwork that had not been properly treated. There have been incidents where someone was merely duplicating what someone else had done and published in the literature, but did not turn out to be as lucky (or wasn't informed of exact techniques). That is one reason not to take for granted everything one sees in print or hears from a colleague. Most accidents can be simply avoided by having the patience to consider safety aspects, in perchlorate chemistry as well as in all other parts of ones life.