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Edited by NORMAN V. STEERE, 140 Melbourne Ave., S.E. Minneapolis, Minn. 5541 4
XCV. Safe Handling of Perchloric Acid in the Laboratory Lowell A. Muse, University of Georgia
Following a perchloric acid explosion in a University of Georgia. Research Laboratory, a report was drawn up outlining t h e hamrdous properties of perchloric acid. The University of Georgia. Safety Services Department, working closely with research and administrative peruonnel, prepared what is felt t o be a practical set of recommendations for the safe handling and storage of perehloric acid.
Safe Handling of Perchloric Acid I n recent ,years perchloric acid has became an indispensible tool in some types of chemical analysis. Although numerous accidents have been reported, many anslysts are not yet, sufficiently aware of the hazards connected with improper use of perchloric acid. This fact was forcefully brought t o the attention of the Public Safety Division a t the University of Georgia recently when a.perchloric acid explosion wrecked a. fume hood and hospitalized a worker. This explosion was caused by t h e use of an excessively large sample and poor technique. The worst known explosion of perchloric acid is described by Schumacher (8). A plastic holder was introduced into an electroplat,ing tank oontaining approximately 150 gal of perchlario acid and 70 gal of acetic anhydride. The ensuing explosion killed seventeen persons, wrecked 116 buildings and caused damages estimeted a t two million dollars. The mushroom cloud resulting w a reminiscent of an atomic explosion. The Handbook of Laboratory Safety, ( 5 ) reporls several accidents caused by ( a ) perchlorate deposits in improperly constructed fume hood systems, ( b ) contact of perchlorates with organic or other flemmitble materials during an analysis, ( c ) contact of perchloric acid with wooden flooring, or absorbent materisl used for cleaning u p spills, with resultant fire or explosion after long lat,entperiods. T h e following observation is found in the above mentioned handbook: "Perhaps the most disturhing features of accidents involving perchlaric acid are ( a ) the severity of the accidents and ( b ) that the persons involved are, in the majority of cases, experienced workers."
In order t o ondersband the hazards of perchloric acid some basic facts are necessary (6). Commercially available perchloric acid is approximately 7 2 7 , acid in water. I t is n strong acid s t room temperature, but is relatively stable and may be stored for extended periods in glass bottles provided there is no contact with oxidizable materid As the temperature of perchloric acid is raked i t becomes an oxygen donor. At the boiling point of t h e 729'' acid, it,s oxidation potential is such t h a t i t will react violently with organic matter or other combustible material. Many Iaboratory explosions have resulted from this property. Under ordinary conditions t h e concentration of perchloric acid will not exceed 72%. At this concentration, a n ," acid-water szeotrope is formed which boils a t 203°C. If, however, in the course of chemical manipulations the acid comes in contact with a strong dehydrating agent, anhydrous perehloric acid may be formcd. The most common dehydrating agents used with perchloric acid are sulfuric acid, qhasphorus pentoxide, and acetic snhydnde. Anhydrous perchloric acid is extremely unstable. I t will explode upon contact with organic material. Fortunately, i t is usually dilnted somewhat by the dehydrating material. The movt dangerous erne is where perchloric acid and acetic anhydride are combined in near equimolsr proportions as in the explosion described by Schomacher. Pure anhydrous perchloric acid will usuitlly explode spontmeously if stored for more than approximately 30 days. Even slight impurities may reduce this period of limited stability. The hazards of perchloric acid may he divided into three categories: 1 . Contact of Organic or Combustible Material wilh the Hot Acid. This ~sllhlly oocurs %,hen perchloric acid is added t o n sample which has not been suiiiciently treated to remove easily oxidized material. Nitric acid is generally used for this purpose. Perchloric acid should be added t o a sample in minimum qnantity after thorough treatment with nitric acid. G. Frederick Smith (9)suggests m alternate
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method. Taking advantage of the fact that perchloric aeid increases stepwise in oxidation potontisl as the temperatnre is raised, Dr. Smith starts with a mixture of nitric and perehloric acid in contact with the sample. The temperatnre rise is carefully controlled by inserting a thermometer in the reaction flask and using a reflux condenser t o control evaporation. The condenser is constructed with a stopcock such that the condensate may be drained off when a rise in temperature is desired. I n this way, t h e nitric acid is allowed to react with t h e sample while the oxidizing potential of the perrhloric acid is law. As water and nitric acid are lost, the temperature rises and perchloric aeid increases in oxidizing potential If the temperatnre is controlled properly, all easily oxidized material is removed by the nitric acid before the oxidizing potential of perchloric acid reaches s. dangemuslevel. 2. Formation of /he A n h & m s Acid. T h e anhydrous acid is somet,imes formed accidentally while manipulating a sample. At other times i t may be formed deliberately in an effcxt t o obtain a higher oxidation potential and speed u p the oxidation of a recalitrant sample. Occasionally the pure anhydrous acid may be prepared. Researchers should be aware of the extreme instability of this compound. Even when diluted in sulfuric acid, it may react explosively with organic material.
3. Formation of Unstable Perchlorates. Perchloric acid, even nl room temperahwe may formunstable perchlorntes. Reagent bottles which have been stored for a n extended period of timc after being opened may contain perehlornto c~.ystali. These crystals, formed around tho bottle cap, have been known l a explode from fl.iction when an attempt was made t o unscrew the cap. Samples contsining fats, oils, glycerin, or other organic materials may form explosive snd unstable compomds when exposed to pexhloric acid. Xetnllic or organic perchlorates may be deposited in a n improperly designed fume hood system. The grouting material ~ a e dt o seal stoneware hoods, and lubricants u e d in the blowcr system may become explosive upon erposnre t o perehlol.ic mid. Some metnls used in doctwork systems may react with perchloric acid to f w m nnstable componnds. Bends, horizontal rnns, and porous construction materials cmws the deposiiion of flammable and explosive materials in the exhaust
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system. These hanerds may be eliminated by using an adequate hood with a. properly designed blower and vent system (7). Of utmost importance are (a) a. vertical exhaust system which will not collect dust or condensate, ( b ) avoidance of the use of organic greases, grouling materials, or other combustibles in the fume hood system, ( e ) installation of a washdown system in the ductwork t o wash out all deposits daily. I t has been estimated that some serious explosions have involved deposits of as little as 1g. Following the ~erchloricacid explosion a t the University of Georgia, the following recommendations have been prepared for safe use of this material:
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FACILITIES
1. Storage
(a) No more than one 450 g (1 Ih) bottle should be s h e d in one hmd. (b) The bottle should be kept in a deep glass tray large enough to hold the cont e n t , of ~ the bottle in case of breakage. ( e ) Large quantities of perchloric acid should be stored in a detached building constrocted of noncombustible material such as brick, tile, concrete blocks, etc.
(d) Shelves in t h k building should be of noncombustible and nonabsorbent material. ( e ) No combustible materials should be stored in the buildine. (f) Electric fixtures should he of the vapor proof type to protect against corrosion fromacid fumes. ( 8 ) Special ventilation is not required. 2. Eqnipment (a) Perchloric acid digestions should be carried out only in an adequate fume hood. No oreanio materials should be stored in the hood. (b) Use of s vapor extractor is recommended where practical, such as that described in references (3) and (4),which draws the acid fumes into water and protects the fume hood from acid vapors. ( e ) An oil bath should never be used for heating perchloric acid. (d) The best heat-resistant glassware should be used t o avoid breakage and spills. ( e ) The laboratory should he equipped with a safety shower and a fire extinguisher. (f) Beaker tongs capable of safely himdling thc equipment should be obtained. Standard laboratory tongs are notoriously poor for this purpose. (g) Portable shields and goggles should be available and should be used in any case where there is a question as t o the camplete safety of a reaction.
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(h) Safety glasses should be worn at all times. (i) A carrier for transporting bottles of scid from the storage area to the laboratory ares should be obtained. This carrier should he capable of protecting the glass reagent bottle and also of containing the acid if breakage does occur. (j) The floor around 8. perohlorie acid fume hood should he of inorganic material and should not be waxed.
B. LABORATORY OPERATIONS 1. Sample size should be generelly limited to 1.0 g; in no case should more than 3.Cb5.0 g of combustible material be used. 2. The use of dehydrating agents should he avoided wherever possible. 3. Samples should he thoroughly treated with nitric acid before the addition of perchloric acid unless s. reflux system such as that described by Dr. Smith in reference (9) is used to carefully control the rate of temperature rise. 4. A minimum amount of perchloric acid should he added (preferably dropwise) after predigestion with nitric acid. 5 . If sulfuric acid must be used, then a moderate excess should be added to take
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advantage of the dilution effect on the anhydrous perchloric acid formed. 6. Special care should be taken to avoid mixing equimolar amounts of perchloric acid and acetic anhydride or any other dehydrating agent. 7. A maximum limit oi 20 ml of perchloric acid should be added per sample. 8. Care should be taken that samples do not boil dry. 9. Perchloric acid spills should he washed up immediately with plenty of water. 10. Organic material (such as paper, wood, or grease) which has been exposed to perchloric acid should be regarded as a fire or explosion hseard. 11. I t should be kept in mind that samples containing enough water to prevent formation of the anhydrous scid are inherently safer than those in which sufficient water is not present. Water also reduces the explosive hazard from perchlorates and perchlorate esters. 12. The preparation and storage of pure anhydrous perchloric acid should be discouraged.
Literature Cited (1) B o n ~ o a H.. , nnrn PRAILL,P. F. G . , Andyst, SO. 4 (1955).
-..... properties,' ~ a & s o t u r e r , and Uses;. American Chemical Sooietv Monograph Series No. 146, Reinhold, N e w York, N. Y . , ~
(4) S a r l ~ x .G. F~enenrc=.Analyst. 80 (1955). (5) Evenem, K., in Handbook of Labomtoy Ssfety. Chemical Rubber Company, Cleveland, Ohio, 1967. (6) Chemical Safety Data Sheet 50.11. Perohloric Acid Solution. Manufacturing Chemists Associstion, Ino.. Washington. D . C., 1955. (7) Design Criteria for Perchloric Acid Fume Hoods. Board of Repents. University of Georgia. 1971.
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