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Edited by NORMAN V. STEERE, 140 Melbourne Ave., SiE. Minneapolis, Minn. 55414
XCXIII. Perchlorate Salts, Their Uses and Alternatives Wayne C. Wolsey, Ph.D., Associate Professor of Chemistry, Macalester College. St. Paul. Minn. 55105
Perchlorate salts renresent a class of compounds of which it.might truly be said "familiarity breeds contempt." The salts are frequently more stable and less reactive than the more generally respected perehloric acid. This unreactivity, coupled with certain unique properties of the perehlorate ion. has led man" chemists to use perchlorates as common laboratory reagents without always keeping in mind the latent explosive property which is always present. Perehloric acid is generally regarded with deference by chemists because of some widely-publicized explosions. Muse (I) recently treated recommended proeedures for safe handling of perehloric acid in the laboratory and Everett and Graf (2) have treated safety aspects of both perchloric acid and perchlorate salts. Little has been said, however, about why perchlorate salts are so extensively used and what possible alternatives there might he to their routine use in certain laboratory procedures. The author was motivated to prepare this report after losing a finger as a result of a perchlarate explosion. The basic chemistry of perchlorates and their applications were extensively reviewed in the American Chemical Society monograph edited by Schumacher (3), and other treatments have been written by Pearson (4) and Addison (5).
UNIQUE PROPERTIES OF PERCHLORATES Perchlorates are of great chemical interest and importance because of several unique properties. Among these properties are the following: 1. High Degree of Ionic Character. The perchlorate has an extremely high electronegativity, 4.244, as estimated by Sanderson (6). This means that the ion should have a very high affinity for both cations and solvent molecules. The former would mean high lattice energies, as reflected in the low solubilities of the heavy alkali metal perchlorates in water. The latter would mean high solvation energies. 2. High Solubilities in a Variety of Salvents. The high solvation energies would be a major factor in the high solubilities of perchlorates in a large num_ber of solvents. Except for the heavy alkali metal perchlorates and a few large cations such as nitron and tetraphenyl arsonium chloride (both used as gavimetric precipitants for
perchlarate), all perchlorates are soluble in water. The high solubility of perehlorates in non-aqueous solvents has made them a logical choice to consider in doing chemicalstudies in the absence afwater. 3. "Inertness" Toward Oxidation Or Reduction. Perchlarates are essentially not electrachemically oxidized or reduced. This property has made them of great value in eleetrochemical studies in a variety of solvents. The perchlorate ion also appears to he kinetically inactive as a chemical oxidizing agent in aqueous solution unles's the solution is acidic and hot. This same chemical stability also is usually observed when one is working with non-aqueous solvents. 4. Poor Camplexing Ability. Although many coordination compounds have been made in which a perchlorate ion is directly bonded to a metal ion, the general rule is that the perchlorate ion is used when a non-complexing anion is desired.
LABORATORY USAGE OF PERCHLORATE SALTS 1. Drying Agents. Anhydrous magnesium and barium perchlorates have been widely used as desiccants and dehydrating agents (8). They give a partial pressure of water in a desiccator as low as phosphoms(V) oxide and avoid the problems of stickiness and scum-formation associated with the usage of the latter. Unfortunately, this has caused some serious accidents when a desiccator charged with a perchlorate was used around certain organic solvents (2). 2. Complex Ion Studies. Perchlorates have been widely used in studying metal complexes in water solution with minimal hazard, but for a variety of reasons the poor coordinating property of perehlorates has led people to continue the studies in the absence of water. In order to avoid competition with the more strongly coordinating water molecules, studies involving the interaction of weak ligands with metal ions necessitate the usage of anhydrous metal salts. Attempts to dehydrate hydrated metal perchlorates by heating under vacuum frequently result in partial evolution of perchloric acid from the salt and formation of mixed metal hydraxy-perchlorate salts (3). This might be expected, due to the high hydration energy of many metal ions.
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2,2-dimethoxypropane has been used to dehydrate metal perchlorates, resulting in the formation of methanal-solvated metal perchlorates (8). A few explosions have occurred in using this procedure, however (9, 10). Recently, the use of triethyl-o-formate has been suggested for dehydrating metal perchlarates (11). Another reaction which was used with some success was the oxidation-reduction reaction between nitrosyl perchlorate and metals in solvents such as ethvl acetate and acetonitrile (12). This reaEtian avoided the use of hydrated metal perchlorates as starting materials. Same of the perchlorates ~ r e ~ a r ebyd this method turned out to be quite covalent in nature. 3. Electrochemical Studies. Perchlorates have been widely used in electrochemical studies for several of the reasons discussed previously. Among the most common uses are alkali metal or tetraalkyl ammonium perchlorates as supporting electrolytes in voltammetry in both water and nonaqueous solvents, and the use of transition metal perehlorates to study the electrochemical properties of the metals.
PRECAUTIONS TO TAKE WHEN WORKING WITH PERCHLORATE SALTS In general the dangerous perchlorate salts are the covalent ones, such as heavy metals and organic perchlorates. These will be shock-sensitive and should he handled only in small quantities. The ionic perchlorates, such as the alkali metals, alkaline earths, and the rare earths are of higher stability. In any case, whenever any perchlorate is used in the presence of an organic material, a potentially shock-sensitive mixture is present. Care should be exercised in using a spatula or stirring rod to mechanically agitate any solid perchlorate. The use of any perchlorate in the presence of an organic material in admixture with a strong acid is strongly discouraged.
ALTERNATIVES TO PERCHLORATES IN NORMAL CHEMICAL STUDIES Since the use of perchlorates is potentially laden with the possibility of an explosive reaction, the use of alternative species should certainly be considered, whenever possible. No one ion will have all of the same unique properties of perchlorate, hut in many cases perhaps nitrates, sulfates, or even halide ions might he used without seriously affecting the nature of the chemical information which is being sought. These all have same undesirable chemical features, but they might be used in some situations. (Continued on page A336)
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A series of ions which might be useful in many situations are the fluoroanions, such as tetrafluomborate B F c and hexafluorophosphate PFe-. These salts have the same poor complexing ability as perchlorates and are soluble in a variety of solvents (13). Many of them are now eommercially available, and they can always be prepared by neutralization of the acids with metal hydroxides, oxides, or earbonates. The problem of dehydration to obtain anhydrous metal salts is still present, hut the explosive problems are circumvented with their usage. It should be noted however, that these salts in the presence of strong acid may evolve hydrofluoric acid, HF, the properties and handling teehniques of which have been extensively covered (14). It would appear that there is no single substitute for the ubiquitous perchlorate ion, but a t least some viable alternatives exist. The usage of an alternative ion cannot be recommended too strongly.
Literature Cited (1) Muse, L. A., J. CHEM. EDUC., 49, A463 (1972). (2) Everett, K., and Graf, F. A., "Handbook of Laboratan, Safety." ed. hv N. V. Steere, 2nd Ed., - ~ h e m i e i l Rubber Company, Cleveland, Ohio, 1971, p. 265.
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Schumacher, J. C., "Perchlorates, Their Properties, Manufacture and Uses," American Chemical Society Monograph Series No. 146, Reinhold, New York, 1960. Pearsan, G. S., in "Advances in Inorganic and Radiochemistry," ed. by Emeleus, H. J. and Sharpe, A. G., Academic Press, New York, 1966, Vol. 8, p. 177. Addison, C. C. in "Supplement to Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistrv." Green and ,. Lanemans " Co., London, 1956, Supplement 11, Part I, p. 596. Sanderson, R. T., "Inorganic Chemistry," Reinhold, New York, 1967, p. 83. Smith, G. Frederick, "Dehydration Studies Using Anhydrous Magnesium Perchlorate." G. Frederick Smith Chemical Co., Columbus, Ohio, 1951. Starke, K, J. Inorg. and Nuclear Chem., 11,77 (1959). Diekinson, R. C., and Long, G. J., Chem. Eng. News, p. 6, July 6, 1970. Cramer, R., Chem. Eng. News, p. 8, Oct. 26, 1970. Mikulski, C. M., Chem. Eng. News, p. 7, Jan. 25,1971. Hathaway, B. J. and Underhill, A. E., J. Chem. Soc. 1960.3705. Sharpe, A. G., in "Fluorine Chemistry," ed. by Simons, J. H., Aca-
demic Press, New York, 1954, Vol. 2, p. 1. (14) Manufacturing Chemists Association, "Guide far Safety in the Chemical Laboratory," D. Van Nostrand Co., Inc., Princeton, N. J., 1954, pp. 101-103.
SAFETY REPORT ON PERCHLORATE EXPLOSION* OF MAY 2, 1972 Background In the course of a study on seetonitrile complexes of lanthanide salts, a batch of nearly anhydrous erbium perchlorate had been prepared. The last stage of the preparation of this salt (from hydrated erbium perchlorate) was an acetonitrile extraction of the Er(CIO& from the insoluble byproduct, basic erbium perchlarate. Evaporation of a small portion of this acetonitrile solution yielded the complex, Er(CH&N)+ (CIOn)s. The infra-red spectrum of this complex showed the presence of a small amount of residual water, presumably still bound to the erbium ion. It was decided to attempt to remove the residual water by heating the erbium perehlarate to 250"C., the highest temperature reached in the initial dehydration procedure. We had found that acetonitrile could be removed by vacuum pumping at room temperature, quantitatively, aecording to an outside microanalytical labarata-
Experimental Procedure The stock acetonitrile solution of erbium perchlorate, which contained about nine grams of erbium perchlorate, was taken to dryness with the aid of a vacuum line a t room temperature. The pink powdery substance, presumably, Er(CH3CNlr (CIOn)a, which was obtained from the evaporation stage was subjected to further pumping a t room temperature for three hours. The removal of considerable acetanitrile was noted by the appearance of frozen liquid in the liquid nitrogen trap. The flask was then placed in a metal can air heater for two hours under vacuum at IW'C. t o speed the removal of the acetanitrile. After this heating, mare acetonitrile had collected in the trap and the material in the flask had set up into a glassy substance. The flask was taken into the dry box and scraped with a metal spatula in an attempt to pulverize it. The flask cracked during the scraping procedure and about 15 of the original material was transferred to a 100 ml. round-bottomed flask. This second flask, containing pulverized material, was placed in the metal can heater and heated to 150°C. overnight. The next morning the flask was removed from the heater and allowed to cool. Again, more acetonitrile had collected in the trap and a glassy substance had formed in the flask. It was also noted that the temperature of the air bath had reached 177'C. The glassy material in the flask was still pink, but had a slight greyish cast. The flask was opened in the drybox after cooling for half an hour and a metal spatula was used to try to break up the glassy ma-
terial. The second or third scrape with the spatula caused an explosion,* 'severely damaging the middle and ring fingers on my left hand, which had been holding the flask. I had a pair of nylon gloves on under the rubber gloves of the drybox. The ring finger was subsequently amputated just below the last joint.
Probable Cause My conclusion is that some residual acetonitrile was still trapped in the glassy erbium perchlarate and that this material was shock-sensitive, as are many organiccontaining perehlorates. I may have erred in trying to speed up the removal of the coordinated acetonitrile. but it was rea~onnhlero assume the absence of arerunirrrlr under the crmdnimc uf heating described above
Recommendations for Future Work 1. The scraping of a potentially shocksensitive material such as this (ifi t must be done a t all) should be done with more protection, such as a shield around the flask, leather or asbestos gloves, or possibly even same remote-control apparatus.t 2. The use of perchlorates in chemical work should be minimized and alternative ions used wherever possible. ..A dmilar e ~ p l o ~ i ahas n been reported by E. J . Birnb ~ u m e n d SShatton . Ilnorg.Chem. 12.379 1197333. t Editor's Note: Quantity limits are discussed by Den"h 0.Nelson in "Safety Techniques for Research and Development of New High Energy Oxidizers." vhich appeaxed in this Journal and in "Safely in the Chemical Laborstory,"Chem. Ed.Pub.Co.. Easton, Pa.. 1967, p63.
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