January 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
one-half of the product has been distilled the process is discontinued and the distillate set aside. A clean dry receiver is then used to collect the second fraction distilled under the same conditions. The pressure may vary between the limits given. A pressure of 5 to 6 mm. is best and most easily obtained. The rate of distillation is of no consequence and the temperature varies between 60" and 95" C .depending upon the rate of application of heat. The product obtained by this process varies * 0.03 per cent in acid content and * 0.0004 in specific gravity. If the starting material is 71 per cent perchloric acid the conditions are the same except that 60 per cent is distilled and discarded. Starting with 70 per cent perchloric acid 65 per cent of the distillate is discarded. Perchloric acid of this standard strength is slightly hygroscopic and fumes very faintly. It should be weighed with the usual precautions to prevent absorbing atmospheric moisture. To prepare a solution of normal perchloric acid using the product obtained in the manner described a sample weighing 136.4201 grams (using brass weights and assuming average humidity and temperature) is weighed and diluted to 1000 cc. Comparison of Preparation of Constant Boiling Hydrochloric Acid with That of Standard Perchloric Acid
The preparation of standard perchloric acid can be compared with the preparation of constant boiling hydrochloric acid in the following manner:
55
The pressure of distillation must be known in the case of hydrochloric acid and the use of a barometer is required. With perchloric acid the pressure may vary over the range 2 to 7 mm. and a very simple pressure-recording device may be used and the barometer eliminated. The preparation of constant boiling hydrochloric acid must be carefully controlled aa regards the rate of distillation and prevention of bumping. The perchloric acid process does not vary with the rate of distillation which may be twice as great as in the former process, and the distillation is free from ebullition. One cubic centimeter of 73.60 per cent perchloric acid (sp. gr. 1.71) equals 2 cc. of constant boiling hydrochloric acid in equivalents of acid contained. Since perchloric acid can be distilled by the process just described at twice as great a rate, and since 50 per cent of the starting product is obtained in comparison with 25 per cent for the hydrochloric acid, the yield of perchloric acid obtained in a given time as compared to hydrochloric acid may be as much as eight times as large, Literature Cited (1) (2) (3) (4) (5)
(6) (7)
(8) (9)
Emster, van, Z . anorg. Chem., 62, 270 (1907). Foulk and Hollingsworth, J . A m . Chem. SOC.,46, 1220 (1923). Goehler and Smith, IND. ENG.CHEM.,Anal. Ed., 2, 48 (1931). Goodwin and Walker, Trans. Am. Eleclrochem. Soc., 40, 157 (1922). Koch and Smith, IND. ENG.CHEM.,Anal. Ed., 2, 41 (1930). Lubs and Clark, J . Wash. Acad. Sci., 6, 609 (1915); 6, 481 (1916). Smith, IND. ENG.CHEM.,18, 1216 (1926). Willard, J . Am. Chem. Soc., 34, 1480 (1912). Wyk, van, Z . anorg. Chem., 32, 115 (1902); 48, 1 (1906).
Dissociation of Concentrated Perchloric Acid during Vacuum Distillation at Moderately Low Pressures' New Method for the Preparation of Anhydrous Perchloric Acid 0. E. Goehler and G. Frederick Smith DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS, URBANA,ILL.
A study of the vacuum distillation of 73.3 to 73.6 The procedure employed conper cent perchloric acid has been made over the prestion has for its object stitutes the only distillation sure range 7 to 18 mm. and an apparatus described method known requiring no the study of the nature for the preparation of anhydrous perchloric acid in desiccating agent to accomof the process and products yields of 7 to 10 per cent of the weight of the starting plish this dehydration. A formed during the vacuum product. distillation of concentrated further distinctive f e a t u r e The anhydrous perchloric acid formed is shown to perchloric acid in the pressure consists in the minimum of be more stable than that prepared by use of previously range 8 to 18mm. Perchloric hazard involved in the prepaknown processes. acid of approximately the r a t i o n of explosive anhyThe mechanism of the distillation described is excomposition of the dihydrate drous perchloric acid. plained, and this expIanation found to be dependent (HC104.2Hz0,73.6 per cent Historical Data upon the implied existence of a surface film of oxonium aeiditv) will be shown under Perchlorate (OHaC104). the coiditiona named to unThe first important study 'dergo a set of dissociations of anhydrous perchloric acid formerly little known. An attempt to explain the mechanism and its monohydrate was made by Roscoe (g). The existence of these dissociations will be made and the design of a special of a constant boiling mixture with water (72.4 per cent a t still particularly well adapted to such a study will be described. 760 mm., b. p. 203" C . ) was shown. The anhydrous acid was Under suitable conditions this process serves as a new method prepared by the reaction between potassium perchlorate and for the formation of anhydrous perchloric acid in somewhat 95 per cent sulfuric acid and by the distillation of constant small but satisfactory yields. The dissociation reactions boiling perchloric acid with 95 per cent sulfuric acid a t 110' involved are represented as follows: C. Perchloric acid monohydrate was prepared by dilution of the anhydrous acid with water or strong perchloric acid. 4HC1042HzO +2HC1043HzO f 2OHsCIO4 (1) This crystalline perchloric acid monohydrate at 100O C. 20HsC104 ---f HC104.2Hz0 HClO4 (anhyd.) (2) gives a distillate of anhydrous acid and a residue correspond1 Received August 23, 1930. Presented before the Division of Physical ing to the composition of the dihydrate. and Inorganic Chemistry at the 79th Meeting of the American Chemical A more comprehensive research on the identification cryoSociety, Atlanta, Ga., April 7 to 11, 1930. A portion of a thesis presented scopically of the various hydrates of perchloric acid was by 0. E. Goehler in partial fulfilment of the requirements for the degree made by van Wyk ( 5 ) . The following hydrates were studied: bf doctor of philosophy in the Graduate School of the University of Illinois.
HE following descrip-
9'
+
ANALYTICAL EDITION
56 HYDRATE HClOd (anhy.) HC104.Hz0 HC104.2HzO 2HC104.5H20 HC104.3Hz0: Alpha Beta 2HC104.7HzO
MELTING POINT
c.
- 112 4- 50 - 17.8 - 28.8 - 37 3.2 -- 441.4
I n the present discussion, for the sake Of simplicity, any hydrate higher than the dihydrate will be designated as the trihydrate. Special Properties Associated with Vacuum Distillation of Perchloric Acid
The vacuum distillation of concentrated (71 per cent) perchloric acid or of the approximate constant boiling composition (72.4 per cent at 760 mm.) up to the composition of the dihydrate (73.6 per cent) shows the distinctive features described in another paper (3). For a correct understanding of the present paper the former should be consulted. Under the conditions of vacuum distillation described below, the perchloric acid being distilled is not in simple equilibrium with the vapors formed. An attempt to explain the pronounced tendency of distilling perchloric to superheat is a secondary objective of the present work. Preparation of Materials and Apparatus Design
Perchloric acid, 70 to 72 per cent, was purchased from the usual sources. The approximate dihydrate of perchloric acid (composition 73.6 per cent) was prepared by vacuum distillation of the former product discarding all but the last 35 per cent of the distillate.
Figure 1-Distillation
Apparatus
The distillation apparatus is shown in Figure 1 drawn to scale, and in Figure 2 set up ready for use. Distilling flask F is provided with receiver flask R, reflux condenser C, and reflux trap T , to by-pass condensate from C into R. A train of three joining liquid air traps (shown without Dewar flasks adjusted) are designated H , B, and D. Each liquid air trap is provided with sealed off intake and efflux tubes to provide for recovery of the products obtained. Liquid air trap D connects directly to a good rotary oil pump of generous capacity (a megavac pump) by means of rubber pressure tubing.
VOl. 3, No. 1
A closed tube differential mercury manometer (not shown) is inserted in this portion of the line. This manometer had been previously checked for accuracy against a McLeod gage. Adjustments in the pressure of the distillation are made by opening stopcock P connected with a high-grade drying agent (anhydrone-granular anhydrous magnesium perchlorate). Thermometers for determining the temperature of the distilling liquid and vapors aresupported by the stopper of flask F which is of the liquid seal type, moist with perchloric acid. Flask is attached using a one hole rubber stopper. Flask F is heated by the use of a burner. Condenser C is cooled using tap water. No capillary air. inlet to F is needed. Distillation Operation
Flask F is charged with 250 to 350 grams of 73.4 to 73.6 per cent perchloric acid and the system evacuated to 2 to 5 mm. pressure. A 3 to 4 inch (7.6 to 10.2 cm.) flame from a Bunsen burner is then applied and at 75" to 85" C., depending upon the pressure, vapors of the dihydrate pass into condenser C , condense and reflux into R. When 15 to 20 cc. of distillate have been collected the operation is discontinued and the flask R emptied, dried, and replaced. The purpose of this preliminary operation is to prepare the distilling liquid for the treatment at the higher pressures and consequent greater degree of superheating. During the preliminary distillation, unless the acid is previously chilled, a slight degree of ebullition will result until dissolved gases are expelled. I n this case some monohydrate may form in the vessels of the condenser. Some few trial runs are advisable to familiarize one with these operations. The distillation is now carried out at pressures varying from 8 to 18 mm. within a 1-mm. maximum variation for any individual run. The pressure adjustment is easily made through use of a stopcock P if the oil pump has sufficient capacity. The traps H , B, and D are immersed in liquid air and the cooling water in C adjusted in rate of flow sufficient to cause a slight deposition of crystals of monohydrate (OH&lO,) at its extreme upper end. These crystals must be maintained in the condenser during the whole distillation process, Perchloric acid dihydrate and trihydrate condense in C and reflux into R a t the rate of 2 to 4 cc. per minute depending upon the degree of superheating. Anhydrous perchloric acid collects in the trap H (with a small amount of monohydrate), in trap B, and a few cubic centimeters in trap D. The yield of anhydrous perchloric acid is highest in the case of the greatest degree of superheating obtained a t the higher pressure of distillation. A favored working range is between 12 and 15 mm. The sampling and analyses necessary for following the various dissociations obtained are accomplished as follows: The distillation is discontinued and the pressure increased to atmospheric, The Dewar flasks containing the liquid air for traps H , B, and D (Figure 1) are removed and the frozen . contents on the side walls allowed to melt and run to the bottom of the trap. The frost is then removed from the outside of the traps and the liquid air replaced over the level of the melted acid in the bottom of the trap until it is again frozen. The drawn-out extensions of the traps H , B , and D are then file-marked and broken open before the acid is melted within, and a tube containing a weighed portion of previously analyzed perchloric acid of 72 to 73 per cent acidity is placed in a position to mix with the anhydrous perchloric acid which quickly melts and runs into the absorbing acid. By previously analyzing and weighing the absorbing acid followed by analysis of the solution obtained from the addition of the known weight of anhydrous acid, the strength of the absorbed acid can be calculated. The absorbing acid
January 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
67
Table I-Distillation of Perchloric Acid Dihydrate a t 7 to 18 mm. Pressure in Preparation of Anhydrous Perchloric Acid CONCN.OF ACIDSIN FLASKS TEMPERATURE OF AMOUNT TOTAL ANHYDROUS YIELD" 3 0 4 PRESSURE Start, F R Residue, F DISTILLATION DISTILLED DISTILLED ACID (ANHYDROUS)
Mm. 16-17 17 5-18 17-17 5 7-7 5 9-9 5
% 73 4 73.30 73.45 73 35 73 45
% 72 26 71 32 71 86 72 83 72 53
% 73 73 73 73 73
05 05 01 30 40
c.
110-117 109-120 108-119 95-102 100-110
should be in sufficient quantity of such strength that the resulting acid concentration of the mixture shall not be greater than 77 or 78 per cent for the reason that above this strength the crystals of monohydrated perchloric acid are not soluble a t room temperature. The sampling of the residue in flask F and receiver R is obviously easily accomplished. That the anhydrous acid obtained was not chlorine heptoxide ( I ) , which would not be apparent by the method of analysis described, was proved by its greater chemical activity, freezing point, and other physical properties. Two representative preparations of anhydrous perchloric acid are recorded in Table I.
Grams 474 0 285 1 302 0 251 7 204 9
Grams 276 0 267 9 216 9 203 1 155 4
Grams 16 4 19 3 13 7 2 9 6 2
of Apparatus
From an examination of this table it is observed that the strength of the acid of the distillate has decreased as well as that of the residue left in flask F. Both of these results would be predicted to accompany Reaction 1 previously cited. A decrease in acidity for the residue in the distilling flask indicates that the dissociation of Reaction 1 takes place in great part in the liquid phase during the process of distillation. The dissociation of Reaction 2 resulting in the formation of anhydrous perchloric acid from the monohydrate is known to result a t temperatures reached by the distillation under the pressures employed ( 5 ) . The separation of the anhydrous acid in the vapor phase in the presence of higher concentrations of the dihydrate and trihydrate is due to the influx of dry air through the water condenser. The hydration of the anhydrous acid in contact with the higher hydrates of the vapor phase is thus prevented. That this reaction would result is shown by the formation of crystals of the monohydrate in the top of the water-cooled condenser. The rate of cooling in the water condenser is adjusted to maintain the conditions leading to the accumulation of the greatest yield of anhydrous acid. An even more completely anhydrous perchloric acid could be obtained if the cooling water of the water condenser were so adjusted as to result in the formation of the deposit of crystals of the monohydrate a t a point further down in the condenser tube, but the yield of anhydrous acid would be lower. The data of Table I show that a larger yield of anhydrous acid is obtained in the case of the higher pressures. However, experiments were made in which the air leak to the flask R remained at 8 to 8.5 mm., while the total pressure on the sys-
% 8 7 10.6 9 8 1.9 5 6
tem was increased by means of a stopcock near the pump side of the system. The results show a slight increase in yield of anhydrous acid at the higher pressure but the data showed a decrease in yield as compared to the previously described adjustment of pressure (9-10 per cent yield dropped to 5-6 per cent working at 16-18 mm.). Possible Explanation of Phenomenon of Superheating
Two characteristics of the dissociation Reactions 1 and 2 for which explanation is desirable are as follows: First, the explanation of the dehydration of perchloric acid accomplished without the presence of a drying agent. Second, the mechanism of the distillation operation accompanied by superheating to such an abnormal extent. It is possible that both these results are explained in the same manner. The dissociation of perchloric acid dihydrate to form the trihydrate and monohydrate (Reaction 1) under ordinary conditions requires a drying agent of the efficiency of sulfuric acid. The dehydration of strong perchloric acid can be brought about by distillation in the presence of concentrated sulfuric acid with the formation of perchloric acid monohydrate (1). While hot concentrated perchloric acid is an efficient drying agent it would be contrary to ordinary experience that the higher hydrates of a given material should be capable of generating a lower hydrate of the same material. Perchloric acid monohydrate has been shown (4) to be in reality of an oxonium structure. The formation of oxonium perchlorate according to the reaction 2HClOg2HzO = OHaC104
Figure 2-Set-up
% 98 97 99 100 97
+ HClOc3H20
might account for a surface orientation of molecules of oxonium perchlorate which would account for the property of Superheating. The retarded liberation of the vapors of the main product of the distillation HClOc2Hz0 as well as the trihydrate of perchloric acid would seem by this mechanism to be explained. That this explanation is plausible is evident from the fact that monohydrated perchloric acid a t 110' C. (approximately) is known to dissociate into the dihydrate and anhydrous perchloric acid. As the oxonium perchlorate molecules dissociate a t the surface of the distilling liquid, more assume their position to maintain the surface film and the superheated condition. The temperature of the vapors of the lower hydrates of perchloric acid forced through the surface film of oxonium perchlorate at the temperatures required (110' to 120' C.) do not react to hydrate the anhydrous perchloric acid in the vapors a t this temperature. That this is true is shown by the fact that when a permanent gas is admitted into these mixed vapors, the mean free path of the molecules is decreased to such an extent that the anhydrous acid molecules do not react as a whole with the lower hydrates a t the lower temperatures of the condenser, nor do they come in contact with the lower hydrated condenser surface. The anhydrous acid does not condense on the first impact with a cold surface because the three liquid air traps, A , B, and C, are necessary to remove all the anhydrous acid from the air stream. All of the lower hydrates are removed by the water condenser which reflux into receiver R. The anhydrous perchloric acid formed is more stable than this product is generally reputed to be. A sample stored under the prevailing outdoor temperatures of the late winter
58
ANALYXICAL EDITION
season did not explode for approximately two months. A sample could be stored a t liquid air temperatures indefinitely without spontaneous decomposition, as shown by the fact that it does not form the usual accumulation of colored decomposition products which after sufficient accumulation bring about the explosion of the sample. A sample was stored in liquid air for 2 months with no decoloration and only exploded after subsequent exposure to room temperature for 4 weeks. If samples of perchloric acid of greater strength than the dihydrate are desired, the monohydrate may be formed by the solution of the anhydrous acid to the point at which the product solidifies. This concentration of perchloric acid can be stored indefinitely without hazard. It must be kept in
Vol. 3, No. 1
mind, however, that even though the process just described for the preparation of anhydrous perchloric acid is entirely without hazard if directions are followed closely, the handling of the product once formed may result in the most violent explosions if it is attempted to store it beyond the point of the formation of an amber color or in case it is allowed to come into contact with organic matter such as dry wood, paper, rubber, cork, cotton, etc. Literature Cited (1) Michael and (2) (3) (4) (5)
Cohn,
J. A m . Chem. Soc., 23, 444 (1900).
Roscoe, J . Chem. S O L , 16, 82 (1863). Smith and Goehler, IND. E N G .CHEM.,Anal. Ed., 2, 48 (1931). Volmer, A n n . , 440, 200 (1924). W y k , van, Z . anorg. Chem., 48, 1 (1906).
Oxonium Structure of Hydrated Perchloric Acid' G . Frederick Smith and 0.E. Goehler DEPARTMENT O F CHEMISTRY,
UXIVERSITY O F ILLINOIS, URBANA, ILL.
Evidence has been supplied substantiating the equals a f u n c t i o n of the HE cryoscopic study oxonium structure of form of perchloric acid melting change in density. A second of the system waterat 50" C., a conclusion previously demonstrated by reference point in the denperchloric acid has others. sity-percentage composition been made by van Wyk (7) The exact melting point of oxonium perchlorate has curve could then be estabover the range Hz0-100 per 0.005° C. been determined and found to be 49.905' lished by determination of cent HClOd with the investiA new form of perchloric acid, oxonium perchlorate the physical constants of the gation among other hydrates monohydrate, is postulated based upon the transimonohydrate. of the mono-, di-, and trition HC10~2Hz0= OH3C10a.Hz0. The alpha and beta The exact determination hydrates, The melting point forms of the trihydrate of perchloric acid previously of the melting point of perof themonohydrate was found known are likewise postulated to result from the chloric acid dihydrate was to be 50" C. and of the ditransition HC104.3HzO= OH1C10~2Hz0.The oxonium found to be complicated by h y d r a t e -17.8' C . Two structure of all the known hydrates of perchIoric the difficulty that a physical forms of the trihydrate were acid is therefore indicated as a natural conclusion transformation (Reaction 2) found, the alpha form, m.p. from the data shown. apparently always takes place -37" C.,,and the beta form, The use of the data of this paper has been suggested upon crystallization of a solum. p. -43.2"C. I n addition for the construction of a density-acid concentration tion of perchloric acid of practo these hydrates the two foltable, the analyses for which have been provided by tically the correct acid comlowing forms were shown: physical rather than chemical means. position 73.6 per cent HC104. 2HC104-5Hz0, m. p. -29.8" The obiect of the Dresent C., and 2HC10~7Hz0,m. p. -41.4" C . The freezing point of anhydrous perchloric acid paper then became the use of this transition to further sGbstantiate the oxonium structure of the hydrated perchloric acids has been found to be -112" C. The great difference in the chemical and physical proper- other than the 50" melting form studied by Volmer. I n thus ties of anhydrous perchloric acid and its monohydrate has strengthening the Hofmann conclusions as carried out in the been explained on the assumption that the hydrate melting at x-ray studies of Volmer, it then became necessary to deter50" C. is in reality of the oxonium structure, OH&104, rather mine the exact melting point of oxonium perchlorate for the than the simple monohydrated form of perchloric acid. purpose of studying transition of Reaction 1. A secondary This explanation of Hofmann has been studied experimentally objective consisted in the explanation of the structure of the by Volmer (0)who demonstrated that the x-ray lattices of the previously known alpha and beta forms of the trihydrate hydrate of perchloric acid m. p. 50" C. and ammonium per- of perchloric acid (7) which is represented in Reaction 3. chlorate are practically identical. The Hofmann conclusion OHaC104 --f HClOd*H20 (1) has, therefore, been rather satisfactorily substantiated and HC104.2HaO +OHaClOa.Hz0 the Hantzsch theory of electrolytes likewise strengthened. HC1043HgO +OHaClOa.2HzO The object of the present investigation was originally As a result of this investigation Reaction 1 was not found the analysis of strong perchloric acid solutions by physical means through the more exact determination of the melting to take place. Reaction 2 was indicated and Reaction 3 point of perchloric acid dihydrate and the simultaneous deter- was previously shown (7). It naturally follows that the two mination of its density. Since the relationship between the hydrates of the form 2HC10~5H20and 2HC104.7HzO probdensity and acidity of strong sohtions of perchloric acid ably exist in the form 20H&104.3&0 and 20H&10~5HzO. has been shown to be linear (7)the analysis in the region of the I n other words the oxonium structure of hydrated perchloric dihydrate could then be found with the density determination acids in general is thus indicated. and a determination of the relationship, per cent acidity 9 The exact chemical analysis of perchloric acid solutions is not prac-
T
+
8
Presented before the Divrsion of Physical 1 Received August 23, 1930. and Inorganic Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio. September 8 to 12, 1930.
ticable for the reason that a satisfactory reducing agent has not been found capable of forming hydrochloric acid which could then be compared with silver in the usual manner.
