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-
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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.
January 15, 1931
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Transition of Crystalline Perchloric Acid Obtained at Freezing Point of Approximate 73.60 Per Cent Solution
The apparatus employed followed the design of Beckman (4) illustrated in Figure 1 drawn to the scale indicated. The large Dewar flask, E , served as the reservoir for a sulfuric acid-ice freezing mixture maintained a t a temperature of -19' to -20' C. A partially evacuated Dewar, D, served as the jacket for the reaction chamber, C, containing the perchloric acid of the determination. Provision was made for the mechanical stirring of the freezing mixture using a platinum wire stirrer. S. moving with a period of' 40' to 50 s6okes per minute inserted through B. The freezing of the sample being examined was followed by use of a 10-junction thermocouple made in the conventional fashion from number 28 wire. The freezing mixture could be seeded out through A using a small capillary tube dipped in the acid used and frozen by brief immersion in liquid air. A stream of dry air was passed into tubes A and B to prevent contamination of F the sample by a t m o s p h e r i c moisture. The whole cryoscopic bath was insulated from changes in temperature due to outside influences by use of wool felt coverings. The t h e r m o c o u p 1e was c a l i b r a t e d a t the freezing point of very pure mercury using the value -38.87' C. The deviation from a standard thermocouple was found Figure 1-Apparatus for Crys- to be $0.00192 microvolts (S). tallization of Perchloric Acid A White sinale Dotentiometer was used for recording the temperature prokded with a compensating resistance equal to that of the thermocouple. The standard cell used (1.0183 volts a t 25 ' C.) was calibrated against a Bureau of Standards cell. The galvanometer was by Leeds and Northrup having a 5-second period, a sensitivity of 5 mm. per microvolt employing a critical damping of 72 ohms. The image from the galvanometer mirror was cast upon a wall a t a distance of 18 feet giving a deflection of 13 mm. per microvolt. The experimental procedure was as follows: Twenty cubic centimeters of perchloric acid varying in composition 72.3 to 73.8 per cent were placed in the freezing point tube, @, and the whole cooled by immersion in a carbon-dioxidesnow cooling chamber. After the acid was undercooled 1 to 2 degrees below its freezing point it was transferred to the vacuum chamber, D, immersed in the Dewar flask, E , containing the sulfuric acid-ice mixture a t a temperature of 1 to 4 degrees above the freezing point of the perchloric acid being examined. The temperature of the well-stirred perchloric acid a t this point was followed closely to observe the point of gradual rise in temperature, a t which point it was seeded as previously described. The crystallization began at this point and a rise in temperature resulted which reached a variable high point, and instead of the temperature reaching a fixed maximum corresponding to the melting point of the dihydrate and remaining stationary for a fixed interval of time, i t began to fall in a direction contrary to the direction (1) and of Kanolt
59
of the bath temperature. At the point a t which the last crystal disappeared in the solution, the temperature then mounted to the bath temperature. The time-temperature plot is shown in Figure 2. The examination of Figure 2 representing the composite data for many duplicate determinations of a given sample shows the similarity in behavior in all cases. The general trend of the data shows that the crystal form first produced by the seeding of the undercooled perchloric acid undergoes a transition before the melting point of this form is reached at B. The transition takes place with the absorption of heat and is complete before the lower melting point is reached at some point lower than A. Reaction 2 above is, therefore, indicated with the stable form represented by the formula OH3C104HZO (oxonium perchlorate monohydrate). That this form is obtained with the absorption of energy is in accordance with the explanation of the decomposition of perchloric acid of the same composition a t approximately 18 mm. and 115' C. in the formation of anhydrous perchloric acid and previously shown ( 2 ) . The dehydration of oxonium perchlorate monohydrate to form oxonium perchlorate according to this explanation was accompanied by the absorption of energy.
/line. Figure 2-Time-Temperature
Plot for Several Determinations
Following the same explanation the beta form of perchloric acid trihydrate previously described (7) is to be correctly termed oxonium perchlorate dihydrate since the latter form results from the former with the consumption of energy. The repetition of the freezing point determination of various perchloric acid samples gave results of the same general type. These data are given in Table I. Table I-Maximum a n d M i n i m u m Transition Temperatures for Various Strengths of Approximately 73 Per Cent Perchloric Acid PERCHLORIC ACID TEMPERATURE OF TRAXSITION Maximum Minimum
% 73.8 73.4 73.2 72.3
c. -19.11 -19.97 -17.85 -19.40
c. -19.53 -20.14 -19.34 -20.01
Determination of Melting Point of Oxonium Perchlorate
The determination of the melting point of oxonium perchlorate would serve a twofold purpose in connection with the present investigation. The possible transition of Reaction 1 above would be examined. Previous determination
ANALYTICAL EDITION
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Vol. 3, No. 1
darkens upon storage due to decomposition. For this reason solutions less strong than that of the maximum melting point were prepared and subsequently concentrated by the addition of anhydrous perchloric acid. The anhydrous perchloric acid used for this operation was stored in liquid air to prevent its gradual decomposition and subsequent explosion. As the oxonium perchlorate composition was approached starting with less strong acid by addition of anhydrous perchloric acid, the crystalline acid was produced in larger and larger crystals until a t the maximum melting point the entire tube consisted of a transparent mass of crystalline acid. Four duplicate determinations of the melting point were carried out upon each tube. Between each pair of determinations 48 hours were allowed to elapse to show any failure to duplicate determinations. The reaction of the concentration of the acid in a given case required a rather extended period of time. To show that equilibrium was established, one sample was studied over a period of 16 days. After a given determination the sample tube was weighed, the tip broken off, anhydrous acid added, the tube then resealed and weighed. The determination of the melting point of the oxonium perchlorate was carried out by two methods of
/
s&& L.
chart over the range of acid composition between 70 and 80 per cent
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Figure 4-Temperature Record for Typical Runs in Determining Melting Point of Oxonium Perchlorate
application of heat to bring about the melting. I n one the bath temperature was maintained a t 0.5" to 1.0" above the melting point of the sample and the sample allowed to melt as a result of this higher temperature employed. I n the other case the bath temperature was gradually raised to a point equal to and slightly above the melting point of the sample. Both methods gave identical results. A graph of the temperature record for a typical run is shown in Figure 4. The results of the determination of the maximum melting point of a group of individual determinations for three different samples of material are given in Table 11. Table 11-Melting
SAMPLE
1 2
3
Point of Oxonium Perchlorate MAX. DEVIATION OF FOURDBTNS.
AV. MAX. M E L T I N G POINT
c.
49.903 49.906 49.910
O
c.
0.017 0.023 0.019
Discussion of Results
The results show a satisfactory individual agreement as well as duplicate agreement. Results are expressed to thousandths of a degree although the thermocouple used
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1931
does not exceed an accuracy of 0.005" C. The melting point of oxonium perchlorateis thus found to be 49.905' 0.005"c. NO evidence of the transition of Reaction 1 above was found. Perchloric acid monohydrate has not, therefore, been shown to exist and the oxonium structure of perchloric acid of this theoretical composition thus indicated.
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Literature Cited Beckman, z. physzk. Chem ,ai, 239 (1896). Goehler and Smith, IND.END.CHEM.,Anal. Ed., 3, 55 (1931). International vO1. I, P. 57* Kanolt, Bur. Standards, Sci. Paper 520. Smith and Koch, IND. CHEM,, Anal Ed,, s, 52 (1931), (6) Volmer, Ann., 440, 200 (1924). (7) Wyk, van, Z . anorg. Chem., 48, 1 (1906).
(I) (2) (3) (4) (5)
Oxonium Perchlorate as Reference Standard for Construction of Specific Gravity-Percentage Composition Table for Strong- Perchloric Acid Solutions' G . Frederick Smith and 0.E. Goehler DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS, URBANA, ILL.
Oxonium perchlorate (OH,CI04, m. p. 49.905" C.) is HE chemical analysis Previous Work used for t h e preparation of known strengths of conof strong solutions of centrated perchloric acid solutions. perchloric acid in the Published work closely reA determination of t h e density of strong solutions region of 65 to 75 per cent acid lated to the present investigaof perchloric acid is described and t h e construction of tion is found in the investistrength is obtainable only by a density-acid composition chart over t h e range 65 to the application of an indirect gations by van Wyk ( 8 ) ,van 75 per cent perchloric acid carried out. The deterEmster ( I ) , and Hantzsch (3). method. A reference acidimination of t h e constants used in t h e calculation of metric standard is first used The data of these investigainterpolated a n d extrapolated values is given. to standardize the solution of tors ( r e c a l c u l a t e d in some The determination of reference points in t h e denB base which is then in turn cases to gain uniformity for sity-acid composition chart established t h e density used for the analysis of the the purpose of easy compariperchloric acid solution in of t h e dihydrate of perchloric acid at 25"/4" C. t o be son) as applied to the deter1.71282. Constant boiling perchloric acid 72.4 per question. This indirect analmination of the density of percent at 760 m m . and 203" C. has t h e approximate ysis is made necessary by the chloric acid of strength 73.60 density 1.6964. f a c t that a s a t i s f a c t o r y per cent (the dihydrate commethod has not been develposition) is given in Table I. oped for the reduction of perchlorates to chlorides which could The values obtained as hereinafter described are included then be determined using silver. The preparation of an accu- to conserve space. rate density-percentage composition table within the range under question is desirable because concentrating dilute Table I-Density of 73.60 Per C e n t Perchloric Acid a n d Related Constants solutions of perchloric acid either a t atmospheric pressure COEFFICIENT OF or under reduced pressure gives rise to acid within this range DENSITY EXPANSION INVESTIGATOR 25'/4O C. Alpha Range d A/dC of percentage composition. Perchloric acid, when distilled c. a t 760 mm., forms a constant boiling mixture with water van Emster 1.70942 0.00121 15-30 ., ., .. ., .. 0.01351 1.7130 0.00121 20-50 0 01390 which consists of 72.4 per cent acid. The formation of the van Wyk Hantzsch 1.71299 . . ... 1.42064 0.01366 approximate dihydrate of perchloric acid (OHaC104.H20) Present work 1.71282 0.00122 20-30 1.42052 0.01343 which has an acid composition of 73.60 per cent results from di - dz Alpha --- (change in density per degree change in acid the vacuum distillation of perchloric acid of constant boiling t r - 11 concentration). strength (2,6,6, 7). The object of the present paper is the construction of a Outline of Procedure density-percentage composition table, the accuracy of which exceeds that of existing data, using a method which can be Samples of oxonium perchlorate (84.794 per cent HClO4) demonstrated to be superior to such data, and employing are prepared by the method of Smith and Goehler and their methods of chemical analysis only. The method of attack exact composition and identity determined by the determinais wholly dependent upon the accuracy with which the melting tion of their exact melting point (49.905' C.). Weighed point of the highest melting hydrate of perchloric acid can be portions of these samples of oxonium perchlorate are then determined (2) (OHaC104, m. p. 49.905' * 0.005" C.). diluted with weighed portions of pure water and a density This product, by consulting the data of van Wyk (8) can determination made of the solutions thus prepared. The easily be seen to serve better than any other known form of identity of the density thus found for solutions of the same perchloric acid as a reference point in the analysis of strong acidity based upon the dilution of the individual preparations perchloric acid following the cryoscopic method, and inde- of oxonium perchlorate serves as an additional evaluation pendent of chemical analysis. of the uniformity in composition of the high melting hydrates as determined by their melting point. Further dilution 1 Received August 23, 1930. A portion of a thesis presented by 0. E. of the samples thus formed serves for the determination of Goehler in partial fulfilment of the requirements for the degree of doctor other points on the density-acid composition curve. From of philosophy in the Graduate School of the University of Illinois.
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