Vol. 3, No. 1
ANALYTICAL EDITION
52
The separation of perchloric acid from phosphoric acid
is not a problem even up to a contamination of 36 per cent if the temperature and pressure of the distillation is low, and probably the separation is equally good under the most unfavorable conditions. The data obtained in connection with the separation of perchloric acid from phosphoric acid prove that no contamination of the distillate resulted from spray
produced by ebullition or otherwise mechanically. This fact proves that the process is particularly adapted to the separation of perchloric acid from solid impurities by distillation. Literature Cited (1) Willard, J . Am. C h m . SOC., 54, 1480 (1912).
Perchloric Acid as a New Standard in Acidimetry' G . Frederick Smith and W. W. Koch DBPARTYENT OF CHEMISTRY,
HE object of this paper is the description of a
T
UNIVE8SITY OF ILLINOIS, U R B A N A , ILL.
A still design to be used in the preparation of standard 73.60 per cent perchloric acid by vacuum distillation is described and a study made showing the variation in product within the pressures 2 to 7 mm. An indirect method of analysis has been employed combining the use of constant boiling hydrochloric acid and a precision type of Foulk chain hydrometer in the analysis of 73.60 per cent perchloric acid obtained by this process. Explanation is made of the effect upon and control of variables known to be related to the process described.
process for the preparation by vacuum distillation of perchloric acid to be used as a standard in acidimetry. The p r o duo t o b t a i n e d corresponds most closely to the d i h y d r a t e (HClOC2Hz0, theory 73.603 per cent HC10J. W i t h i n the l i m i t s of -'the p r e s s u r e range used in this process, 2 to 7 mm. of mercury, the product obtained agrees with the composition of the true dihydrate as well as the analysis of the product can be determined, 73.60 * 0.03 per cent, or an accuracy of one part in approximately 2500, It will be shown that the conditions of preparation are conveniently attained and easily duplicated. Briefly the process is as follows: Perchloric acid of 72 per cent strength is distilled in a properly designed still at 2 to 7 mm. pressure until half the product has been fractionated. The distillation is interrupted and the distillate discarded. The residue is then distilled under the same conditions t o obtain the standard product.
The apparatus, shown in Figure 1, consists of a modified Claisen flask, A , 500 cc., fitted with a ground glass stopper with hook for supporting an Anschutz thermometer 50" to 100" range. The side arm of the flask is sealed a t the top and the condenser jacketed portion provided with the ground glass terminal shown, to which the 500-cc. receiving flask, C, is attached. The side arm of the flask A should provide an opening leading to the condenser tube approximately 12 mm. wide instead of 20 to 25 mm. as is ordinarily the case with the unmodified flask. Condenser B should provide 200 to 225 mm. of cooling surface length and the inner tube should be 10 mm. inside diameter. Flask C connects through its side arm to a good rotary oil pump. Between flask C and the oil pump is a differential mercury gage of the closed tube type reading pressures over a range of 0 to 20 mm. and should be accurate to *0.5 mm. Between flask C and the oil pump is placed a soda-lime tube 25 to 35 mm. in diameter by 200 mm. long provided at each end with a 13 to 15 mm. adaptor. An air release inlet tube should be placed between the flask C and the soda-lime tube. Rubber tubing is used to make connections on the pump side of flask C. The ground glass union in the neck of flask C may be replaced by a rubber 1 Received
August 23,1930.
Distillation Operation
The unusual features of the distillation operation in the case of perchloric acid have been fully described in another paper (3) and reference to this DaDer must be made for a correct understanding of this operation. The vacuum distillation of perchloric acid is not a hazardous operation. All pure perchloric acid of commerce is prepared by vacuum distillation. It is well to conduct the operation out of possible contact with wood or other inflammable material as fire may result in case of breakage. The distillation of perchloric acid under atmospheric pressure is accompanied by some decomposition to form chlorine, oxygen, and water. Such decomposition does not occur in the process described. Table I-Boiling HC104
% bywt.
Distillation Apparatus
stopper. Condenser B employs a moderate stream of cold tap water.
56.65 61.2 65.2 70.06 71.0 72.4
Point a n d Density of Various Perchloric Acid Solution8 DENSITY Hc101 B. P.760 MM. 15'/4O c . 25'/4' c. DISTILLATE c. % b y wt.
148.0 162.3 181 2 198.7 200.8 203.0
1.49 1.55 1.61 1.67 1.69 1.71
1.48 1.54 1.60 1.66 1.67 1.69
...
0.9 6.06 40.11
...
72.4
Selection of Perchloric Acid Used as Starting Material
Perchloric acid is usually purchased in the form of a 60 per cent solution. A solution of 72 per cent acid is more satisfactory and can now be obtained from the better supply houses. It may be made conveniently following the Willard (8) method of the oxidation of pure ammonium perchlorate with nitric and hydrochloric acid. Pure acid is preferred but relatively impure or "commercial" acid may be purchased and purified by distillation under reduced pressure. The distillation of constant boiling perchloric acid (the 72.4 per cent acid of Table I) may be carried out at atmospheric pressure but the loss by decomposition is appreciable. Acid less concentrated than 72 per cent may be converted to the constant boiling strength by boiling at 760 mm. until the temperature reaches 203" C. Decomposition begins a t 60 per cent acidity and increases in amount as the reaction temperature increases. Two methods serve for its rough analysis, the determination of its density or its boiling point. Values for use in this connection are given in Table I. These
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1931
53
values were found by reference to the work of van Wyk and van Emster (Q,l).
calibrated by the Bureau of Standards and buoyancy corrections were applied. PREPARATION AND TITRATION O F SAMPLES-The filled Preparation of Materials sampling ampules, together with the capillary tube from the same, were added to a titration beaker. The sampling bulb PERCHLORIC Am-Prepared by the method of Willard (8). and the capillary tube were broken and crushed under 100 to For other method see Goodwin and Walker (4) (anodic 150 cc. of water and a measured portion of the indicator oxidation of hydrochloric acid). solution added. The acid was then titrated with the standBARIUMHYDROXIDE-Prepared approximately 0.1 N from ard barium hydroxide solution using weight burets ( 7 ) . pure barium hydroxide dissolved in conductivity water with The weight of acid taken corresponded to from 100 to 200 filtration into resistance glass bottles containing air free from grams of titrating solution. Both the standardization of the carbon dioxide. It was preserved out of contact with carbon barium hydroxide solution against constant boiling hydrodioxide a t all times and was standardized before use and chloric acid and the titration of the unknown perchloric acid again after most of it had been used up. Precisely concordant samples were carried out in exactly the same way. results were obtained for both standardizations. Yield of Perchloric Acid as Function of Strength of Starting Material
A
In order to obtain data concerning the rate of concentration resulting from the vacuum distillation of various strengths of starting material, a fractional distillation of three different starting concentrations was made. The data are recorded in Table 11. Table 11-Fractional Distillation of Perchloric Acid 1-5 to 7.5 mm. pressure; 2000 cc. still FRACTION Av. HClOi FRACTION WEIGHT DISTILLED HClOa % Grams % b y ool. % 70 PER CENT ACID
Figure 1-Distilling
Apparatus
STANDARD ACID-constant boiling hydrochloric acid was prepared following in every detail the directions of Foulk and Hollingsworth (2). INDICATOR-The :odium salt of dibromothymolsulfonphthalein (bromothymol blue) was prepared as described by Lubs and Clark (6). The color change from yellow a t a pH of 6.0 to blue a t pH of 7.6 with green at the transition point, serves both in natural and in artificial light. Correction was made for the small amount of standard solution necessary for the end point color change. WATER-COndUCtiVitY water was used throughout. Method of Analysis
SAMPLIliG-Both~the constant boiling hydrochloric acid and the samples of perchloric acid were weighed in small 2-cc. glass bulbs having finely drawn capillary openings (miniature Dumas vapor density bulbs). These were filled by bending the capillary openings into a V shape, placing the open end within the sample to be taken contained in a small test tube, and warming the bulb to displace the air within by the acid of tthe sample. The sampling ampules were then cooled to room_temperature and the capillary opening sealed by fusion with a fine blast flame. The difference between the weight of the sample tube and contents and the empty sampling tube, corrected for the weight of the fine capillary tube sealed off, gave the weight of the contents. The weight of the sample was not corrected for the weight of air over the sample which was somewhat different before and after sealing because of temperature effects. This correction was in all cases negligible for the reason that the sample tubes were nearly full in each case. WEIGHING-A~~ weighings were made using a Troemner balance accurate to *0.02 mg. The weights used were
1
190
9.0
2
290
12.0
3
300
11.0
4
230
9.0
5
220
8.6
6
225
8.8
7
113
4.4
Residue
932
36.5
51.03 51,07 66,72 66.75 72.49 72.52 73.03 73.08 73.02 72.98 73.31 73.35 73.09 73.09 73.65 73.65
51.05 66.74 72.51 73.06 73.00 73.33 73 09 73.65
71 PER CENT ACID
1
190
7.8
2
230
9.1
3
240
9.3
4
260
9.9
5
250
9.5
6
260
9.8
7
99
3.5
Residue
1070
40.5
1
191
7.9
2
253
9.8
3
254
9.5
61.09 61.13 67.15 67.16 71.31 71.30 72.90 72.86 73.45 73.45 73.52 73.54 72.64 72.63 73.55 73.55
61.11 67.16 71.31 72.88 73.45 73.53 72.63 73.55
72 PER CENT ACID
4
254
9.5
5
294
10.9
Residue
1417
52.5
60.02 60.03 69.24 69.21 72.00 72.00 73,08 73,05 73.02 73.00 73.59 73.54
60.02 69.23 72.00 73.06 73.01 73.57
As shown in Table I1 the yield of acid corresponding most closely to the dihydrate (HC104.2HzO) from 70, 71, and 72 per cent perchloric acid as starting material is 36.5, 40.5, and 52.5 per cent by volume, respectively. It is therefore advantageous to start with the 72 per cent acid in order to arrive at the best yield of the finished product. Concentration takes place rapidly in all cases up to the 73 per cent range after approximately 35 per cent by volume has been distilled. Less rapid concentration results after 73 per cent acid concentration is reached, but the yield from all three starting strengths is excellent for this type of preparation.
54
ANALYTICAL EDITION
Factors Influencing Selection of 2 to 7 m m . as Pressure Range of Distillation Yielding Standard 73.60 Per Gent Perchloric Acid
The complex nature of the various possible dissociations of perchloric acid to form the different known hydrates during vacuum distillation over the pressure range 0 to 18 mm. has been fully discussed in another paper (3). From this work, which should be consulted for a proper understanding of the reactions involved, i t is apparent that above 7 mm. pressure the distillate starting with 73.6 per cent acid strength contains the following hydrates of perchloric acid: HC104.3Hz0, HC104.2H20, and HC104 (anhydrous). Depending upon the ratio of the various hydrates formed, which in turn depends upon the pressure of distillation, i t would be predicted that a distillate of varying composition would be obtained. The distillate obtained a t 7 mm. pressure and below most nearly corresponds to 73.60 per cent perchloric acid (3). Below 7 mm. pressure any variation in the acid concentration as a result of the formation of other hydrates than the dihydrate (HC104.2Hz0)is corrected for in the use of the still described in Figure 1. The side arm of the Claisen flask A provides a reflux chamber in which the other two forms HC104.3Hz0 and HClOl (anhydrous) are caused to combine and reflux into the distillation chamber. Variations in pressure below 7 mm. result in the formation of a distillate of such uniform composition, therefore, using the apparatus described, that the process conforms practically to the requirements leading to a method for the preparation of a new standard for acidimetry. Determination of Lower Limit of Pressure Permissible in Process Employed If the upper limit of pressure as indicated (3) is 7 mm. for
all practical purposes, there remained the determination of the lower limit below which the distillation could not be carried out without appreciably affecting the acid concentration of the distillate, This was determined using the apparatus of Figure 1 with the 73.60 per cent perchloric acid and gradually decreasing pressures starting with 7 mm. The lower limit found by the analysis of the distillate a t various pressures below 7 mm. was 2 mm. This is not a serious handicap since without the use of special equipment a pressure less than 2 mm. is hard to attain with the apparatus described. Furthermore the magnitude of the possible error is small as shown in the following manner: The 73.6 per cent perchloric acid was distilled in an apparatus similar to that of Figure 1 except that all ground joints were eliminated by glass seals and a mercury vapor pump and liquid air trap used to increase the vacuum produced by the oil pump. Distillations were carried out using a pressure range of 0.02 to 0.1 mm. as determined using a McLeod gage. Three hundred and twenty-three grams of acid were distilled and 150 grams of distillate obtained. A second portion of 360 grams of acid was distilled and 130 grams of distillate obtained. The two residues were then combined and 200 grams from these distilled, A distillate of 115 grams was thus obtained and the residue of 85 grams analyzed. It was found to be 73.8 per cent perchloric acid. The analyses of the preliminary residues showed that most of the concentration that has resulted took place in the early stages. The analyses of the distillates showed them to be correspondingly weak in acidity, 73.27 per cent. The temperature during these distillations varied between 60" and 70" c. There is, therefore, little danger of error due to distilling at too low pressure.
Vol. 3, No. 1
prepared in the apparatus described using approximately 72 per cent perchloric acid as starting material and various pressures between 2.2 and 7.0 mm. Most of these preparations were carried out at 3.5 to 5.0 mm. pressure since this range represents the mean between the two extreme pressures permissible; 300 to 400 cc. of 72 per cent perchloric acid were distilled and the samples taken from the second fraction after the original material had been half distilled. The samples of standard perchloric acid were analyzed by an indirect method depending upon the determination of their relative density. Two reference samples, one greater and one lower in density, were then carefully analyzed as previously described. Assuming a linear relationship to exist between the density and percentage composition of samples intermediate in density between the two reference solutions, the percentage composition of the sample being analyzed was determined graphically. That such a linear relationship exists was determined by van Emster (1). The relative density of the samples of perchloric acid to be analyzed was determined using a precision type of Foulk chain hydrometer previously described (5). The two reference samples by analysis were 73.49 and 73.51 per cent, average 73.50 per cent, perchloric acid, and 73.69 and 73.71 per cent, average 73.70 per cent, perchloric acid. The chain hydrometer float position differed in equilibrium with these two reference solutions from reading 60 to reading 150 on a millimeter scale. Thus each displacement of 1 mm. in the 0.20 position of the chain hydrometer float corresponded to 90 or 0.0022 per cent change in acidity in the sample being analyzed. The analysis of the samples of perchloric acid intended to represent standard material is given in Table 111. Table 111-Analysis
SAMPLE Ref ?No. l Rel:"o:-2 1 2 3 4 5 6
of Perchloric Acid b y Graphical Interpolation of Relative Densities
PREPARATIONHYDROMETER CALCD. P R Essu R E READINQ Hc101 Mm. H e Mm. % 73.50 (by analysis) 73.70 (by analysis) 73.58 97 3.5-5.0 102 73.69 3.5-5.0 73.63 3.5-5.0 120 73.58 99 3.5-5.0 73.67 91 2.2 73.59 100 6.5-7.0 Mean 73.59
F By reference to Chis table it will be seen that if the pressure range 3.5 to 5.0 mm. is maintained, the average strength of the perchloric acid, obtained by distillation of approximately 72 per cent perchloric acid in the apparatus described, results in the preparation of perchloric acid of composition 73.60 per cent. This value is obtained if the distillate from the last 50 per cent of the distillation is taken. At pressures between 2 mm. and 7 mm. the variations in percentage composition of the perchloric acid obtained is inappreciable. It is at once apparent that slight variations in the pressure during distillation or slight inaccuracies in the reading of the pressure-recording device are of no appreciable influence upon the composition of the final product. Specific Gravity of 73.60 Per Cent Perchloric Acid
Relative density measurements only were required for the purpose of this investigation. The determination of the specific gravity of 73.60 per cent perchloric acid gives 1.71282 a t 25'/4O C. The ideal pressure for the distillation of 73.60 per cent perchloric acid has been shown to be 5.7 mm. ( 3 ) .
Preparation and Analysis of Constant Strength Perchloric Acid for Use as New Standard of Acidimetry
Summary of Directions for Preparation of 73.60 Per Cent Perchloric Acid
To test the application of the apparatus described and the pressure range determined, a series of acid samples were
Pure 72 per cent perchloric acid is distilled under a vacuum of 2 to 7 mm. in an apparatus described in Figure 1. When
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'
+
