ANALYTICAL EDITION
48
Tests run on the same sample of reclaim by different operators show that this method is not subject to the personal error which is caused primarily by the squeezing. The old method, however, cannot be depended upon to give reproducible results by the same or different operators as Table VI11 shows. I n order to compare the amount of alkaline material actually present with that extracted by the old and new method, a complete extraction was made on reclaims A and B. It was necessary to run this for 55 days until the percentage of alkaline material removed corresponded to the blank. Twenty-five-gram samples were digested in 800 cc. of boiling water continuously. At first the water was changed every 24 hours, while near the end of the experiment it was changed every 48 hours. Kilbourne and Miller (1) required about 60 days for determination of total alkalinity. Samples of the same reclaims were also run by the old and new method. The results, given in Table IX, show that the old
Vol. 3, No. 1
method extracts 14.31 per cent and the new method 29.29 per cent in reclaim A, and in the case of reclaim B, 3.93 per cent of the total was extracted by the old method and 14.57 per cent by the new. While it is possible to achieve as high results as are given by the benzene-ethanol method by continued extraction in water for a t least 24 hours and upwards to 48 hours, this length of time is hardly practicable. The benzene-ethanol method may be completed in approximately 5 hours. Acknowledgment
The authors are indebted to 0. C. Moyer for his helpful suggestions and analytical work. Literature Cited (1) Kilbourne and Miller, IND. END. CHEW.,22, 69 (1930). (2) Shepard, Palmer, and Miller, IbAi., 20, 143 (1928). (3) Stafford, Trans. Insl. Rzrbber Ind., 6, 340 (1930).
Purification of Perchloric Acid b y Vacuum Distillation' G. Frederick Smith and 0. E. Goehler DEPARTMENT OR CHEMISTRY, UNIVERSITY OF ILLINOIS, URBANA, ILL.
HE preparation of pure
T
Special designs of stills for the purification of perchloric acid by vacuum distillation are described including besides conventional designs a cold target type. Special features involved in the vacuum distillation of perchloric acid are shown to govern the design of the still to be used in its purification. The variation in the acid concentration of both distillate and residue with change in pressure over the range 1 to 7 mm. is given. The separation of perchloric acid from various amounts of sulfuric and phosphoric acids by vacuum distillation is subjected to a careful quantitative examination, and the conditions under which complete separations result are described. The time rate of distillation of various designs of stills is determined and the rate of distillation shown to be directly proportional to the degree of superheating employed.
perchloric acid by all methods of manufacture requires that it be purified by distillation. Distillation of perchloric acid at atmospheric pressure results in too great a loss through the formation of chlorine, oxygen, and water. The vacuum distillation of perchloric acid is com'plicated by the pronouqced t e n d e n c y the distilling liquid shows to superheating. This phenomenon of s u p e r h e a t i n g , together with c o n s i d e r a t i o n zf the pressure applied during distillation, is closely related to the rate of distillation. The object of the present paper is the study of the various factors influencing the design of vacuum distillation equipment for the purification of perchloric acid and its use in the separation by distillation from sulfuric and phosphoric acids. Special Properties Associated with Vacuum Distillation of Perchloric Acid
The vacuum distillation of concentrated perchloric acid of approximately the constant boiling composition (72.4 per cent a t 760 mm.) up to the composition of the dihydrate (73.60 per cent) shows the following distinctive features: The distilling acid can be superheated to the extent of 30" to 60" C. depending upon the pressure and the rate of application of heat. A thermometer placed in the liquid being distilled and another placed a t varying heights in the vapors over the Presented before the Division of Physical 1 Received August 23, 1930. and Inorganic Chemistry at the 50th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12, 1930.
liquid will show wide differences in temperature depending upon the amount of superheating and the difference in level of the thermometer bulbs within and above the distilling liquid. The acid, therefore, is not in simple equilibrium with the vapors formed. The complex nature of the mixture of different forms of perchloric acid in this vapor phase is the subject of a s u b s e q u e n t paper. By regulating the p r e s s u r e between the limits 0.0 to 18.0 mm., the temperature of the distilling liquid can be varied between 20' and 120" C. The distillate under these conditions collects a t the rate of 0.5 to 10 cc. per minute. No ebullition accompanies such distillations. Even with agitation of the strongly superheated distilling acid, no ebullition results. If the perchloric acid being distilled under reduced pressure contains dissolved gases some preliminary ebullition results. There are two methods of eliminating these dissolved gases: Chill the strong (71 to 72 per cent) acid before distillation to a temperature of -5" to - lO",C. or lower. This method has the additional advantage that solids in solution, such as sodium, potassium, or other perchlorates, are also removed. Or first concentrate the perchloric acid to be distilled at atmospheric pressure until a temperature of approximately 200" C. is attained and the acid concentration has reached 71.5 to 72 per cent using the same still for the concentration as that to be employed in the vacuum purification. The temperature of the concentrated acid is then allowed to fall to from 80" to 90" C. and the distillation continued a t reducedCpressure.
January 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
Control of Factors Leading to Rapid Distillation by Changes in Still Design
Several factors govern the design of a still for the purification of perchloric acid. The main ones are: First, the efficient condensation of the distilling vapors. This problem is made the more important by the fact that the vapors are at a temperature much higher than minimum distillation temperature because of the superheated condition of the distilling liquid. Second, the use of a target type of condenser-can be predicted as most efficientin both condensation
m
AA
88 Figure 1-Stillheads of Conventional Design '
of the vapors emitted and in avoiding return flow of condensate. Third, the control of the degree of superheating by regulation of the pressure. This last factor is influenced in turn by the first two. Dimensions Applied to Stills of Conventional Design
Figure11 shows two stillheads with dimensions included. The important dimensions are those of the inside diameter of the condenser tube and its length. As shown, the point of contact between the condensing side arm and the neck of the flask is placed as close to the balloon of the flask as indicated. This is possible because the liquid is distilling without ebullition and is advisable to prevent return flow of condensed acid vapors. A 5-liter flask has been used successfully in connection with this design of still. This design does not provide for recovery of the condensed acid forming on the inside of the neck above the side arm. The length of the condenser arm was found to be suited to the use of ordinary tap water for cooling.
49
As a result of the tests upon stills A A and BB given in Table I, it is observed that the latter gave both the best yield of finished product per unit of time and resulted in the greatest degree of superheating. A maximum application of heat which just failed to produce ebullition was employed. Modification of Cold Target Design of Still
According to the explanation of the mechanism accounting for the phenomenon of superheating to be given in a later paper, the molecules of gaseous perchloric acid are projected through a probable film of oxonium perchlorate (OHsC104) upon the surface of the distilling liquid. In taking advantage of this phenomena and for the additional advantage of making the distillation apparatus more compact the cold target type of still illustrated in Figures 2 and 3 were designed and tested. These are of two types, those of A and B, Figure 2, which provide for the application of the pump connection at the end of a vertical condensing column and a refluxing of the condensed acid vapors to be drained out a t the lower horizontal outlet shown. Neither type A nor B was found to be suited by comparison with the conventional form of still shown in Figure 1. This type of still has a further disadvantage that if operated a t pressures greater than 7 mm. there is some probability that anhydrous perchloric acid may be formed in some quantity and fail to be condensed before passage through the pumping system. The stillheads C and D, Figure 2, and the design shown in Figure 3 were found more efficient in condensing the distilling vapors and in addition are more compact. The drawings of Figure 2 do not show the condenser chamber water-cooling jackets in place. The performance of these (Figure 2) stills, with the exception of type A , is shown in Table I, The most satisfactory still design of all those studied is shown in Figure 3. This type differs from those of Figure 2 in that the double ring seal a t the top of the distilling flask is eliminated and the length of the tubular from the top of the distilling flask into the condensing chamber is changed. It was observed in the cold target type of condenser that practically all of the vapors condensed a t the upper end of the cooled target which is in accordance with the fundamental principles underlying the construction of such a still. The cooling surface is thus materially diminished. The ground stopper used to charge the still and support the thermometer (an Anschuta of range 50" to 105" C . ) also provides for the use of a little concentrated perchloric acid to seal the stopper against leaks. The receiving flask is attached using a rubber stopper and the connection made to the vacuum pump using rubber pressure tubing. This form of still provided for the greatest degree of superheating and the best time rate of distillation, as shown in Table I. It is the most easily charged and recharged of those designs shown and is by far the most compact. It is more expensive than the conventional type of Figure 1, which i s practically its only disadvantage. All the subsequent tests of the present paper were made using this type of distillation equipment. Table I-Rate of Distillation and Working Temperature of Various Types of Perchloric Acid Vacuum Stills a t 1 to 2 mm. Pressure DISTILLAT~ON TEMPERATURR DISTILLATION RATE50' C. TYPE RATR Maximum Average Kg. Per hour Kg. per hour C. O C.
An examination of the data of Table I shows the superiority of the distillation apparatus designed according to Figure 3. The maximum rate of distillation is seen to accompany the greatest degree of superheating and the highest
.
50
ANALYTICAL EDITION
average temperature of distillation. The strength of the acid distilled in these tests was 71 to 72 per cent. Further Considerations in Purification of Perchloric Acid
Crude perchloric acid made from technical grades of ammonium perchlorate after concentration to from 70 to 72 per cent strength at atmospheric pressure will have, after cooling and gravity separation of the solid perchlorate impurities (potassium, sodium and ferric perchlorates, and de-
VoI. 3, No. 1
oxides of chlorine. Concentration to 72 per cent crude perchloric acid has the additional advantage that soluble impurities become less soluble to be removed later by chilling. I n the distillation of even the crudest of perchloric acid, the process can be carried out until two-thirds of the starting material is distilled, and with more pure forms until 90 per cent of the starting product has distilled. This process of distillation which includes no ebullition makes it particularly suited to the freeing of perchloric acid from soluble solids because of the absence of spray from the distilling liquid.
Figure 2-Types of Cold Target Stills
hydrated silica), variable amounts of soluble impurities to be removed by vacuum distillation. The degree of dehydration of silicic acid and the extent to which soluble impurities are removed will depend upon the temperature applied during the concentration a t atmospheric pressure and the temperature to which the acid is chilled prior to the separation of solid impurities. Crude ammonium perchlorate is made by a large number of technical modifications of essentially the same process, and the degree of purity depends upon the particular process employed and upon the question of its purification by recrystallization. Varieties of crude ammonium perchlorate containing appreciable amounts of potassium perchlorate are particularly objectionable, but varieties having as high as 5 to 6 per cent of sodium perchlorate are profitably employed in the manufacture of perchloric acid. The natural tendency is to fail to concentrate the crude acid to a temperature of approximately 200" C. at atmospheric pressure in the preparation of 72 per cent acid for vacuum distillation. This is because appreciable decomposition of the hot acid results, a great portion of which decomposition takes place at 190" to 200" C. This decomposition can be almost completely eliminated by using large enough vessels in the concentration a t ordinary pressures, Twenty-liter, round-bottom, Pyrex flasks serve well. Starting with such a flask of dilute (20 to 30 per cent) perchloric acid, a t the point of concentration to 72 per cent acid the flask is approximately one-third full. Under these conditions the known tendency ( I ) of the concentrating acid to form lower hydrates which decompose, is offset by their reaction with the water vapor being expelled which counteracts their decomposition into chlorine and
Variation in Acid Strength as Governed by Pressure of Distillation
The distillation of 73.60 per cent perchloric acid was carried out using the still design of Figure 3 a t various pressures from 0.5 to 7.0 mm. The variation in the composition of the distillate and undistilled residue in the distilling flask was then examined by the determination of the density of the products obtained. The data obtained were of importance because of their bearing upon a process for the preparation of perchloric acid dihydrate by vacuum distillation. The method of determining the density of the various acids thus obtained will be given in a subsequent paper. The results of these determinations are given in Table 11. Table 11-Variation i n Acid Concentration Resulting from Distillation of 73.60 Per Cent Perchloric Acid a t Pressures between 0.5 and 7 mm. DISTILLATE RESIDUE ACIDIN DENSITY DENSITY DIFFERENCERESIDUEPRESSURE 25°/4a C. 25"/4O C. DENSITY DISTILLATE Mm. 0.5-1
2.0-2.6 4.0-4.6 6.5-7
%
1.71600 1.71545 1.71383 1.71211
1.71521 1.71520 1.71416 1.71190
0.00021 0.00025 0.00033 0.00021
+0.15 +0.18 f0.23 -0.15
An examination of this table shows that the maximum difference obtained in acid composition between distillate and residue between the range 0.5 to 7 mm. is 0.30 per cent. Thus by plotting the values obtained from the density of the acid of the distillate a t various pressures of distillation as a function of the acid composition, it is observed that the pressure producing acid of the exact dihydrate composition is 5.7 mm. The plot of the data is shown in Figure 4. The-
January 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
n
It would be predicted from a general knowledge of the variation in vapor pressure with the temperature that the separation of perchloric acid from both sulfuric acid and phosphoric acid would be more complete the lower the pressure of distillation. It would also be predicted that the sulfuric acid contamination would be the most difficult to remove. I n the case of the mixed solutions of sulfuric and phosphoric acids with perchloric acid studied, the contamination was purposely made far more extensive than would ever be the case in the use of any serviceable stock of ammonium perchlorate for the manufacture of perchloric acid. The data resulting from the study of this purification are given in Table 111. Table 111-Separation of Perchloric Acid from Sulfuric and Phosphoric Acids a t Several Temperatures and Pressures Perchloric acid used, 71-72%; volumes distilled, 600 to 3000 grams, per cent acid distilled, 50% CONTAMINATING ACID TEMPERATURE PRESSURE At start In distillate % Gram c. Mm. Sulfuric acid 0.2 0.0000 65-80 1 2-4 0.0039 10 105 4-5.73 0.0042 10 105 2-4.1 0.0013 1 75-85 4.1-6.2 0.0024 1 75-85 6.2-10.4 0.009 1 75-85 Phosphoric acid 3,5-12.8 None 1 75-85 1 12.8-19.9 None 75-85 19.9-36 Trace 1 75-85
Three L / t e r f / a s k .
Figure 3-Design
51
for Cold Target Still
density of the exact dihydrate was taken from unpublished data as 1.71282 a t 25"/4O C. Separation of Perchloric Acid from Sulfuric and Phosphoric Acids
The following conclusions can be drawn from the experiments listed in Table 111. For the separation of perchloric acid from sulfuric acid, if the contamination of the distillate is to be restricted to 0.001 per cent, the original impurity of sulfuric acid must not exceed 4 per cent and the pressure and temperature of the distillation must be as low as possible. The possible contamination is slowly increased by allowing the pressure to increase together with the temperature. The amount of
The cheapest method of manufacture in the preparation of perchloric acid consists in the oxidation of ammonium perchlorate ( I ) . By this process any ammonium sulfate or phosI I I I I phate impurity in the raw materials would result in a conI 1 tamination of perchloric acid produced, which would require the purification of the same by a distillation process capable of separating perchloric acid from sulfuric and phosphoric acids. The study was therefore made by this separation using the vacuum still of Figure 3, a t varying pressures and with varying amounts of contaminating sulfuric and phosphoric acids. The process in the case of both sulfuric and phosphoric acid was as follows: 71 to 72 per cent perchloric acid was adulterated with known quantities of sulfuric and phosphoric acids in separate determinations starting with 2 per cent by weight of the particular acid as impurity. The distillation was then carried out until a given portion of distillate was collected a t a given pressure of distillation and the distillate set aside for further examination. The residue in the still, assuming the amount of contaminating acid carried over in the first operation was small, has a new ratio of contaminating acid which can be tested for complete separation by a further distillation. The second distillate is then set aside for analysis and the process continued. The extent of the conFigure 4-Plot of Data Used t o Determine Exact Dihydrate Composition of Perchloric Acid tamination of the acid being distilled may then be taken as the mean of the starting and finishing concentrations. The extent of the contamination of the distillate of per- sulfuric acid is not more than 0.004 per cent up to a concentrachloric acid by the sulfuric or phosphoric acid was deter- tion of 6 per cent of sulfuric acid in the product purified even mined in each case by its second distillation under the same under the most unfavorable conditions of temperature conditions with an analysis of the total residue in the still and pressure. The amount of sulfate in crude ammonium for the contamination involved. The accuracy of this perchlorate is never great enough to require that the perprocess depends upon the practically complete separation chloric acid made from it be twice distilled in vacuo in order to completely free it from sulfuric acid. of the contaminant in one operation. I
I
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 and 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 1.49 1.48 162.3 1.55 1.54 0.9 181 2 1.61 1.60 6.06 198.7 1.67 1.66 40.11 200.8 1.69 1.67 203.0 1.71 1.69 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
