INDUSTRIAL A N D ENGINEERIXG CHEMISTRY
676
Vol. 17, No. 7
Conc6ntration of Argon from Air by Fractional Liquefaction' By G. R. Fonda, F. W. Reynolds, and S. Robinson GENERAL ELECTRIC Co , SCHZNEWADY, N . Y.
I
F A mixture of gases Of Experiments by Baly have demonstrated to what exdioxide a n d moisture, tent the oxygen content of air may be concentrated by through a differential flow different boiling points is gage, and finally into a deep subjected to a partial liquefaction. The present work extends his data by determining the variation in argon content as well. glass trap, consisting of a ture within the range of their boilingPoints,comPleteliqueThus, if only 5 per cent of a volume of air is liquefied, central tube 11 mm. in the liquid will contain 40 per cent oxygen and 2.35 per diameter, from the lower end faction will not occur, but of which the air entered the there will result a liquid of difcent argon, At the other end of the series an 80 per bottom Of a larger inclosferent composition, richer in cent condensation leaves a gas having a composition ing tube 27 mm. in diamthe less volatile constituents. of 1.6 per cent argon. The -composition of the gaseter, and passed up between the two, leaving a t the top to eous phase will show a corresponding difference. I n the case of air, for instance, the a gasometer. The trap was immersed to a depth of about boiling points of the pure gases vary from 77.3" K. for ni- 18 cm. in a large volume of liquid air of known temperature, trogen to 90.1" K. for oxygen, with argon intermediate a t which remained constant during the period of a run. The 87" K. The composition of the liquefied air will be near 1 mm. wide outlet of the central tube was constructed so that of atmospheric air for any temperature below about close to the bottom of the trap that the entering air was forced 78" K., but will predominate in argon and oxygen at higher to bubble through whatever liquid was formed, and therefore to acquire the same temperature. I n this way the comtemperatures. This relationship has already been demonstrated for ni- position reached an equilibrium characteristic of the temtrogen and oxygen mixtures by Baly,2 who analyzed the perature, as shown by preliminary experiments with variable liquid and gaseous phases of liquid air during its evaporation. amounts of air. More definite verification of this is given in the discussion of the experimental results of Table 11. He did not consider the argon. In the present instance it was particularly desired to follow Table I-variation of Condensate with Temperature a t 106 c m . Absolute Pressure the variation in argon content as well. The aim of the work Temperature of was to find to what extent the 0.95 per cent argon of the air liqui: air bath Per cent K. condensed might be concentrated by subjecting the air to a temperature a t 83 9 6 2 which only fractional liquefaction would occur. Because of 83 7 12 7 83 0 26 4 the difference in boiling points between argon and nitrogen, 82 5 41 5 81 8 55 4 this concentration would be most marked relatively to the 81 0 80 3 latter, as the argon and oxygen would be the first to condense. Experiments were made under the conditions cited in Experimental Table I, which show the extent of liquefaction a t various The apparatus is shown in Figure The air was passed points in the temperature range used. For atmospheric pressure and a flow of 860 cc, of air a minute, the critical through soda lime and caustic to remove temperature below which no air would liquefy was 830 K. 1 Received November 8, 1924. At a pressure of 30 em. of mercury above atmospheric and 2 Phil. Mag., 49, 517 (1900). with the same flow, this mas raised to 84.5" K.
'*
Gasometer
for unliquified gas
Gasometer for condensate
Mercury gage
Condensation trap
Flow gage
Purifiers
4 ir c
Figure 1-Apparatus for Partial Liquefaction of Air
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1925
These results are shown graphically in Figure 2. The operation of the experiments, which were carried out likewise a t 106 em. absolute pressure and a flow of 860 cc. per minute, consisted in filling the bath with liquid air of the temperature desired, running compressed air through the trap and allowing the unliquefied portion to pass directly into a gasometer. At the completion of any experiment the bath was slowly lowered, and the portion of the air which had liquefied in the trap, the condensate, was vaporized into a second gasometer, after first flushing out the line.
Y
12 E 2
The complete analyses of the two portions of air are given in Table 11,and are shown graphically in Figure 3. Results
The amount of air used was varied so that the volume of liquid collected in the trap a t the end of a test might show wide fluctuations from test to test. Despite this fluctuation, it is apparent from Figure 3 that the results fall on sb smooth curve. This is the best evidence that an equilibrium in composition was attained as the gas bubbled up through the liquid, for otherwise the effect of using too small a volume of liquid in the trap would have been to give an abnormally high value for the percentage of oxygen in the gas escaping, deviating noticeably from the curve. The following two pairs of results for about the same percentage oxygen, but with widely differing extremes in the amounts of air used, illustrate this maintenance of constant composition during the partial liquefaction, as seen by their agreement with the curve of Figure 3. Liquid in t r a p cc 5 1 14 6 10 0 28 2
Figure 2-Variation
Per cent condensate of Condensate with Temperature
The analysis for oxygen was made over water in waterjacketed burets by the copper absorption method, using absorption bulbs filled with fine copper wire and a solution of 1.52 cc. of saturated ammonium carbonate, 100 cc. of concentrated ammonia, and 48 cc. of water. Table 11-Composition of Gaseous a n d Liquid Phases i n Fractional Liquefaction of Air a t 106 C m . Absolute Pressure Percentage composition Percentage composition of unliquefied air of condensate Temperature of bath Per cent air A in A in ’K condensed 0 2 A (Nz A) 0 2 A iN2 A ) ... ... 84.2 4.2 19.8 ... ... ... 19.7 46.8 2.33 4.44 84.2 4.5 0.88 1.10
+
84.1
83.9 83.9 83.8 83.7 83.5 83.2 83.0 82.5 82.4
81.8 81.6
81.3 81.2 81.0
6.2
8.2
8.5 10.8 12.7 14.5 21.0
26.4
37.0
41.5 55.4 61.8 71.4 71.8 80.3
... ,..
...
...
0.84
...
1.03
...
...
...
... ... ...
... ... ...
...
1.17
19.0 18.3 17.2 17.7 15.0 14.3
11.6 9.6 10.1 9.0 8.5
... ...
... ... ... ... ...
1.28
...
...
+
...
...
42.5 38.5
33 29 27 26
8 7 0 0
2.21
2105
4.20
.. ..
...
... ...
3.71
1 44 1 34 1.12
0.98 0.84
2.23 2.02 1.59 1.34 1.13
0.74
0.97
...
...
... ...
...
677
Per cent air condensed 14 5 21 0 41 5 55 4
Per cent 0 s in unliqueBed air 17 7 15 0 11 6 9 6
Particular care had to be used in analyzing the gas obtained by evaporation of the condensate. The nitrogen in it would, of course, evaporate off first and accordingly collect in the upper part of the gasometer, while the oxygen and argon, whose evaporation was more delayed, would predominate in the lower. The time required for diffusion to give a uniform mixture, even with only a few liters of gas, was found to be several hours, and in cases of extreme differences in composition 24 hours was allowed. The presence of any error due to this or other causes was determined for each element by noting if its percentage with reference to the entering air was so divided between unliquefied and liquid air that the sum of the two added up to a constant value, that of its normal percentage in air. This has been done in Table 111, which is calculated from the experimental results of Table 11, so as to show for each element what percentage it bears a t any point to the total volume of air entering the apparatus. Values in parentheses are taken from the curves of Figure 3 drawn through the experimental results for those cases where only one phase of the mixture had been analyzed.
... r
u .* bL
z Y
E
8
f Y
il I.
i
B * EI
9
* Y
2 L
k
Per cent condensate Figure 3-Composition of Gaseous and Liquid Phases
Per cent condensate Figure &Division of Oxygen and Argon between Gaseous and Liquid Phases
The argon content was determined by measuring the density of the gas in a 550-cc. glass bulb. With the oxygen percentage known, and also the densities of the three pure gases, nitrogen, argon, and oxygen, the percentage of argon could be readily calculated.
Figure 4 is based on these results and shows graphically how the amount of each element is divided between the gaseous and liquid phases. Thus, the curves cross a t 50 per cent, which denotes thaB a t this point the amount originally present in the air is equally divided between each of
INDUSTRIAL AND ENGINEERING CHEMISTRY
678
the phases. When 27 per cent of the entering air has been liquefied, for instance, one-half of the original oxygen is present in it, while the other half is in the unliquefied air; similarly the liquid resulting from a 29 per cent liquefaction contains one-half of the argon originally present in the air. Table 111-Partition of Oxygen a n d Argon between Liquid a n d Gaseous Phases in Fractional Liquefaction of Air PERCENT OZIN ENTERING PERCENT ARGONIN ENTBRAIR ING AIR Per cent air UnliqueConUnliqueConcondensed fied densed Total fied densed Total
10.8 12.7 21.0 26.4 37.0 41.5 55.4 61.8 71.4 80.3
16.3 15.0 11.8 10.5
10.2
20.9 20.4 20.7 20.7
6.8 4.3 3.9 2.4 1.7
14.0 16.5 16.7 18.6 (19.5)
20.8 20.8 20.6 21.0 21.2
...
‘s” :); ...
...
...
... ... *..
(0:69)
0.26
0.95
to: 42) (0.39) (0 35 (0:33{ 0.33 (0.36)
0.56 0.62 0.61 0.60 0.59
0.95 0.95 0.97 0.94 0.93 0.95
...
... ... 0.53
Note that the values add up to the normal content to an extent within the experimental error.
The results of Table I1 represent, then, the equilibria a t various temperatures between gaseous and liquid phases, starting from a mixture of constant composition-namely, that of air-78.05 per cent nitrogen, 21.00 per cent oxygen,
Vol. 17, No. 7
and 0.95 per cent argon. Baly’s results show the equilibria resulting on starting with a mixture of variable composition, ranging from 100 per cent nitrogen to 100 per cent oxygen. The present experiments cover only the portion of this range in which the lower limit is given by the composition 21 per cent oxygen in the gaseous phase and 48 per cent oxygen in the liquid and in which the upper limit is the equilibrium between 7 per cent oxygen in the gaseous phase and 21 per cent oxygen in the liquid. The results agree with Baly’s curves for oxygen within this range, and show in addition the corresponding relations for argon. I n Table I1 the argon content is stated also in terms of its relation to the nitrogen and argon residue that would result on removing the oxygen. It is this figure that is of particular interest in the purification of argon mixtures and for oxygen can be removed quite readily, whereas nitrogen offers more difficulty. Evidently, the most favorable condition for obtaining argon from a mixture in nitrogen lies in condensing only a small fraction of the incoming air. To take an extreme case, when 4.5 per cent of the air is liquefied, the liquid contains 47 per cent oxygen and 2.35 per cent argon, which yields after removal of the oxygen 4.5 per cent argon in the nitrogen and argon mixture. This represents an amount equal to 10 per cent of the total argon present in the original air.
An Inexpensive Method for Determining Lead’p2 By Wilfred W. Scott UNIVERSITY OF SOUTHERN
CALIRORNIA, LOS ANOELES,CALIX?.
HE high cost of potassium iodide makes the chromateiodide method for the determination of lead very expensive when used with large classes in quantitative analysis. For this reason the author devised a method using Knop’s reaction with diphenylamine, ferrous sulfate, and chromate solution in connection with the isolation of lead as a chromate salt according to the well-known chromate-iodide method. I n place of potassium iodide being added to the lead chromate solution, the liberated chromic acid is titrated directly by means of ferrous sulfate in presence of diphenylamine. Chromium is reduced from hexavalent to trivalent form giving the ratio 3 Fe = Cr = Pb. The normal equivalent of lead, therefore, is one-third its atomic weight, as in case of the chromate iodide procedure. This method equals the iodide procedure in accuracy, a t about one-eighth the cost. A trial of the method in the hands of students showed that a large amount of hydrochloric acid interferes with the end point to the extent that, instead of producing a deep blue color, the green color deepens to a dark green end pqint, which does not give the contrast desired. Sodium chloride interferes slightly, but to a much less extent. It was thought that the difficulty might be due to the yellow color of iron but this proved to be not so. Later it was discovered that a large excess of hydrochloric acid was responsible and that its effect could be counteracted by addition of an acetate salt. Approximately 1 gram of ammonium acetate per cubic centimeter of free hydrochloric acid is required. In a volume of 200 cc., 50 cc. of free hydrochloric acid (sp. gr. 1.2) required 45 grams of ammonium acetate to give the desired blue end point. I n testing the tolerance of free hydrochloric acid it was found that up to 10 cc. caused no difficulty. In the usual chromate procedure this is more than is required 1 2
Received March 18, 1925. Work done at Colorado School of Mines, Golden, Colo.
in the solution of lead chromate by the hydrochloric acidsodium chloride solution. If a large amount of this solution is used, ammonium acetate must be added to counteract the free hydrochloric acid. The procedure was tested with pure metallic lead and with lead ores. Comparison was made with the chromateiodide method and with the permanganate-oxalate method. The following are typical of results obtained: SAMPLE Pure metallic lead Lead ore
Chromateiodide Per cent
Permanganateoxalate Per cent
Chromateferrous Per cent
99.80 76.58
7k: i 0
99.76 76.95 76.80
PREPARATION OF SAMPLE-The solution of the material, isolation of lead sulfate, and conversion to lead chromate are in accordance with the standard procedure used in the chromate-iodide method. Solution of the lead chromate is effected by adding 50 to 100 cc. of a mixture of hydrochloric acid and saturated salt solution. (1000 cc. saturated salt solution, 120 cc. water, and 100 cc. HC1, sp. gr. 1.20.) TITRATION-The solution is diluted with water to about 150 cc. Ten cubic centimeters of phosphoric-sulfuric acid solution (1:l) and four to six drops of diphenylamine indicator (1 gram salt in 100 cc. HzS04) are added. A 0.1 N solution of ferrous sulfate is now added in excess and the excess determined by titration with 0.1 N potassium dichromate, or potassium permanganate, until the green color changes to blue. If a precipitation of lead occurs the end point will be a navy blue instead of the deep violet blue. Should an excess of hydrochloric acid be present a dark green color will beobtained. Ammonium acetate added to this solution will cause a change from dark green t o blue. A 0.1 N solution is equivalent to approximately 0.0069 gram of lead per cubic centimeter of solution.
