T H E DETERMIXATIOS' OF T H E DEGREE OF PURITY OF LIQUID SUBSTANCES AND AZEOTROPIC MIXTURES
W. SWIETOSLAVSKI Institute of Physical Chemistry, Polytechnic High School, Warsaw, Poland Received February 22, 1934
During the last few years (1) the differential ebullioscopic apparatus has been so improved that it now seems possible to propose a standardized type of apparatus and to state the conditions for carrying out the determination of the degree of purity of either single liquid substances or of azeotropic mixtures. Figure 1 represents the standardized type of ebullioscope for such purposes. This apparatus differs from the former one in that it has two dropcounters,fl andf2, instead of one. To the drop-counterfi is sealed a reflux tube by means of which the liquid can be heated with either a small microburner, 1, or an electric heater. This device enables one to collect a certain quantity of relatively volatile impurities in the upper part of the apparatus and thereby to increase the difference between the boiling point, tl, measured in the lower part of the ebullioscope, and the temperature of condensation, tz, measured in the upper part. The two drop-counters enable one to fix the intensity of the heating and to standardize the conditions of the experiment. The degree of purity of a single liquid substance or of an azeotropic mixture is characterized by the difference At = tl - tz between the two above-mentioned temperatures. It is evident that, in the case of an absolutely pure single substance or of an ideal azeotropic mixture, this difference At is practically zero, because the correction for the difference of pressures at the levels tl and t z , where the temperatures are measured, does not exceed 0.001"C. But since every liquid, even the most highly purified one, contains impurities, certain differences of At > 0 are noted, which depend on the nature and the amounts of the impurities. Since the difference At depends upon the degree of dephlegmation of the vapors present in the whole apparatus, the intensity of the heating should remain constant throughout. This condition may be fulfilled by adjusting the rate of flow through the two drop-counters measured by the numbers n1 and n2of drops falling from the latter per unit time for a given intensity of heating; the number of drops depends upon the heat of vaporization and upon the boiling point of the liquid. Our experiments with water have led us to fix 1169
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for nl a value of 40 drops per minute. For other liquids which boil at < 100°C. the following empirical relation can be used: 900
711
= -
d.L
where L represents the molecular heat of evaporation expressed in kilogramcalories, and d the density of the liquid. When 21 > 100°C. t,he number of drops should be correspondingly greater. Good results can be obtained
FIQ.1. THE EBULLIOSCOPE
with n2 = 0.9 nl,or nl = n 2 . The size of the flame of the microburner, 1, should be regulated so that this relation can be obeyed. For the estimation of the degree of purity of liquid substances the following measurements should be carried out: (1) the determination of the boiling point and the temperature of condensation of the substance under atmospheric pressure and the comparison of both points with the boiling point of water as the primary standard substance, or of another substance chosen as secondary standard; (2) the deterniination of the same temperatures after removal by distillation of a small quantity of the liquid.
PURITY O F LIQC-ID SUBSTASCES ASD AZEOTROPIC 3IIXTURES
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This last operation seems to be necessary in alniost all cases. Indeed, it often happens that very pure liquids contain niiriute traces of moisture or other impurities which can easily be removed b y evaporating several cubic centimeters of tlie liquid (4). For this purpose the water is removed from the reflux condenser D, tlie small condenser, d , is joined to the tube provided with a ground joint, and a gradual distillation of 2 cc., 2 cc., and 2 cc. of the liquid is performed. After each distillation the difference At is measured. I n this way it can be found whether the moisture present in the liquid or introduced as vapor adsorbed b y the internal surface of the ebullioscope can be removed by this simple operation. The investigation should be carried out as follows: (1) Before use the walls of the apparatus should be thermally isolated. Tube 1 should be wrapped with wet asbestos strips; after they are dry they fit closely to the surface of tlie Pyrex glass. The tubes in ~diiclithe temperatures are measured are also wound with strips which are fastened round the tubes by means of thin copper wire. On top of the burner L an asbestos roof of conical shape is provided. ( 2 ) The ebullioscopic apparatus should be carefully dried and filled with a given amount of liquid under investigation (15 cc.). After allowing the liquid to boil for ten to fifteen minutes the apparatus is emptied, then refilled with a new portion of the same liquid; the same process is repeated, the liquid being once more poured out. Finally the apparatus is filled with 50 cc. of tlie liquid t o be actually measured. (3) The temperatures should be determined Tyitli the accuracyof =tO.OOl"C. For this purpose the electrical resistance thermometer or the Beckmann mercury thermometer (modified by me) or that of Roberteau, etc., may be used. When Roberteau's thermometer is selected, the enlargement between tlie scale and the bulb called "grain" must be imniersed in mercury. Each thermometer should be imniersed t o tlie same extent in both tubes, tl and tz. (4)The intensity of tlie heating with both burners should he regulated as already stated. ( 5 ) The determination of the tn-o abovementioned temperatures should be carried out b y using the same thermometer. For this purpose the method of successive measurements should lie applied (an example of this method is given in table 1). The boiling point of water or other substance chosen as the standard liquid ( 2 , s ) is measured simultaneously. If another substance than water is used as standard, its boiling point should differ only slightly from the boiling point of the liquid under examination, so that its boiling point may be measured by transferring the same thermometer from one ebullioscope to another. (6) After having determined the difference At (first difference) the small condenser d is joined to the side-tube of the condenser D and the water is removed from this condenser. Afterwards three 2-cc. portions of the liquid are carried over into the small test tube b y successive distillation. After each distillation the deterrniiiatioii of the difference At is carried out. If all the siiccessive differences, A f, I t l , At?, At3, or at least tlie last two remain
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practically constant, At > At, = At, = At3 or At > Atl > At2 = Ats, the value Atz may be accepted as a basis for characterizing the degree of purity of the investigated liquid. If, however, the above-mentioned differences gradually decrease, At > At, > At, > A h , it proves that the impurity cannot be removed by this operation and that consequently the second difference, At] ought to be accepted as the characteristic value for the degree of purity of the sample investigated. In both cases all the successive differences, At, At,, Atz, At3, should be stated in order to give some idea as to the nature TABLE 1 D e f e m i n a t i o n of the p u r i t y of isopropyl alcohol
11.00 11 02 11 04
4 978 4 979 4 979
11 10 11 12 11 14
(4 979)
'
I 1
I '
0 196 0 193 0 199
First difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.196
(4 979) (4 979)
11.18 11.20 11.22
4.979 4.979 4 979
11.28 11.30 11.34 11.36 11.38 11.40 11.42 11.44
(4.979) (4.979) (4,979)
1
,
4 783 4 786 4 780
' 1
2 430 2 430 2 429 2.430 2 430 2.430 2 430 2 430
I ~
I
1
1 3.013 3.013 3.013
1.966 1.966 1.966
4.979 4.979 4.979
* The condensation temperature varies between relatively large limits on account of the presence of a large amount of volatile impurities. of the impurit'ies present in the liquid under exaniinat'ion. For the present it appears t'hat actually t'he following scale may be accepted for characterizing the degree of purity of liquid substances: Degree of p u r i t y
First degree of purity ( I ) .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Second degree of purity (11).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Third degree of purity (111).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fourth degree of purity (IV). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fifth degree of purity (V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
L i m i t s in degrees difference Ai
JOT the
1.00 t o 0.10 0.10 t o 0.05 0.05 t o 0.02 0,020 t o 0.005 0.005 to 0,000
PL-RITT O F LIQUID S U B S T A S C E S A N D AZEOTROPIC M I X T U R E S
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If the liquid i s , however, characterized b y the fifth degree o f p u r i t y , a more caref u l investigation ought to be carried out with the help of a more sensitive diflerentia1 ebullzoscope. At the present time the differential ebullioscope with several sections (3) seems to be the most convenient for this purpose. The degree of purity of isopropyl alcohol (Schering-Kahlbaum) was determined by means of a standardized ebullioscope (table 1). Benzene, selected as a secondary standard, has a boiling point t~~~ = 80.122"C. at 1 atmosphere. The first difference, At, between the boiling point and the condensation temperature was determined. The difference between the boiling points of the two liquids examined is equal to At = 1.966. After the introduction of the correction for the true value of the degree, the difference, tBar - tl = AT, becomes equal to 2.060, therefore the boiling point of isopropyl alcohol, t,, equals 82.18"C. By successive distillation of three 2-cc. portions of the liquid, the following results have been obtained: = 0 195 At2 = 0 144 At3 = 0.083 At1
ATi
=
2.098
AT2 = 2.108
AT3
2.124
tu' t," to"'
= 82 22 = 82 23 = 82.25
CONCLUSIONS
Control of the purity for the given sample of isopropyl alcohol proves that the impurities cannot be removed by distillation of a small quantity of the liquid. I n accordance with the scale proposed above, the degree of purity seems characterized by the first difference, At = 0.195. The boiling point of the liquid, t,, is equal to 82.22"C. at 1 atmosphere, which is 2.10' higher than the boiling point at 1 atmosphere of benzene, the secondary standard substance. After distillation by help of a rectification column with twenty-one bulbs the determination of the degree of purity of the purified isopropyl alcohol has been carried out. The following result has been obtained: At = 0.010; At1 = 0.009; I t 2 = 0.009;& = 0.008;ATa = 2.17; t,'" = 82.29" ( p = 1 atmosphere). It follows that in accordance with our scale, the sample of the purified isopropyl alcohol can be considered as having the IT--th degree of purity, At being equal to 0.009", and the boiling point to 82.29'. The boiling point of the investigated isopropyl alcohol is 2.17" higher than the boiling point of benzene, the secondary standard substance ( p = 1 atmosphere). SUhlMARY
The application of a standardized differential ebullioscope for determining the degree of purity of single liquid substances and azeotropic mixtures has been described. Brief instructions are given for executing the investi gation. A scale for characterizing the relativc degree of purity of liquid substances and azeotropic mixtures has been proposed.
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REFERENCES (1) SWIETOSLAWSK~, W.:Bull. intern. acad. PolonaiseA1930, 504; A1933,177; Compt. rend. 192, 1457 (1931); Roczniki Chem. 13, 176, 227 (1933); Chem. Listy 26, 442 (1932). (2) SWIETOSLAWSKI, W.: J. chim. phys. 27,496 (1930). (3) SWIETOSLAWSKI, W. : Chem. Listy 26, 442 (1932). W.,AND USAKIEWICZ, J.: Roczniki Chem. 13,494 (1933). (4) SWIETOSLAWSKI, (5) ZMACZYKSKI,A . : J. chim. phys. 27,503 (1930).
