August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
shows the greater change in entropy with pressure and with temperature. Even with the significant deviation from the perfect gas law shown in Figures 1 and 4,the change in entropy is nearly directly ProPortional t o the logarithm of the Pressure in both cases, as would be strictly true in the case of a perfect gas.
1459
ACKNOWLEDGMENT
Financial support and cooperation were received from t h e Hercules Powder Company. The company’s permission to publish the results is appreciated. The assistance of L. Katz and P. S. Farrington in connection with the numerical thermodynamic calculations is acknowledged.
NOMENCLATURE
b
c,
=
specific gas constant
= isobaric heat capacity, B.t.u. per pound
(O
H = enthalpy, B.t.u. per pound n
LITERATURE CITED
R.)
(1) Davis and Johnston, J. Am. Chm. SOC.,56, 1045 (1934). (2) Gibbs, “Collected Works,” Vol. I, New York, Longmans, Green
= weight fraction
P = pressure, pound per square inch absolute
and Co., 1931. (3) Holborn. Ann. Phwsik. 63,674 (1920). (4) Johnston and Davis, J . Am. Chem. SOC.,56,273 (1934). (5) Kassel, Ibid., 56,1841 (1934). (6) Sage and Lacey, Trans. Am. Inst. Mining Met. Engrs., 136, 136 (1940). (7) Sage and :me,, “Volumetric and Phase Behavior of Hydrocarbons, p. 41, Stanford Univ. Press, 1939. (8) Smith and Taylor, J . Am. Chem. SOC.,45,2107 (1923). (9) Vold, Zbid., 57,1192 (1935). (10) Wiebe and Gaddy, Ibid., 60, 2300 (1938).
s =
entropy, B.t.u. per pound ( O R) t = temperature, O F . T = thermodynamic temperature, a R. specific volume, cu. ft. per pound residual specific volume, cu. ft. per pound compressibility factor, PV/bT Superscripts * = infinitevolume ’ = referencestate Subscripts N = nitrogen H = hydrogen k = componentk ~
v = v = z =
RECEIVED Maroh 10, 1947
Liquid-Vapor Equilibrium Relations in Binarv Svstems J
d
Ethylene-n-Heptane System W. B. KAY1 Standard Oil Company (Indiana), Whiting, Ind. T h e P - V - T - X relations at the liquid-vapor phase boundaries for the ethylene-n-heptane system were obtained from measurements of a series of mixtures varying in composition from nearly pure ethylene to nearly pure heptane. T - X diagrams of the coexisting liquid and vapor at constant pressure were constructed, from which data were obtained for calculating the phase-equilibrium constants for ethylene and n-heptane. From a comparison with previously published data on binary paraffin systems it is concluded that the substitution of an olefin for a paraffin in a binary mixture of paraffin compounds has relatively little effect on the P-V-2’-X relations if the olefin has about the same boiling point as the replaced paraffin compound.
T
HIS paper presents the P-V-T-X relations at the liquid-vapor phase boundaries of the ethylene-n-heptane system. The work is a continuation of a research program designed t o discover the factors affecting the P-T-X relations in the critical region in mixtures of petroleum hydrocarbons. The three binary systems t h a t have already been investigated in accordance with this program-ethane-heptane ( 4 ) , ethane-butane (6),and butane-heptane (6)-have provided quantitative data on the effect of a difference in the physical and thermal properties of the components in mixtures of the n-paraffin series. With the study of the olefin-paraffin system, ethylene-heptane, information is obtained on the effect of a differenoe in the chemical nature of the components. 1 Present address. Department of Chemical Engineering, Ohio State University, Columbus, Ohio.
The apparatus and experimental procedure were the same as those employed in the study of the ethane-heptane system (4). Before the experimental measurements were begun, the calibrations of the pressure gage and thermocouple were carefully checked. The check on the pressure gage was made by determining the vapor pressure of pure water at several temperatures corresponding t o pressures u p t o 1200 pounds per square inch. The pressure-temperature relations for water given in the International Critical Tables (2) were used t o determine t h e true pressure from which the correction for the gage was calculated. PURIFICATION OF MATERIALS AND PREPARATION OF MIXTURES
Cqmmercial ethylene of high purity was further purified by fractional distillation in a silvered and vacuum-jacketed column filled with Stedman packing and having a n efficiency equivalent to about 100 theoretical plates. Eighty milliliters of liquid ethylene were charged t o the still and a middle cut of 35 ml. was collected. The sample was then transferred t o the high vacuum degassing apparatus where i t was alternately frozen and melted and the noncondensable gases over the solid were pumped off each time. Finally, the purified sam le was distilled into a steel storage bomb. The high purity of tEe sample was indicated by the fact t h a t the pressure change between the boiling and dew points amounted t o only 0.2 pound per square inch a t 32.17” F. where the vapor pressure was found t o be 598.2 pounds per square inch. The n-heptane was from the same sample t h a t was used in the work on the ethane-heptane and butane-heptane systems. The pressure change during condensation of the samples used in the preparation of the mixtures amounted to about 1 pound per square inch %hen t h e vapor pressure was approximately 192 pounds per square inch. Mixtures of ethylene and heptane were prepared by loading the experimental tube with a sample of pure heptane, calculating 6
Yd. 40, NQ. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
1460
r
-180 b’igure 1.
0
100 xx) 300 400 TEMPERATURE O F ;
500
I
I
I
I
Uensity of S a t u r a t e d Liquid and Vapor, E t h y l e n e - H e p t a n e System
1qigiii-e 3.
P r e s s u r e - T e m p e r a t u r e Relations, EthyleneHeptane System 16s I\ eight troni the measured volume and density, and addlug i; tneasuied volume of ethjlene gas a t a known temperature anti pressure to make a mixture of known concentration. Th(3 apparatus and procedure were the same as t h a t used in the 11 orl, on the ethane-heptane system.
T o deteimirir \+hetl-rei the higher tcmprratuies to m hich t h r l mixtures mere subjected in the coursc of the measurements causrti any chaugr in the composition, the low-temperature meaburrments were carried out first and 1% ere then checked after the completion of the run. T h a t the agieement between the t a o sets o+ data was within the experimental error indicated no measurable change in composition. HNHAFIOH OF ETHYLENE
ur rrs C H r r i c A i , t w w r
Because of the anomalous behavior of ethylene a t its critical point, reported by McIntosh, Dacey, and Maass ( 7 ) ,the critical point was determined. The behavior ycas found to be different from t h a t observed in the case of other pure substances. J i t h i a * 10 pounds per square inch and 1 0 . 5 F. of the critical point the mercury surface and walls of the glass tube appeared to be covered with a sticky layer, judging by the behavior of the steel stirring ball. It was found impossible t o move the steel ball ii B liquid layer was present and very nearly impossible even when n u liquid was visible. However, nothing unusual was noted in the appearance of the critical phenomena. It would seem that ethylene forms a viscous complex in the vicinity of the critical point The values of the temperature and pressure at the critical point-Le., a t the disappearance of the meniscus-were found t o be 48.65 F. and 735.6 pounds per square inch, whereas Naldrett and Xaass (8)and McIntosh, Daceg, and Maass (7) found 48.58 O F, and 731.5 pounds per square inch for this point, which they tcsrmed “the crit>icaldispersion point.” EXPERIMENTAL RESULTS
F i g u r e 2.
Pressure-Composition Relations, Heptane S y s t e m
Ethylene-
Eight mixtures of ethylene and heptane containing approximately 2, 6, 10, 20, 30, 50, 70, and 90 weight yoof ethylene were studied. Their exact compositions expressed in both weight and mole per cent are given in Table I. For each of these mixtures the P-V-T relations were determined along the border curve from
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1948
1461
.06 .I .2 .4 6 .8 I EQVlLlBRlUM CONSTANT K=Y/X Figure 6. Comparison of Equilibrilml Constant Curl-es for Heptane with Different Hydrocarbnns
-10%
I
20
I
I
I
60 80 COMPOSITION MOLE 70 ETHYLENE 40
I 3
32' F. rhrough tlitb t.i.itical rryioli. A summary o f t Iir pressure~lensit\.-tempcrtttuI.1. dara aluiig rlie plrase boundaries of cach oi 1 h t . mirturc..i: is given i i i Table 11. The v a l u c ~werc read from stnoorlied cuIves of large scale p101+ of tlie ~)re-srire-tcmpelaturr and deniityt.cwpcwture border curves. The acetiracy of thes(L h t : i ir c~stiniaredto be as follows: pressure to within = 1 pound per squnrt. inch: tcmpt.ratwe -0.2" F.; dcnsiry -0.10 pound per cubic foot ; aiitl coinposition ro ivithin 0.1 mole c ; . Thc iiiterrelations of t h e t~finipt.i'atuw, prc.ssure, deiibity, anti compo4tiuii Stw 4ion.u graphically in tlie charts. l'igurc 1 ShO\\-:j the vapor prcusurt. curve' 01 pu1.c. rrhylcsiie atid /Acytwne, the P-T borclr.r ( ~ r v 01'e ~rllc mixtures that tverv sturlictl, :ant1 t l i c ci.iric,al locus c w v c ~ ior the systt*ni. Thi. vapor ~ I T S ~ U I Yd 1 ~1 1 1 1 for cqhylene, PuC('L1T f o r tht. cririciil point, are froln the Intcrniltional C'riti(8:tl Tables ( 3 ; ; rhe \ d u e s oi r h c s prcssuw and tc~mptwture at the critical point wrr deierniini~clexper iinentsllv. Tlie vapor prrssurc data ior EQUILIBRIUM CONSTANT I < = Y / X n-heptane are those of Young ( 1 1 ) . The boiling point curvps of the mixtures have Figure 5. Experimental and Calculated Pressure-Equilibrium Constant been extrapolated below 32" F. by plot. Curves a t Constant Temperature, Ethylene-Heptane System ting the experiniental pressure and temperature data above 32" F. as the logarithm of pressure against the recipTABLE I. CoMPOSITION 4P;D CRITICAL CONSTANTS O F ETHYLENE-n-HEPTANE MIXTURE3 rocal of the absolute temCritical Point Max. Pressure Point Max. Temp. Point perature and extending the Pt lb./ Po, Pe, pp, lb.) Pt, straight line through these hhxComposition sq. lb./ sq. lb./ sq. lb./ data t o temperatures below Lure Wt. % Mole % ' Tcinch cu. .!If: n:Sh cu 2'1, inch cu. No. CzH4 CZH4 F. abs. ft. it. F. abs ft 32" F., using as a guide in 0 0 513.3 396 14.6 ... ... ... ... ... the extrapolation the- boiling 1 2.08 7.04 504.0 472 15.07 499 482 18.1 565'.1 460 12.4 2 5.96 18.46 485.4 613 15.73 468 650 20.7 490.2 548 11.0 point pressures a t -130" F. 3 10.07 28.57 464.3 758 16.33 438 815 21.9 477.0 622 10.2 which were calculated on the 4 20.05 47.25 410.8 1081 17.37 370 1167 23.1 ... ... .5 30.03 60.52 357.0 1316 18.00 321 1374 22.3 4iS:o 844 7.6 assumption that the ideal solu6 49.83 78.01 253.3 1516 18.60 259 1516 18.2 350.2 937 6.3 7 70.06 89.31 157.2 1319 18.17 213 1448 13.3 278.0 875 5.0 tion laws are valid at the low 8 90.01 96.99 82.9 932 16.00 135 9.9 179.1 685 3.7 pressures c o r r e s p o n d i n g t o 100.00 100.00 48.7 . 736 13.11 . . . 1121 ... ... ... ... ,.. this temperature. The comFigure 4.
Temperature-Composition Relations, Ethylene-Heptane System
Pi
O
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1462
TABLE 11. SU3IMARY O F PRESSURE TEXIPER~TURE AND DENSITY DATAAT PHASE BOUXDARIES FOR ETHYLEXE-WHEPTXXE SYSTEM
180
150 200
250 300 350 400 450 500 550 600 650
Temp.,
F.
Density, Density, Ib./cu. Temp., Lb.icu. it. O F. ft.
183.0 260.2 317.1 360.7 39e.o 426.7 454.6 480.0
... ... ... ...
1150
300 350 400 450 500 550
600 650 700 750
800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500
...
... ... ...
-5.on
32.0 64.0 93.9 122.2 149.8 176.7 203.2 230.1 257,2 284.6 312.5 341.0 372.0 411.3
... ... ... ...
... ...
...
...
. I .
-54.60 -29.8Q -10.5a
6.3a
21.55
35.8 49.0 61.7 73.8 85,5 96.8 108,l 119.1 130,l 141.0 151.8 162.7 173.8 185.2 197.1 209.1 222,l 236.0 251.2 268.5 292.6
... ...
...
...
...
...
ii:i3 .,.
...
... ...
... ...
... ... ...
1.12 1.68 2.25 2.90 3.55 4.28 5.05 5.93 6.89 8 07 9.50 11.28 13.40 16,04 19.40
... ..,
.., ... ...
...
... ... .,. 70Ethylene ..,
0.80 1.17 l,55 1.90 2.30
271.0 300,O
I . .
... ...
0
F.
36 0 86 8 131 5 173.4 214.2 253.0 289.6 325.1 359.1 392.0 424.9 468.0
...
2.70
3.10 3.55 4.00 4.45 4.90 5.40 5.95 6.50 7.10 i .75 8.40 9.15 9.95 10.80 11.75 12.85 14.10 15.55 17.30 19.60
...
... ...
, . .
...
a
-75.00 -53.0" -36.0a -21.55 -9.2a 1.5a 12.05 21.5a 30.75
39.0 47.1 54.8 62.0 69.0 76.0 82.8 89.5 96.1 103.0 109.8 117.0 124.9 133.0 142.3 153.0 165.5 181.1
Extrapolated values.
... ... ... ... ...
184.5 205.2 219.6 230.8 239.8 247.5 .. . . . 253.9 259.1 263.5 29: 70 267.2 29 .oo 28.40 270.2 27.80 272.7 27.25 274.6 26.70 276.1 26,15 277.2 25.55 278,O 24.95 278.1 24.30 278.0 23.65 277.2 23.05 276.0 22.35 274.3 21.60 271.5 20.75 268.0 19.76 263.9 18.65 258.3 17.40 251.1 16.05 241 .O
...
-13.5" 9 0:
, . .
28.9 47 .O 64.2 80.5 96.2 111.4 126.2 141 .O 155.8 170.5 186.0 201.3 217.0 233.5 250.7 269 .O 288.2 310.0 343.0
ft.
...
... 38:13
37.47 36.85 36.28 35.70 35.15 34.55 33.90 33.25 32.50 31 , 7 5 31 .OO 30.10 28.98 27.50 25.50
I . .
...
...
... ... ...
:
46b 0 479.5 487.0 490,s 487.6
...
, . .
...
... ... ...
7.29 8.80 11.10 14.60
Ethylene
... , . .
... , . . ... ... ...
...
... ...
. .
... ... ...
... ... . . ... ... ... ...
,..
... ...
... . . ... . .
t . .
...
...
:
433 0
426.1 417.2 407,5 390.0
... ifi:is 18.15 20.70
78.01 3Iole % Ethylene -67.2a -44.5Q -27.1a -12.0Q 1.0:
12.6 23.14
33.5 43.3 52.8 61.6 70.1 78.4 86.8 95.0 102.8 110.3 118.0 125.8 133.8 141.9 150.2
158.7 167.3 176.5 186.6 198.4 212.5 233.0
..
....
...
33:75 33.20 32.65 32.20 31 .70 31.25 30.85 30.40 30.00 29.60 29.15 28.65 28.15 27.65 27.15 26.60 26.05 25.40 24.60 23.65 22.50 20.55
0.62 0.90 1.20 1.50 1 .so 2.10 2.42 2 .73 3.03 3.40 3.70 4.10 4.45 4.80 5.20 5.60 6.00 6.50 6.95 7.40 7.96 8.50 9.05 9.75 10.45 11.30 12,30 13,55 15.70
96.99Mole yo Ethylene
89.31 Mole % Ethylene
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400
...
- 40a
Denbity, Ib./cu.
, .
4 0 : 72 39.37 37.98 36.55 35.10 33.58 31.96 30.27 28.35 26.04 20.63
47.25Mole
...
321.3 338.2 352.4 364.0 37:55 36.90 374.0 36.30 382.6 35.80 389.8 35.40 396.0 35.00 401.3 34.55 406.0 34.05 409.5 33.55 411.3 33.10 412.6 32.60 412.8 32.15 412.4 31.60 411.0 31.10 408.4 30.50 404.9 29.85 400.0 29.20 393.9 28.45 385.3 27.55 374.8 26.35 362.0 345.0 24.70
...
Temp.,
yo Ethylene
321.9 357.3 383.9 405 1 422.6 3 7 , 60 437.0 36.61 449,l 35,63 458,8 34.60 486.2 33.51 472 , 5 32.39 4i6,2 31.10 476 . O 29,55 471.9 27.65 465.3 24.75 452.9
... ... ... ... ...
... ... ... ... ...
...
4i:34 40.45 39.52 38.57
60.52Mole 100 150 200 250
4g3:7 504.3
...
28.57Mole 100 1.50 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100
... ... ... ... ...
38.64 35.90 33.70 31.66 29.73 27.81 25.61 22.76
Density, lb./cu. Temp., F. ft.
0.50 0.78 1.03 1.30 1,56 1.82 2.09 2.37 2.65 2.93 3.21 3.54 3.85 4.18 4.50 4.82 5.20
5.55 5.95 6.40 6.85 7.30 7.80 8.35 9.00 9.85 10.90 .
- 79a -57.5a -41.0n -27.4a -15.5a -4.8C ... 4.95 ... 13.5Q ... 21.8" 29.4Q 24:90 24.15 36.8 23.30 44.0 22.40 50.7 57.4 21.45 64.1 20.30 18.95 70.9 78.1 17.15 16.45 86.2 14.10 94.8 12.95 104.8 11.45 119.0
...
TABLE111. COXPOSITION AT POIYTo r ?yl~ X I M C X CRITICALPRESSURE FOR BINARYHYnROC.4RBON
18.46 Xfole Yo Etbylene
7.04N o l e V0 Ethylene Pressure, Lb./Sq. Inch h b s .
... 0.71 0.95 1.20 1.47 1 .71 1.98 2.27 2.56 2.87 3.17 3.50 3.86 4.20 4.60 5.00 5.46 6.00 6.59 7.35 8.55
Vcl. 40, No. 8
SYSTEMS
Composition, l l o l e % of &[ore
Eyqtein Ethane-wheptane ( 4 ) n-Butane-n-heptane ( 6 ) n-Pentane-n-heptane (1) Eth>lene-n-heprane Ethane-n-butane ( 6 ) Propane-n-butane (S)
Volatile Component 77 1 77 0 77 70 7 69 0 68 5
bination of the border and vapor pressure curves forms a space diagram representing the P-T-X relations for the system. The shape of this space diagram is similar to the space diagrams of other binary systems of n-paraffin hydrocarbons. The critical locus curve contains a point of maximum critical pressure with the following coordinates: 1516 pounds per square inch, 264" F., and 76.7 mole yo ethylene. I n all binary hydrocarbon systems that have been studied containing n-heptane with a more volatile hydrocarbon the composition of the mixture n i t h the greatest critical pressure is approximately 23 mole yo heptane and 77 mole yGof the more volatile component. T h a t the composition of the mixture with the maximum critical pressure might be constant and characteristic of the less volatile component in all binary hydrocarbon mixtures is further indicated by the data for the ethane-n-butane ( 5 ) and propane-n-butane (9) systems, where the composition is approximately 31 mole yC n-butane and 69 mole 7,of the more volatile component. The data are shown in Table 111. The temperature, pressure, and density at the critical point, point of maximum pressure, and point of maximum temperature of the mixture studied are given in Table I. Figure 2 shows the relations between pressures and the composition of the coexisting liquid and vapor phases a t constant temperature. The envelope curve (the dashed line) is tangent to the P-X diagrams a t the point of maximum pressure-ix., the point of maximum pressure on the P-T border curve. The effect of pressure on the solubility of ethylene gas in heptane at constant temperature is given by the liquid branch of the P-X diagrams. The density-temperature curves of the saturated liquid and vapor of the eight mixtures and of pure ethylene and heptane are shon n in Figure 3. They are similar in all respects to those found for the ethane-heptane system (4). An inflection point at the critical point will be noted in the 50, 70, and 90 weight 7' ethylene mixtures, as was also the case for the ethaneheptane system. I n Table I V the relations between temperature and composition of the coexisting liquid and vapor phase a t constant pressure are given for a series of pressures ranging from 100 to 1500 pounds per square inch. These relations are shown graphically by the diagrams in Figure 4. From large scale plots of Figure 4 data were obtained from which vapor-liquid equilibrium K-values ( K = y/z) for ethylene and heptane
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1463
$3 c oNb N
.. ..
.. .. .. .. ....
Figure 7. Deviation of Critical Pressures of Hydrocarbon Mixtures from Values Calculated by Mixing Rule
were calculated. The K-values and their relation t o the pressure at constant temperature are shown graphically by the series of isothermal curves in Figure 5 . For comparison of the experimental values with those calculated from the fugacity of the hydrocarbons, curves for ethylene and n-heptane constructed from data given by Sherwood (IO)are added. The agreement between the experimental and theoretical curves for ethylene is fair up t o 800 pounds per square inch a t 100' F. but very poor in the same pressure range a t temperatures above 200 O F. ; in the case of heptane the agreement is satisfactory up t o 200 pounds per square inch over the temperature range from 100 a t o 500 F. but becomes noticeably poorer at higher pressures. These results are in agreement with the results of previous investigations-namely , that the difference between the theoretical and experimental values is a function of the deviation of the mixture from a perfect sblution. +I
m
0
COMPARISON WITH OTHER HYDROCARBON SYSTEMS
I n Figure 6 a comparison is shown of the vapor-liquid equilibrium K-values of heptane in ethylene, ethane, and n-butane. All the curves at a given temperature coincide approximately at low pressures but deviate at the higher pressures in a manner that is a function of the critical pressures of the respective systems of mixtures. The effect of the physical properties of the components on the critical pressures of mixtures of the components mag be shown by comparing the differences between the actual critical pressures, PcBxpI., and those calculated as the product of the composition expressed as weight fraction and the critical pressure of the pure components, Pcoelod,.These differences for the ethylene-heptane, ethane-heptane, ethane-butane, and butane-heptane systems in relation t o the composition are shown by the curves in Figure 7.
1464
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Thus, the maximum value in tile ethane-heptane s’i stem 16 ahout 3.4 times greater than the. maximum value in the c~thaiic~-butane Yystem. I n other words, the greater the differerice in boiling - PCLvlid ). 011 the othet points the greater the value of (Pcexpt hand, in mixtures of ethane and butane Tvith a boiling p i n t difference of 88’ C. the value of Po,:,,+ - Pcoaicdis 1 .B times greater than in mixtures of n-butane arid n-heptane with a boiling point difference of 99” C. I n othei nords, compounds with a high volatility-that is, low boiling point--sho.i\- larger values of P,,,,,, Pconicd, for a given difference in boiling points than the higlier boiling compounds. These generalization5 based 011 the behavioi of biiiai )- paiaffiri hj-drocarbon systems hold, a t least qualitative11 , for the ethyleneheptane system, Thus, because ethylene is more volatile than ethane, it is not surprising that the maximum value of PCrxptPccaicd of ethylenemheptane mixtures is about 1.3 h i e s greater than in mixtures of ethane and heptane. It would appear, therefore, that the P-V-7’-X relationq for ethvl~,ne-heptanr mixtures
Vol. 40, No. 8
M ould bc, veij- near13 the same as for inixtuies of heptane and et hypothetical parafin compound of the same standard boiling point as ethylene.
C uniiiiiiigs, Stoneq, and \ olante,
I\:>. Fsc: CHavr., 25, 7 2 8
(1933). ( 2 ) International Critical Tablea, 101. 111, p. 210, NPW York. McGraw-Hill Book (’o., 1928 13) Ibid., p. 230. (4) Ka?, W. €3.. I h D . h\G. C H > \ r . , 30, 454 (193X) (5) Ibad , 32,353 (1940). (6) I b z d , 53,690 (1941).
(7) McIntosh, Dacey, andMaass, Pan. J . Kaseuich, 178, ,241 (1939). ( 8 ) Naldrett and Maahu, I b t d , 18B,118 (1940). (9) Nysewander, Sage, and Lace>, INDEKG.CHEX.,32, 118 (1940). (10) Sherwood, “Absorption and Extraction,” p 105 Ne- York. MrGIaw-Hill Book C o , 1937. (I1) Young S., J Chew S o r 73, 675 (1898) H
omposition o
Strong Phos KUSSELL N. BELL V i c t o r Chemical W o r k s , Chicago H e i g h t s , i l l .
&I liquid phosphoric acids b e t w e e n HLO.PA), arid 3H20.PzOj were found to he mixtures. Orthophosphoric and triphosphoric acids were present at all phosphorus pentoxide levels; pyrophosphoric acid was present up to 859’0 P205and the poljmer of metaphosphoric acid commonly known as hexame taphosphoric acid was present aboie 8370 P20jn A n unidentified acid, indicated bj difThe sodium ference, was present hetweeri 78 and 8870 1’2Qa. ialts of three of the acids present in one of the mixture9 were prepared and identified. I l i x t u r e s cwtitaining hetween 79 and 8070 P ? O , ,appvovimatel> theoretical f o r p>rophosphoric acid, crj stallizetl on standing. The crj stallized acid contained more pyrophosphoric acid than the same acid before crystallization but, on renielliiig, the romposition retirimed t o that of the origiiixl liquid.
HQS13H01ZlC acids having a pliosphoiur peiitoudr iwiitent higher thair 01 thophosphoric acid have hcen commerimlly available for several yeais. Although these ale frequentlv called “pyrophosphoric,” “poly phosphoric,” or “metaphosphoiic” acids, no claiins ale uiua1l.i. made as to their actual composition. Attempts have been made ( 1 , 5, 7 , 9, 11) to deteimine the various acids present, but the methods employed n r r s not ,idequate as they did not recognize the existence of ti iphosphoric acid, Ahich is present and vihicli iriteiieies in some procedures tor pyrophosphate detei miiiation (4). loyama (1) and Dworzak and Rcich-Rohim ig (6) neutiahzcd the acid TTith 0.1 K sodium hydrouide, precipitated the phos))hatesn i t h a measured amoiint of silver nitrate, and titrated thc acids libeiated from the precipitate by hydrogen sulfide ( 1 ) oi hydrochloric acid (5) Stollenneike and Baurle (11 ) prccipitated oitho- and pyrophosphates with a measuied amount of haiium h>droxide, then dctrrmined the total phosphorus in the piecipitate, and from these figures calculated the composition o t the acid. Britske and Draguiiov ( 4 ) calculated the pyrophosphoric acid from a titration of the sulfuric arid lihciatctl
he11 m i c sulfate M dtkd to >L solutio11 ol thr pho,phat r and Miles ( 7 ) deteirnined ortho-, pH around 3 8. G and mrtaphosphoric acids by colorimetric pH titration6 and used a sliding scale to correct the end points for varjing proport i o r i i of the acids present. Kone of these procedures rrcognized tiiphosphorie acid. Salts of this arid are now ell knonn and it\ prcwnce in the system €120 PZO, would be expected. Lum, \raloi\an, and Durgin (9) checked the methods of Gerber and \Lilt+ agaiiist those of Britske and Dragunov and found fair npicvrnent between them nhen acids having less than 789; P20, I \ C J C analyzed or when mixtures of sodium ortho-, pyro-, and riicxtaphosphates n ere used. Ho~5 ever, the methods showed convderable variation when acids of high phosphorus pentoxide cwitrsnt or fused mixtures of the sodium salts were analyzed. Thi- they attributed t o t h e presencc of unidentificd polyphoptiatea or polyphosphoric acids. The purpose of this research was to dctcrmine the compositiori ot acid mivtures betxveen H20.P20,and 3H20.P20,,wing new riiethodi Cor the determination of tri- and pr rophosphoiir acid.: ( 2 ) . Information was also desired as to mhhcther or not th(A method of pieparing the acid matc4allv affertrd itr ultimatrb wmpo’ition it
PROCEDURE O B A 4 c ~ u s Acids were p r e p a i d b\ heating acid to tirivc off ~ a t e and r by t h e addition of phosphoi us pciitoudc, to 01 thophosphoric acid.
C’oiiimeicial S50,lC oithophosphoric acid vTas hcated until the desiied phosphorus pentoxide content x a s attained. The acid pas transfeired to a n ell-stoppered Pyrex container, cooled, and analyzed. A s strong phosphoric acid attacks glass above 200” C., the thermally prepared acid having 82 87, PZO; was made In graphite. I n the preparation of strong acids by the addition of phosphorus pentoxide, the calculated amount of the oxide was added to commercial 85y0 orthophosphoric acid and agitated until the reaction was complete. The phosphorus pentoxide was added s l o ~ l yso , that the tempeiature of the acid remained below SO” C. Inasmuch as the first two acids prepared in this manner
