LITHIUM CHLORIDE AMMONIA COMPLEXES BY S. C. COLLINS AND F. K. CAMERON
Introduction From the literature it appears that lithium chloride absorbs ammonia to form a series of definite compounds containing one mole lithium chloride (LiC1) to one, two, three, four, five or six and one half moles of ammonia (NH3). Bonnefoil reports preparing the one, two, three and four ammonia complexes by passing ammonia, a t atmospheric pressure, over dry lithium chloride. The stable solid formed at 85°C is monammonio-lithium chloride (Li.Cl.NH3). Below 85°C and above 60°C the stable solid is the di-ammoniolithium chloride, according to Bonnefoi; between 60°C and rs°C the triammonio and below 12°C the tetra-ammonio-lithium chloride is the stable solid. Between -4jOC and -78.j°C, Bilby and Hausen* found as stable solids the quinta-ammonio-lithium chloride and the complex containing one mole lithium chloride to six and one half moles ammonia. Ephraim3 confirmed the existence of the tetra-ammonio-lithium chloride, finding its vapor pressure to be 760 mm. at 12°C. I n the present investigation, the possible existence of the di-ammonio-complex is not confirmed, although many efforts have been made to obtain it. Nor do the vapor pressure measurements for monammonio-lithium chloride accord with those of Bonnefoi. If has been found that, a t temperatures between 20°C and 3ooCmonammonio- and tri-ammoniolithium chlorides are stable in the presence of water.
Materials The lithium chloride was prepared by dissolving “Baker’s Analyzed,” strongly acidifying the solution with hydrogen chloride and recrystallizing. The ammonia was taken directly from a cylinder of commercial liquid ammonia, there being no volatile components present which might interfere with the formation of the ammonio-lithium chloride complexes. The System: Lithium Chloride-Ammonia Attempts to dry lithium chloride by heating it in the absorption flask resulted in a dense cake with a glazed surface very poorly adapted to the absorption of ammonia. The di5culty was overcome by immersing the flask in a bath at 180°C while dry ammonia was passed through the flask. There is no absorption of ammonia at this temperature. The ammonia was dried by passing it through a train consisting of a four-foot tube containing flakes of sodium hydroxide, a second four-foot tube containing chips of metallic sodium and finally, an eight-inch column of molten metallic sodium. That ‘Ann. Chim. Phys., (7) 23, 317 (1901). I (1923). Ber., 52, 236 (1919).
* Z. anal. Chem., 127,
1706
S. C. COLLINS AND F. K. CAMERON
the drying of the lithium chloride was completed was determined by the flask coming to constant weight, and the content of lithium chloride was then known quite accurately. The flask was suspended in ice water and dry ammonia passed through until there was no further absorption, the process requiring ten to twelve hours. Four trials were made in this way and in each case there were absorbed per mole of lithium chloride (LiCl), thus confour moles of ammonia ("3) firming Bonnefoi's results. Also, a determination of the vapor pressure a t o°C accorded perfectly with that of Bonnefoi. On standing a t room temperature for a few hours, the tetra-ammonia complex loses exactly one mole of ammonia. And when the dry lithium chloride is brought into contact with the dry ammonia a t room temperature, again the stable solid persisting is the tri-ammonio complex. Thus, a sample of the tetra complex kept for some hours a t zs0C and then analyzed, gave 2.99 moles NH3 per mole LiCl. A number of trials were made to obtain the complex LiC1.z NH3 by heating the tri-ammonio complex to 6ooC. Always, two moles of ammonia escaped, leaving the monammonio complex, LiCl.NH3. Similarly a sample of the tetra-ammonio complex heated to 66.4OC gave a residue which, on analysis, yielded 0.992 moles ammonia per mole lithium chloride. Bonnefoi states: iiLiC1.1NH3 se produit tuutes les jois que l'on fait absorber N H 3 par L i C l entre 60" et 88", au bien e n maintenant les composds LiC1.SNH3, ou LiC1.4NH3 entre ces deux temperatures." Hence, further efforts were made by causing lithium chloride to absorb ammonia between 60°C and 8s0C, but is no case was the di-ammonio complex obtained. Typical analyses of the products obtained by absorption of ammonia, in moles per mole lithium chloride, gave 3.97 a t o°C, 2.93 a t 23OC, and 0.99 a t 64OC. Isotherms, showing the successive pressures as ammonia is removed from these complexes, were determined ; and, for this purpose, special equipment was designed and gradually assembled until a satisfactory form was developed. Since the accumulation of the data for any one isotherm required a period of several days, during which the temperature of the absorption complex must be kept continually quite constant, a special form of constant temperature bath or thermostat was developed, which has already been described.' Various experimental difficulties, particularly leakage of ammonia kept long in contact with stop-cocks and rubber joints, and corrections for meniscus in tubes and vessels of various diameters, were finally eliminated in the assembly illustrated in Fig. I. A long-necked flask, A, of Pyrex, to contain the lithium chloride, carries a two-hole rubber stopper, cut to fit very snugly and crowded far down the neck. Above the stopper mercury forms a seal. The glass tubes B and B' are 4 mm. in diameter, without joints and terminate in mercury wells, C and C'. C is an extraction thimble, fitted closely with a stopper carrying, besides the tube B, another bent tube of the same diameter, E, carrying the mercury a t atmospheric level to the scale, for comparison with the mercury in B. Being of the same diameter, com1
S.C. Collins: J. Phys. Chem., 31, 1097 (1927).
LITHIUM CHLORIDE AMMONIA COMPLEXES
I707
parison can be directly made without correction for meniscus or capillarity. The Gooch funnel, C’, forms a trap through which ammonia can be drawn without breaking the seal, and is fitted with a mercury reservoir D, for which an ordinary separatory funnel serves. The closely fitting stopper carries a tube with accurately ground glass stop-cock connecting with the absorption
Apparatus for determining Vapor Pressures.
flask, H, which in turn is fitted with a burette, I, through the stopper as illustrated. The volume of the flask A, together with the two tubes entering it, is approximately 65 cc. The entry of the two tubes into A permits one to pass dry ammonia over the lithium chloride, contained therein, and prepare the complex desired after assembly is set up. An experiment may be repeated a number of times, without dismounting the assembly. The only outlets being mercury sealed, leak-
1708
S. C. COLLINS AND F. K. CAMERON
age is practically eliminated. A constant temperature in A is maintained by immersing it in the bath as noted above. An accurately weighed sample of lithium chloride, approximately z .5 grams, is dried in the flask A, as described above, that is by passing a current of dry ammonia through the flask, while the latter is kept a t a temperature of 180'C or higher. The flask is then assembled with the tubes and submerged in the bath a t a coo1 temperature, while dry ammonia passes through the flask for several hours. There is an absorption of ammonia the course of which is followed by bringing the ends of the tubes under mercury from time to time and noting if the mercury rises in the tubes. When
FIG.2 Vapor Prassure-Concentration Curves for the System : LiCl -XHs.
the absorption is completed, the system is brought to the pre-determined temperature and maintained at that temperature continually. The regulator is adjusted and successive small quantities of ammonia are removed by aspirating through standard hydrochloric acid solution and then determined. A vapor pressure reading is made between each removal of ammonia. The total amount of ammonia absorbed in the complex is computed from the sum of the successive small portions removed. The results obtained in several series determined as just described are given in Table I and plotted in Fig. 2 . Consider the isotherm for o°C. There being two components in the system, lithium chloride and ammonia, then with two solid phases in contact with the gas phase ("8) the system is, in general, univariant. But, the temperature being fixed, the system becomes invariant, and the pressure remains constant until one of the phases disappears. Thus, starting with tetra-ammoniolithium chloride, LiC1.4KH3, in equilibrium with gaseous NHI and at a pressure of 750 mm. Hg, and keeping the temperature of o°C, when ammonia is withdrawn, the pressure quickly falls to 387 mm., with the appearance of a
LITHIUM CHLORIDE AMMONIA COMPLEXES
I709
second solid phase, tri-ammonio-lithium chloride, LiC1.3KH3, and the pressure remains constant with subsequent withdrawals of ammonia, until all tetra-ammonio complex disappears. With further withdrawal of ammonia there is an abrupt fall of the vapor pressure to 2 2 mm. with the appearance of a new solid phase, the monammonio complex. The tri-ammonio complex persists together with the monammonio complex in contact with the gas phase a t the constant pressure of 2 2 mm. until, with successive withdrawals
Vapor Pressure-Temperature
FIG.3 Curves for the System : LiCl-N Ha.
of ammonia, there is no longer sufficient ammonia present to form the triammonio complex. With a further withdrawal of ammonia, there is again an abrupt fall in pressure to 5 . 5 . mm. with the appearance of lithium chloride as one of two solid phases in contact with the gas phase, the other solid phase being, of course, the monammonio complex; and this pressure persists until all of the ammonia is withdrawn from the system, when there is another sudden drop of pressure to a point approximating zero, and the only phase remaining is solid lithium chloride, although theoretically, there is also a gas phase instead of the very complete approach to a vacuum actually realized.
1710
6. C. COLLIN6 AND F. K. CAMERON
TABLEI Vapor pressures in the system: LiCI-NH3. NH, in moles per mole LiCl in the system. Pressures in mms. Hg. 33O 45.2O
O0
-
54.5O
NH3
mm.
iXH3
mm.
KH3
4.01
750
3.05 556 3.01 368 2.98 372
7 50
387 387 366
600 185.8 185.8 185.8 185.8 185.8 185.8 185.8 185.8 185.8 185.8 185.8 92.0 47 ' 5 47 ' 5 47 ' 5 46.0 47.0
3.1
3.75 3.30 2.98
2.93 2.9 2.6
2.99
2.75
372
1.7
2.48
1.4 1.21
1.99
37 2 372 372 372
1.75
372
.98 .90
611 612 611 611 611 611 156
1.30
372
'75
I49
I. IO
372 372
.38 .04
152
1.04 I .oo .99 .97
2.5
2.75
a8.o
2.3
2.15
22.0
2.05
1.99
22.0
1.75
1.30
22.0 22.0
1.97 1.9 I .6
1.10
22.0
1.04
20.0
1.00
.65 .35 .OS 00
.
6.0
1.3 1.25
1.03
5.5
.99
5.5 5.5
.83
0.0
.97 '51 IO
.
. 00
0.0
mm.
2.15
2.03
.80
.65 ,35 .OS
. 00 58.3O 2.98 745.5 2.47 745.5 1.95 7 4 5 . 5 1.60 745.5
66.4' 755
1.02
1.00
420
.99 .97
1.03
745.5
,85
281 281 281
1.00
358.0 189.0 185.0 185.0 185.0
.66
285
.60
282
.33
282
.99 .j5 .4j .IO
. 00
.o
.14 282 .08 274
. 00
0
372 371
mrn.
"3
2.1
1.05
.00
15' 0
126 92 92 92 92 0
74.9O 0.81 423
86.9' 0.85
0.66 82O
0.81
152
598
750.5 750.5
LITHIUM CHLORIDE AMMONIA COMPLEXES
1711
I n a similar way, and quite as sharply, the data obtained define the isotherms for 33', 45.z0,54.5', and 58.3' C, commencing with the removal of ammonia from a system of tri-ammonio-lithium chloride in contact with gaseous ammonia. The tetra-ammonio complex is not stable a t these temperatures a t the pressure of an atmosphere. The data obtained a t 66.4' leave no reasonable doubt that at that temperature, the only stable complex is the monammonio-lithium chloride. Since there was, relatively, a fairly large volume of gas phase in contact with the mixture of solids in the absorption flask and connections, data were realized for a few points on the vertical portions of the isotherms, these vertical sections being pressure-volume curves for the gas remaining in the flask during the transition from a system with a high pressure level to the system with the lower pressure level. From the data in Table I the pressure-temperature relations are plotted in Fig. 3 , and for convenience in comparison also the data of Bonnefoi. The data for the tetra- and tri-ammonio complexes are in very satisfactory accord. The data for monammonio lithium chloride yield a curve almost coinciding with one plotted from Bonnefoi's data for the di-ammonio complex. It might be assumed that Bonnefoi obtained the di-ammonio complex as an unstable solid, since the data here reported show the monammonio complex to be stable over the temperature-pressure range concerned. But if the data here reported for the mono complex be correct, the question arises as to what was the complex which Bonnefoi supposed to be the mono complex. Possibly it was another modification. Roozebooml cites a case where a hydrate exists in two modifications with two different vapor tensions for each particular temperature. Janecke2 has shown in the system K2POa - H 2 0 - NH,, within temperatures from o°C to zs0C, a t least, there can exist two liquid phases in contact with a gas phase, so that, metastability may be anticipated in systems in which ammonia is a component. But Bonnefoi's modification of the monammonio complex should be more stable than the one obtained in the present investigation. The results of the present investigation are offered with confidence that they are approximately correct in absolute magnitudes and that they do represent stable equilibrium conditions. The System: Lithium Chloride-Ammonia-Water
4 series of solutions was prepared of varying concentrations with respect to ammonia, and with lithium chloride present in excess. When i t appeared that equilibrium had been attained, anaiyses of the clear liquid phase and of the corresponding residue of liquids and solids were made, the results being assembled in Table 11. When plotted on a triangular diagram in the conventional way, Fig. 4, the data indicate that this system yields three solid phases a t a temperature of z s 0 , namely, lithium monohydrate (LiCl.H20) in contact with solutions containing up to I 5Yc ammonia, monammonio-lithium 2. physik. Chem., 4, 43 (1889). 2. physik. Chem., 127, 82 (1927)
S. C. COLLINS AND F. K. CAMERON
1712
chloride (LiCl.NH3) in contact with solutions containing from about 15% to somewhat more than 2 7 % ammonia, and tria mmonio-lithium chloride (LiC1.3NH3) in contact with solutions containing 32 % to 40% ammonia.
Li Cl FIG.4 Isotherms and corresponding Solids for the System :LiCl-
TABLE I1 Percentage composition of liquid and of residue in system : LiCl-NHs-H20, a t 23°-250C Liquid Residue LiCl "8 HsO LiCl NE HzO 45.4 46.7 48.1 50.84 54.90 50.73 50.81 51.9 54.7 57.0 52.57 46. I 45.3 41.52 39.3 38.28 39.11 37.9 45 ' 9
0.00
3.' 7.9 13.88 14.16 14.86 15.34 16.70 20.0
21.64 27.31 32.0 32.7 35.7 35.8 36.53 38.6 40.4 33.2
54.6 50.2
44.0 35.28 30.94 34.41 33.85 31.4 25.3 21.36 20. I2
21.9 2 2 .o 22.8 24.9 25.29 22.3 21.7
20.9
49.5 54.4 56.57 54.52 53.92 55.12
60.0 58.2 58.88 54. 47. 45.1 44.3 42.8 39.95 41.26 43. I 45.3
2.5
5.7 10.27
12.41 12.67 12.16 16.5 19.1 23.0 28.3 35.3 39.3 4=.7 39.4 44.21 43.9 44.9 39.6
47.9 39.9 33.16 33.07 33.41 32.72 23.5 22.7 18.12 17.7 17.7 15.6 14.0 17.8 15.64 14.84 12.0
15.1
1113
LITHIUM CHLORIDE AMMONIA COMPLEXES
The data in Table I11 (charted in Fig. 5 ) were calculated from Table 11. The corresponding numbers in the two columns indicate the amounts in moles of ammonia and of lithium chloride respectively that will jointly dissolve in IOO grams of water. The curves in Fig. 5 show that there is a steady increase in the amount of lithium chloride dissolved in water when ammonia is added until there is present a mole of ammonia per mole of lithium chloride. At this point monammonio-lithium chloride becomes the stable solid phase. Con-
,a.
MOLES NII, Y
b.7
6
100 Gms.IilO. E
10
FIG.5 Mutual Solubilities of Lithium Chloride and Ammonia in Water at 23'-2joC.
TABLE I11 Mutual solubility of Lithium Chloride and Ammonia in Water. Moles solute per IOO grams water. Temperature 23O-25OC SHa
LiCl
0.00
1.96 2.19 2.58 3.40 3.47 3.53
0.36 1.oj
2.32 2.53 2.66
LiCl
"*
LiCl
2.70
4 . I9
3.12 4.65 5.97 7.98 8.47
3.90 5.09 6.30 5.88 3.73
8.49 8.59 8.73 9.20 9.35
3.58 4.99 4.87 4.29 5.19
"1
tinued addition of ammonia to a high concentration precipitates a solid phase which, as pointed out in the preceding paragraph, seems to be the triammonio-lithium chloride. The existence of the above cited solid phases in contact with the solutions was confirmed by vapor pressure measurements made with the apparatus shown in Fig. I . I n these experiments a relatively large proportion of the solid phase was present, a condition attained by first preparing triammoniolithium chloride in the reaction chamber A, and then adding water. Vapor pressure readings were made between each withdrawal of ammonia. The results are assembled in Table I V and charted in Fig. 6.
6. C. COLLIXS AND F. K . CAMEROS
1714
The addition of a small amount of water increases the vapor pressure of the mixture. But it is only that part of the solid which dissolves that is affected, for, the less the water added, the smaller the amount of gaseous ammonia that must be withdrawn before the pressure falls to the value due
6
600
b
QOO
x Y)
E
E ZOO
n
.A
1
FIG.6 Vapor Pressure-Concentration Curves at zg°C for the System : LiCl - S H 3 -H*O. Solid Phases, LiC1.3XH3, LiCl.KH3,and LiC1.H20
to the dry complex. $fter bringing about the reaction with the liberation of a certain amount of ammonia from the complex, the further effect of the water is analogous to greatly increasing the volume of the container. Thus, there is significance in the sections of the pressure-concentration isothermals lying between the horizontal sections. The more water present, the greater the “effective” volume of the system and the less abrupt the change in pressure with the disappearance of one solid phase and the appearance of another.
LITHIUM CHLORIDE AMMONIA COMPLEXES
TABLE IV Vapor pressures for the system: LiC1-NHsH20, hloles NH3 Moles H 2 0 per Mole LiCl per 0.47 0.74 1.14 Mole LiCl mm. Hg mm. Hg mm. Hg
2.69 2.59 2.45 2.4 2.3 2.2
212
187 186
2.16 2.0
1.95 1.8; 1.81 1.73
184
190
186
165
Moles H10 per Mole LiCl
MoiYiC1
0.47 0.74 1.14 mm. Hg mm. Hg mm. Hg
I.
428
1.5
1.62 51
I49 I22
146
164 103 85 68 55
I . 40
3'5 260
2.04
Temperature 2 5°C
Moles NH3
738 568 300 258 215
1715
230 I95 171
1.30 1.16 I .03 .94 .90 .6
40 111
65
45
45 40
.4
46
.2
44
.oj
25
36 30
The data assembled in Table I1 and charted in Fig. 4 are far from satisfactory. It was impracticable to obtain truly representative samples for analysis. There was always a loss of ammonia when withdrawing samples of either liquid or residue, since, generally, the solutions were highly charged with this gas. More serious was the impracticability of obtaining residue samples reasonably low in adhering mother liquor. Furthermore, the concentration of lithium chloride in the solutions ranged from 40% t o 60%, while the range in the solid phase was from 45% to 70%. Hence there could be no great difference in the analysis of the two. It is evident from the plot that the corresponding samples of mother liquor and residue in no case differed sufficiently to justify an approach to accurate drawing of tie lines. Xevertheless, in view of the supporting evidence just adduced, and the fact that no practicable way of overcoming the experimental difficulties can be realized a t present, the data are offered as demonstrating with reasonable certainty that the system presents but three isotherms a t 2 j"C, each corresponding to the solid phase indicated.
summary Satisfactory apparatus has been developed for studying vapor pressures in the systems : lithium chloride-ammonia and lithium chloride-ammoniawater. 2. Vapor pressure measurements for these systems have been tabulated for wide ranges of temperature and concentration. 3. It has been shown that, in the system lithium chloride-ammonia, a t temperatures from 0°C to 66°C either tetra-, tri- or monammonio-lithium chloride may be formed. And the pressure-temperature limits for the CYistence of each have been determined. I.
1716
S . C. COLLINS AND
F. E, CAMERON
4. Vapor pressure measurements for tetra- and tri-ammonio complexes with lithium chloride accord well with measurements by Bonnefoi. Measurements for the monammonio complex do not agree with the published results by Bonnefoi. 5 . No evidence could be found for the existence of a diammonio-lithium chloride described by Bonnefoi. 6. It has been shown that in the system: lithium’chloride-ammoniawater, either lithium chloride monohydrate, monoammonio-lithium chloride, or triammonio-lithium chloride may be a stable solid phase. The pressuretemperature-concentration limits for the existence of each have been approximated. No evidence was found for the possible existence of a diammonio complex. 7. The significance of the graphs for various isotherms has been discussed. 8. The effect of varying concentrations of ammonia on the solubility of lithium chloride a t 25’ has been determined.
University of A’orth Carolina, Chapel Hill, North Carolina.
