VOl. X X I X .
No. 8
AUGUST1907
THE JOURNAL OF THE
American Chemical Society [COSTKIBUTIOK FROM THE CHEMICALLABORATORY
OF T H E USIVERSITY
O F ILLIXOIS.]
DECOMPOSITION O F HYDRATED AMMONIUM SALTS. BH W I L L I A M If. DEHN A S D EDWARD 0. Received May $1, 1907.
HEUSE.
Under the influence of rising temperatures, diammonium oxalate' decomposes as follon~s: I.' (COONH,),.H,O :== (COOSH,), H,O 11.' (COONH,),.H,O =- (COOH), H,O -- 2NHs III.3 (COONH,),.H,O = HCOOH -L CO, -t 2NH, H,O IV.' (COONH,),.H,O = (CONH,), jH,O V.5 (COONH,),.H,O = CO, $- CO 2NH3 2H,O CO, 7-N H , 3H,O (COONH,),.H,O = H C N VII.' (COONH,),.H,O = C,N, -k jH,O Some of these reactions take place at approximately the same temperatures ; others only at successively higher temperatures. That equation I represents the initial decomposition is established with certainty'; the end-products at high temperature are shown to be largely cyanogen and water. I t w 1 e observed that only equations I and I1 represent reversible reactions which are rapid and complete ; the products of reaction 111 condense to ammonium formate and ammonium bicarbonate ; some of the products of reaction V and V I condense to ammonium carbonate and animoniuni bicarbonate ; reaction IV and VI18 are practically nonreversible. f Dupre, Analyst, 30,266; Ber.: 18, 1394; Gillot, Bull. Acad. Roy. Belg., 1900,744.
+
+ + + +
-
+
+
Gillot; Gay Lussac, Ann. chlm. phys., 46, 218. Turner, Schweigg, Jour. 62, 444; Pogg. Ann., 24, 166. Dumas, Ann. chim. phys., 44,.129 (1830). Ibid, 54. 240. Dumas; Gay Lussac; Lorin, Cotnpt. rend., 82, 750. Dumas. Dumas; Michael, Ber. 28, 1632. fi Peleuse and Richardson (Ann. 26, 63) show t h a t water and cyanogen yield irnmonium oxalate. Zellet (Monatshefte 14, 224) finds that when cyanogen is heated with water a t IOOO, h e obtains oxalic, hydrocyanic and azulmic acids, urea, carbon dioxide, and ammonia.
I
138
W I L L I A M 31. D E H X A X D E D W h R D 0 . I-IEVSI.:
Frolil consitleration not onl!- of the nuni1)er of tlicle pussil)le rcactio:i.+ but also oi the diversity of the products formed, it ilia>-he supposctl that the decoinpositioii t i t ' this simple salt involves a hopeics.; coiiiplesity : however. the follo\ving stuclics seen1 to indicate that the o r h r or' sLrccc5sive decoriipositions is largely as shmvn in the equations : C O U S H , ) . - €f.O X O X H , ) ? - 2H2O
(COOSH,),.H1O
. I
i COOSH,)I
CONH,))
-
C,S,
2H?O
Samples of pure diaminonium oxalate in open criiciblcs I\ ere iieatctl in air baths whose temperatures \\ ere held constant during one hour. 'rile total loss in \\.eight and the residual ammonia were tleterniined in each e s perimeiit Jvith the follo\ving results : Loss per ceiit of Experiment
Temperature
Total
I
YO0
9 51
2
95'
12.17
Ainnio i i i.1
....
....
.... ....
1180
13.17
143'
14.02
153' 168' I is0 193O
63. j S jS.5j 87.89
0.33 6.75 13.86 13.92
2G0
S5.21
18.95
15.42
Water
9.51 12.17
13.17 14.02 15.q j6.03 64. i 3 74.0; 70.26
These experiments show that \\.hen dry diammonium oxalate is heated : I . It evolves one molecule of water below I O O ~ ? (Equation I ) ; and, to I 50' at least, decomposes according to Equation Ii-. 2. Below 168" the loss of water is even greater than that represented by equation 11- (38.02 per cent.) : hence osainide is largell- formed at these temperatures. 3 . At 1 5 0 3 ammonia begins3 to be evolved (Equation 11) and at higher temperatures it continues to be evolved or else the substance sublimes. Since oxamide sublimes but, as sho\m below, does not decompose into cyanogen and water (Equation 7-11,) below zPo", it may be concluded that the lower temperatures represent only two main decompositions ( I and IV). Efforts were made to confirm this by vapor pressure curves. And other products a t higher temperature; the total per cent. of water .i 63.37 per cent. ?
One molecule of viater represents 12.67 per cent.
Gillot (1.Chem. Soc., 1901, A I IS) shows that ammoilia is completely hydrolyzed and expelled from boiling solutions of diammonium oxalate.
DECOMPOSITIOS O F HYDRATED AMMONIL'hl
SALTS
1139
VAPOR PRESSURES O F D I A M M O N I C M OXALATE. Temp. Pressure Temp. Pressure 7' I1 I45 2 I34
74
156 161 168 171
I 76 I11
I21 I22
314 597 827
180
IS2
187
'95 197
131 138
999 1271 1616
200 205
141
181s
210
I 26
'Temp.
2748 3144 3877 4319 482I 5233 5546 6064 7103 7326 7682 8219 8518
VAPOR I'RESSGRES O F OXAMIDE. Pressure Temp. Pressure
265'
45
270 274
71
86
277
I18
283
241
290 291 292
2;s
1182
I353
293 294 294.5 295 295.5 296.5 297 297.5
1596 1911 2115 2157
2301 2412 2536" 2752-
I t will be observed that the vapor pressure curve of diammonium oxalate is represented by three distinct segments. Segment A unquestionably represents the partial aqueous decomposition ; segment B evidently represents the elimination of the molecule of water of crystallization (equation I) ; and segment C represents the decomposition into oxamide and probably the simultaneous decomposition represented by equation 11, 111, V and VI. That reaction VI1 does not take place below zgo" is sufficiently indicated by the curve of oxamide. Monoammonium Oxalate. This salt, prepared by the methods of NicholsS and Walden', was found to be pure NH,HC?H,.H,O. I t is reported that when heated, this salt is stable to 70" ; at higher temperatures, it begins to lose its water of cryst a l l i ~ a t i o n at ~ ; 140' it forms oxamic acid6; at more elevated temperatures, it yields carbon dioxide, carbon monoxide, formic acid and oxamide ; finally, it expels hydrocyanic acid and ammonium carbonate; the residue contains oxamic acid and oximide . When samples of the salt were heated in open crucibles in the manner indicated above, the following data were obtained : Sublimation was observed at this temperature. I I 82 mm. ; a large quantity of cyanogen was found. :' Chem. S e w s , 22, 14. Am. Ch. J., 34, 147. Balard, Ann. chim. phys., (3)4, 94;Ann., 42, 197. Ost. and Mente, Ber., 19, 3229.
The non-reversible pressure was equ a1 to
I 140
WILT.I.IZ1
M . D B H N .\?;I3 E D W A R D 0 . I I E Y S H
Loss per cent. o f
_--_-*-
Experiment
'Temperature
I
75 94
2
3 4 5 6 S 9 IO
105
115 I34 155
'Total
.
..
2.6s
A I 1 1 1llotl:il
. .. . ..
8.54
. .. . ..
9.09 10.99
0.3"
7.47
0.13
.o:,
Water
. ..
2.6g 7.17 s 54 8.96 10.69
11.45
169
l:.jI
I
I S3
1.71
200
AS.36 88. j I
s.9s
46.65 79.53
227
99.64
12.24
h j.Ai)
DECOMPOSITION O F HYDRATED A M M O N I U M SALTS
I
141
I t is seen that monoammonium oxalate : I . IS stable to 7 j o , 2. Parts with its molecule of water of crystallization (14.40 per cent.) below 170". 3. Loses two other niolecules of water at 183" and simultaneously incurs a secondary decomposition or sitb!imes. Therefore the main successive decompositions are probably indicated by the equations: 'NH,HC,O,.H,O = NH,HC,O, H,O NH,HC,O, = HOOCCONH, H,O HOOCCONH, r : CO I \&-IHz0
'+
co/
The above data of decomposition are closely confirmed by the vapor pressures of the substance. Temperature
81 58 95.8 106
Pressure
7.7 15.9 50.2
110
469 686
11.5
575
I22
I102 I 230 1420
125
130 I35 140
I 623
I 340
Temperature
Pressure
145
2131 2370 26S2 2964 3c96 3268 4009 5862 7427 8159 9546
150
I55
I 60
165 170 175 I 76 I so
182 185
I t will be observed (see plate I ) that ( I ) the general form of the tivo curves are much alike, ( 2 ) the molecule of water in each salt is completely eliminated below 1oo-17o0, and ( 3) the upper segments represent the second stages of decomposition. Decompositions of the above organic compounds indicate that the initial and predominating reactions involve the expulsion of water ; and that simultaneously, particularlp at higher temperatures. secondary reactions indicated by the dissociation of ammonia, are involved. I t was hoped that studies of inorganic hydrated ammonium salts, along the lines indicated above, would lead to a more intimate knowledge of water of crystallization and of the structure of hydrated salts. That this hope has been partially realized is evidenced by the following studies. It was found, for instance, that certain hydrated ammoniuni salts decompose so as to yield both water and ammonia at most temperatures above the initial temperature of decomposition. Studies of the rate of expulsion of water and ammonia have shown that abundant yields of ammonia usually accompany the largest yields of water; and, though the last trace of aininonia is given off only with the last trace of water, it is given off simultaneously with it at most of the lower temperatures. I n
WILLIAM ,If. D E H S A X D ED\I‘:\KD
I 142
0 . HECSE
other words, ciirves of expulsion of ammonia. as \veil as of water, estencl from the temperatures of initial decomposition to those of complete decomposition ; consequently tracing the course of ammonia through the ioiiipositc decompositions leads t o Iiiio\~IetIgeof the respective individual decompositions and, as will be shoi\n, thro\vs light upon individual s!mctures in the complete structure. For instance, suppose it can be shoi;.n that highly polyhydrated ammonium salts are largel!. decomposed below 1 0 0 ~ . while the residues of ammonia and of ”ivater of composition” a r e completely expelled only at considerably higher temperatures, it may then be concluded that the ~itiiorzof nririiioiiiu rcsriiiblcs i i i o r c riosclji tlic iiuiori of “zLatcr of cori~positioiz” tlzaiz tlze iiiiioii of “water of f.y-~isfff~~i.~a~~o?z. ” -Again suppose it can be shown that ammonia is given off at all lo\ver temperatures, it may also be concluded that both “zcuter of crj’stnllizntiori” a n d 6‘zc~ater of cornposifioii” are gizwi off at nil of tliesc l o ~ i wtci>ipcrcrtiircs. In respect to the methods used to differentiate the respective dissociations, it has been found that vapor pressure curves (z’itfc plate I -ff ) are not necessarily indicative of the q z d i t a t k y decompositions of conipounds ; in the case of polyhydrated salts they a r e a measurement only of the composite effect of a number of co-temporaneous dissociations. For instance i f each molecule of water and ammonia in the original compound has a definite vapor pressure for each temperature, it may easily be seen that the resultants of their pressures may so blend as to indicate no definite breaks in the vapor pressure curve, therefore, recop1itioii of poifits of dccoiizjositioii may fail nztireZ3, liCfheti otily riapor prcssitrc il(ri‘es a.rc sttidicd. For this reason other methods of investigation have heen employed. Decomposition of Inorganic Salts. 1-arious investigators] have represented partiall!. dehydrated salts, for instance, hydrated ammonium salts, by very contradictory and, as shown below, by very erroneous formulas. 11-e find in the periodicals and the text-books that free use is made of formulas : (h”,MgAsO,),.H,O and (SH,hlgPO,),.H,O to represent ammonium magnesium arsenate and ammonium magnesiuni phosphate dehydrated at 100-I IO’: Though most investigators agree t h a t the composition of ammonium magnesium arsenate at ordinary temperature is NH4MgAs0,6H,O, a considerable difference of opinion as to its composition a t temperatures between 98’ and 100’ is expressed. For instance Bunsen’concludes that nearly $ of 31noleciile of water is held a t 98’. Rose’, Puller4, Fields, and Lefevres, affirm that exactly % mol. H,O is retained
’
U’ach, Schweigger’s J. Chem. Physik, 59, zS8; Rose, Z. anal. Chem., 1, 417; Ann. Physik., 76, 20; 2. anorg. Chem., 23, 146. Puller, Z. anal. Cheni., 10, 68. * Ann. Pharni., 192. 311. :’2. anal. Chem., 1, 417. Ibid, 10, 68. Jahrsb., 1858. 170. “ A n n . chim. phys., ( 6 ) 2 7 , jg.
’
114s
DECOMPOSITION O F HYDRATED A M M O N I U M SALTS
by the salt when it is dried at IOOO on the water bath ; Fuller holds that i t is practically dehydrated at 103' ; and Bunsen' further states that is conipletely dehydrated at 104.j". These data do not appear to be particularly discordant; but in view of the fact, as shown in this research, that 3-4 per cent. of ammonia-equivalent in weight to about mol. H,O-are lost at these temperatures, none of the conclusions drawn are correct. For when the salt is dried at these temperatures less ammonia and more water are present than are represented by the formula (NH,MgAsO,),.H,O. A more correct representation would be a mixture in equal proportions of HMgAsO,.H,O and NH,MgAsO,.H,O. However, even this formulation will be shown to be incorrect, for one conclusion of these studies is that 110 d e f i n i f e f o r m d a can be given to many hydrated amnioniiim salts dried a t temperatures befweefz4o0-2oo0. This is clearly illustrated by data obtained on heating samples of these salts at definite intervals of temperature for equal lengths of time and determining both the total loss in weight sustained and also the weight of ammonia evolved. T h e salts, contained and weighed in U-shaped tubes, were heated in baths controlled by thermostats, while air, dried and freed from carbon dioxide, was passed continuously through the tubes and into flasks containing standard sulphuric acid. T h e total loss of weight in the U-tubes represented, of course, the loss of both water and ammonia ; this weight, less than the weight of ammonia, determined by titration, gave the loss of water. I t was found that quite different.resu1ts were obtained when we varied t h e following conditions : I . Temperature. 2 . Time. 3. Kind of drying gas. 4. Quantity of drying gas. 5 . Size of salt crystals. 6. Preservation of salt crystals. j. Manner of heating. T h e effect of temperature is the most important aud it was on temperature a s a basis that the following studies were made. T h e influence of time was soon found to be a very disturbing factor, for these salts do not dry to definite composition, therefore briefer or longer desiccation gave very widely different per cents. of decomposition. This is seen in the following table : Substance
Weight
NH4MgPO,.6H,O. ................0.7473 NH,MgP0,.6H20.. .0..$636 HNaNH,P0,.4Hz0 ............ .o.Sor6 HNaNH,P0,.4H,O ...............+ 7094 NH,MgAs0,.6Hz0.. ..............0.7941 .0,7958 NH4MgAs0,.6H,0.. NH,MgAs0,.6H,01 ............... 0 . 2 9 2 0 NH,MgAs0,.6H20 ................ 1.1127
.............. .............
Loss
0.0423 0.1655 o.11i.S 1.0784 0.0136 0.0372 0.1 108 0.4556
Loss percent. Time
j.66 35.76 14.70 22.89 0.35 4.67 37.95 39.61
4
Tenip.
70°
40
7oo
4
76O 75O
57 4 20
50° 50°
4 40
I100
I IO0
It will be observed here that heating for four hours invariably gave lower per cents. of decomposition than when heating for 20-57 hours. An exloc. cit.
* ‘44
\VILLIAB.I &I. D E H N A N D EI)W-\RD
0 . HEYSE
planation of these great differences of results on short and protractetl heating is conceivable when one recalls that “water of crystallization” is more easily expelled than “water of composition”. T h e former is usually eliminated a t temperatures belon 100’ ; the latter, at teniperxtures above rooo. By protracted heating at low temperatures, however, “water of composition” may be removed completely, therefore too prolonged heating a t low temperature does not reveal the iiornial deconiposition a t these temperatures. On the other hand too brief a heating a t low temperatures does not insure complete removal of the decomposition products. I t was to avoid on the one hand incomplete dehydration, and on t h e other excessive secondary decompositioii that periods of 4-7 hours heating were finally chosen. It was found, moreover, that heating the salts progressively, that is heating the same sample to successively higher temperatures, did not yield the proper results, for, on comparing the per cents. of decomposition obtained by heating different samples at the respective temperatures, very different results were obtained. T h e method of heating individual samples a t different temperatures for the sanie lengths of time was adopted, in preference to heating the same sample successively to higher temperatures, for the reason t h a t t h e former method really eliminates the element of time and t h u s minimizes secondary decompositions. For instance when a sample of a salt is heated a t 6 jofor four hours and then another sample of t h e same salt i s heated a t i o o for four hours, all other conditions remaining t h e same, the difference of effect is the result of temperature alone. A further reason for employing the method of separate samples for each interval of temperature, was to avoid the accumulative errors of analysis involved in the other method. T h e effect of using different gases to carry off t h e decomposition products may he seen in the use of hydrogen and of air. I n t h e following table, the data were obtained on heating microcosmic s a l t for periods of four hours each, ‘i‘en:perature
Per cent.
Carrier
so3
0.35
Hydrogen Air .Air Hydrogen ;\ir
\ioo .ioo
;6O
7.5’
0.60 0.59
9.64 16.55
T h e results here indicate that hydrogen is not so efficient a carrier as air, and this undoubtedly is owing to the fact that it possesses a much more rapid rate of diffusion. T h e effect of speed, or rather t h e quantity, of carrying gas, is seen i n t h e following experiment. Dry air was passed for 2 7 hours over 4.4334 grains of NH,MqAsO,.GH,O; it lost 0 . 2 1 1 4 grams or 4.89 per cent.
DECOMPOSITION O F H Y D R A T E D A X M O N I U M SALTS
I
745
whereas another sample of 3.4729 grams exposed to quiet air for the same time lost scarcely a Teighable quantity. T h e air used in the following experiments was regulated so that 60-70 bubbles per minute passed from the exit-tube dipping into the flasks containing the standard acid. A t first considerable difficulty was encountered in regulating this passage of air but after a number of trials the following, quite satisfactory system was adopted. T h e air from the main supply was passed through a bottle connected on the one hand with the drying-train and on the other with a shunt tube dipping into a definite depth of water. T h e object.of the shunt was to force through the drying system air backed by a constant pressure, equal always to the height of water in the shunt system when air was constantly passing o u t of the latter. T h e quantity of air passing through the drying system was controlled by a screw clamp attached to a rubber tube in connection with the exit tube. By this means the number of bubbles per minute could be regulated to a nicety ; the size of the bubbles passing through the normal sulphuric acid, was limited, of course, by the size of the exit tube dipping into i t . Fifthly, the size of the salt crystals used was found to exercise a very appreciable effect on the results ; mass varying as the cube, and radiating surfaces as the square of the diameter. T o reduce this influence to a ininiinum the crystals were pulverized so as to pass through a particular, fine-mesh sieve. Sixthly, efflorescence of some salts, for instance with a~nnionium calcium arsenate, was found to introduce large factors of error. T h e weathering, that is the decomposition of these salts a t ordinary temperature, is often so great, particularly in summer, t h a t only freshly prepared samples could be used. Finally, the manner of heating, for instance, whether in open crucible, i n desiccators over sulphuric acid, or in the U tubes mentioned above, was found to yield different per cents. of decomposition. Of course the same method of heating was employed throughout any given experiment, nevertheless at some temperatures certain abnormal results were often obtained and can be explnined only on the basis of “suspended transformation”. For instance in the following table : Substance
$J,(SO 4,?‘) ISH,O ..............................
............................... .............................
K A I ( S O , ) , . I ~ H , O .............................
Temperature
S3.0
Per cent. volatilized
90.5 71.5
.79 4.69 Y.3j
83.0
2.2[
II
i t is seen t h a t higher temperatures yield lower per cents. of decomposition though the experiments were carried out under similar conditions. Heating at once to the higher temperatures seems to induce this “suspended transformation.” ’ See page 1161.
Though recognizing the above mentioned influences and observing i n the experiments every precaution necessary to avoid their disturbing effects, concordant results mere often obtained only after repeated trials. This difficulty of obtaining concordant results was particularly noticeable a t temperatures a t which more than one molecule of water was given off, for instance i n the case of ammonium magnesium arsenate, a t 70-goo. T h e following table includes data obtained in studying the deconiposition of ammonium magnesium arsenate prepared i n the usual manner and found by analysis to be strictly SH4hlgXs0,.6H-O. I Ill IV v I! Ten]. Weight Loss of weigh: of persalt 1I:O -SH, S H ; Ii?O :it lire
1.0344 0.2692 0.287 j 0.6945 o.64b2 0.SroS 0.269s 0.2332 0.ji66
.oo;o
.C l 0 C
, oc42
,0003
13.0
oc05
10.6
1.56 2.01
.~027
S.8
3.so
.c)c63 ,0093 .0039 ,003s .or64
s.9
Y . 7
s.0
1 0 21
7.h
12,71
00 j;
0.3 I 64
,0264 ,0629 .os2s .0343 ,0379 . I j63 .os3 2 , I 162
0.3247
,
0.29
9.0
s.9
.00s;.
S.j
16.35 30.26 34.64
,0122
8. j
36.j2
,1212
,0127
0 2920
.IIOS
.OI
0 070s
,029
.oci;o
S.6 56 h.6
0.1156
,047s .06jS ,1166 ,1670 .303j
.00jU
S.5
, LO7 2
s.0
0.2402
0.14:s
225
1-1 \'I1 VI11 IS 1 XI XII' Xatio Total per cent. l < > i a t'mctioiial per cent. of Of' 109s of 110th I i 2 O + S H , I H . llLo Ii,O.-SH, S H ; I1,O
0.2496 0.3641 0.65j1
S .5
16
. 0 I 2s
.oI 96 ,2651
.0jS4
.;i..i1
3i.95 40.6s 41 .35 43.:7 44.32 4j.S6 46.40
7.6 i.5 6.9
j SO
40 5"
0.jJ
o 50
0.04
When NH41Ighs0,.6H,O is heated it may sustain an!' of the following losses or their intermediate per cents. : I,oc< of
hhlecules of
Per ceiit. of IOSS
XES,, . . . . . . . . . . . . . . . . . . . . . . . . j.SS H,O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . 12.46 2H.,O rS.69 24.92 .jH,O . . . . . . . . , . . . . . . jI.I\j 37.37 6;$H,O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , . . . . . . . . , . . 40.49 ~ . .... ...... . . . . . . . . . . . . . ..... . ..... . ... .. . 46.37 614H20 N H,.... .
.
.
.
.
I
I
.
T h e above experimental data plotted with per cents. as ordinates and degrees of temperature as abscissas, give from columns V I I I , I X and \''I1 respectively, the curves of evolution of nnimoiiia, water. and both ammonia and water. I These fractional per cents. are obtainetl by subtracting acljacent total per cents. : t h e y indicate t h e effect of the i u u e i u e i c t of teniperature.
DECOTvfPOSITXOX O F H Y D R A T E D AMJIONIUM SALTS
1147
It- will be observed that: I. Both water and ammonia begin t o be given off at 40' and are com.pletely removed a t z z j o . 2. F u l l y one-half of all the ammonia is expelled below Soo, the remainder, between temperatures 8 5-22j o . 3. About t h e same ratios' of water and ammonia are given off a t temperatures 60-2IO". Between 60-1j o o the ratio of weight of water to ammonia averages S. j : I , which is equal to a molecular ratio of 8.0 : I . T h e mass ratio of water to ammonia in KH,MgAsO, is 6.9 : I. I n determining t h e data for the above table, wide variations from the ratio of 8.5 : I always indicated experimental errors. This approxinlate constancy of ratio is not in evidence 2 See column V I above. with other salts (see NH4I\.IgP0,.6H,O) except a t high temperatures.
I
P4S
4. j.
WILLIAM 31. DE"
A S D EDW.4RD 0. HEC'SE
Water is gradually given off below 65'. T h e n below 80' the remainder of fozw molecules of water is given
Off'.
6. T h e m.rt two molecules are given off a t teniperatuies between 80-1jo". 7 . T h e Zast one-half molecule of water, derived from the ammoniumoxygen group (XH,O-), is slowly expelled a t temperatures I jo-225'; a t the last of these temperatures, magnesium pyroarsenate is formed. T h e fact that two molecules of water are given off, finally and independently of each other, and of t h e other four molecules is confirmed by t h e following experiment. T h e salt containing the six molecules of water was heated for three hours on the mater-bath in closed vessels with a large quantity of ordinary alcohol. After cooling, washing by decantation, first with alcohol, then with ether, and finally drying for a short time i n a vacuum dessicator, it was found that dehydration and removal of amtiionia from the salt had resulted. This is shown in t h e following analyses : Per cent. loss
= ~. . . . . . . . . 25.59 grams Xg,As,O, . . . . . . . . . . . 25.63
n.~;j41 grams substances gave o.zojg grams Jfg,;ls,O;
o.41I I granis substances gaye o.;ogS
0.0021 grains X X , . ...... 0.0j24 grains substances gaye 0.0015 grams SH:,. . . . . . . . . . . . . . . . . . 2.92 .\verage . . . . . . . . . . . . . . . . . . 2.8s
0.o7jS grams substances gave
Xow the total loss by ignition less the ainmonia is equal to t h e water , 2 2 . j 4 per cent. H,O. Theory a H M g A s 0 4 . 2 H , 0= 22.50 per cent. H,O. Therefore, after dehydrating S H , M g A s 0 , . 6 H 2 0 by nieans of ordinary alcohol, two molecules of water remain, so they must be different from the other four molecules. T h i s differeiice of :he last two molecules of water froin the other four molecules evidently must involve a difference i n structure, that is, there must exist for these water molecules different forms of union i n t h e parent molecule. If it is tenable that such dif/rrren(-es oJrohemire ofmoleconversely it may be r u l e s of rider iii7~ohvsd i f e w n c e s of- s f i ~ c I i ~ i . rthen , held that 7?zo/t.rides simzdta?zeoiis!if e.@clicd iizvoive s i m i i a r i ~pf~ sfrurizms. Now since it is true, as was showii above, that SH,Mghs0,.6H,O possesses two molecules of water differing from one another and from the 2 5.61-2.88 -
I t may :ippe:ir froin the curve of evolutiou of water t l l a t j i w and not j b r i r . molecules of water are expelled siiriultaneously; but it must be remembered that each of the six molecules of water contributes a t all lotver teiiiperatures its quota, conseqiiently at the decomposition point for four niolecules, II surplus derived from \ l i e method other t i r o and a half molecules will be olitaiiie(1. The :ilcoliol-del~~-tlrating establishes beyond a rloul)t the dissimilarity of four iiislecule~of water, (water of crystallization \ from t h e remainder of water water of cotilpc1sl:ioii ) .
DECOMPOSITION O F HYDRATED AMMONIUM SALTS
1149
other four-it is interesting to see what structures will account for all of t h e facts. I n t h e first place there can be little doubt that water of crystallization is held in definite molecular structures, and that the structural formula of arsenic acid is : (HO), E AS = 0 and that its ammonium magnesium salt is :
Mgwhile in suspension aiid powdery when &>.. By dissolving it in h)-drochloric acid. adding ammonia to incipient precipitation. filtering aiid letting stand. t h e solution j-ielded beautiful, sinall. transparent crystal>. I r.o:,3o grams powder heated a t 350' lost o.i 1 7 0 grams I1 0 . 3 3 2 0 grams crystals heated a t 2 2 5 ' lost o.og6.j ~ I - Z I W 'Theory HSrAs0,.€i20 1;i
HL0
I I .(io
r~ol~ll~l
I
11
11.29
10.94
"lie bariuni salt prepared as abo1.e >-ielded sinall. pearly crystals, 0.6660 grams substance \.ieltled o.I. D E H N . \ N D E D W . \ K D 0 . HEUSE
Though the general forms of these two curves are siniilar. it is observed ( I ) that the normal pressures are not exerted when the salt is heated a t
I 163
DECOMPOSITION OF HYDRATED AMMONIUM SALTS
once io 70' and ( 2 ) that this condition of suspended transformation is lost at 99'. Evidently the cause of this condition is the formation of a superficial impervious layer of dehydrated substance that protects the inner portions from immediate decomposition. T h e vapor pressures of the other curves are given in the following tables : AMMOKIUM CALCIVMARSESATE~. Temperature
Pressure
31.5 34.0 39.4 40.0 42.5
25.6 29.5 40.8 45.3
44.0 46.5
50.5
60.8
Temperature
50.0
55.0
Pressure
Temperature
55.2 61.2 73.6 93.6 124.5
62.7 65.I 67.4 70.2 70.8
Pressure
136.9 161.6 188.3 222.7
246.3
AMMOXIUM MAGNESIUMPHOSPHATE'. Temperatiire
Pressure
Temperature
Pressure
Temprrature
Pressure
28.0 31.5 34.0 39.4
10.36 10.79 11.57 12.43
40.0 42.5 44.0 46.5
12.82 13.26 14.06 15.27
50.0
16.54 '7.92
55.0 58.5
20.02
VAPOR PRESSURES' Temperature
30.6 32.8 35.0 39.0 42.2 45.0 47.8 48.0 52.5 55.0
55.3 56.8 59.0 61.0 64.5 68.7 70.8 71.2 75.o 78.0 82.5
HNaNH,P04.4H20 NH4MgAs0+6H20
... ... ...
12.0
17.0
... ...
14.5 23.0
...
30.0
...
...
30.3 33.0 58.0
...
44.0 45.5
... ...
... ... ...
NH4MgP046H:O
30.0 32.0
37.0
47.0 54.3 61.5 75.0 77.0
...
...
19.5 25.3 34.0 45 .o
...
65.6 69.0
...
89.0
106.0
95.5
117.0 141.0 217.0 246.5 390.0 407.0
120.0
65.0 73.0 85.0 113.0 134.0 148.3 170.0
70.0 84.0 85 .o 125.0 137.5 154.0 212.0
176.5 215.0 239.0 240.6 273.0
201.0
267.0 405.0
418.0
252.5
NH,CaAsO4.6H~0
'32.5 151.0
330.2
...
... ... ...
...
All vapor pressures mentione thus far were :tennine in Dehn's tensimeter. (This Journal,29,1052.)The break in the curverepresented by t h e expulsion one of molecule of water from ammoilium magnesium phosphate is not plainly marked ; however t h e vapor pressures here were not determined with t h e greatest of care. These vapor pressures were determined in t h e Bremer-Frowein form of tensimeter (Phys. Chem., 1, 5 ; 17, 52), a n d , being only approximately correct, are useful here only in showing relative pressures at higher temperatures.
I
164
WII,LI.~3I 31. D E H S .\SD E D W . \ K D 0 . HEUSE
Further studies by the methods herein expressed are being made; a t present one may safely draw the following:
Conclusions. Hydrated ammonium salts upon being partially or largely dehydrated yield products of indefinite composition, for the reason that 2. These salts at elevated temperatures undergo primary or secondary decompositions of all of the different dissociating molecules of water and ammonia, consequently I.
I
SPECIFIC HEAT OF SOLIDS
r65
3. Drying on the water-bath or “drying to constant weight” cannot yield homogeneous products, and therefore, 4. Many of the empirical formulas of such compounds given in the literature are necessarily incorrect. 5. T h e affinity and manner of union of water of composition do not differ largely from the affinity and manner of union of ammonia. 6 . Water of crystallization, conforming to the law of definite proportions, must be held in definite niolecular structures, through the agency of valency, as in other compounds. 7 . Tetravalent oxygen, necessary to express these structures, is loosened at temperatures above IO^', therefore salts usually expel1 water of crystallization below this temperature and water of composition above this temperature. 8. Finding dissimilar molecules of water in hydrated salts, leads to a conception of their structure. URBANA,ILLINOIS. May 29, 1907. ~CON’I’RIBUTION F R O M T H E
RESEARCH LABORATORY O F PHYSICAL CHEMSTRY
THE MASSACHUSETTS INSTITUTE O F
NO. TECHNOLOGY.
15.
OF
]
THE SPECIFIC HEAT OF SOLIDS AT CONSTANT VOLUME, AND THE LAW OF DULONG AND PETIT. B Y GILBERT NEWTON LEWIS.
Received Jan. 28, 1907.
T h e study of the relation between the specific heat of gases at constant volume and at constant pressure has led to a number of important theoretical conclusions, but in the case of solids and liquids we have been familiar hitherto only with the specific heat at constant pressure. To determine experimentally the specific heat at constant volume of a liquid or solid would be a difficult undertaking. Fortunately it is possible, assuming nothing more than the validity of the two laws of thermodynamics, to calculate this important quantity from existing data. T h e internal energy at constant temperature is nearly independent of t h e volume, in t h e case of a gas, but may vary considerably with the volume in the case of a liquid or solid. Except in rare cases i t increases with increasing volume. T h e difference between t h e specific heats a t constant pressure and constant volume is chiefly due to this change of internal energy with the volume, as I have shown in a previous paper‘ w h e r e the following purely thermodynamic equation was obtained. are the two specific heats, E and z’ are respectively the inE w i s . The Development and Application of a General Equation for Free Eiiergy and Physico-Chemical Equilibrium Equation 38. Pr. Am. Acad., 35, I (1899) ; J. physic. Chem., 32, 364 (1900).
cp and
