April, 1960
~ E H Y D R A T I O S O F CALCIUM HYDIiOGEN
PHOSPHATE DIHYDRilTE
491
Licniineccent J l n f c r i n l c Lnhointory, Lnmp Jlefnls ck Conzponentc I ) e p n r f m ~ n tGcncrril . Elecfizc Coniprrnzi, i O f / ' / 11or1110~Road, Cleveland 10, Oh20
T h e dehydration of CaHP04,2H?Oin dry air, humid air, and at various pressures of N? was studied by means of thermogravimetric, c!ifferential thermal and X-ray diffraction methods. It mas found that, in humid air or a t three or more atmospheres of ?;2 the drhydration is catalyzed by moisture and proceeds in a single reaction a t 135" or lower t o form a n h y k o u s C a H P 0 4 . I n the absence of moisture the reaction is complex involving several changes including formation of a n amorphous phase. This phase persiPts on further heating even after the formation of Ca2P2Oia t 430" until recrystallization occt1r.s at 530".
Introduction In 1948, BoullB' noted that thermogravimetric analyses indicated stepwise dehydrat'ioii of CaHP04.2f120hetn-een 100 and 225" in dry air. S'ol'fkovicli and U r o s o ~ ?have found by diff ereiitial that, at, vapor pressures of water 100 mm. the dehydration occurred in t,hree stcps xt 100-110", 127-142" and 172-175". BoullB and duP011t3 haye also found that the dehydrat,ioii proceeds similnrly in cucuo. Nore recently BoullB and duPont4 have report'ed that the anhydrous CnHPOI obtained by heating CnHI'04.2Hz0 in humid air shom well-defined crystals x ~ i dmore distinct X-ray diffraction patterns compared t80t,he dry air dehydrat'ed product. The present iiivestigation was undertaken to c1:irify further the mechnnism and kinetics of the dehydration of CaHPO4.2II2Ohot'h in dry air and in Li humid atmosphere. The unique effect of pressure of dry gases 011 the dehydration mechaiiism of C:iHP04.2H20also was studied.
Experimental Thermogravimetric Analyses (TGA).--A pen-recording Chevenard xype thermobalancr T V ~ R used for the TGr\ studies. A heating rate of 5" per minute was used for all analyses e s w p t those pcrformed a t constant temperitture. The performancr c>h:Lr:rcteristics of the Chevenard thermoA special vacuum h l a n r c h;iv(. heen evaluated :ind p r e ~ w r cthri ~ i n o h : r l : i n r ~ cW~ ~: ~ P iised for the pressure TGA rtiidies. sample sizc of 0.5 g. was iised for all runs. The thermoh,zi,e :L wnsitivity of rk0.2 mg. For humid air on stii(Iios, a s t w m generator was used a t a stram 0 t o 40 p r r minute. This flow rate had no I(. i ~ f f ( v ~oin the i'iirn:tw temperature because of ?it!- o i the, iarn:iw :md fast recovcry of the heating A\
r i b .
The resu1t;j :we reported in the figures as weight per cent. chxnge :is a t'iinction of temperature or time. Differential Thermal Analysis (DTA).-The DTh studies pressiirc and vacuum D T A ~(-rcxpc'rfornird 1)y mean!: of s r o n s t r u c t d at i h Laboratory.' The heating 7" per minute for 1 runs. Chlcined A41?0,4 was he incrt rcferenre mztterial. The sample size \vas :i!)out 0.4 t o 0.6 g. pa,ckcd into a cylindrirnl ravity of 11'~'' tlcapth an(! l , ,,," tli:imc~tr~r.The thermocouple was Pt-Pt/ ~
(1) .i. B o u l l ~ C. o m p ! . r c n d . , 226, 1017 (1948). ( 2 ) S. I. Vol'fkorich a n d V. V. Urosov, Izccsl. 4 k n d . Znuk S B S R , O'del Kim. S n ? ( k . 4 , 311 (1931). 1 2 ) .I.Boull' and A I . diiPont, Comp!. rend., 240, 860 (1'255). (1) -4, Borill6 and ?>l.ilul'ont. ibid., 241, 12 (19.55). (ri) E. L. Siuirins. A . E. S e w k i r k a n d I. Aliferis, ; i n a l . Chem., 29, 18 ( I '3.57). (61 .J. G. Rnimtin n n d C . S. C a r d , bid., 3 1 , 1689 (19.59). ( 7 ) J . G . Rohatin a n d C'. S. C n r n . J . Elecfrocliem. S o c . , 104, ci8c (1937).
10% R h , KO. 32 wire with the junctions ( e i i t m d in thc :$lumina and samplr cavities. T h e tcmpcrr,+urP T : ~ Pis reproducible to 1 2 ' and the endothermic :in$l exothcrnmic, peaks are reproducible to f3'. Additional iniormatinii on DTA may be found in a recent te.tt.8 X-Ray Diffraction Studies.-The X-rar tliffr,zrt inn p:itterns were obtained by means of a G F XRD-5 :tpli:trLtiis iising Cn Ka radiation. The phases identified awording t'o the A S T N X-Ray Ponder Data, File are reported on Tnhle I. Density Measurements.--The absolute tlmsities w r e measured with a 10 ml. pycnometer a t 27.5" lising h1ityl acetate as the liquid medium. The values arrl rt'prodririhle t o f O . l g./cc. Kinetic Studies.-Some kinetic data n-ere oiitnined h>direct analysis of the thermobalanre curves m:ttlr :tt constant temperature. Reproducibility as determined b y making several runs at, each temperature v a s =to005 min.-' for t h r first-order velocity constants. For t,he othor dzita rc'ported a special vaciiiim oven was used. With the oven set :it 130", samples n-ere introduced and the c h a n i h v cv:rcuated. Then the desircd pressure of steam rapidly. The samples were held for vnrioris ti air quenched. The kinetic data obt:iined Ci,oni tlic. rate ciirvrs were rcproduciblp t o f 0 . 0 0 5 min. Materials.-The CaHPOa.2H.0 for these rsptrinients was a special electronic grade product used in phosphor m:tnufnctiire. The % impurities are lrss than those, fonnci agent g r d e CaHPOl. The Fisher Suh S i c w 8izc.r gc' particle di:rmeter n-as 2.2 p .
Results In t8hediagrams, Figures 1-2, corresponding DTh and TGA thermograms vere placed 011 the same graph for ease in comparing results. The convention is used tBh:ztm an exothermic reaction is indicated by an upward deflect'ioii of the DTh curve from the haseline (shon-ii ns a 1,rokeii horizontal line) and a11 endobhermic re:ictioii by clownward deflectmion. Typical T G A and DTX ciirves for t'he dehydration of CaI-11'04.21-120 in dry air show (Fig. 1) characteristic endotherm penks :it 13.5, 135 and 195". The TG,1 curve shorn n sh:irp change in slope at ahout, 180". hut cannot, he resolved into three distinct reactions corresponding to the DTh peaks. The forniat'ioii of C:i?P207 occurs between 360-450" as indicated hF an endotherm peaking at 430" aiid a correspofitiing 4 . 8 5 weight loss on the TGX curve (t,heory 5.2::{). The difference in weight' loss m:iy he explained hy chemical analysis for CazP20i according t o the method of Gee and diet^,^ v-hich s h o w S-SC:;. CazP20iin the air dehydrated product, for dehydrxtion temperat,ures as low as 1'70". -1chnmcteristic exotherm appearing a t 530" (where 110 \wight loss ( E ) XI-. .J. Smothers and Yao Chiang, "Differential l'heruinl -4nni::sis," Chemical Publishing Co., Inc., l-ew Tork, N . I-,, 1'358. (9) A . Gee and V. R. Dietz, d n a l . Chem., 25, 1.320 (1953).
,J. G. RABATIS,R. H. GALEASD A. E. SEWKIRK
492
100 200 300 400 500 Fig. 1.--CzHP04.2H20 dehydration in dry air.
1
II Y
1350
I' I
100 200 Fig. :!.--CaHPOd.2H20
I
I
I
300 400 500 dehydration in humid air.
or gain occurs) must be attributed to some physical change as mill be brought out later. The DTA and TGA curves reproduced in Fig. 2 for the dehydration of CaHPO4-2H20in a humid atmosphere show a single sharply peaking endotherm at 135" in contrast to 3 peaks found in the dry air dehydrations (Fig. 1). The TGA curve shows a rapid weight loss of 20.2% between 120 and 160" compared to a theoretically expected weight loss of 20.9% for complete removal of hydrate water. The DTA curve shows an endotherm peaking a t 430" but unlike the air dehydration (Fig. 1) no exotherm appears. Dry COz and Szwere substituted for dry air for the studies on the effect of pressure on the dehydration mechanism. At one atmosphere Nz or COz the TGA and DTA results were identical to the dry air dehydration as shown in Fig. 1. At 3 atmospheres S 2 (or CO,) the DTA curves are similar to the humid air dehydration results shown in Fig. 2 except that the endotherm at 135" is split into a very sharp peak at 132" and a broader endotherm at 148". At 25 atmospheres Sz the sharp endotherm h3s shifted to 112" and the broad endotherm remained relatively invariant at 150". At intermediate pressures the sharp peak falls between these temperatures, with the peak temperatures approximately inversely proportional to the pressures.
T'ol. G4
The TGA curve for 3 atmospheres S?shows a weight loss of 20.G% between 100 arid 200" Fimilar to the humid air dehydration (Fig. 21. For higher pressures the thermobalallce showed a shift to higher temperatures for the weight loss due to the increase in the boiling point of water and a decrease in the vaporization rate of water. Thus it could not be used to show the rate of release of crystal water which mas faster than the vaporization rate of free water at these pressures. In humid air the loss of hydrate water proceeds rapidly according to the first-order reaction with a velocity constant of 0.11 min.-' a t 130". On the other hand, in dry air the dehydration is much slower with the total amount of hydrate xater lost dependent on the temperature. For iwtance, at 130", about of the hydrate m t e r is lost even after many hours of heating. Thus evaluation of the reaction kinetics of the dry air dehydration was possible for only the initial phase of the reaction. Using first-order equations also for dry air dehydrations, the velocity constant as 0.009 rnin.-l a t 130". Because of the dependency of the reaction kinetics on the presence of moisture, a study T ~ made on the effect of various partial pressures of mater vapor on the reaction rates. The vacuum oven technique described in the experimental section was used, although accurate data could not be obtained below pressures of 0.05 atmosphere of ,vater vapor. The velocity constants were determined at 130" for several pressures. I n z'aczio, the velocity constant, is close to that for dry air (0.009 min.-l). The results show a direct dependency on the dehydration to the vapor pressure up to about 0.2 atmosphere, after which the rate is constant at about 0.11 min.-'. Attempts to identify new phases by the X-ray diffraction method were unsuccessful. The :Lctual phases present were identified by use of the ASTM X-Ray Powder Data File and by reference to the cvork of McIntosh and Jablonski.I0 These rewilts together with the absolute densities are listed in Table I. TABLE I X-RAYAND DENSITY DATA FOR THE PRODI-cT5 O F CnHPOz' 2H20 DEHYDRATION Dehydration proeedure
Dehydration temp.,
Dry air 1 a t m NZ 3 atrn Nz 2 5 a t m . Nt Humid air
170 170
OC.
115 110 135
Absolute density
2 33-2 40 2 31 2 81 2 82 2 82
Phases by X-ray CaHP04.2H20 CaHPOA 2H20
+ CaHPOa (weah)
+ CaHPOi (weah)
CaHPoa CaHPOk CRHPOI
In dry air most of the CaHP04.2H20 X-ray diffraction lines were present even after heating at 250", although the weight loss indicated all of the dihydrate water should have been lost. These lines were of low intensity and broadened indicating poor crystallinity. M7eak anhydrous CaHP04lines were also present but did not intensify even at 400" where the Ca2P207begins to form. -kn amorphous phase could be seen under microscope examination. At 3 atmospheres or more Xz and in humid air, the (10) A. D . McIntosh and N. L. Jablonshi. Anal. C h e m . 28, 142.2 (195G).
S
April, 1960
DEHYDRATIOS OF C$LCIUM HSDROGEN I'HOSPHATE DIHYDRATE
X-ray diffraction patterns of the dehydration products showed sharp, intense lines of anhydrous C!k€IP04. Discussion The dehydration of CaHP04.2H20 in humid air (Fig. 2) is susceptible to a straightforward iiiterpretatioii. On heating iii the presence of sufficient \F Liter, the dihydrate decomposes according to equation 1 CaHPO, 2Hz0
+C ~ H P O I ( S )+ 2H20(g)
(1)
yielding a step iii the TGA curve and the 135" peak in the D T S curve. On further heating, the anhydrous salt remains stable to just below 400", then decomposes nccordiiig to equation 2 yielding a second step in the TGA curve and the 430" DTA peak ~ C L ~ H P O I---+ ( S ) Ca,P,O;(S)
+ HzO(g)
(2)
The weight loss for each step is near that expected. When the dihydrate is dehydrated at 133" in humid air, the anhydrous CaHP04 formed gives a sharp X-ray pattern and has a deiisitv of 2.82, somewhat less than the ideal value of 2.92." If we consider next the dehydration under nitrogen pressure, there are three new features to be explained : (1) the lowering of the temperature of the 133" DTX peak with increased pressure; (2) the appearance of a 1-18" DTA peak; and (3) the shift of the TGA4curve to higher temperatures with increased pressure. The differences can be accounted for by reactions 3 and 4. The liquid water would, of course, teiid to be saturated with CaHPOd. CaHPC)4.2H20 ---+ CaHPOI(S) &W1) +HzO(g)
+ 2H20(1)
(3) (4)
K e would wpect liquid water to form under iiitrogeri pressure for two reasons; first the nitrogen would cut down the rate of diffusion of mater away from the sample, and second since the atmosphere is relatively st:ttic, the higher pressure of water vapor near the snmple would tend to reduce the rate of e~-apor:ttioii. The presence of liquid water would serw to increase the rate of reaction. Also the rate of heat transfer would be increased between the sample 2nd the crucible. On all these grounds, n-e xould expect reaction 3 to occur at lower te1nperaturt.s. the higher the S, pressure and thus the 135" DTZ4 peak would move toward lower temperatures approaching the equilibrium temperature of 36" :IS a limit.'? The TGA curve, of ( 1 1 ) C, .\Iacl,ennan and C 4. Bewrrs, Acta (12) H. Bassvtt. J . Chem. Soc., 2939
(1958).
C ~ u s f .9,, 579 (1955).
493
course, detects only weight losses, and since water will evaporate more slowly the greater the nitrogen pressure, me observe a shift of the TGA curve to higher temperatures. The 148" DTA peak may be accounted for as simply the evaporation of water (equation 4). Similar effects have been rioted by Borchardt and Daniels13and by Reisman and Karlak14 with other hydrates. The formatioii of anhydrous CaHP04 with a good sharp X-ray diffraction pattern and a density of 2.82 by heating at 110" under 25 atmospheres ?$, serves to confirm this picture. The dehydration of CaHP04.2H20 in dry air presents a most complex picture relative to the first two cases discussed above. The presence of 3 endotherms at 135, 155 and 195" may be explained as follows. Since prolonged heating of CaH2P04.2H20in dry air a t 135" results in only about of the hydrate water lost, it can 1 e assumed that the 135" endotherm results from the reaction CeHPO4 2HzO
+C a H P 0 4 . z H L 0+ ( 2 - r)HzO(l) ( 5 )
ryhere z is about 0.9 to 1.2 moles of H,O. The second endotherm is less easily explained but may be due to the boiling off of the water released from the hydrate. The 195" endotherm results from further release of the hydrate water according to reaction 6 CaHPOa.zH20
+amorphous CaHPOJ + LHBO
(G)
The X-ray diffraction pattern of the amorphous CaHP04 resembles the origiiial CaHP04.2HsO pattern. On continued heating the X-ray diffraction lilies become less iiiteiise and the absolute densities remain close to the values for CaHPOd. 2H20. According to X-ray diffraction data at 475" this amorphous phase remains even after weight loss data (Fig. 1) show that C:d?20, should be present. At 600" the X-ray diffraction data shows the presence of crystalline r-C:~920i. Thus the exotherm at 530" results froni the reaction CazPz07(amorphous) -+ -,-CaJ'nOi (crgst:tlline)
(7)
The same y-form of Ca2P207results regardless of the dehydrating conditions. Acknowledgments.-The authors wish to express their appreciations to Dr. Dallas T . Hurd and to Rlr. Clyde S. Card for their valuable discussions and comments. (13) H. J. Borchardt and F. Daniels, Tim J O L R Y ~61, L ,917
(19571.
(14) A. Reisman and J. Karlak, J . Am. Chern. SOC.,80, 6500 (1958).
