1938 egan dissociation pressure carbamate

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454 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 4acids occurs a t t,he outer region and increases as it approachcs th e

heartwood. When the heartwood is reached, a material in-

crease in resin acid is noted, but whether the inner or outer heart-

wood has the larger or smaller percentage of resin acids is dc-

pendent upon their respective cxtractivc contents.

This constituent iiicreases in per (wit as

it passes from the newly formed sapwood to the inner sapwood

where it reaches a maximum; then the percentage of f att y acids

decreases as it approachcs the pith.This fraction usually iiicreases in per cent as it

approaches the pi th of the tree.

In heartwood th e percentage of esters decrenses f roni

t,he outer heartwood to the pith. I n some instances t he per-

centage continues t o increase as it approachcs the out,ersapwood;

in others it ten ds to decrease.

UXSAPONIFIABLES.n general, the perccntage of unsaponifiable

is the leas t a t the outer sapwood and iiicreases, in some <maximum in the outer heartwood and then decreases as it ni l -

proaches the center of t he trce.

I t is interesting to note tha t t he coniposit.ion of thc outrr niitltll(~

hear,tnood in stan d I11 indicates that it is apparently in the trail-

sition stage from sapwood to heartwood. The relatively lo\\-

rwin acid content, together w it hi he slight increase in unsaponifi-

able material in this outer heartwood region, suggests that this

transitional deposition of extrac tives was t,aking place.

FREE ATTYCIDS.

VOLATILES.

ESTERS.

APPLICATION OF RESULTS

I11seeking means for effectively utilizing the potentia l c.lieriiicw1

products from ponderosa pine, it was found that this wood coii-

tained a sufficient quan tit y of extractives to warrant t he possible

removal and recovery of these materials from the lumber and

from forest and mill wood waste. Preliminary investigation\

have shown th at it is possible t o extract all or a large portion 01

the extractives from lumber; the result is 8 further improveiiicnt

in the lumber offered by manufacturers; in addition, a com-

mercial volume of extractive products may become available

from this wood. Th e amount of recoverable extrac tables is not

uniformly distributed throughout the trunk of the tree. The

average extractive content in sapwood is usually within tlie limit.

2.0 to 9.8%.of the weight of the dry wood, whilc th e heartwood

extractables are usually within the limits 3 .5 to 31.5% of tho

weight of the dry wood. Tho greater quantities of extractivc i

are obtained from the lumbci antl wood waste originating from

the but t portion of th e trunk and from tha t portion of th e trcl

containing massed pitch arcs?.

The acetone extractives, R Iictlier from the heartwood or \ u p

wood, contain, in addition to rebin acids and terpenes, free fatly

acids, fats, and unsaponifiablc msteiia l. Thus the extractive.

differ from gum oleoresin foiIncd by wounding the tree by bhe

presence of these aliphatic and un5aponifiable substances. T h e

percentage of each of thesc erilitics is not uniformly dis tributedthroughout the tree but depcntls from which part of the log thc.

extrac tives arc obtained. In tlic case of heartwood extrac tive<,

the products found in approxiinaic order of quanti ty preicnt are

resin acids, free fatty acids, unLaponifiable, esters, volatile, wntri

soluble, and water and ether insoluble. In sapwood extractive.

these entitios are found in thc following older: free fat ty acidi,

r e h acids, water soluble, estcrs, uninponifiable, mater an d eth6.r

iiisoluble, and volstile. Since th r conimercial value of t h e x-

tractives is contingent in part upon th e quantity and exact r i i

tule of its entities, t he identificdtion o f ~x c*k if thesc produvt

iindo1 inviLhtigatiori by thi- l a h l r i ( o r r

ACK\0% J,LI)( ;Rf E N 1

The author is indebted to Gcorgc Schroeder and C. V. Zaayeu

for collecting the wood sections ant l t h e advice of Albert H c r r n ~ nI > greatly appreciated.

LITEHA7'UL{ 1.: c;t'rl?;l)

(1 ) Adariis, J . INU. NO.C m x . , 7, 957 (1913),( 2 ) A4nderson, b i d . , 3 6 , 662-3 (1944).

( 3 ) Assoc. of Official Agr. Chein . , Methods of Analysis, pp. 469-71

141 Benson and Jones. .J. ISD. $hi;.HEM.. . 1096 (191'7i

(1940).

, ,

Dore, I t ~ i d . , 1, 556-63 (1919).

(6) Hihbert and Phillips, C a n . J . l i c s s n r c h , 4 , 1-34 (1931)(7) Koch an d Kricg, Chem.-ZtN., 15, 1 4 0 - 1 (1938).

(8)Kiajriiiovic, I b i d . , 55, 894 (1931).

(9) Kurth, IXD. NG. HEM. , 3, 1156 (1931).

(10) Schorger, U. S. Forest Service, Bull. 119 (1913).(11) Trendelertburg and Schailc, P a p ie r - F n h r . , 35, 221-30 (1937).(12) Tiertelak and Garbaczowiia, lsn. RSG. CHBM., NAL.ED., a ,

(13) JTise, "Wood Chemistry", pp . Xi3-4 , A.C.S. Monograph 87

(14) \TOMan d Scholae, Chern ,-Zlg . , 38, 3G!)-70 (1914).

110-11 (1935).

- N e w York, Reinhold Pub. Gorp., 1944.

issociation Pressure ofAmmonium Cark. P. EG-kN, JR . , J. E. POTI'S, JR . , AND GEQRGETTE 1). PO n ' S

T e n n e ss e e V a l le y A u t h o r i t y , Wilson D u m , Ala.

EMPERATURE-pressu re relations for th e dissociation ofT olid ammonium carbamate into gaseous ammonia and

carbon dioxide have been measured by several investigators.

(1-5) . The reported values are divcrgeiit; at a total pressure

of 40 atmospheres the divergence is as much as 17 atmospheres.

The present paper cover5 .t study of the dissociation preism c

of solid ammonium carbama te over the temperature range 35' $0

83' C. and in the absence of 'til excess of ei ther gaseous reactvnf

From the vapor pressure data tlie lree energy of dissociatioii

and th e heat of dissociation h a w bwn derived.

Briggs and Migrdichian (1)-measured the dissociation pressure

of ammonium carbamate over temperature range 10" to 49" C. PREPARATION OF SOLID A3tMOh'IUR.I CARBAMATE

and obtained very consistent data . They also studied the

effect of excess ammonia or carbon dioxide and found excellent

agreement mith the mass Ian accoiding to t he equation:

Solid ammonium carbam i te ~ v n s repared directly in a 50-c

sample bulb (Figure 1) which later was connected to th e pre?siirci

measuring system. Stoichiometric proportioning of the r('ii<

tant s, as was employed by Briggs and Migrdichian ( f ) , proved ta

br unnecessary. Thc carbamatc n a s deposited in the bulb fron,II&OZ1\"2 (solid) =BTH, gas) +COz (gas) (1 )



April, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 455

Figure 1. Apparatus for Preparing Ammon ium Carbamate it r i d hleasuring1 I Dissociation Pressure

ti roughly equimolar mixture of ammonia and carbon dioxide

which was charged a t a rate of 1 liter per minute . Deposition of

carbamate in the inlet and outtet tubes was prevented by main-

taining the temperature of t he tubes a t 100" C. Th e gases, which

were of commercial grade, were dried thoroughly t o prevent th e

formation of ammonium carbonate and bicarbonate. The

carbon dioxide first was washed with acid permanganate and

then was dried successively with sulfuric acid and Dehydrite.

The ammonia was dried with freshly pulverized fused potassium

hydroxide. I n preliminary trials in which the sample bulb waa

cooled with dr y ice, expansion of t he produc t on warming to

room temperatur e shatter ed the bulb. Thi s indicated either a

high coefficient of expansion or a phase change in solid am-monium carbamate between -78" C. and room temperature.

In subsequent work, therefore, the bulb was cooled in an ice-salt

bath.

The ammonium carbamate was purified by alternate partial

vaporization and evacuation through a vacuum line that con-

tained a tr ap cooled with dry ice. The sample was first warmed

until t he dissociation pressure reached 700 to 800 mm. of mercury,

then wm cooled w<th a mixture of dry ice and acetone, and the

T h e pressure of dissociation of solid ammonium car-

bamate into gaseous ammonia and carbon dioxide was

measured over the temperature range 35' to 83' C. Th e

results, when plotted a s log P against 1/T, fall on th e samestraight line as the data of Briggs and Migrdichian for

the range 10' to 45' C. The combined data of the two

studies are represented by, the equat ion:

log P (mm . Hg) = -2741.9/T 4- 11.1448 (283' to 355"K.)

The slope of th e curve defined by th is equ ation indicates

that the heat of dissociation of ammonium carbamate,

if assumed to b e consta nt over the experimental range of

temperature, is 37.6 kg.-cal. per mole. This value agrees

with calorimetrically determined values reported pre-

viously. The equation is applied also in the derivation of

the free energy of dissociation.

system was evacuated t o 10-4 mm.

of mercury. After three such cycles

of degasification, th e sample was as-

sumed to be free of foreign gases. Oncompletion of the purification step , all

the tubes joining the sample bulb were

sealed off, except a short control

manometer. The purified sample vas

riot weighed but was estima ted t o be

0.2 to 0.4 gram.

Each preparation was checked for

quality by comparison of i ts dissocia-

tion pressure at an arbitrari ly selected

temperature of 34.5 O C. with the value

interpolated from the da ta of Briggs

and Migrdichian (1). About one third

of th e preparations were discarded

because their dissociation pressures ex-

ceeded the adopted tolerance of 1mm .

of mercury deviation from the in-

terpolated value of 170 mm. of mercury .

None of the preparations yielded a pres-

sure of less th an 170mm. at 34 .5"C.

MEASUREMENT OF DISSOCIATIONPRESSURE

In the technique used for the

pressure measurements. the autog-

eiious dissociation pressure was allowed to come to equilibrium

at a given tempera ture with the gases exposed only to t he sample

bulb and the short control manometer. Th e control manometer

was a null-point instrument with sealed-in electrical contacts

in a circuit th at automatically balanced th e dissociation pressure

with nitrogen pressure. Th e measuring manometer was in the

nitrogen system, as Figure 1 shows.

The control-manometer used for pressures above 1000 mm

was a glass Bourdon gage that established electrical contact at

the tip of th e free end of t he elastic element. Th is gage had the

advantage th at it presented only glass surface to th e products of

dissociation, but its sensitivity to temperature introduced a

significant correction factor a t pressures below 1000 mm. At

these lower pressures, therefore, a mercurial control manometer

was used. A correction factor represen ting th e pressure re-

quired to establish electrical contact was determined for each

type of gage and was added algebraically to the observed disso-

ciation pressures.

The balancing pressure was obtained from a nitrogen cylinder

through a reducing valve. The dissociation pressure was bal-

anced by continuously bleeding a small amount of nitrogen from

the system to a vacuum and intermittently introducing nitrogen

under pressure through a solenoid valve that would pass a slow

stream of gas under a differential pressure of about 10 cm. Th e

valve consisted of an 8-mm. tube that terminated with a fritted

glass disk slightly above a pool of mercury in a n integral jacket

to which th e nitrogen source was connected. A cylindrical iron

plunger surrounded the 8-mm. tube and floated on the mercury.

A solenoid 'surrounding the jacket pulled down the plunger and

thereby raised th e mercury surface sufficiently to seal the fritted

disk and prevent th e passage of nitrogen. Th e solenoid waa

actuated through a vacuum tube relay and the contacts in the

null-point manometer. At equilibrium th e intermit tent action

of t he solenoid caused fluctuations of less than 0.2 mm. in the

level of mercury in the measuring manometer.

Pressures up to 1000 mm. were measured on a mercurial

manometer with B 12-mm. bore. One leg of th e manom6ter waa

evacuated t o <10-4 mm., which made the manometer absolute

within th e accuracy of th e readings. To eliminate parallax, a

glass mirror scale and a sliding hairline index were employed for



456 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No . 4

Figure 2 . Dissociation Pressure of Solid A m r n o t i i i i i r i (:arbamate

reading the manometer. The pressurc reading5 u t'w c7orrertPd

to 0" C. for the expansion of glass and mercury.

Pressures above 1000 mm. Kere read on a three-s tage compound

mercurial manometer in which the interstage spaces were filled

with freshly boiled distilled water. Ss with the qimple manome-

ter, the final leg of the compound manometer was evacuated

The manometer was mounted on a heavy paper scale that

differed from the glass scale by less than 1 part in 1000. Th e

pressure differentials in the three s tages of the manometer , cor-

rected fo r th e head of water in each stage, were added t o give th e

tot al pressure. The tot al pressure as corrected to 0" C. for

th e expansion of mercury.

Th e

temperatu re of th e bath was estimated to th e nearest 0.01 O C.

with a calibrated thermometer; the over-all accurary probably

was +=0.1"C. The attainment of equilibrium x-as ensured by

The sample was heated in a thermoregulated oil bath.

TABLE. DISSOCIATION PPESSrRY: O F d l lM oN IUM 4 RB \lATE

Temp., P, T y p . , P, 'Temp., P ,OC. him. Hg C. Mm. Hg C. \ Im. H g

34.4934.5534.5942.9943.1846 .484 7 , 5 350 ,9252.1954.73

55 0055.78

170.1171.2170.9294.2295.1366.8388.9479.2517.2603.2

610.2641.6

56 9858 2659.3159 8361 246 2 0962 0962.1365 2066 19

67 2067 56

688 7743.7790 .1812.5881 .9924 2925 7927.2

10961158

12231248

68.9170.4371.7473.1874.4076.4777.3078.8280.2781.81

83.3383.38

1327145015511673180.519982093226424562659

28562864

TABLE 1. FREE EXERGY F DIssocraTrox OF SOLIDA\rvon-~r-arCARBAMATE

P , Aim. X p , Atm." Ak' ', Cal./hIole.I errip., C

0"20406080

1 O O b120b140b

0 01670,08110.32121 , 0 7 83.1538 , 2 2 3

19.4542.33

6.90 x 10-7

4 91 x 10-37 . 9 0 X 10-6

1.86 X 10-14.648 . 2 3 X 10

10.90 x 10 2

11.23 X lo 3

7700550033101110

- 080-3270- 460- 660

a K p = A P s , where P s dissociation pressure in atmospheres.

b Extrapolated.27

approaching equilibrium from both the

high- and the lorn-pressure sides. Th r

pressure w as read at half-hour interval5

until three surressive readings agreed within

0.5 mm.

DISCUSSION O F RESULTS

Table I gives the measured dissocia-

tion pressures. Of the da ta previouslj

reported, only those of Rriggs and

Rligrdichian (1 ) fall on the same straight

line with the present measurements when

plotted as the logarithm of the pressure

in millimeters of mercury against th e

reciprocal of the absolute temperat,urr

Application of the method of least squarca

to the combined d at a of Briggs and

RIigrdichiaii and of the present work

yields t he following equation €or thc

dissociation pressure of solid ammoniiirri

carbamate:

lo g P = -2741.9/T $-

11.1448 (283" o 355"K .) (2 )

w h w e P = pressure, mm. of I l gT = absolute temperature

The average deviation of the present measuremrnts from

Equation 2 is *0.47, and the maximum deviation is 1.4Cr0; the

corresponding values for the data of Briggs and htigrdichian are

~ 0 . 3nd 1.3%, which are of the same magnitude as for an

equation representing Briggs and hIigrdichian's d at a alone

Figure 2 compares calculatrd and r n p n w r e d dissociation pres-

qui-es.

If it is assumed th at d i d ammonium carbamate dissociateq

according to Equa tion 1 and that the vapor is a perfect gas w e -

tem,

T-dues of t he dissociation constant, Kp , as calculated f r u m

smoothed dissociation pressures derived from Equation 2 , are

presented in Table I1 together vith values for the free encrgy of

dissociation as calculated from the relation:

AFo = - R T l n K ,

The heat of dissociation corresporiding to Equat ion 1, as c-ulcu-

lated from th e slope of the vapor pressure line in Figure 2 on the

assumption of constancy of th e hea t of dissociation, is 37.6

kg.-cal. per mole of solid ammonium carbamate. This value

agrees with calorimetrically determined values (3)but is lower

than the heat of dissociation calculated by Krase (3 ) from the

dissociation pressure data of Briner (9).

In this paper no correction of measured pressure to fugacity

has been made. A trial calculation, assuming the absence of

mixture effect, indicates that, at the melting point, the fugacity

of the mixed gases mould be about lOyolower th an th e pressiire.

n-hich is within t.he error int,roduced by extrapolation .

LlTERATUHE CITED

( I ) Bnggs, T. R. , and Migrdirhian, V ., J . Phus. Chem.. 28, 1121-35

121 Briner. E.. J.chim. hws.. 4. 267-84 (1906'1.

(1924).

{3j Curtis,' H.'A., "Fixed Nitrogen", Chap. XIII, N ew York, Cherni-

(4)Xlatignon, C., and Frejacques, M., ull. SOC . chim., 31, 307-16

c a l Catalog Co., 1932.

(1922).

( 5 ) Tokuoka,M., . Agr. f 'hmr i . .SOC. Japan , 10, 1333-44 (1034).

1938 egan dissociation pressure carbamate

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