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I ) l I JOURNAL OF RESEARCH of the Nati onal Bureau of Standards-A. Physics and Chemistry Vol. 66A, No. 2, March- April 1962 The Vapor Pressure of Palladium R. F. Hampson and R. F. Walker (Dece mb er 6, 1961) Th e vapor press ur c and h eat of sublim atio ll of palladium were measur ed using a ,·acllum microbalanc e technique. The m ean h eat of subl imat i on obta ined was 89.2 ± 0.8 kca l/ mo l c. Over the tempe ratur e range of 1,294 to 1,488 o J( the m eas ur ed vapor prcss llrcs mav bc represented by: Log P mm = 8.749 - 18655/ 7'. Th e normal boiling point is est im ated to be 3,200 ole Th e vapor pressure of palladium was measured by HaeHing and Daan e [lV using a modified Knudsen effusion technique which incorporated a micro- balance. Th eir dat a yielded a m ean heat of subli- mation at 298 o K, (298), of 80 kcal /mole. Walker , Efimenko, ,md Lofgren [2], u sed the Lang- muir technique wit Ii . a microbalance to obtain a mean (298) of 103 k e" l/mole. The latt er authOTs point ed out that sin ce boLh sets of dat a yielded values of (298) which tended Lo vary with the temperatures at which Lhe measurements were made, unresolv ed s:\·s Lel1hLLie errors were prob- ab ly involved in both techniques. MOTe recently, Dreger andlIargrave [3] measured the rate of sublimation by suspending a sample from a microbalance inLo a graphiL e tube furnace. Th e furnace approximated blackbody co ndi Lions. The mean value o[ (298) and its ayerage deviation were 91.0 and ± 0.8 lecal/mole. These authors sug- gested that elToneous ass umptions wC're made by Walker et al., eoncel'lling Lhe rmissivi ty of Pel and that these errors could account for the disagreement betweell the respecLive s C'ts of data. However, furLher iuvestigation in this laboraLory sLrongly indicated that a slow chan ge in the c alibration o[ the microbalance was responsible for the errors in the measurements of Walker et al. Sub- sequent improvements in t li e design of the a pp a ratus eliminaLed this problem. Th e technique, sampl e, and experimental pro- cedure used for the present m easuremenLs were gencrally similar to those recently described [4 ]. Two significant c]l anges were maele Lo reduce Lhe unccrLainty in the Lernperature measllremen ts: a small hole 0.02 in. diam X 0.16 in. deep was drilled into the wall of the 0.0 '5 in . di a m X 0.75 in . long sample rods at an angle o[ 75° to the normal (the angle of sigh ti ng with the optical the U -shaped hooks on the sapphir e-rod suspension were replaced with V-shaped hooks. The flr st e1hl,nge permitted observations of the temperature of the sampl e undel" approximately black body conditions to be co rnpar ed wiLh observations of the surface 1 Figures in b,·"ckcts inclicate the literature references at the encl of this paper. brightness temperature. The second ell ange pre- venLed the small sight-hole twisLing out of the line of sight during the co ur se of a l'l111 . It was assumecl that 1'01' a s ub stance having a ublimation coe ffi cient of unity the sight-hole would not conLribut e to the efI ective area o[ the vaporiz ing surface. :MeasUl'cments of the rates or s ublimation were made according to the pro cC' clul' e described [4], and were co nverted to equilibrium vn, por pressures usi ng the well-known Langmuir eCjlmLion. The sublim a- tion coeffi ei enL was assumed to be unit) ·, and palla- dium. was assumecl to vaporize solely to the mono- meric gaseous species. SpecLroch cmical analysis of tlle sa m pIc showC'd it to be better than 99.9 percent pme with sJllall },monnts of t 11 e following as inlpll rities: 8i, B, Cu, Fe , Pt, a nd Rh. The vapor pressures calcul ated from the me asured rates of vaporization are given in the tab l e, together with Lhe eOlTesponding values of (298). The free functions of Stull and Sinke [5] were used to obtain L1lC heats of sub limation. OnlY those te mperatures obtained from obsel"\rations the sight-hole nrc reported in the table. The relative values of these temperatures and the observed sur- face brightness temperatures were generally con- sistent wi th normal spectral emissiviLi es assumed in the previous work [2]. Figure 1 shows a Clausius- Clapeyron plot of the data given in the tabl e. The m ean value and standard d eviation of /:::,.I1 ; (298), obtained from the data of table 1, are 89.2 and 0.2 kcal /mole respectively. The normal boiling point is estimated to be 3,200 O K. The least squar es line through the dat a shown in figure 1 is represented by log P mm = 8.749 - 18655 j T. If the valu e of the sublimation coefficient is less than unity , the equilibrium vapor press ur es given in table 1 are too low, and the corresponding values of (298) are too high. Neglecting un certainties in th e free energy functions and assuming a sublima- tion coefficient of the overall limits of error in the absolute value of (298) are estimated to be ± 0 .8 keal /mole, ""hich corresponds to an error of ± 35 percent in the vapor pressures. 177
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