Humidity fixed points of binary saturated aqueous solutions

1 JOURNAL OF RESEARCH of the National Bureau of Standards-A. Phys ics and Chemistry Vol. 81 A , , January-February 1977 Humidity fixed points of binary saturated aqueous solutions Lewis Greenspan Institute for Basic Standards, National Bureau of Standards, Washington, D. C. 20234 (October 22, 1976) An e va lua ted compilati o n of equilibrium relative humidities in air ve rs us temperature fro m pure phase to approximately lOS pascal (1 a tm) in pressure is presented fo r 28 bina ry saturate d aqueous solutio ns . The re lative humidities of th e solutio ns ra nge fro m about 3 to 98 pe rcent. Using a da ta base from 2 1 sep a ra te in vestigatio ns compris ing 1106 individual measure me nts, fit s we re ma de by th e me th od of least s qua res to regul a r po lyno mia l equatio ns with two thro ugh four coeffic ie nts.

2 Equations and ta bles a re present e d a lo ng with the estima te d uncerta inties in th e corre la te d results. Key words : aqueous solutio n ; equilibrium; Humidity ; re lative Humidity ; salt ; satura ted salt solution; vapo r pressure; wa te r vapor. 1. Introduction Researc h, hygrome te r calibration, testing and material conditioning ofte n require th e accurate control o f Humidity in a wo rking space. The common me th ods of controlling the Humidity accurately use e ithe r a Humidity gene rator [lA]t or th e equilibration of a closed s pace with a chemical syste m [lB] whic h produces th e d es ired equilibrium vapor pressure . Humidity gene ra tors te nd to be expe ns ive a nd complex whe reas equilibration with c hemical syste ms tha t provide fixed po ints is a relatively in expe nsive a nd simple me thod of Humidity control.

3 A mo ng th e chemical syste ms used for this purpose are aqueous sulphuric acid solutions , glycerine and water solutions a nd sin gle a nd binary salt solutions . Eac h s uc h solution offe rs a degree of Humidity adjustme nt that can be a chi eved by c ha nging its concentra tion. On th e othe r hand, s pecial proble ms are assoc ia ted with th e use of solu-tions because their concentrations mus t be mea sured and controlled. Not only must the concentration of the solution be de termined initially but the pres ence of any Humidity sources or sinks in th e controlle d space and e ve n the initial equilibra-tion process of the space can alter the solution conce ntration. An especially useful me thod of Humidity control by c hemi-cal syste m involves th e use of binary saturated aqueous solutions (primarily of s in gle salts) in which the solute is hi ghl y non-volatile.

4 At a ny te mperature , the concentration of a saturated solu-tion is fixed and does not ha ve to be de te rmined . By provid-ing excess solute, th e solution will re main saturated e ve n in the presence of modest sources or s inks . Whe re th e solute is a solid in the pure phase, it is easy to de termine tha t th ere is indeed saturation. Due to th e ease of its use, this is a popula r method of Humidity control. Since a give n saturated salt solution provides only one re lative Humidity (RH) at any desired te mpe rature, a differe nt re lative Humidity mus t be a chi e ved by selecting anothe r appropriate salt. Though muc h data on saturated salt solu-1 Figures in brac ke ts indicate the lite rature re fe re ncL'S al the end of this pape r.

5 Tions have been produced a nd ma ny compila tions of th e e quilibrium re la tive humidities of selected saturate d salt solutions exis t, th e re are no compilations fo r whic h the d a ta have been c riticall y analyzed a nd estima tes of th e uncerta in-ties involv ed g iven , a s tep whic h is a bolutely essential to th e implime ntatio n of the concept of fixed po ints. We ha ve moved to fill this gap by compiling, fro m th e lite ra ture, d a ta on a s uffic ie nt vari ety of satura ted salt solu-tions to cover th e entire ra nge of r e la tive Humidity at r easona-bly close intervals. We have adjus ted th ese da ta [1- 21] to be cons is te nt with te mpe ra tures on IPTS-68 a nd th e mos t recent e quations for th e vapor pressure of wa ter [22].

6 We have also a na lyze d th e expe rime nta l techniques used in obtaining th e orig inal da ta and ha ve mad e estima tes of th e uncerta inties in the orig inal d a ta . We ha ve th e n used th ese da ta to calc ul a te " best" values of relative Humidity in air as a function of te mperature from pure phase to a pproximately 105 pascal (1 atm) in pressure for these satura te d solutions . 2. Background The methods used by investigators to d ete rmine th e wa te r vapor in equilibrium with saturated salt solutio ns a re dive rse. A short description of the vario us me th ods used in th e refe re nced pape rs is of inte rest. (1) The direct measurem ent of the vapor pressure. A cham-ber containing a saturated salt solutio n a t a controlled tem-perature is firs t evac uated to re move all gases.

7 E vaporation from the solution is th e n allowed to proceed until th e ambie nt va por, essentiall y all wate r, has come to equilibrium with th e solution and a direct de te rminatio n of th e to tal pressure within th e cha mbe r is made by conve ntional pressure mea-sure me nt techniques. (2) Dew point measurement. The de w po int of th e gas within a chamber conta ining a satura ted salt solutio n a t controll ed temperature is measured by means of a cooled mirror within the c ha mbe r. Using vapor pressure ta bles or equatio ns, this d ew point is conve rted to the vapor pressure of wa ter. (3) Isopiestic vapor pressure measurement. The vapo r pres-89 sure of a saturated salt solution in one cell or chamber is allowe d to come to equilibrium with a cell or chamber containing a reference solution at a fixed temperature.

8 The refe re nce solution must be well characterized as to vapor pressure as a func tion of concentration at the reference te mperature. Under the equilibrium condition, th e equilib-rium vapor pressure of th e saturated salt solution is ide ntical to th e equilibrium vapor pressure of the reference solution. After the two cells have reache d equilibrium, the concentra-tion of the re fe re nce solution is determined (usually by weighing) and th e vapor pressure is calculated . (4) Relative vapor pressure measurement. A chamber con-ta ining a saturated salt solution a nd a chamber conta ining pure wate r or othe r well charac te rized solution are each evacuated to re move all non-water vapor gases. The two chambe rs are maintain ed at the same temperature and the absolute pressure of the saturated salt solution is measured as in the first me thod.

9 In addition th e pressure diffe rence be-tween th e two chambe rs and/or the pressure of th e reference solution is de te rmined. The ratio of the vapor pressure of the saturated salt solution to th e vapor pressure of th e wate r i s th e activity (or re lative Humidity ) of th e satura te d salt solution. (5) Measurement with a calibrated Humidity sensor. A chambe r conta ining a saturated salt solution a nd a Humidity sensor are brought to equilibrium at a controlled te mpe rature. Calibratio n of th e sensor be fore or/and after the measure ment provides the means of determining the equilibrium vapor pressure. (6) Gravimetric determination. Dry gas is passed through th e binary satura ted solution at a fixed te mperature.

10 The wate r vapor in th e efflue nt gas is absorbed by a desiccant and measured by weighing. The volume of th e gas is also de ter-mined. From these the vapor pressure or th e mixing ratio can be d e termined. As one would imagine , th e errors associate d with these me th ods diffe r as to source and mag nitude. The en'ors in any of th e me thods are also functio ns of the le vel of vapor pressure be in g measured as well as th e temperature of the saturated salt solution. The re is, the re fore, probably no one method that gives a best measureme nt unde r all conditio ns. 3. Method We have accumula ted experimental data from various researc he rs [1-21] and calc ulated " best" values of re la tive Humidity and th e assoc iate d uncerta inties of those values.