Journal of Chemical and Engineering Data, Vol. 23, No. I , 1978
69
Solubility of Krypton in Water and Seawater Ray F. Weiss* and T. Kurt Kyser Scripps lnstitution of Oceanography, University of California at San Diego, La Jolla, California 92093
The solubility of krypton in distilled water and seawater has been measured microgasometricallyover the range 0-40 "C. The data have been corrected for nonideality and are fitted to equations in temperature and salinity of the form used previously to fit the solubilities of other gases. The fitted values have a precision of 0.2% and an estimated accuracy of 0.4% and are given in units of the Bunsen solubillty coefficient as well as the solubility in milliliters/ liter and milllllters/kilogram from moist air at 1 atm total pressure.
in C of -0.14% per atmosphere deviation from 1 atm total pressure. Thus, for krypton solubility at total pressures near 1 atm, the exponential term in eq 1 may generally be neglected, reducing the equation to the simple form C = Pf. In the following discussions, gas volumes and fugacitypressure corrections are based on the virial equation of state. For the pure gas (neglecting the small contribution of water vapor) the solubility measurements have been corrected using the approximations VP,T) = V'(P,T)
+ €45)
(2)
and This paper is one of a series dealing with the solubilities of Nz, Oz,and Ar ( 74, 76)-He and Ne ( 77), 3He ( 73,and COZ( 78) in pure water and seawater. The methods used here for the solubility of krypton follow closely this earlier work. The measurements were made using the same analytical techniques, and the data were fitted to equations in temperature and salinity using the same numerical methods. This work was prompted by a desire to evaluate solubility relations for krypton concentrations in the sea (2,3, 70).Existing measurements of krypton solubilities were not adequate for this purpose. The only existing measurements of Kr solubility in seawater were by Koenig ( 7 7), whose data on He, Ne, and Ar solubilities showed low precision and poor agreement with other workers, and by Wood and Caputi (79),who made measurements at only three temperatures and one salinity. In distilled water, the measurements of Morrison and Johnstone ( 72)were the most detailed, although they did not agree well with the distilled water data of Koenig ( 7 7) or Wood and Caputi ( 79).
Nonideality At standard conditions, 1 mol of krypton occupies -0.3 YO less volume than 1 mol of ideal gas. This difference is about twice the precision of the solubility measurementsreported here, so that correction for nonideality is essential. The application of Henry's law to real gases has been reviewed by Weiss ( 74,who showed that the solubility of COz over the range 0-40 OC, and at partial pressures up to 500 atm, is well represented by a modified form of Henry's law expressed in terms of the fugacity of the solute and the total pressure. The same approach is followed here for krypton. A more thorough discussion of the pertinent thermodynamics is given in the earlier work ( 78). The solubility of krypton is given in terms of the Bunsen coefficient P, defined here as the volume of gas (STP) absorbed per unit volume of the solution, at the temperature of the measurement, when the total pressure and the fugacity are both l atm. According to the modified form of Henry's law ( 78), C, the volume (STP) of gas dissolved per unit volume of solution at the temperature of the measurement, is given by the relation C = Pfexp[(l
- f)V/RT]
(1)
where f is the fugacity, P is the total pressure, V is the partial molal volume of the gas in solution, R is the gas constant, and Tis the absolute temperature. Although V for krypton in aqueous solution has not been measured, values of V for He, Ar, N2, Oz,and COPdo not differ greatly (8). In our calculations, we have used a value of V for krypton of 33.5 cm3/mol, which lies between the measured values for Ar and COP.This value of V corresponds to a variation 0021-9568/78/1723-0069$01 .OO
f = Pexp[B(T)P/RT] (3) where Vis the volume of 1 mol of real gas, V' is the volume of 1 mol of ideal gas, and B ( T ) is the second virial coefficient. Values of 6(T ) in cm3/mol for Kr in the range 270-320 K ( 73) are well represented by the power series B( 5) = -516.66 4- 3.3769 T - (8.3056 X lod3)F?
4- (7.41404 X 10-6)p
(4)
In order to calculate the solubility of atmospheric krypton, the fugacity of krypton in air must be evaluated. Because the mole fraction of krypton in air is ((1, the atmosphere may be regarded for these purposes as a binary Kr-air mixture. The following equation gives the fugacity of each component in a binary mixture ( 78): fl = xlPexp[(Bll
+ ~ x ~ ~ ~ ~ ~ ) P / R T(5)]
where the subscripts 1 and 2 refer to the two components of the mixture, x is the mole fraction, B11 is the second virial coefficient of pure gas 1 , and 612 equals the cross virial coefficient €Il2 for interactions between the two gases minus the mean of 6, and 8 2 2 for the two pure gases. Following the treatment used earlier for C02 in air ( 78), the quantity &-air has been evaluated using the Lennard-Jones (6-12) potential model. Over the temperature range -2 to +40 OC, may be taken as a constant +3.5 f 0.3 cm3/mol. The magnitude of this correction, as opposed to assuming 6 = 0 (Lewis and Randall rule) is -0.03 %, or about one-eighth of the total deviation from ideal gas behavior. Although we have used the calculatedvalue of = 3.5 cm3/mol to prepare our fitted equations and tables for the solubility of atmospheric krypton, the 0.03% error introduced by assuming 6 = 0 may be tolerable for many applications.
Experimental Section Krypton solubilities were measured by the Scholander microgasometric technique as refined by Douglas (6, 7)and Weiss (77,78). This method was chosen because of its simplicity, precision, and proven agreement with other high-precision techniques in measuring the solubilities of a number of other gases ( 74, 76,78). A detailed description of the apparatus and technique is given in the cited literature. Measurements were made using krypton supplied by Matheson Gas Products and specified >99.995% pure. Independent chromatographic checks showed
