Carbon dioxide critical density

Solvent Strength of Pure Fluids. The density of a pure fluid is extremely sensitive to pressure and temperature near the critical point, where the reduced pressure, P, equals the reduced temperature, =1. This is shown for pure carbon dioxide in Figure 2. Consider the simple case of the solubihty of a soHd in this fluid. At ambient conditions, the density of the fluid is 0.002 g/cm. Thus the solubiUty of a soHd in the gas is low and is given by the vapor pressure over the total pressure. The solubiUties of Hquids are similar. At the critical point, the density of CO2 is 0.47 g/cm. This value is nearly comparable to that of organic Hquids. The solubiHty of a soHd can be 3—10 orders of magnitude higher in this more Hquid-like CO2. 

Catchpole-Kinp examined binaiy diffusion data of near-critical fluids in the reduced density range of 1 to 2.5 and found that their data correlated with average deviations of 10 percent and a maximum deviation of 60 percent. They observed two classes of behavior. For the first, no correction fac tor was required R = 1). That class was comprised of alcohols as solvents with aromatic or ahphatic solutes, or carbon dioxide as a solvent with ahphatics except ketones as solutes, or... 

Most supercritical fluid chromatographs use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3°C and 72.5 atmospheres. Nitrous oxide, ammonia and n-pentane have also been used. This allows easy control of density between 0.2g ml-1 and 0.8g ml-1 and the utilization of almost any detector from liquid chromatography or gas chromatography. 

A supercritical fluid (SCF) is a substance above its critical temperature and critical pressure. The critical temperature is the highest temperature at which a substance can exist as a gas. The critical pressure is the pressure needed at the critical temperature to liquify a gas. Above the critical temperature and critical pressure, a substance has a density characteristic of a liquid but the flow properties of a gas, and this combination offers advantages as a reaction solvent. The liquidlike density allows the supercritical fluid to dissolve substances, while the gaslike flow properties offer the potential for fast reaction rates. Supercritical carbon dioxide (scC02) has a critical temperature of 31°C and critical pressure of 73 atm. 

Carbon dioxide has a conveniently low critical point (31 °C, 7.39 MPa), and supercritical CO2 has become the most widely used fluid where supercritical solvent properties are required, as it is also inexpensive and nontoxic. The solvent powers of supercritical fluids generally increase with increasing density, which can be regulated at will by varying the pressure. The absence of a gas-liquid interface and associated surface tension in a supercritical fluid enables the fluid to penetrate porous solids freely, and also to... 

Figure 6.2—Density for carbon dioxide as a function ofpressure at four different temperatures. At the critical point, the density of CO2 is 0.46 g/cm3. The figure on the right represents the variation of the retention factor k for three alkaloids analysed under identical conditions and pressure, fixed downstream from the column by a restrictor (T = 4"C, modifier 5% water and 15% methanol 1 codeine, 2 thebaine, 3 papaverine). As the pressure increases, k decreases.

The above equations were obtained from twenty non-polar gases including inert gases, hydrocarbons and carbon dioxide (but not hydrogen and helium). Hence, possible errors can be as large as 20%. The maximum pressure corresponds to a reduced density of 2.8. In the above equations, Zc represents the critical compressibility factor. The value of gamma is calculated using Eqn. (3.4-26). 

CARBON DIOXIDE. (CAS 124-38-91. CO., formula weight 44.01. colorless, odorless, nonloxic gas at standard conditions. High concentrations of the gas do cause stupefaction and suffocation because of the displacement of ample oxygen for breathing. Density 1.9769 g/l (O C. 760 ton). sp gr 1.53 tair — 1.00). mp -56.6"C (5.2 atmospheresl. solid CO sublimes at -79°C (760 torr). critical pressure 73 atmospheres, critical temperalure 3I C. Carbon dioxide is soluble in HiO (approximately I volume CO. in I volume H.O at 15 C. 760 tom. soluble in alcohol, and is rapidly absorbed by most alkaline solutions. The solubility of CO in H 0 for various pressures and temperatures is given in Table I. 

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