Cox chart : chart : The chart, known as the Cox chart, uses a logarithmic scale for the vapor pressure and an entirely arbitrary arbitrary scale for the temperature in °F. The vapor pressure curve for any particular particular component, as as shown in Figure 15-1, can be defined as the dividing line between the area where vapor and liquid exists. If the system system pressure exists at its vapor pressure, pressure, two phases can coexist in equilibrium. Systems Systems represented by points located below that vapor pressure curve are composed only of the vapor phase. Similarly, points above the curve represent systems that exist in the liquid phase. These statements can be conveniently summarized by the following expressions: • p < pv → system is entirely in the vapor phase • p > pv → system is entirely in the liquid phase • p = pv → vapor and liquid coexist in equilibrium equilibrium where p is the pressure exerted exerted on the pure component. Note that the above expressions are valid only if the system temperature T is below the critical critical temperatur temperaturee Tc of the substance. substance. The vapor pressure chart chart allows a quick quick determination of pv of a pure component at a specific temperature. For computer and spreadsheet applications, however, an equation is more convenient.
1) A straight-line graph of the logarithm logarithm of vapor pressure against a special special nonuniform temperature scale; vapor pressure-temperature lines for many substances intersect at a common point on the Cox chart.
2) Cox chart Molecular formula : CAS : nature : Linear said the liquid vapor pressure and temperature of a map. If elected to vapor pressure and temperature on a straight line, said the coordinates, then it is by a minority of the data, it is easy to figure out a wide range of vapor pressure values. The plan by E. R. Cox in the 1923's, its longitudinal coordinates of the vapor pressure values, and the abscissa used by the water vapor pressure press ure (on 1
numerical) and the temperature linear drawing from the special temperature calibration. The coordinates of the other liquid vapor pressure (on numerical), the relationship between temperature and become straight. Cox called this straight line
Making a Cox chart: 1. Mark on the horizontal scale values of log p* so as to cover the desired range of p* for the compound of interest 2. Next draw a straight line on the plot at a suitable angle, say 45 ⁰, that covers the range of T that is to be marked on the vertical axis 3. To calibrate the vertical axis in comm. on integers such as 25, 50, 100, 200 degrees, and so on, you use a reference substance, namely water. for the index tic mark for the first integer, say 100⁰F, you look up a vapor pressure of water at 100 ⁰ F in the steam tables, or calculate it from the Antoine equation, to get 0.9487 psia. Locate 0.9487 psia on the horizontal axis, and proceed vertically until you hit the 45 ⁰ straight line. Then proceed horizontally left until you hit the vertical axis. Mark the scale at the intersection at 100⁰F 4. Pick the next temperature, say 200 ⁰F, for which the vapor pressure of water 11.525 psia. From 11.525 psia on the horizontal axis proceed vertically to the 45 ⁰ straight line, and then horizontally to the vertical axis. Mark the scale at the intersection with the vertical axis at 200⁰F 5. Continue as in 3 and 4 until the vertical scale is established over the desired range for the Temperature 2
Here are two possible forms of the Antoine's equation:
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