Similar plots were made for 0, 10, 20, 30, 40, and 50 degrees. The values of the slope were:
The problem is now essentially solved except for unit conversions. DuPont's liquid phase concentration is expressed in weight percent, and I wish to express that value in molarity (M). The first step is to calculate what volume of water contains 1 mole of HCN. The molar mass of HCN is 27.03 g. The mass of water (MH2O) can be expressed as:
(MH2O) = (100/C -1) x 27.03where C is the concentration in weight percent HCN. The density of water is 1.0 g/mL and will be treated here as independent from temperature. HCN density as a function of temperature was found by a linear extrapolation of the densities found on page 2 of the same DuPont document. The fit in g/mL yielded:
pHCN=0.715-0.00133 x Twhere T is expressed in degrees Celsius (° C), and pHCN is the density of HCN. The volume of solution that contains 1 mole of HCN can now be calculated, if one neglects the small effect of mixing on changing the volume. Figure 11 on page 31 of DuPont's handbook displays the specific gravity of HCN solutions as a function of the weight percent HCN. Inspection of this figure is sufficient to show that neglect of the volume of mixing is warranted. The volume of solution in milliliters (mL) that contains 1 mole HCN is therefore:
V= 27.03/pHCN + MH2O/1.0I convert to molar concentration:
[HCN] = 1000/VThe gas phase concentrations of in the gas chamber were in the range of 8-16 g/m3. I therefore chose an array from 1-20 g/m3 converted that array to Torr and calculated from the above relationships the equilibrium water concentration at the given Temperatures. Conversion from g/m3 is straightforward:
P=R x T x (C/27.03) x (760/101325)Here P is the partial pressure of HCN in Torr. R is the universal gas constant (8.31441 m3Pa/mol K (SI units!). T is the temperature in Kelvin (273.15 plus the temperature in Celsius); C is the concentration of HCN expressed in g/m3; 27.03 is the molar mass of HCN in grams (not SI units, but grams cancel); there are 760 Torr in an atmosphere and 101,325 Pascals (Pa).
ResultsThe results of these calculations are shown if Figure I.2. The temperatures in the gas chambers were most likely between 20 and 40 ° C, but even if they got down to 10 ° C, and even if the higher concentrations reported were used, the equilibrium concentration of HCN is on the order of 0.1-0.2 M. In other words, these are the maximum concentrations that could be achieved. More likely, the concentration was limited by kinetics and never reached equilibrium.
Last modified: October 16, 1998