THE RUDOLF REPORT

c(HCN) [mol/l]0.250.20.150.10.0506. FORMATION AND STABILITY OF IRON BLUE0 5 10 15 20 25 30Temperature [°C]Graph 2: Saturation concentration ofhydrogen cyanide in water as a function oftemperature at 1 mol% HCN in the air (partialpressure 348 of p(HCN)=0.01, e.g., 10 mbarHCN at 1,000 mbar total pressure).air, 346 which correspondsto approximately13 g hydrogencyanide per m 3 air. 347 Itincreases, as with anygas, with decreasingtemperature and liesbetween 0.065 mol per lat 30°C and 0.2 mol perl at 0°C.These high concentrationsprove the extremesolubility of hydrogencyanide in water.320 It decreases by approximately half every 20°C. It is thereforeapproximately 10,000 times more soluble in water than oxygen (O 2 )and approximately 250 times more soluble than carbon dioxide (CO 2 ).The latter is not without importance, since the opinion is sometimesexpressed in the literature that the carbon dioxide content of air couldhave an influence on the amount of hydrogen cyanide soluble in water.57 But since hydrogen cyanide is considerably more soluble in waterthan CO 2 , and since CO 2 , furthermore, is hardly converted into carbonicacid in water at all, this influence may be disregarded. 349B: The moisture content of masonry is very strongly dependent onthe relative humidity of air and the temperature. Since the tendency ofwater to evaporate (water vapor pressure) increases with rising tem-346 mol is a standard amount of particles: 1 mol = 6.023 × 10 23 particles, according to the definition,the number of atoms contained in 12 g Carbon.347 Landolt-Börnstein, Eigenschaften der Materie in ihren Aggregatzuständen, part 2, volume b,Lösungsmittelgleichgewichte I, Springer, Berlin 1962, pp. 1-158.348 The partial pressure of a gas is it fraction of the total gas content.349 The influence is supposed to be based on the fact that carbonic acid (H 2 CO 3 , pK A = 6.37) displacesthe cyanide ion (CN – ) from the equilibrium: 1. CO 2 + H 2 O ⇌ H 2 CO 3 ; 2. CN – + H 2 CO 3⇀↽ HCN + HCO – 3 . Since, however, CO 2 is only soluble in water with difficulty and since, inaddition, the equilibrium of the reaction CO 2 + H 2 O ⇀↽ H 2 CO 3 (carbonic acid) lies almostcompletely on the left side, the concentration of carbonic acid in the moisture of masonry itselfis several orders of magnitude lower than that of the hydrogen cyanide, even if the contentof carbon dioxide in the room under consideration, exposed to hydrogen cyanide, is similar tothe hydrogen cyanide content. HCO –3 (pK A =10.25), finally, is a weaker acid than HCN(pK A =9.31) and would therefore be displaced by the latter: HCN + HCO – 3 ⇀↽ CN – + CO 2 +H 2 O. Therefore, even a higher carbon dioxide content in air can hardly influence the absorptionof hydrogen cyanide in masonry.165