Hydrocyanic acid and the cyanide ion

The value of Af// (HCN, g, 298.15 K) listed in the JANAF Tables [98CHA] and NBS [82WAG/EVA] is 135.14 kJ-moP. It is based on Thomsen s experiment. The reason for the large difference from the value obtained here is unknown, but might indicate that the combustion reaction was considered to have been carried out as a bomb calorimetric measurement at constant volume. 

Badger [24BAD] studied the reaction NH,-)(g) -n C(cr) HCN(g) + H2(g) and Horiuchi, Yano, and Kanai [51HOR/YAN] investigated the reaction 2CO(g)+ NH3(g) HCN(g) 3- C02(g) + H2(g). The data were re-evaluated in [98CHA] and the results obtained there are accepted. Badger s data yielded an enthalpy of reaction at 

15 K that differed by about 50 kJ-mol between the evaluations by the second and by the third law, respectively. Hence his results will not be considered further. The data for the second equilibrium reaction lead to an enthalpy of reaction at 298.15 K of (12.1 10.9) kJ-moP from the second law and (4.60 + 2.10) kJ-moP from the third law. From the last result Af//°(HCN, g, 298.15 K.) = (131.11 +2.16) kJ-mol is obtained. 

Several evaluations of Af/7° (HCN, g, 298.15 K) from spectroscopic determinations of the enthalpy of atomisation of HCN(g) and from quantum mechanical ab initio calculations of enthalpies of reactions are available. Martin [96MAR] calculated Af//° (HCN, g, 298.15 K) = (128.24 2.54) kJ-moP from experimental spectroscopic data. From ab initio calculations of the enthalpy of atomisation of HCN(g) at 0 K and the enthalpy of the reaction 2HCN(g) (CN)2(g) + N2(g) he obtained with auxiliary 

The review selects with equal weight the estimates of the standard enthalpy of formation of HCN(g) from [1882THO], [51HOR/YAN], the spectroscopic data in [96MAR] and [93F1N/GAR], and the quantum mechanical calculations in [96MAR] 

---

There is a discrepancy between the cyanide criteria for both aquatic and drinking water standards and the current analytical technology. The criteria are stated for free cyanide (which Includes hydrocyanic acid and the cyanide ion), but the EPA approved analytical methodology for total cyanide measures the free and combined forms (11). This test probably overestimates the potential toxicity. An alternative method (cyanides amenable to chlorination) measures those cyanide complexes which are readily dissociated, but does not measure the iron cyanide complexes which dissociate in sunlight. This method probably tends to underestimate the potential toxicity. Other methods have been proposed, but similar problems exist (12). The Department of Ecology used the EPA-approved APHA procedure which includes a distillation step for the quantification of total cyanide (13,14). A modification of the procedure which omits the distillation step was used for estimation of free cyanide. Later in the study, the Company used a microdiffusion method for free cyanide (15). 

Hydrocyanic Acid and the Cyanide Ion When hydrocyanic acid, HCN, ionizes, it produces the cyanide ion, CN . Both the acid and the anion are exceptions to the nomenclature rules. The acid name suggests a binary acid, and the anion name suggests a monatomic anion. 

---