EPA Method 9013A

The toxicity and mobility of cyanide in soil is governed by its chemical form. Simple cyanide, or the cyanide ion (CN- ), can be weakly adsorbed onto soil particles at pH>9.2. Weak metal-cyanide complexes ([M(CN4)]-2) and strong metal – cyanide complexes ([M(CN)6]-3 or [M(CN)6]-4) have an affinity for metal oxides and organic matter that decreases with increasing pH, however, other salts in solution tend to inhibit adsorption1.

Simple cyanide, weak metal-cyanide complexes, and strong-metal cyanide complexes are readily soluble in water. Metal – Metal cyanide complexes, such as Prussian Blue (Fe4[Fe(CN)6]3), are insoluble in water and are the most common forms of cyanide found in sediment and soil2. Metal-metal cyanide complexes are insoluble in acid solution, and solubility increases with pH.

A common practice for the extraction of cyanide in soil and sediment is acid distillation3. This approach is valid for all cyanide forms except the metal-metal cyanide complexes that are most likely to be present.

The quality control practice of spiking a soil sample with simple cyanides, or strong metal-cyanide complexes to validate acid distillation is misleading because these cyanide complexes are readily soluble. Acid distillation of cyanide in soil results in low and irreproducible recoveries for the metal-metal cyanide complexes most likely to be present. Acid distillation of cyanide in soil is ineffective and does not accurately measure “total” cyanide nor does it estimate cyanide toxicity.