Cyanide (CN-)

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Cyanide refers a group of all cyanide ions (CN) and is usually found in the form of alkali metal compounds as white solid. These heavy metal compounds are found in plants in the form of hydrocyanic acid and can be analyzed in the form of cyanide ions. Cyanide can be analyzed and obtained by means of distillation measurement.

Types of Cyanide
1. Free cyanide refers to cyanide in the form of hydrogen cyanide gas or cyanic acid (HCN) and cyanide ions (CN-). The ratio of cyanic acid to cyanide ions depends on pH and acid ionization constant. Naturally, this substance is therefore usually found in the form of hydrogen cyanide.

HCN → H+ + CN

2. Cyanide and alkali metals are cyanide combined with alkali metals, such as potassium cyanide (KCN), thiocyanate (SCN-) and ammonium cyanide (NH4CN). Their general formula is A(CN)x where:

A = alkali metal
X = amount of cyanide ion

KCN

Cyanide compounds and alkaline metals are often unstable and volatile. They are a well soluble solid and when dissolved in water they form cyanide ions (CN-), which are highly toxic and an almond-like odor.

3. Cyanide and heavy metals are cyanide compounds combined with various types of heavy metals without any alkaline metal as their component, such as copper (II), cyanide (Cu (CN)2), silver cyanide (AgCN) and zinc cyanide (Zn(CN)2). These compounds are least soluble.

4. Complex cyanide is cyanide compound combined with alkaline metals and heavy metalin their molecules, such as potassium ferrocyanide (K4Fe(CN)6) and potassium hexacyanocobaltate (III) (K3Co (CN)6). Their general formula is AyM (CN)x where:

A = alkaline metals
y = amount of alkaline metals in cyanide compounds
M = type of heavy metals
x = amount of cyanide ions (A + M)

These compounds are well soluble in water and decompose to form free cyanide ions.

5. Other cyanide compounds include, for example, cyanogen chloride (CNCl), and evaporate easily and slightly soluble in water. There is a very high toxicity due to the addition of chlorine in the solution containing cyanide compounds.

NaCN + Cl2 → CNCl + NaCl

Reactions and decomposition
Cyanide ions well react with other substances ;  for example, cyanide ions and alkaline metals form potassium cyanide (KCN), cyanide ions and heavy metals form silver cyanide (AgCN). Some parts of these compounds are decomposed by microorganisms or oxidized as cyanate (CNO) and they will experience a chemical decomposition to form carbon dioxide, ammonia and formate evaporating into the air.

Industrial applications
Industrials using cyanide compounds

  • Cadmium cyanide (Cd(CN)2) is used in the electronics industry.
  • Calcium cyanide (Ca(CN)2) is used as disinfectant to smoke the products, production of HCN and ferrocyanide, and preservation of cements.
  • Cuprous cyanide (CuCN) is used in electronics industry and production of medicines and pesticides.
  • Cyanogen bromide (CNBr) is used to extract gold and production of pesticide pesticides.
  • Hydrogen cyanide (HCN) is used in the electronics industry, metal catching agent, pharmaceutical industry, and component of pesticides and ratsbanes.
  • Nickel cyanide (Ni(CN)2.4H2O) is used in the electronics industry.
  • Potassium cyanide (KCN) is used in the electronics industry, steelmaking process, mineral extraction, component of polishing solution, and film development process.
  • Potassium ferricyanide (K3Fe (CN) 6) is used in the film development and blueprinting processes, heating of metals, electronics industry.
  • Potassium ferrocyanide (K4Fe (CN)6) is used to heat the steels and paint industry.
  • Silver cyanide (AgCN) is used in the electronics industry.
  • Sodium cyanide (NaCN) is used in the electronics industry, film development process, and synthesis or extraction of substances.
  • Lead cyanide (Pb(CN)2) is used as component of pesticides and electronics industry.

Plant sources
Cyanide is mostly found in plants, including cassavas and asparaguses, which are in the form of hydrocyanic acid that has the chemical formula of HCN. It can be found in all parts of the cassavas, including their heads used to make consumption flours.

casava

Hydrocyanic acid in cassavas is created by two amino acids, namely, firstly, valine creates linamarin from acetone. Cyanohydrin that is found more than 93 percent, and secondly, isoleucine creates lotaustralin from methylethylketone cyanohydrin that is found approximately 7 percent.

Cyanide forms in cassavas

  1. Bound cyanide is connected to the starch molecules or called cyanogenic glycoside.
  2. Cyanohydrin cyanide is connected to other molecules other than starch molecules.
  3. Free cyanide is hydrocyanic acid.

The amount of cyanide in cassavas depends on its species and growth period. Some species may contain only cyanide of 10 mg/kg while other species may contain cyanide as high as 500 mg / kg. The safe amount of cyanide in cassavas required by FAO shall not exceed 10 mg/kg.

Toxicity
Cyanide ions can suspend the growth and inhibit the production and degradation of living cells as well as inhibit the respiratory process. In addition, they can degrade nitrogen and phosphates and prevent the tissues in the central nervous system from using oxygen, which can finally make the creatures to die.

Toxicity is divided into two types:
1. Acute toxicity
• Obtaining in a high level can cause death.

2. Chronic toxicity
• iodine deficiency
• Body haggard due to lack of protein

Toxicity of cyanide compounds (U.S.EPA, 2000)(1)
1. Hydrogen cyanide (HCN)
– Physical state : Gas
– TLV : 5 mg/m3
– LD50 : 1 mg/kg (human)

2. Potassium cyanide (KCN)
– Physical state : Solid
– TLV : 5 mg/m3
– LD50 : 10 mg/kg (rat), 2.85 mg/kg (human)

3. Sodium cyanide (NaCN)
– Physical state : Solid
– TLV : 5 mg/m3
– LD50 : 6.44 mg/kg (rat), 2.85 mg/kg (human)

4. Cyanogen chloride (CNCl)
– Physical state : Gas
– TLV : 0.3 ppm
– LD50 : –

5. Sodium cyanate (NaCNO)
– Physical state : Solid
– TLV : –
– LD50 : 260 mg/kg (rat)

6. Potassium cyanate (KCNO)
– Physical state : Solid
– TLV : –
– LD50 : 320 mg/kg (rat)

7. Potassium ferricyanide (K3[Fe(CN)6])
– Physical state : Solid
– TLV : –
– LD50 : 1,600 mg/kg (rat)

Note:
TLV (Threshold Limit Value) is the highest concentration of human capacity and is often used as a standard of occupational safety at 8 hours/day or 40 hours/week.

LD50 is concentration in mg/kg of body weight, which causes 50 percent of experimental animals to die.

References
1. U.S. EPA, 2000. Office of Research and Development. Capsule Report Managing Cyanide in Metal Finishing.