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Fast and Simple Solutions
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Fast and Simple Solutions
for Chemistry
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Storage of cyanide (CN) standard solutions is related to study with CN standard solutions properly. To understand this relation, we have to look over its chemistry first:
When you prepare cyanide stock solution from KCN or NaCN in water, you should consider that you obtain a weak acid reaction with water. After salts are dissolved in water, cation and anion are formed. If anion form of the salt in water is a weak acid with proton ion of water, that means an equilibrium reaction is occured. And equilibrium of this hydrolysis reaction depends on pH of solution. See the example below:
(a) KCN(s) → K(+) + CN(-)
(b) CN(-) + H(+) ⇔ HCN(aq)
Hydrogen cyanide (HCN) is a colorless, highly poisonous gas or liquid (below 26.7 °C) and it is weak acid [1]. HCN is also at equilibrium between gas and liquid phase under normal conditions because of its low boiling point or Henrys Law Constant: 1.33e-04 atm-m3/mole [2].
(c) HCN(aq) ⇔ HCN(g) (Under Room Conditions)
The term cyanide refers to a singularly charged anion consisting of one carbon atom and one nitrogen atom joined with a triple bond, CN-. The most toxic form of cyanide is free cyanide, which includes the cyanide anion itself and hydrogen cyanide, HCN, either in a gaseous or aqueous state. At a pH of 9.3 - 9.5, CN- and HCN are in equilibrium, with equal amounts of each present. At a pH of 11, over 99% of the cyanide remains in solution as CN-, while at pH 7, over 99% of the cyanide will exist as HCN. Although HCN is highly soluble in water, its solubility decreases with increased temperature and under highly saline conditions [3].
As we see from the information above, concentration of CN- depends on pH of the solution. At high pH unit of solution, H+ concentration of solution decreases according to reaction (b). This means equilibrium between CN- and HCN changes and amount of CN- increases. At low pH unit of solution causes increasing of HCN(aq) amount vice versa.
Besides HCN solution is light sensitive and only stable under room temperature [2]. So, at low pH units (<= 9), more HCN(aq) forms and HCN(g) forms from HCN(aq) according to temperature and light. It is obvious that forming of HCN is the main reason of changing of CN concentration in solution.
Preparing CN solution with destilled water or dilution of CN standard solution with destilled water causes some storages problems according to pH of solution. CN stock solution from KCN in water should be stored at 5-8°C no longer than 2 weeks before use [4].
Aqueous samples for cyanide analysis should be preserved under alkaline conditions by adjusting the pH to 12 or greater using sodium hydroxide [5]. Sample must be preserved with sodium hydroxide (target pH ≥ 12; pH ≥ 11 is acceptable) and cooled to ≤ 10°C [6]. This solution is stable for 3 months at 1 - 8 °C. [7].
Summary:
Using of CN solution as stock solution or standard solution for a long time period highly depends on acidity of solution, temperature and light. With light and high temperature (higher than 10°C) effects the concentration of CN-. The other important thing is pH of the solution. Preparation of CN solutions or dilution of CN solutions with pure water decreases the stability of solution (No longer than 14 days). Using NaOH or KOH to reach target pH of the solution (target pH ≥ 12; pH ≥ 11 is acceptable) while preparing solution or diluting solution with water, increases the stability of solution for 3 months (after opening the bottle).
Sources:
[1]. National Research Council (US) Subcommittee on Acute Exposure Guideline Levels. Washington (DC): National Academies Press (US); 2002. Acute Exposure Guideline Levels for Selected Airborne Chemicals: Volume 2.
[2]. U.S. National Library of Medicine. National Center for Biotechnology Information.
[3]. International Council on Metals and the Environment (ICME): https://www.cyanidecode.org/cyanide-facts/cyanide-chemistry
[4]. FDA: BAM Media M126: Potassium Cyanide (KCN) Broth: https://www.fda.gov/food/laboratory-methods-food-safety/bam-media-m126-potassium-cyanide-kcn-broth
[5]. Principles of Analytical Chemistry: A Textbook Writter: Miguel Valcarcel
[6] SM 4500 Analysis of Cyanide (Total, Weak Acid Dissociable, and Free) - PBM
[7]. ISO 14403-2 Water Quality
