Nowadays, cyanide has many uses in different industries. It is used in the manufacture of plastics, paper, and textiles. It is also among the chemicals used in developing photographs. Cyanide gas is used as well to exterminate pests and vermin in buildings and ships (12). One of the essential applications of cyanides is metallurgy for electroplating, metal cleaning, and gold extraction from its core (8). In the mining industry, removing the precious metals from the ore bodies is achieved by having cyanide form complexes with gold and silver, resulting in their dissolution.
The toxicity and the control of cyanide concentration in the extractions of gold and silver require precise monitoring and detection of this compound (9). The efficiency of the cyanidation processes for silver and gold extraction and meeting the environmental regulations depend heavily on the measurement and control of free cyanides. Cyanides are classified into three categories: free, weak acid dissociable, and total cyanides. Free cyanides include cyanide ions CN- and hydrocyanic acid HCN. Weak acid dissociable cyanide has the weakly complex forms of cyanide (copper, nickel, zinc, and cadmium cyanide complexes). Total cyanide consists of all forms of cyanide (free cyanide, weak and robust complexes such as iron and cobalt).
Titration is considered the most common method to determine the free cyanide concentration in the gold extraction industry. This technique consists of adding a titrant whose concentration is known to a known volume of a sample whose concentration is unknown. The completion of titration is marked by a change of color or the electrode's potential, and it is known as the finishing point or endpoint. These changes can be detected either visually or instrumentally. Procedures involving titration are generally used for monitoring large cyanide quantities when no weakly complexed metal cyanides or other interferences are present (4).
Silver nitrate titration is considered the most common method for monitoring free cyanides based on the number of silver nitrates required to transform the entire cyanide present into silver cyanide or one of the derivative compounds. As usual, the titration is performed, introducing the silver nitrate standard solution to a known quantity of the material with cyanide. In potassium iodide in the solution, the silver nitrate would react with forming silver iodide, which causes yellowish turbidity that is easily recognizable. This method is used with either potassium iodide or rhodanine as indicators.
Effect of the Presence of Other Metals on the Titration Results
Silver nitrate titration is considered reliable for determining free cyanides when no other interfering compound is present. However, since more complex gold ores are being treated with the addition of cyanide (usually with reactive sulfide minerals or soluble copper present), significant interferences are noted with the visual and instrumental finishing point determination giving increasingly different data (5). Researchers reported that the presence of copper, zinc, iron (especially when the cyanide/metal ratio is low), thiosulfate, and sulfide ions might cause interference and, therefore, a wrong estimation of the cyanide concentration. Specifically, thiosulfate is known to cause overestimation of the concentrations of free cyanide. At the same time, sulfides present in the solution produce black precipitates (silver sulfides) and make the detection of the finishing points difficult (6,7). These interferences cause a decrease in the leaching efficiency, the contamination of the final precious metal by copper, weak cementation or activated carbon adsorption, and inefficient precious metal dissolution (3).
Moreover, the presence of metals such as cadmium, mercury, copper, silver, zinc, or/and nickel causes the quantification of both free cyanide and a portion of the cyanides complexed with metals. While zinc-cyanide complexes can be fully titrated using silver cyanide if the pH is higher than 12, copper-cyanide complexes can only be partially titrated by this method. Therefore, it would be incorrect to assume that this method determines the concentration of free cyanides when zinc or copper are present (11).
Another disadvantage of this method is that silver ions can react with complex cyanides, leading to the disappearance of the color after reaching the endpoint, leading to erroneous results. This method needs, therefore, users with experience and special equipment (6).
Importance of Evaluation of the Titration Method
The composition of the sample is crucial for the choice of the cyanide monitoring method. Even though silver nitrate titration may deliver well in known sample composition and suitable equipment, correct industrial settings may not be guaranteed. Using titration with a complex sample may lead to interference, wrong estimation of cyanide concentrations, redundant treatments, and unaware disposal of cyanide to surface waters. Complex matrices with known or unknown interfering agents are treated in industrial environments, and therefore, special care is required (6).
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3. Nava- Alonso F, Alonso-Gonzalez O. Thermodynamic analysis of free cyanide determination by silver nitrate titration in copper bearing solutions. Canadian Metallurgical Quarterly.
4. Pohlandt C, Jones E, Lee A. A critical evaluation of methods applicable to the determination of cyanides. Journal Of the South African Institute Of Mining And Metallurgy. 1983.
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12. CDC | Facts About Cyanide [Internet]. Emergency.cdc.gov. 2021 [cited 18 July 2021]. Available from: https://emergency.cdc.gov/agent/cyanide/basics/facts.asp