Controlling Cyanide

Situation and Criteria A method was to be developed to determine trace amounts of cyanide (CN ) in waste water. The nature of the task means precision is not so much of an issue as are the limits of detection and quantitation (LOD, LOQ), and flexibility and ease of use. The responsible chemist expected cyanide levels below 2 ppm. 

Experimental A photometric method was found in the literature which seemed to suit the particular circumstances. Two cyanide stock solutions were prepared, and an electromechanical dispenser was used to precisely prepare solutions of 20, 40,. .., 240 respectively 10, 30, 50,. .., 250 fig CN /100 ml. 10 ml of each calibration solution were added to 90 ml of the color-forming reagent solution and the absorbance was measured using 1-cm cuvettes. (See Table 4.17 (left and middle panels) and data file CYANIDE.dat.) 

Absorb. No. Cone. Absorb. No. Cone. Absorb.  

Legend No number of measurement. Cone concentration in fig, CN /100 ml Absorb absorbance [AU] slope slope of regression line t CV intercept see slope res. std. dev. residual standard deviation Srts -n number of points in regression LOD limit of detection LOQ limit of quantitation measurements using a 2-fold higher sample amount and 5-cm cuvettes—i.e., measured absorption 0. .. 0.501 was divided by 10. 

Data Analysis The results were plotted at first glance a linear regression of absorbance versus concentration appeared appropriate. The two dilution series individually yielded the figures of merit given in Table 4.17, bottom. The two regression lines are indistinguishable, have tightly defined slopes. 

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Centers for Disease Control, Cyanide poisonings associated with over-tbe-counter medication— Washington Sate, 1991, JAMA, 265, 1806-1807, 1991. 

Van Weert, G., and Choi, Y., Silver impregnated activated carbon A new tool to control cyanide in effluents, Chem. Eng. Commun., 134, 87-92 (1995). 

While the thermodynamically more stable c/s-cyano ketone was obtained under aprotic conditions (Et2AICN-CgHg), the desired trans isomer was virtually the sole product of kinetically controlled cyanide attack in the protic medium (Et3AI-HCN-THF). 

Hat No. Cyanide, concentration, M Control, cyanide absent Cyanide present ... 

Other salts include lead arsenates and lead arsenites (see Insect control technology), lead chromates and lead sihcochromates (see Pigments), lead cyanide (see Cyanides), lead 2-ethyIhexanoate (see Driers and metallic soaps), and lead fluoroborate (see Fluorine compounds, inorganic). 

Electroplating. Most silver-plating baths employ alkaline solutions of silver cyanide. The silver cyanide complexes that are obtained in a very low concentration of free silver ion in solution produce a much firmer deposit of silver during electroplating than solutions that contain higher concentrations. An excess of cyanide beyond that needed to form the Ag(CN)2 complex is employed to control the concentration. The silver is added to the solution either directly as silver cyanide or by oxidation of a silver-rod electrode. Plating baths frequently contain 40—140 g/L of silver cyanide... 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

Electroplating. Aluminum can be electroplated by the electrolytic reduction of cryoHte, which is trisodium aluminum hexafluoride [13775-53-6] Na AlE, containing alumina. Brass (see COPPERALLOYS) can be electroplated from aqueous cyanide solutions which contain cyano complexes of zinc(II) and copper(I). The soft CN stabilizes the copper as copper(I) and the two cyano complexes have comparable potentials. Without CN the potentials of aqueous zinc(II) and copper(I), as weU as those of zinc(II) and copper(II), are over one volt apart thus only the copper plates out. Careful control of concentration and pH also enables brass to be deposited from solutions of citrate and tartrate. The noble metals are often plated from solutions in which coordination compounds help provide fine, even deposits (see Electroplating). 

The pH of the pulp to the flotation cells is carefliUy controlled by the addition of lime, which optimizes the action of all reagents and is used to depress pyrite. A frother, such as pine oil or a long-chain alcohol, is added to produce the froth, an important part of the flotation process. The ore minerals, coated with an oily collected layer, are hydrophobic and collect on the air bubbles the desired minerals float while the gangue sinks. Typical collectors are xanthates, dithiophosphates, or xanthate derivatives, whereas typical depressants are calcium or sodium cyanide [143-33-9] NaCN, andlime. 

Recovery of hydrogen cyanide from coke-oven gases has been dormant in the early 1990s, but new methods involving environmental control of off-gas pollutants may be lea ding the way for a modest return to the recovery of cyanide from coke-oven gases (see Coal conversion process, carbonization). 

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