Cyanide ions precipitation with bromide

In Example Problem 12.2, we saw that cyanide ions precipitate with silver. Bromide ions show similar behavior. The two pertinent equilibria are... 

Detection of halide ions. The alkaline solution from the sodium fusion is first treated with dilute nitric acid and boiled in the hood to remove cyanide and sulfide ions that may be present. These ions form insoluble precipitates with silver ion and interfere with the detection of halide ions. On boiling with dilute nitric acid they are volatilized as hydrogen cyanide and hydrogen sulfide hence care should be exercised, and the hood must be used in heating the test solution. The addition of silver ion forms a precipitate of silver halide which varies in color from yellow (iodide) to white (chloride and bromide). If it is necessary to identify the halide present, a portion of fresh solution is acidified with a few drops of dilute. sulfuric acid and a thin layer of chloroform (2-3 ml) is added, followed by a drop of freshly prepared chlorine water. On shaking, the chloroform layer becomes colorless if the ion is chloride, brown if it is a bromide, and violet if it is iodide. 

Sodium chromate can serve as an indicator for the argentometric determination of chloride, bromide, and cyanide ions by reacting with silver ion to form a brick-red silver chromate (Ag2Cr04) precipitate in the equivalence-point region. The silver ion concentration at chemical equivalence in the titration of chloride with silver ions is given by... 

The potentiometric detection of the endpoint of precipitation titrations is very often used because not many visual indicators are available, in particular when mixtures of analytes are titrated. Halides, cyanide, sulfide, chromate, mercaptans, and thiols can be titrated with silver nitrate, using the silver sulfide-based ISE. Also complex mixtures, such as sulfide, thiocyanide, and chloride ions, or chloride, bromide, and iodide ions, can be titrated potentio-metrically with silver(I) ions. When the solubility of a compound formed during titration is too high, nonaqueous or mixed solvents are used, for example,... 

Cyanide and sulfide ions interfere with the test for halides. If such ions are present, they must be removed. To accomplish this, acidify 2 mL of the stock solution prepared above with dilute nitric acid and boil it for about 2 minutes. This will drive off any HCN or H2S that is formed. When the solution cools, add a few drops of a 5% silver nitrate solution. A volumitwus precipitate indicates a halide. A faint turbidity does not mean a positive test. Silver chloride is white. Silver bromide is off-white. Silver iodide is yellow. Silver chloride will readily dissolve in concentrated ammonium hydroxide, whereas silver bromide is only slightly soluble. 

This exercise is particularly interesting because it permits us to specify the conditions that must be respected in order to carry out the titration of chloride, bromide, thiocyanate, and cyanide ions with a mercuric salt according to Votocek-Dubsky s method (see Chap. 27). The titrant solution is a mercuric nitrate (or sulfate) solution in which the precipitation of HgO resulting from the hydrolysis of Hg2+ is precluded by addition of nitric acid. 

Recall that silver chloride is soluble in an excess of chloride ions by the formation of the ion complex chlorosilver(I) (see Chap. 25). Bromide ions give a clear yellow color of silver bromide that dissolves in the same reagents as the chloride ion but with more difficulty. Iodide ions give a silver iodide precipitate insoluble in ammonia but soluble in potassium cyanide and sodium thiosulfate. From another standpoint, Ag+ ions give silver dithizonate with dithizone. The... 

Ammonia possesses similar physical properties to that of water, which is similarly highly associated. It is a good solvent for many compounds. Owing to the lower dielectric constant (NH3 16.9, H2O 78.3 at 298 K) of ammonia in comparison with water, less polar compounds are more soluble in ammonia and polar compounds, for example, salts, are more soluble in water. Organic compounds tend to have a higher solubihty in ammonia than in water. Armnonium salts, nitrates, nitrites, cyanides, and thiocyanates dissolve readily in ammonia. The solubihty increases from fluorides to chlorides, bromides, and iodides. Salts with higher charged ions dissolve only poorly in ammonia. This results in the reversal of some precipitation reactions in ammonia compared to water. 

Sensitive extraction-spectrophotometric methods are based on the extractable (into CHCI3, 1,2-diehloroethane, benzene, or toluene) ion-associates of basic dyes and anionic Ag complexes with cyanide [35,36], iodide [37,38], and bromide [39]. In these methods, use has been made of such dyes as Crystal Violet [35,39], Brilliant Green [38,39], Malachite Green [39], Methylene Blue [36], and Nile Blue A [37]. In some of these methods the molar absorptivities are elose to MO [36,39]. A flotation method has been proposed, based on the addition compound [R6G ][Ag(SCN )2] [R6G ][SCN ] which is formed by silver ions (at pH 2-5) in the presence of thiocyanate and Rhodamine 6G (flotation with DIPE, the precipitated compound is washed and dissolved in acetone, e = 1.5-10 ) [40]. The complex Ag(CN)2 , associated with Crystal Violet, has been utilized in another flotation-spectrophotometric method of determining silver [41]. Silver has been determined also in a system comprising thiocyanate and Rhodamine B, as an aqueous pseudo-solution, in the presence of poly(vinyl alcohol) [42]. 

The application of direct potentiometry with silver bromide precipitate-based ion-selective electrodes for bromide measurements in water samples as been investigated [65]. Cyanide, sulfide, and iodide ions represent the major interferences. A 20 times higher concentration of chloride also can cause positive error. TTierefore, the applicability of direct potentiometry in the analysis of bromide concentration of water samples is limited. 

Colloidal Silver Halides. — Hydrosols of silver chloride, bromide, and iodide were prepared by Lottermoser and E. v. Meyer 1[ by treating colloidal silver with the corresponding halogen. They were very sensitive to electrolytes. Another theoretically interesting method has also been worked out by Lottermoser. 11 It is based on the effect of silver nitrate on the silver halides. By this method have been prepared colloidal solutions of silver chloride, bromide, iodide, cyanide, ferro and ferricyanides, phosphate and arsenate. The reaction is particularly interesting because it can be followed by measurement. It may be carried out in two ways. Either a measured amount of silver nitrate is treated Avith dilute solutions of the alkali halides, or silver nitrate from a buret is added to a known amount of the halide. In the first there must be an excess of the silver ion and in the second an excess of the halide ion in order to obtain a hydrosol. If too much of the lesser constituent is added precipitation results. 

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