Cyanide ions precipitation with silver

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. 

In the former, it gives precipitates with halides (except the fluoride), cyanides, thiocyanates, chromates(VI), phosphate(V), and most ions of organic acids. The silver salts of organic acids are obtained as white precipitates on adding silver nitrate to a neutral solution of the acid. These silver salts on ignition leave silver. When this reaction is carried out quantitatively, it provides a means of determining the basicity of the acid... 

Sulphur, as sulphide ion, is detected by precipitation as black lead sulphide with lead acetate solution and acetic acid or with sodium plumbite solution (an alkaLine solution of lead acetate). Halogens are detected as the characteristic silver halides by the addition of silver nitrate solution and dilute nitric acid the interfering influence of sulphide and cyanide ions in the latter tests are discussed under the individual elements. 

Silver Chloride. Silver chloride, AgCl, is a white precipitate that forms when chloride ion is added to a silver nitrate solution. The order of solubility of the three silver halides is Cl" > Br" > I. Because of the formation of complexes, silver chloride is soluble in solutions containing excess chloride and in solutions of cyanide, thiosulfate, and ammonia. Silver chloride is insoluble in nitric and dilute sulfuric acid. Treatment with concentrated sulfuric acid gives silver sulfate. 

This complex ion dissociates to give silver ions, since the addition of sulphide ions yields a precipitate of silver sulphide (solubility product 1.6 x 10 49 mol3 L 3), and also silver is deposited from the complex cyanide solution upon electrolysis. The complex ion thus dissociates in accordance with the equation ... 

A simple example of the application of a complexation reaction to a titration procedure is the titration of cyanides with silver nitrate solution. When a solution of silver nitrate is added to a solution containing cyanide ions (e.g. an alkali cyanide) a white precipitate is formed when the two liquids first come into contact with one another, but on stirring it re-dissolves owing to the formation of a stable complex cyanide, the alkali salt of which is soluble ... 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

Determination of silver as chloride Discussion. The theory of the process is given under Chloride (Section 11.57). Lead, copper(I), palladium)II), mercury)I), and thallium)I) ions interfere, as do cyanides and thiosulphates. If a mercury(I) [or copper(I) or thallium(I)] salt is present, it must be oxidised with concentrated nitric acid before the precipitation of silver this process also destroys cyanides and thiosulphates. If lead is present, the solution must be diluted so that it contains not more than 0.25 g of the substance in 200 mL, and the hydrochloric acid must be added very slowly. Compounds of bismuth and antimony that hydrolyse in the dilute acid medium used for the complete precipitation of silver must be absent. For possible errors in the weight of silver chloride due to the action of light, see Section 11.57. 

A familiar example is the reaction of dilute ammonium hydroxide with silver ions to give Ag(NH3)2+, which reduces the concentration of free Ag+ to a value below that needed to exceed the solubility product of AgCl (but not that of AgBr or Agl). If ammonia is replaced by cyanide ions, which form more stable complexes, only the more insoluble Agl can be precipitated. Hence by choice of the masking agent used a cation can be selectively masked towards some reagents but not others. 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

The potential of a silver electrode during the course of the titration of silver nitrate with potassium c an-ide is shown in Fig. 78 the first marked change of potential occurs when one equivalent of cyanide has been added to one of silver, so that the whole of the silver cyanide is precipitated, and the second, when two equivalents of cyanide have been added, corresponds to the complete formation of the Ag(CN)2 ion. It will be seen that the changes of potential occur very sharply in each case this means that the silver cyanide is very slightly soluble and that, t+vo complex ion is very stable. 

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... 

For the determination of higher quantities of cyanides (2-40 mg 1 ) titration against silver nitrate can be used. After transforming all cyanides into the complex [Ag(CN)2] the excess of silver ions is indicated by p-dimethylbenzilidene rhodanine which reacts with silver to form a red precipitate [14]. 

Given the vride variety of ion-selective electrodes already commercially available and the many more specialized ones that can be fabricated, titrations involving the precipitation or complexation of ions are widely used. Halides, cyanide, thiocyanate, sulfide, chromate, and thiols can be titrated with silver nitrate, using the appropriate... 

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,... 

Earlier we pointed out that when cyanide ions encounter silver ions, a precipitate forms. There are relatively few cations that precipitate cyanide, but other anions such as phosphate or carbonate precipitate readily with a wide range of cations. Eor the engineers and chemists who design industrial processes, solubility often is vital in both the isolation of desired products and the treatment of waste streams. Because industrial processes are often carried out at high temperatures, the characteristics of solubility must be viewed within the context of the equilibrium (and the equilibrium constant) of the reactions at those temperatures. 

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. 

The presence of halide is determined by first acidifying a portion of the FAQS with dilute nitric acid and boiling the solution in the hood to remove any sulfide or cyanide ions as hydrogen sulfide or hydrogen cyanide, respectively. This is necessary because sulfide and cyanide interfere with the test for halogens. Silver nitrate solution is then added, and the formation of a precipitate of silver halide signals the presence of halide in the FAQS (Eq. 25.1). 

Some of these complexes are very stable —the stability of the argento-cyanide ion, Ag(CN)2, for example, is so great that addition of iodide ion does not cause silver iodide to precipitate, even though the solubility product of silver iodide is very small. The ferrocyanide ion, Fe(CN)e, ferricyanide ion, Fe(CN)e, and cobalticyanide ion, Co(CN)e—, are so stable that they are not appreciably decomposed by strong acid. The others are decomposed by strong acid, with the formation of hydrocyanic acid, HCN. 

The familiar test for Cl ions through precipitation of silver chloride on the addition of nitric acid to the ammoniacal solution cannot be used in the presence of IO3- ions because the latter behave like Cl ions under these circumstances. However, a reliable differentiation is based on the finding that a solution of mercuric cyanide that has been acidified with sulfuric acid (or some other oxyinorganic acid) gives hydrogen cyanide when alkali chloride is present, whereas alkali iodate has no such effect. The hydrogen cyanide is volatile and readily detected. 

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. 

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... 

Iodide ions reduce Cu to Cu , and attempts to prepare copper(ll) iodide therefore result in the formation of Cul. (In a quite analogous way attempts to prepare copper(ll) cyanide yield CuCN instead.) In fact it is the electronegative fluorine which fails to form a salt with copper(l), the other 3 halides being white insoluble compounds precipitated from aqueous solutions by the reduction of the Cu halide. By contrast, silver(l) provides (for the only time in this triad) 4 well-characterized halides. All except Agl have the rock-salt structure (p. 242). Increasing covalency from chloride to iodide is reflected in the deepening colour white yellow, as the... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

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