Bismuth/ions/salts

Bismuth. 25.0 mL bismuth ion solution + solid hexamine to pH about 4.6 the precipitate of basic bismuth salt dissolves as the EDTA solution is added but the titration is slow. 

Bismuth forms tervalent and pentavalent ions. Tervalent bismuth ion Bi3+ is the most common. The hydroxide, Bi(OH)3 is a weak base bismuth salts therefore hydrolyse readily, when the following process occurs ... 

The bismuthyl ion, BiO+ forms insoluble salts, like bismuthyl chloride, BiOCl, with most ions. If we want to keep bismuth ions in solution, we must acidify the solution, when the above equilibrium shifts towards the left. 

In very acidic solutions, bismuth(III) exists in the form of the nonaaquo ion [Bi(H20)9] +, which is similar to the aquo complexes of the lanthanide ions, but partial hydrolysis of bismuth(III) salts leads to the formation of bismuth oxo clusters. The core structure of these complexes is often based upon a Bie octahedral core with oxide, hydroxide, or alkoxide functions bridging the edges and/or faces of the octahedron. The [Bi6(OH)i2] + ion (11) has been studied spectroscopically. In oxo clusters, the octahedron is face-bridged by eight oxo or alkoxide functions (12). Such core structures have been found in the hydrolysis of bismuth nitrate or perchlorate. ... 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Bismuth trioxide is practically insoluble in water it is definitely a basic oxide and hence dissolves in acids to form salts. Acidic properties are just barely detectable, eg, its solubiUty slightly increases with increasing base concentration, presumably because of the formation of bismuthate(III) ions, such as Bi(OH) g and related species. 

Cations forming insoluble chromates, such as those of silver, barium, mercury (I), mercury(II), and bismuth, do not interfere because the acidity is sufficiently high to prevent their precipitation. Bromide ion from the generation may be expected to form insoluble silver bromide, and so it is preferable to separate silver prior to the precipitation. Ammonium salts interfere, owing to competitive oxidation by bromate, and should be removed by treatment with sodium hydroxide. 

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. 

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

The amount of precipitated bismuth decreased as the concentration of bismuth salt increased (Table 9.16) and the duration of sonication required to bring about hydrolysis also increased. The initial reaction was spontaneous as per Eq. (9.111), which, however, seemed to be facilitated by ultrasonic cavitation at high concentration of bismuth. Since the H+ ions were also produced during the formation of bismuthyl ion, at the point where the sum of concentration of H+ ions present initially and formed by Eq. (9.110) was equal to the concentration required to shift the equilibrium of Eq. (9.111) towards left side, the hydrolysis did not occur even after sonication. 

Two salts involving bismuth complexes as both ions have been structurally characterized (116). [Bi2Cl4(tu)6][BiCl5(tu)] consists of a binuclear centrosymmetric dication (117), and bismuth adopts a six-... 

Quaternary bismuth compounds are generally unstable. When the anionic ligand is chloride or bromide, the compounds decompose spontaneously on standing azides and selenocyanates decompose more rapidly. The perchlorates, tetrafluoroborates, and hexafluorophophates, however, are considerably more stable but eventually decompose. The vibrational spectra of the latter compounds show the presence of the free ion and are consistent with a tetrahedral BiC4 skeleton for the cation. The acetonyltriphenyl compounds, [(C(5H5)3BiCH2COCH3]Y, where Y is C10- 4 or BF- 4, also appear to be true bismuthonium salts. 

Many other powerful oxidants are used in redox titrations. Often a metal ion may be present in more than one oxidation state which must be oxidized or reduced into the desired oxidation state. For example, a salt solution of iron may contain both Fe2+ and Fe3+ ions. Peroxydisulfates, bismuthates, and peroxides are often used as auxiliary oxidizing reagents to convert the ion of interest into the higher oxidation state. The half-reactions for these oxidants are as follows ... 

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