Precipitation complexing agent

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. 

The formation of these additional precipitates can usually be minimized by carefully controlling solution conditions. Interferents forming precipitates that are less soluble than the analyte may be precipitated and removed by filtration, leaving the analyte behind in solution. Alternatively, either the analyte or the interferent can be masked using a suitable complexing agent, preventing its precipitation. 

Silver Bromide. Silver bromide, AgBr, is formed by the addition of bromide ions to an aqueous solution of silver nitrate. The light yellow to green-yeUow precipitate is less soluble in ammonia than silver chloride, but it easily dissolves in the presence of other complexing agents, such as thiosulfate ions. 

In Ni—P electroless deposits, there can be as much as 10% by weight of phosphoms. The amount depends on the added complexing agents and the pH. The Ni—P deposits are fine-grained supersaturated soHd solutions, which may be precipitation hardened by heat treatment to form dispersed Ni P particles in a nickel matrix. 

Inorganic heavy metals are usually removed from aqueous waste streams by chemical precipitation in various forms (carbonates, hydroxides, sulfide) at different pH values. The solubiUty curves for various metal hydroxides, when they are present alone, are shown in Figure 7. The presence of other metals and complexing agents (ammonia, citric acid, EDTA, etc) strongly affects these solubiUty curves and requires careful evaluation to determine the residual concentration values after treatment (see Table 9) (38,39). 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

A similar procedure may also be used for the determination of antimony(V), whilst antimony (III) may be determined like arsenic(III) by direct titration with standard iodine solution (Section 10.113), but in the antimony titration it is necessary to include some tartaric acid in the solution this acts as complexing agent and prevents precipitation of antimony as hydroxide or as basic salt in alkaline solution. On the whole, however, the most satisfactory method for determining antimony is by titration with potassium bromate (Section 10.133). 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

In the presence of a metal ion (M " ), a metal chalcogenide phase M2Sen will be precipitated upon exceeding the solubility product of [M and [Se ] (or [HSe ]). The concentration of free metal ions must be controlled by an excess of complexing agent, determining the applicable solubility of the metal and the overall competitive chemical reaction, in order to prevent the formation of sulfite, sulfate, and... 

Precipitation of Fe(IIl) compounds from acid solutions as the pH increases above 2.2 is a particular problem. Complexing agents that have been used include 5-sulfosalicylic acid and citric acid (136) dihydroxymaleic acid (137) ethylenediaminetetraacetic acid (138) lactic acid (138) blends of hydroxylamine hydrochloride, citric acid, and glucono-delta-lactone (139) nitriloacetic acid blends of citric acid and acetic acid lactic acid and gluconic acid (140). 

Successful methods entail precipitation of the amylose from solution as an insoluble complex, which is removed by high-speed centrifuging the amylopectin is isolated from the supernatant liquor by precipitation with alcohol or, more satisfactorily, by freeze-drying. (Precipitation with alcohol does not always appear to be quantitative, and the physical form of the product obtained by freeze-drying is more satisfactory for subsequent dissolution and esterification.) The amylose can then be further purified by reprecipitation with the same or, preferably, a different complexing agent. 

The most widely used complexing agents are alcohols (butanol, n-propyl alcohol and n-pentyl alcohol1). Schoch33 now recommends the use of Pen-tasol, a commercial mixture of pentyl alcohols, for the first precipitation, and 1-butanol for recrystallizations. For com (maize) starch, this avoids contamination of the amylopectin with an intermediate fraction which is sufficiently linear to be precipitated with Pentasol and yet has a degree of branching which prevents complex formation with butanol. 

The category of builders consists predominantly of several types of materials -specific precipitating alkaline materials such as sodium carbonate and sodium silicate complexing agents like sodium triphosphate or nitrilotriacetic acid (NTA) and ion exchangers, such as water-soluble polycarboxylic acids and zeolites (e.g., zeolite A). 

The electrolyte effect for the adsorption of anionic surfactants which leads to an enhancement of soil removal is valid only for low water hardness, i.e. low concentrations of calcium ions. High concentrations of calcium ions can lead to a precipitation of calcium surfactant salts and reduce the concentration of active molecules. Therefore, for many anionic surfactants the washing performance decreases with lower temperatures in the presence of calcium ions. This effect can be compensated by the addition of complexing agents or ion exchangers. 

Definitions. Titrimetric Reactions. Acid-base Titrations. Applications of Acid-base Titrations. Redox Titrations. Applications of Redox Titrations. Complexometric Titrations. Ethylenediaminetetraacetic Acid (EDTA). Applications of EDTA Titrations. Titrations with Complexing Agents Other Than EDTA. Precipitation Titrations. ... 

EDTA titration solutions frequently need to be buffered to a high pH to ensure stoichiometric formation of the complex. Unfortunately many metals will precipitate as hydroxides or hydrated oxides under these conditions. Use can be made of secondary or auxiliary complexing agents to retain the metal ion in solution. Typical reagents for this purpose are... 

Eq. (a) shows that the quaternary salt gets quantitatively precipitated by sodium tetraphenyl boron as the complexing agent. Eq. (b) depicts that quaternary compounds shall readily react with certain anionic dye, such as bromophenol blue, to yield a blue, chloroform-soluble complex. 

The spherical or disk-like particles of hematite dissolved and large, octahedral crystals of magnetite precipitated. The dissolution process involved electron transfer between hydrazine and the Fe of the hematite and was promoted by complexing agents such as TEA. 

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