Anions in concentrated acids

This is just one example how a method may be developed. There are other examples. The best method determination of trace anions in concentrated acids either requires removing the matrix anion, selective detection, or choosing a column that has sufficient capacity and selectivity to allow the matrix to travel quickly through the... 

A. Siriraks, CA. Pohl and M. Toofan, Determination of trace anions in concentrated acids by means of a moderate capacity anion-exchange column,... 

Figure 10.142 Valve switching for the trace analysis of anions in concentrated acids of low acid strength, (a) Loading the sample loop, (b) preseparation on an ion-exdusion column, (c) preconcentration of the heartcut fraction, and (d) separation of the preconcentrated ions.

Kaiser, E., Rohrer, J., and Ausserer, W. (1999) Determination of trace anions in concentrated weak acids by ion chromatography. Presentation No. 962, Pittsburg Conference, Orlando. 

Dionex Corporation (2002) Dionex Technical Note No. 44 The Determination of Trace Anions in Concentrated Phosphoric Acid. Dionex Corporation (now part of Thermo Fisher Scientific), Sunnyvale, CA, USA. 

E. Kaiser, J. Rohrer and W. Ausserer, Determination of Trace Anions in Concentrated Weak Acids by Ion Chromatography , Presentation No. 952, Pittsburg Conference, Orlando 1999. 

The acid error is different from the alkaline error in that it changes very little with temperature and is a negative error where a greater number of millivolts per pH unit is observed. With high acid activity, the water activity is reduced and some reduction in the hydrated layer occurs. The magnitude of this error may vary between 0.1 pH in a 2 M acid solution to more than 8 pH units in concentrated acids. The magnitude of this error depends on pH, temperature, exposure time, and size of the anions in the test solution. 

The surface tensions of some perfluorinated anionic surfactants with a CsFi7— hydrophobe in strong acids are given in Table 4.9 [56]. The surfactant CgFivSOsK is very effective in concentrated acids, although its solubility in 37% hydrochloric acid is limited to 0.2 g/L. 

In concentrated hydrochloric acid solution, the reaction is GeCl -p Cr [GeClj]-and salts of this anion are known. 

Therefore the extent of extraction or back-extraction is governed by the concentration of X ia the aqueous phase, the distribution coefficients, and selectivities depending on the anion. In nitrate solutions, the distribution coefficient decreases as the atomic number of the REE increases, whereas ia thiocyanate solutions, the distribution coefficient roughly increases as the atomic number of the REE increases. The position of yttrium in the lanthanide series is not the same in nitrate and thiocyanate solutions, and this phenomenon has been used for high purity yttrium manufacture in the past. A combination of extraction by carboxyUc acids then by ammonium salts is also utilized for production of high purity yttrium. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

The increased acidity of the larger polymers most likely leads to this reduction in metal ion activity through easier development of active bonding sites in siUcate polymers. Thus, it could be expected that interaction constants between metal ions and polymer sdanol sites vary as a function of time and the sihcate polymer size. The interaction of cations with a siUcate anion leads to a reduction in pH. This produces larger siUcate anions, which in turn increases the complexation of metal ions. Therefore, the metal ion distribution in an amorphous metal sihcate particle is expected to be nonhomogeneous. It is not known whether this occurs, but it is clear that metal ions and siUcates react in a complex process that is comparable to metal ion hydrolysis. The products of the reactions of soluble siUcates with metal salts in concentrated solutions at ambient temperature are considered to be complex mixtures of metal ions and/or metal hydroxides, coagulated coUoidal size siUca species, and siUca gels. 

Stannous Sulfate. Stannous sulfate (tin(Il) sulfate), mol wt 214.75, SnSO, is a white crystalline powder which decomposes above 360°C. Because of internal redox reactions and a residue of acid moisture, the commercial product tends to discolor and degrade at ca 60°C. It is soluble in concentrated sulfuric acid and in water (330 g/L at 25°C). The solubihty in sulfuric acid solutions decreases as the concentration of free sulfuric acid increases. Stannous sulfate can be prepared from the reaction of excess sulfuric acid (specific gravity 1.53) and granulated tin for several days at 100°C until the reaction has ceased. Stannous sulfate is extracted with water and the aqueous solution evaporates in vacuo. Methanol is used to remove excess acid. It is also prepared by reaction of stannous oxide and sulfuric acid and by the direct electrolysis of high grade tin metal in sulfuric acid solutions of moderate strength in cells with anion-exchange membranes (36). 

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