Dissolution in EDTA

Onken and Matheson (1982) studied kinetics of phosphorus dissolution in EDTA (ethylenediamine tetraacetic acid) solution for several soils. They examined eight kinetic models (Table 2.2) and found that phosphorus dissolution in EDTA solution was best described using the two-constant rate, Elovich, and differential rate equations as indicated by high r2 and low SE values. None of the models best described the dissolution for all soils. 

TABLE 2.2 Summary of r2 and SE of Eight Kinetic Models for Phosphorus Dissolution in EDTA Solution from Six Test Locations Varying in Plant Response to Applied Phosphorus"... 

As the result of the performed investigations was offered to make direct photometric determination of Nd microgram quantities in the presence of 500-fold and 1100-fold quantities of Mo and Pb correspondingly. The rare earth determination procedure involves sample dissolution in HCI, molybdenum reduction to Mo (V) by hydrazine and lead and Mo (V) masking by EDTA. The maximal colour development of Nd-arsenazo III complex was obtained at pH 2,7-2,8. The optimal condition of Nd determination that was established permit to estimate Nd without separation in solution after sample decomposition. Relative standard deviations at determination of 5-20 p.g of Nd from 0,1 g PbMoO are 0,1-0,03. The received data allow to use the offered procedure for solving of wide circle of analytical problems. 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

Can also be determined by precipitation as BaS04 and dissolution in excess EDTA (Section 10.73). 

Sulphate can also be determined by an exactly similar procedure by precipitation as lead sulphate from a solution containing 50 per cent (by volume) of propan-2-ol (to reduce the solubility of the lead sulphate), separation of the precipitate, dissolution in excess of standard EDTA solution, and back-titration of the excess EDTA with a standard zinc solution using solochrome black as indicator. 

Fig. 12.28 Dissolution-time curves for magnetite, maghemite and hematite in EDTA at 30 °C and pH 3 in the presence or absence of Fe and UV light ( = 254 nm) (Litter Blesa, 1992, with permission).

Rueda, E.H. Grassi, R.L. Blesa, M. (1985) Adsorption and dissolution in the system goethite/aqueous EDTA. J. Colloid Interface Sci. 106 243-246... 

Chelators such as EDTA, nitrilotriacetic acid (NTA), 1,2-aminocyclohexane 7V,7V,7V ,N7-tetraacetic (DCyTA), and ethylene glycol-bis(2-aminoethyl)-(V,(V, 7V ,7V -tetraacetic acid (EGTA) have been studied extensively and are well summarized (Peters, 1999). Chelator concentration and reaction pH influence metal complexation and the success of removal from soils. Sun et al. (2001) observed that batch extraction methods result in 1 1 molar extraction ratios of EDTA/metal (Pb, Cd, Zn, Cu) and reveal which metal is more or less soluble in EDTA solutions. Column leaching studies, however, relate the elution patterns and recalcitrance of the metals to desorption and dissolution by EDTA. There is concern over the detrimental effects on soil quality from using chelators because of their biotoxicity, persistence in soil environment, and their removal of beneficial micro-and macronutrients, which leave the washed soil infertile for revegetation when it is backfilled. 

Among the multi-class methods reported in the literature, a procedure that involves sample dissolution with EDTA under mildly acidic conditions (pH 4.0) followed by SPE with Oasis HLB cartridges has been applied for the simultaneous analysis of macroiides, tetracyclines, quinolones, and sulfonamides in honey samples Separation and determination by UPLC-MS/MS enabled the analysis of 17 compounds in <5 min. Mean recoveries ranged from 70% to 120%, except for three compounds (doxycycline, erythromycin, and tilmicosin), which had recoveries of >50%. Application of the method to the analysis of honey samples obtained from different beekeepers and local supermarkets... 

Clearly some form of sample pretreatment is required for soils and sediments. Total levels may be obtained following sodium carbonate-boric acid fusion and the dissolution in hydrochloric acid employing lanthanum as a buffer and releasing agent. If the determination of silicon is not required, it may be volatilized as silicon tetrafluoride using hydrofluoric acid, although some calcium may also be lost as calcium fluoride. For many samples, however, it may be more appropriate to determine the exchangeable cation content of the sample. Here, the sample may be shaken with an extractant solution, for example, 1 mol 1 ammonium chloride, ammonium acetate, or disodium EDTA, prior to filtration and analysis. Where final solutions contain more than - 0.5% of dissolved material, the standards should also contain the major constituents, even where no chemical interference is expected, in order to match the viscosity and surface tension and avoid matrix effects. 

Figure 9.19 Normalized integral passive dissolution kinetics for titanium thin films immersed in EDTA/SIE (simulated interstitial electrolyte) (a) real time data empirically fitted with two-phase logarithmic law relationship (b) a semiloga-rithmic plot of the data demonstrating the two-phase logarithm relationship. The correlation coefficient for the least-squares fit of the linear functions are given. (Healy and Ducheyne, 1992.)...

Shoesmith et al. has made an extensive study of oxide-covered iron electrodes in EDTA and citric acid solutions. Three distinct potential regions were observed. In Region I ( > -100 mV vs. SCE), little Fe + was released, and there was only minor oxide dissolution. This is considered the induction period for pore formation. In Region II (-450 mV < E < -100 mV), potential values were between those of magnetite reduction (reductive dissolution) and metal dissolution, suggesting that autoreduction. 

Other work on the mechanism of magnetite dissolution in chelants was recently reported. Hausler looked at magnetite dissolution in ammonium EDTA at pH values from 4.2 to 7.0, and found that hydrazine accelerated the dissolution rate at a pH of 7 but not at 4.2. Instead of invoking the accepted reductive dissolution mechanism, he proposed an unusual N2H4-Fe(lil)EDTA complex to explain his results. 

Rubio and Matijevic looked at the dissolution of FeOOH in EDTA. The maximum rate occurred at a pH of 9.7. This was interpreted to mean that dissolution of FeOOH involves adsorption of EDTA and stabilization of the ferric EDTA species. Thus, different oxidation states of iron in the scale may lead to completely different dissolution mechanisms. 

The dissolution of magnetite in EDTA was described by Blesa and Maroto, whose pH vs. reaction rate data were significantly different from those obtained by Chang and Matijevic for hematite. Both authors found decreased dissolution at a very low pH (4.5) compared with moderate pH values (6) however, Blesa and Maroto found even lower rates ata pH of 8, while Chang and Matijevic found just the opposite effect. Their studies were conducted at 77°F (25°C). As the temperature was raised, the chelants in both cases became much more effective at lower rather than at high pH values. Blesa and Maroto claimed that the adsorbed EDTA blocks the surface and thus retards dissolution. As the temperature is raised, this blocking action presumably becomes less effective, and the rate of oxide bond scission exceeds that of chelant/oxide formation. 

The data from these three experiments indicate that reductive dissolution occurs in ammonium EDTA solutions. Although some magnetite may directly dissolve, releasing ferric ions that subsequently reduce on the bare steel, reduction of ferric iron in the scale by base metal or N2H4 appears to predominate. The accelerative effect of reductive dissolution in acidic solution is frequently attributed to potential changes... 

Rate Constants for Iron Oxide Dissolution in Na and NH EDTA... 

The effect of temperature change on the rate of magnetic dissolution in (NH4)4 EDTA is shown in Figure 5 and Table 8. While the data are insufficient to calculate an activation energy with precision, the temperature dependence is definitely small ( 15 kcal/mol) in the range tested. This small number suggests a diffusion limited, rate-controlling step. 

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