EDTA titrations amperometric

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

Several analytical methods will differentiate the "free" (hydrated) metal ions from dissolved complexed metal ions. These methods include specific ion electrodes, polarographic, and other amperometric and voltammetric methods and various types of spectroscopy (see Section 7-10). Specific ion electrodes only respond to the free metal ion for which they are "specific." To determine the relative amounts of complexed and uncomplexed metal ion in a solution, we can use a "wet chemical" method to measure the total concentration of "free + complexed" ions, and then an ion-specific electrode to determine the free metal ion concentration (activity). Care must be taken to eliminate interferences that may affect these measurements. We deduce the concentration of the "complexed ions" by the difference between these two measurements. For example, in the EDTA titration method for hardness, free and complexed calcium and magnesium ion s are measured. 

We must also keep in mind that equivalence points of EDTA titrations can be detected by using several instrumental methods. A first method, potentiometry, was just mentioned. There are also other potentiometric methods, based on other principles than the previous one, that may be used. Amperometric and conductometric methods have been proposed equally (see electrochemical methods of analysis). Finally, we ll mention photometric and spectrophotometric indications. 

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. 

Figure 4.64. Flow-injection measurement a) and flow-injection amperometric titrations (/ , c, and d) of Cu(II) with EDTA. Executed at a potential of -300 mV (against a saturated Ag/AgCl reference electrode), the volume of the injected Cu(II) solution was in all cases 150 xL, while the concentrations were (a) 1 x 10" M, (b) 7.5 x 10" M, (c) 5 x 10" A/, and (d) 2.5 x W M. The titrant carrier stream solution in the three experiments was (b) 1 X 10" M EDTA in 0.1 M NH4OH/0.I M NH4NO3 buffer, (c) 2.5 x 10" M EDTA in the same buffer, and (d) 5 x 0 M EDTA. In the manifold (e), G is the gradient tube.

A square-wave amperometric titration has been used for the determination of the total available heavy metals in water samples. This method involves the direct anodic oxidation of mercury in the presence of excess EDTA. The resulting mercury wave is used to detect the endpoint of the amperometric titration by running a polarogram after each successive addition of an aliquot of EDTA. The successful utilization of this method lies in the ability to discriminate between Ca(n) and heavy metals, such as Cu(II) and Zn(II). Thus, in practice, it involves 1 1 dilution of samples with 0.2 mol 1 acetate buffer (pH 4.8), prior to the amperometric titration. At this pH, heavy metals, such as Fe(III), Hg(II), Ni(II), Cu(II), Pb(n), Zn(II), Cd(II), Co(II), and Al(III), are completely ( 99%) converted to EDTA complexes. Furthermore, the presence of Ca(II) does not interfere with the determination of the available heavy metals under these conditions. As little as 1 pmol 1 of available heavy metals has been successfully determined in water samples by this method. 

After converting silica to alpha-12 molybdosilicic acid, the latter can be titrated with ferrous sulfate in the presence of EDTA, tartaric acid, and chloroacetic acid buffer at pH 2.5, The end point is determined amperometrically with two platinum electrodes (332). 

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