EDTA titrations potentiometric

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. 

Variamine blue (C.I. 37255). The end point in an EDTA titration may sometimes be detected by changes in redox potential, and hence by the use of appropriate redox indicators. An excellent example is variamine blue (4-methoxy-4 -aminodiphenylamine), which may be employed in the complexometric titration of iron(III). When a mixture of iron(II) and (III) is titrated with EDTA the latter disappears first. As soon as an amount of the complexing agent equivalent to the concentration of iron(III) has been added, pFe(III) increases abruptly and consequently there is a sudden decrease in the redox potential (compare Section 2.33) the end point can therefore be detected either potentiometrically or with a redox indicator (10.91). The stability constant of the iron(III) complex FeY- (EDTA = Na2H2Y) is about 1025 and that of the iron(II) complex FeY2 - is 1014 approximate calculations show that the change of redox potential is about 600 millivolts at pH = 2 and that this will be almost independent of the concentration of iron(II) present. The jump in redox potential will also be obtained if no iron(II) salt is actually added, since the extremely minute amount of iron(II) necessary is always present in any pure iron(III) salt. 

POTENTIOMETRIC EDTA TITRATIONS WITH THE MERCURY ELECTRODE 15.24... 

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold amalgam electrode. When this electrode dips into a solution containing the analyte together with a small amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by (b) and (c). 

The number of reversible metal-metal ion electrodes is limited so that the accurate direct potentiometric measurement of the activity of a metal ion with an electrode of the same metal usually is not feasible, except perhaps with the Ag/Ag,(OH2)4 system. However, a number of metal ion-metal half-reactions are sufficiently reversible to give a satisfactory potentiometric titration with a precipitation ion or complexing agent. These couples include Cuu(OH2>6+/Cu, Pbn(OH2>4+/Pb, Cdu(OH2)l+/Cd, and Znn(OH2)i+/Zn. However, all these metals can be determined by EDTA titration and the mercury electrode that is described in the preceding section. 

Potentiometric Methods Potential measurements can be used for end point detection in the EDTA titration of those metal ions for which specific ion electrodes are available. Electrodes of this type are described in Section 21D-1. In addition, a mercury electrode can be made sensitive to EDTA ions and used in titrations with this reagent. 

Both metallic and membrane electrodes have been used to detect end points in potentiometric titrations involving complex formation. Mercury electrodes are useful for EDTA titrations of cations that form complexes that are less stable than HgY- . See Section 21D-1 for the half-reactions involved and Equation 21-5 for the Nernst expression describing the behavior of the electrode. Hanging mercury drop and thin mercury film electrodes appropriate for EDTA titrations are available from a number of manufacturers. As always, whenever mercury is used in experiments like these, we must take every precaution to avoid spilling it, and it must be stored in a well-ventilated hood or a special cabinet to remove the toxic vapors of the liquid metal. Before working with mercury, be sure to read its Materials Safety Data Sheet (MSDS), and follow all appropriate safety procedures. 

Another advantage of potentiometric titrations is that substances to which the electrode does not respond can be determined, if the electrode responds to the titrant or to some low level of an indicator substance that has been added to the solution. For example, low levels of Al can be determined by titration with standard fluoride solution, using a fluoride electrode [22]. EDTA and other chelates can be determined by titration with standard calcium or copper solution. Manganese(II), vanadium(II), or cobalt(II) can be determined via EDTA titration if a small amount of CuEDTA indicator is added to the solution and a copper electrode is used. The electrode responds directly to the Cu activity which, however, is dependent on the activities of the EDTA and the other metal ion in solution. 

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. 

CrP" -selective and Ni " -selective electrodes have been used to detenuine the copper and nickel ions in aqueous solutions, both by direct potentiometry and by potentiometric titration with EDTA. They have also been used for detenuining the CiT and Ni " ions in indushial waters by direct potentiomehy. 

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. 

The hydrogen ions thus set free can be titrated with a standard solution of sodium hydroxide using an acid-base indicator or a potentiometric end point alternatively, an iodate-iodide mixture is added as well as the EDTA solution and the liberated iodine is titrated with a standard thiosulphate solution. 

Mercury(II) chloranilate 700 Mercury(II) nitrate standard soln. of, 359 Mercury/mercury( II )-EDTA electrode (mercury electrode) 586 potentiometric titration of metallic ions with EDTA and, 588 prepn. of, 587 Mercury thiocyanate 700 Metaphosphoric acid in homogeneous precipitation, 426 Metal apparatus 93 Metal ion buffer 53... 

Fig. 5.17 shows the curves for the potentiometric titration of Ca2f in the range 5 10 3-5 10 2 M with a titrant carrier stream of 5 10 4 M EDTA using a calcium ion-selective electrode each titration is initiated by an abrupt increase in the potential, followed by an S-shaped decrease in which the inflection point marks the end of titration. According to eqn. 5.12, where the titration product is AB , the mixing volume V, the original concentration of A in the sample Cl and the titrant concentration CB, can be calculated. In the experiments in Fig. 5.17 the sample volume was 200/d and/ = 0.84 ml min-1 by... 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

The magnesium will be liberated quantitatively and may then be titrated with a standard EDTA solution. Where mixtures of metal ions are analysed, the masking procedures already discussed can be utilized or the pH effect exploited. A mixture containing bismuth, cadmium and calcium might be analysed by first titrating the bismuth at pH = 1-2 followed by the titration of cadmium at an adjusted pH = 4 and finally calcium at pH = 8. Titrations of this complexity would be most conveniently carried out potentiometrically using the mercury pool electrode. 

Epoxy-based membrane of 2-[(4-chloro-phenylimino)-methyl]-phenol reveals a far Nemstian slope of 43 mV per decade for Pb+2 over a wide concentration range CIO 6 to 10 1 mol dm-3). The response time of the electrode is quite low (< 10 sec) and could be used for a period of 2 months with a good reproducibility. The proposed electrode reveals very high selectivity for Pb(II) in the presence of transition metal ions such as Cu2+, Ni2+, Cr and Cd2+at concentrations l.()xl() 3 M and 1.0><10 4 M. Effect of internal solution concentration was also studied. The proposed sensor can be used in the pH range of 2.50 - 9.0. It was used as an indicator electrode in the potentiometric titration of Pb+2 ion against EDTA. 

Potentiometric titration is actually a form of the multiple known subtraction method. The main advantage of titration procedures, similar to multiple addition techniques in general, is the improved precision, especially at high determinand concentrations. ISEs are suitable for end-point indication in all combination titrations (acid-base, precipitation, complexometric), provided that either the titrand or the titrant is sensed by an ISE. If both the titrant and the titrand are electro-inactive, an electrometric indicator must be added (for example Fe ion can be titrated with EDTA using the fluoride ISE when a small amount of fluoride is added to the sample solution [126]). 

Mg in fertilizers is based on such proceedings thereof has been applied on multiple occasions. In milk fermentation, where the samples were dried, calcined in a furnace at 600 °C, the ash was dissolved in 0.03 M HCl, the solution was centrifuged and the supernatant was thus analyzed . The complexometric method for determination of Ca(II) and Mg(II) can be carried out in a single titration with EDTA in alkaline solution, using a Ca-ISE for potentiometric determination of two endpoints. This is accomplished on digitally plotting pCa values measured by the ISE as a function of the volume V of titrant added to the aliquot of analyte the first and second inflection points of the curve mark the Ca(II) and Mg(n) equivalences, respectively. ... 

The existence of a wide variety of zirconium(IV)-aminopolycarboxylate complexes in solution was established by the potentiometric titration studies of Intorre and Martell.675-677 This work and related solution studies have been reviewed by Larsen.5 Zirconium(IV) forms 1 1 and 1 2 complexes with tetradentate aminopolycarboxylates, 1 1 complexes with hexadentate and octadentate aminopolycarboxylates, and mixed ligand complexes with aminopolycarboxylates and various bidentate ligands, for example 1 1 1 Zr-edta-Tiron and 1 1 2 Zr—nta-Tiron complexes (Tiron = l,2-dihydroxybenzene-3,5-disulfonate). The stoichiometries of these complexes are consistent with the tendency of zirconium to achieve a coordination number of eight. 

Figure 12-9 Titration of Ca2+ with EDTA as a function of pH. As the pH is lowered, the end point becomes less distinct. The potential was measured with mercury and calomel electrodes, as described in Exercise 15-6 in Chapter 15. [C. N. Reilley and R. W. Schmid, "Chelometric Titration with Potentiometric End Point Detection Mercury as a pM Indicator Electrode Anal. Chem. 1958, 30. 947.]...

Although changes in pM can readily be followed by physical means (e.g. potentiometrically), following colour changes associated with the formation and dissociation of metal coordination complexes visually or spectrophotometrically is a more versatile and convenient procedure. The serendipitous discovery of so-called metallochromic indicators made by G. Schwarzenbach (1945) led immediately to the introduction of murexide (50) as an indicator in calcium titrations and initiated the search for indicators for other metal-EDTA systems. It will be realized that the chosen metal indicator must be considerably less stable than the metal-EDTA complex, but not so weak as to dissociate appreciably in the vicinity of the end-point when the concentration of free metal... 

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