Types of EDTA titrations

The most important procedures for the titration of metal ions with EDTA are the following. 

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. 

Back-titration. Many metals cannot, for various reasons, be titrated directly thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-tit ration. 

Replacement or substitution titration. Substitution titrations may be used for metal ions that do not react (or react unsatisfactorily) with a metal indicator, or for metal ions which form EDTA complexes that are more stable than those of other metals such as magnesium and calcium. The metal cation M + to be determined may be treated with the magnesium complex of EDTA, when the following reaction occurs  

The amount of magnesium ion set free is equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator. 

29 Complexometiy III Metal Cation Indicators and Types of EDTA Titrations 

Murexide forms eomplexes with numerous metal eations such as Cu, Ni +, Co, and Ca + and ions deriving from lanthanides. The complex s color depends on the pH and the nature of metallie ions. Murexide permits Ca + titration with EDTA at pH = 11. As the figure shows, the eolor turns red from violet blue. 

There are also metal eation indicators that are unieolor. 

Surprisingly, at first sight, redox indioators may also be used in some cases to detect the endpoint of a complexometric titration with EDTA. In fact, the endpoint of an EDTA titration may be accompanied by a ehange in the redox potential of the solution. When a mixture of Fe + and Fe + is titrated with EDTA, Fe + disappears before Fe + since Fe gives more stable eomplexes with EDTA than Fe + does. A simple inspection of Nernst s equation shows that in these conditions, the solution s redox potential decreases markedly, in particular at the equivalence point. The sharp change may be detected by potentiometry with a platinum electrode or with a redox indicator such as Variamine blue. 

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. 

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The scope of the present treatment does not include details of the various instrumental methods for the detection of EDTA titration end points. Nevertheless, we may mention spectrophotometric detection methods, which are of two types. The first is based on instrumental observation of the color changes of metal ion indicators. The second is based on the absorption of radiation in the visible or ultraviolet regions of the spectrum by the metal-EDTA complex. For example, MgY shows appreciable absorbance at a wavelength of 222 nm, whereas the reagent HjY ... 

The vast majority of complexation titrations are carried out using multidentate ligands such as EDTA or similar substances as the complexone. However, there are other more simple processes which also involve complexation using monodentate or bidentate ligands and which also serve to exemplify the nature of this type of titration. This is demonstrated in the determination outlined in Section 10.44. 

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. 

The visual metallochromic indicators discussed above form by far the most important group of indicators for EDTA titrations and the operations subsequently described will be confined to the use of indicators of this type nevertheless there are certain other substances which can be used as indicators.11... 

In addition to these kinetic studies, there has been a considerable amount of work on the thermodynamic parameters associated with this type of reaction. Thus the interaction of [M(edta)aq] with anions of 8-hydroxyquinoline-5-sulfonic acid (oxs ), ida2- and nitrilotriace-tic acid (nta3-) was thoroughly studied for M = Y, La-Lu (except Pm) by calorimetry and pH titration,419 and AG, AH, AS and K found. Some of these values are given in Table 5. It was, however, concluded from the variation of these parameters with atomic number that all M3+ in aqueous solution have the same coordination number (nine) but that the coordination number of the [M(edta)aq] species changes between Sm3+ and Tb. ... 

Another major step in many analyses is separation (Chapters 22 to 25). When, because of the method chosen or the nature of the sample, this unit operation is not required, much effort can be saved. For example, if a masking agent will complex an interfering metal ion in an EDTA titration, a separation step may be avoided. Where a separation is essential, a choice among several techniques is usually available. In general, separation involves the formation of two phases, physically separated, one containing the material of interest and the other the interference. Either phase may be a gas, liquid, or solid. Thus six major types of separation processes are possible. Once separation has been effected, the quantitative determination by physical means is often straightforward. 

EXAMPLE 11-4 Calculate the relation between the potential of an indicator electrode of the type (11-28) and pNi near the end point of an EDTA titration of nickel(II). 

Several different types of titration methods can be used with EDTA, as described next. 

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. 

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. 

Chelating reagents have been used in chelatometric titrations and as masking agents for colorimetric determinations and for separations by precipitation or solvent extraction. This investigation has provided the first indication that the acids are an integral part of the complexes formed with Ti(III). It was found that Ti(III), EDTA, and oxalic acid form a very stable complex of equimolar composition which shows an absorbance maximum at 120 mil. Tartaric acid appears to form a similar type of complex which is much less stable and also shows an absorbance maximum at 720 mp. With this complex, however, the optical absorbance and EPR signal intensity... 

The photometric end point has been applied to many types of reactions. For example, most standard oxidizing agents have characteristic absorption spectra and thus produce photometrically detectable end points. Although standard acids or bases do not absorb, the introduction of acid-base indicators permits photometric neutralization titrations. The photometric end point has also been used to great advantage in titrations with EDTA (cthylenediamineietraacelic... 

Defect concentrations of Fe in Fei xO are usually measured by chemical analysis. It is impossible to determine the compositions of non-stoichiometric compounds, because the error of an ordinary quantitative analysis is about 10, while the deviation of a crystal with intrinsic defect from its stoichiometric composition is about <10. Nevertheless, it is possible for chemical analysis to determine if the metal atoms in non-stoichiometric compounds are excessive or less. Because a non-stoichiometric compound, in common, is a multi-component solid solution in which the different components have different valences, e.g., Fei xO can be viewed as a solid solution which consists of Fe +O and Fe2" 03. Deviation of those types of compounds can directly be determined by measuring of the concentration of an atom that shows an abnormal valence in it. For example, it forms the solutions containing large amounts of Fe + ion and less amounts of Fe + ion, when Fei xO (catalyst) is solved by hydrochloric acid solution under conditions with the absence of air or oxygen. Among these ions, the contents of both Fe + and Fe + can be determined by a titration of EDTA, but the Fe + needs to be oxidated to Fe + by ammonium persulfate prior to titration. The volume ratio of EDTA solutions that are consumed by Fe + and Fe + ions respectively is the ratio of Fe + and Fe + in sample, namely defect concentration of Fe, i.e., x = 1/(3 - - 2Fe +/Fe +). There is also Kulun titration or polarographic analysis except for oxidation-reduction titration which can be used for such measurement of an ion with abnormal valence in the solution of a solid sample. 

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