Metal-titrant complexes

Of course, the metal-indicator complex must be less stable than the metal-EDTA complex, or else the EDTA will not displace it from the metal. On the other hand, it must not be too weak, or the EDTA will start replacing it at the beginning of the titration, and a diffuse end point will result. In general, the metal-indicator complex should be 10 to 100 times less stable than the metal-titrant complex. 

The indicator H I and the titrant H Y are considered to be in fully dissociated anionic forms I and Y (charges are omitted for convenience). The reaction will proceed if the metal-indicator complex MI is sufficiently less stable than the metal-titrant complex MY. Here it is assumed that the indicator acid forms only a 1 1 metal complex MI, and that it is not involved in any side reaction. The stability constant Kmi is... 

For analytical purposes, metal indicators can be smdied from two standpoints, which, additionally, are interrelated. The first concerns their ability to complex metallic ions as a function of pH. The second standpoint concerns the stability of metal-indicator complexes compared to that of metal-titrant complexes. The latter standpoint will be studied in Chap. 29. 

Spectrophotometric titrations are particularly useful for the analysis of mixtures if a suitable difference in absorbance exists between the analytes and products, or titrant. Eor example, the analysis of a two-component mixture can be accomplished if there is a difference between the absorbance of the two metal-ligand complexes (Eigure 9.33). 

You can see from the example that a metal-EDTA complex becomes less stable at lower pH. For a titration reaction to be effective, it must go to completion (say, 99.9%), which means that the equilibrium constant is large—the analyte and titrant are essentially completely reacted at the equivalence point. Figure 12-9 shows how pH affects the titration of Ca2+ with EDTA. Below pH 8, the end point is not sharp enough to allow accurate determination. The conditional formation constant for CaY2" is just too small for complete reaction at low pH. 

The data in Figure 12.24 show the potentiometric titrations of a metal-exchange synthetic resin (Dowex 50W-X8) known to produce relatively weak metal-surface complexes (outer-sphere complexes). These data show that when the resin was saturated with Ca2+, no titration plateau (region of Na+, NH4 or NH3 adsorption) was exhibited. When the resin was saturated with Cu2+, up to two titration plateaus were exhibited, depending on the type of titrant used. When NaOH was the titrant, one apparent titration plateau was exhibited, whereas when NH4OH was the titrant, two... 

The most widely used titrant for such determinations is diamino-ethane-tetra-acetic acid (ethylenediamine tetra-acetic acid) which is conveniently employed as the disodium salt, referred to throughout this book as EDTA. (The titrant is referred to in the B,P, and B,P,C, as sodium edetate, but this synonym has not gained universal acceptance.) Many other amino-polycarboxylic acids have been used and in certain special applications they may have some advantage for routine pharmaceutical work, however, it has not been found necessary to use any titrant other than EDTA. This substance reacts stoichiometrically with most metals to form a 1 1 complex and, usually, the reaction is instantaneous (but see Aluminium, p. 32). pH has a marked effect on the stability of the complexes formed the alkaline earth metals form complexes that are stable in alkaline solution but decompose in neutral and acid solution aluminium, copper, lead and mercury all complex under mildly acid conditions while bismuth and ferric iron form stable complexes in a solution as acid as pH 1. The monovalent ions of sodium, potassium and silver form complexes that are too weak to be used for titration purposes whilst mercurous mercury forms no complex,... 

There are now many metallochromic indicators which may be used in complexometric titrations. For successful end-point indication the pK value of the metal-dye complex should be at least 4 units less than that of the metal-EDTA complex which is formed during the titration if the difference is less than 4 units the titrant is unable to compete satisfactorily with the indicator dye and a sluggish end-point results. The indicators which are most widely used in the methods described in this book are solochrome black (for titrations carried out in ammoniacal solution), xylenol orange (for titrations carried out between pH 4 and 7) and catechol violet (for titration in mineral acid solution at pH 1 to 2). Table 63 lists the various indicators called for in methods in the monographs. Discussion of the merits of different indicators for specific purposes will be found in various parts of the book. 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

The utility of complexation titrations improved following the introduction by Schwarzenbach, in 1945, of aminocarboxylic acids as multidentate ligands capable of forming stable 1 1 complexes with metal ions. The most widely used of these new ligands was ethylenediaminetetraacetic acid, EDTA, which forms strong 1 1 complexes with many metal ions. The first use of EDTA as a titrant occurred in... 

The equivalence point of a complexation titration occurs when stoichiometri-cally equivalent amounts of analyte and titrant have reacted. For titrations involving metal ions and EDTA, the equivalence point occurs when Cm and Cedxa are equal and may be located visually by looking for the titration curve s inflection point. 

Selection and Standardization of Titrants EDTA is a versatile titrant that can be used for the analysis of virtually all metal ions. Although EDTA is the most commonly employed titrant for complexation titrations involving metal ions, it cannot be used for the direct analysis of anions or neutral ligands. In the latter case, standard solutions of Ag+ or Hg + are used as the titrant. 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Alkaline-earth metals are often deterruined volumetricaHy by complexometric titration at pH 10, using Eriochrome Black T as indicator. The most suitable complexing titrant for barium ion is a solution of diethylenetriaminepentaacetic acid (DTPA). Other alkaline earths, if present, are simultaneously titrated, and in the favored analytical procedure calcium and strontium are deterruined separately by atomic absorption spectrophotometry, and their values subtracted from the total to obtain the barium value. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Majer65 in 1936 proposed measuring, instead of the entire polarographic curve, only the limiting current at a potential sufficiently high for that purpose if under these conditions one titrates metal ions such as Zn2+, Cd2+, Pb2+, Ni2+, Fe3+ and Bi3+ with EDTA66, one obtains a titration as depicted in Fig. 3.55 i, decreases to a very low value, in agreement with the stability constant of the EDTA-metal complex and the titration end-point is established by the intersection of the ij curves before and after that point correction of the i values for alteration of the solution volume by the titrant increments as in conductometric titration is recommended. 

In actual practice, an excess of the standard solution of disodium edetate is added to the sample, pH is adequately adjusted for the residual titration with a metal-ion solution e.g., ZnS04 and employing an appropriate indicator which is sensitive enough to the respective titrant. However, the metal ion under estimation remains firmly complexed with the EDTA and offers little interference with the Zn-EDTA complex formed. It has been established experimentally that bismuth readily yields a highly stable complex which may be titrated conveniently between pH 1 and 2. Bismuth forms a stable complex by reacting with EDTA quantitatively at pH 4.0 and, therefore, dithizone is employed as an indicator to detect the end-point for it has a transition state of colour at pH 4.6. 

These titrations arc used in the estimation of metal salts. Ethylenediamine tetracetic acid (EDTA) shown in Figure 3.10 is the usual titrant used. It forms stable 1 1 complexes with all metals except alkali metals such as sodium and potassium. The alkaline earth metals such as calcium and magnesium form complexes which are unstable at low pH values and are titrated in ammonium chloride buffer at pH 10. The general equation for the titration is ... 

Volumetry uses the formation of complexes between the metal ions and a complexing reagent (the titrant). This reagent is added in the same manner as in... 

Masking agent — An auxiliary - ligand that is added to a sample in order to avoid unwanted interference of a metal ion in a complex formation analysis. A suitable masking agent has to react selectively and form a sufficiently stable complex with the interfering component so that its reaction with the ligand used as - titrant is prevented [i]. 

Back titrations based on the addition of excess EDTA followed by back titration of the excess reagent are useful when reactions are slow or a suitable direct indicator is not available. The excess generally is determined by titration with standard solutions of magnesium or zinc ion. These titrants are chosen because they form EDTA complexes of relatively low stability, thereby preventing the possible titration of EDTA bound by the sample metal ion. Examples of the indirect method are the following ... 

Formation or disappearance of a soluble colored complex can also indicate an endpoint. Many reagents that form colored complexes with certain metals have been developed only for use as indicators in these titrations. If the cation being titrated produces a color with the indicator, the endpoint will be characterized by the disappearance of this color. When the cation does not give a colored complex, a second cation that does is introduced, and the first excess of titrant then decolorizes this complex. 

Complexation reactions have many uses in analytical chemistiy, but their classical application is in complexometric titrations. Here, a metal ion reacts with a suitable ligand to form a complex, and the equivalence point is determined by an indicator or an appropriate instrumental method. The formation of soluble inorganic complexes is not widely used for titrations, as discussed later, but the formation of precipitates, particularly with silver nitrate as the titrant, is the basis for many important determinations (see Section 13F). 

Solutions of EDTA are particularly valuable as titrants because the reagent combines with metal ions in a /. / ratio regardless of the charge on the cation. For example, the silver and aluminum complexes are formed by the reactions ... 

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