Selective EDTA titration

One mole of the complex-forming H2 Y2 reacts in all cases with one mole of the metal ion and in each case, also, two moles of hydrogen ion are formed. It is apparent from equation (o) that the dissociation of the complex will be governed by the pH of the solution lowering the pH will decrease the stability of the metal-EDTA complex. The more stable the complex, the lower the pH at which an EDTA titration of the metal ion in question may be carried out. Table 2.3 indicates minimum pH values for the existence of EDTA complexes of some selected metals. 

Calcichrome. This indicator, cyclotris-7-( l-azo-8-hydroxynaphthalene-3,6-disulphonic acid), is very selective for calcium. It is in fact not very suitable as an indicator for EDTA titrations because the colour change is not particularly sharp, but if EDTA is replaced by CDTA (see Section 2.26), then the indicator gives good results for calcium in the presence of large amounts of barium and small amounts of strontium.13... 

DETAILS FOR THE DETERMINATION OF A SELECTION OF METAL IONS BY EDTA TITRATION... 

Table 10.6 Summarised procedures for EDTA titrations of some selected cations...

Discussion. In mixtures of magnesium and manganese the sum of both ion concentrations may be determined by direct EDTA titration. Fluoride ion will demask magnesium selectively from its EDTA complex, and if excess of a standard solution of manganese ion is also added, the following reaction occurs at room temperature ... 

Saturated solutions of some reagents (T) 829 Schoniger oxygen flask see Oxygen flask Schwarzenbach classification 53 Screened indicators 268 Sebacic acid 469 Secondary pH standards 831 Selective ion meters 567 Selectivity coefficient, 559 in EDTA titrations, 312 in fluorimetry, 733 of analytical methods, 12 Selenium, D. of as element, (g) 465 Semi-log graph paper 572 Sensitivity (fl) 834, (fu) 732 Separation coefficient 163, 196 Separations by chromatographic methods, 13, 208. 233, 249... 

The most common technique to detect the end point in EDTA titrations is to use a metal ion indicator. Alternatives include a mercury electrode (Figure 12-9 and Exercise 15-B) and an ion-selective electrode (Section 15-6). A pH electrode will follow the course of the titration in unbuffered solution, because H2Y2 releases 2H+ when it forms a metal complex. 

Figure 12-14 Guide to EDTA titrations of common metals. Light color shows pH region in which reaction with EDTA is quantitative. Dark color shows pH region in which auxiliary complexing agent is required to prevent metal from precipitating. Calmagite is more stable than Eriochrome black T (EB) and can be substituted for EB. [Adapted from K. Ueno, Guide tor Selecting Conditions of EDTA Titrations." J. Chem. Ed. 1965,42,432.]...

Thiourea masks Cu2+ by reducing it to Cu+ and complexing the Cu+. Copper can be liberated from thiourea by oxidation with H202. Selectivity afforded by masking, demasking, and pH control allows individual components of complex mixtures of metal ions to be analyzed by EDTA titration. 

For end-point detection, we commonly use metal ion indicators, a glass electrode, an ion-selective electrode, or a mercury electrode. When a direct titration is not suitable, because the analyte is unstable, reacts slowly with EDTA, or has no suitable indicator, a back titration of excess EDTA or a displacement titration of Mg(EDTA)2- may be feasible. Masking prevents interference by unwanted species. Indirect EDTA titrations are available for many anions and other species that do not react directly with the reagent. 

How Masking and Demasking Agents Can Be Used to Enhance the Selectivity of EDTA Titrations... 

Ion-selective electrodes have a role for monitoring milk both for its calcium content and as a possible vehicle for health fluoridation programmes. The ion-selective electrode method appears to give higher ionic calcium levels (< 2.71 mmol dm" ) than the method of EDTA titration of ion-exchange eluates ( 2.52 mmol dm ) possibly because of the smaller pH changes in the former case. However, in passing from raw milk to sterilised milk ( 2.27 mmol dm ) and pasteurised milk ( 2.04 mmol dm" ) there is a fall in the ionic calcium level [275]. 

The method described here is based on the difference between measurements of total alkaline earths by complexometric titration with EDTA (ethylenediamine-N,N,N, N -tetra-acetic acid) and selective measurement of calcium described in Section 11.2.1. The simultaneous EDTA titration of calcium, strontium and magnesium involves Eriochrome Black T (EBT) as indicator and was originally applied to seawater analysis by Voipio (1959) and Pate and Robinson (1961). To eliminate subjective errors in the determination of the endpoint, Culkin and Cox (1966) used photometric endpoint detection. A slight modification of this procedure, including the standardization of EDTA by magnesium is reported here. 

EDTA titrations are still widely used because of their great versatility with respect to the analysis of a large number of different metal cations. Furthermore, the technique can be made more selective by adjusting the pH or by the use of compounds that effectively remove interfering cations from the titration (masking agent). The method is inexpensive and reasonably accurate. 

Procedure. Select a volume of sample requiring less than 15 mL of titrant to keep the analysis time under 5 min and, if necessary, dilute the sample to 50 mL with distilled water. Adjust the pH by adding 1-2 mL of a pH 10 buffer containing a small amount of Mg +-EDTA. Add 1-2 drops of indicator, and titrate with a standard solution of EDTA until the red-to-blue end point is reached. 

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. 

Sketch the spectrophotometric titration curve for the titration of a mixture of 5.00 X 10 M Bi + and 5.00 X 10 M Cu + with 0.0100 M EDTA. Assume that only the Cu +-EDTA complex absorbs at the selected wavelength. 

Solutions containing both Le + and AF+ can be selectively analyzed for Le + by buffering to a pH of 2 and titrating with EDTA. The pH of the solution is then raised to 5 and an excess of EDTA added, resulting in the formation of the AF+-EDTA complex. The excess EDTA is back titrated using a standard solution of Le +, providing an indirect analysis for AF+. 

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. 

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... 

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. 

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