Titration, complexometric

Complexometric titrations are mainly used to determine the concentration of cations in solution. The method is based on the competition between a metal ion (for example) and two ligands, one of which acts as an indicator and the other is a component of a standard solution. 

Some knowledge of the principles of metal-hgand binding is required in order to understand this method. 

Ligands are chemical species that co-ordinate with metal ions to form a complex. They are classified on the basis of the number of points of attachment to the central ion. 

The metal ion under investigation is bound to an indicator in solution (under strict pH control). This solution is then titrated against a standard solution of EDTA. This can be expressed in the form of an equation  

Metal-indicator -I- EDTA metal-EDTA -I- indicator 

Metal-indicator + EDTA — metal-EDTA + indicator [23.1] 

The stability of the alkaline-earth EDTA complexes decreases down the group as expected from the increased ionic radius. However, Ca ions are exceptional in forming the most stable EDTA complex. For this reason, calcium cannot be titrated using the Erio T indicator, since no colour change to steel blue is observed. However, when a suspension of magnesium complexone is added, the calcium replaces magnesium in the complexone releasing Mg ions which can be titrated. This replacement titration may be used to standardise EDTA solution. 

Eriochrome black T (or solochrome black) is a satisfactory indicator in EDTA titrations. Between pH 7 and 11, the indicator is blue due to the predominance of the anion (HD ). Over this pH range, Mg, Zn, Mn or (M=lanthanide) give red complexes. On titrating these metal ion solutions, die solution is red until the end point is reached, when it assumes the blue colour of HD.  

With the exception of Be, the four alkaline-earth salts in solution can be determined by direct titration with standard EDTA solution at pH 12 using bromothymol blue indicator. 

---

Among the complexing agents that find use as titrating agents, ethylenediamine-A,A(A, A-tet-raacetic acid (acronym EDTA, and equation abbreviation, H4Y) is by far the more important, and it is used in the vast majority of complexometric titrations. The successive acid values of H4Y are pKi = 2.0, pisTj = 2.67, = 6.16, pTCt = 10.26 at 20°C and an ionic strength of 0.1. The fraction... 

How to sketch an EDTA complexometric titration curve see text for explanation. 

Sketching the Titration Curve As we have done for acid-base, complexometric titrations, and redox titrations, we now show how to quickly sketch a precipitation titration curve using a minimum number of calculations. 

Three experiments involving complexometric titrations are described in this second block of experiments. 

The concentration of cyanide, CN, in a copper electroplating bath can be determined by a complexometric titration with Ag+, forming the soluble Ag(CN)2 complex. In a typical analysis a 5.00-mL sample from an electroplating bath is transferred to a 250-mL Erlenmeyer flask, and treated with 100 mL of H2O, 5 mL of 20% w/v NaOH, and 5 mL of 10% w/v Kl. The sample is titrated with 0.1012 M AgN03, requiring 27.36 mL to reach the end point as signaled by the formation of a yellow precipitate of Agl. Report the concentration of cyanide as parts per million of NaCN. 

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. 

Such reactions are not available for Bi but this can readily be determined by complexometric titration using ethylenediaminetetraacetic acid or similar complexones ... 

The formation of a single complex species rather than the stepwise production of such species will clearly simplify complexometric titrations and facilitate the detection of end points. Schwarzenbach2 realised that the acetate ion is able to form acetato complexes of low stability with nearly all polyvalent cations, and that if this property could be reinforced by the chelate effect, then much stronger complexes would be formed by most metal cations. He found that the aminopolycarboxylic acids are excellent complexing agents the most important of these is 1,2-diaminoethanetetra-aceticacid (ethylenediaminetetra-acetic acid). The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. The values of pK are respectively pK, = 2.0, pK2 = 2.7,... 

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

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. 

The following points should be borne in mind when carrying out complexometric titrations. 

E. Detection of the colour change. With all of the metal ion indicators used in complexometric titrations, detection of the end point of the titration is dependent upon the recognition of a specified change in colour for many observers this can be a difficult task, and for those affected by colour blindness it may be... 

The hardness of water is generally due to dissolved calcium and magnesium salts and may be determined by complexometric titration. 

The analysis of low-melting alloys such as Wood s metal is greatly simplified by complexometric titration, and tedious gravimetric separations are avoided. The alloy is treated with concentrated nitric acid, evaporated to a small volume, and after dilution the precipitated tin(IV) oxide is filtered off heavy metals adsorbed by the precipitate are removed by washing with a known volume of standard EDTA solution previously made slightly alkaline with aqueous... 

Full details are given for the determination of aluminium by this method. Many other metals may be determined by this same procedure, but in many cases complexometric titration offers a simpler method of determination. In cases where the oxine method offers advantages, the experimental procedure may be readily adapted from the details given for aluminium. 

Complexometric titration general discussion, 258, 309, 311, 322 see also EDTA Complexones 55 Computers 133... 

Complexometric titrations, 2,782 Complexones, 2,777-791 applications, 2,790 macrocyclic, 2,789 metal complexes... 

The sum of calcium and magnesium ions in the mud determines the total hardness. These ions are analyzed with complexometric titrations using EDTA (versenate). 

Pyridylazo-ligands (Scheme 2) have widely been used in colorimetric analyses of various metal ions. For example, l-(2-pyridylazo)-2-naphthol (Hpan) is one of the most well-known reagents for the colorimetric determination and complexometric titration of... 

Braun and Richter [923] have described an application of CE in additive analysis, namely quantitative analysis of heat stabilisers in PVC, such as Irgastab 17A and 18 MOK-N, which are metal-based (in the past usually Cd, Ba and Pb, now nontoxic Ca, Zn and Sn). Quantitative metal analysis is of interest for PVC recycling purposes. Various alternative approaches are possible for such quantitative analysis, such as XRF [924], polarog-raphy [925] and AAS [923], The performance of AAS, CE and complexometric titrations in the analysis of the heavy metal content in PVC was compared [923]. For all methods investigated the metals must be separated from the polymer and transferred into an aqueous phase. 

Applications Quantitative dry ashing (typically at 800 °C to 1200°C for at least 8h), followed by acid dissolution and subsequent measurement of metals in an aqueous solution, is often a difficult task, as such treatment frequently results in loss of analyte (e.g. in the cases of Cd, Zn and P because of their volatility). Nagourney and Madan [20] have compared the ashing/acid dissolution and direct organic solubilisation procedures for stabiliser analysis for the determination of phosphorous in tri-(2,4-di-t-butylphenyl)phosphite. Dry ashing is of limited value for polymer analysis. Crompton [21] has reported the analysis of Li, Na, V and Cu in polyolefins. Similarly, for the determination of A1 and V catalyst residues in polyalkenes and polyalkene copolymers, the sample was ignited and the ash dissolved in acids V5+ was determined photo-absorptiometrically and Al3+ by complexometric titration [22]. 

Fig. 12. Titration curves for complexometric titrations. Titration of 60.0 ml of a solution that is 0.02 mol dm-3 in M curve A represents a 0.02 mol dm-3 solution of the tetradentate ligand D to give MD as product curve B represents a 0.04 mol dm-3 solution of the bidentate ligand B to give MB2 and curve C represents a 0.08 mol dm-3 solution of the un-identate ligand A to give MA4. The overall formation constant for each product is 1.0 x 1020. Adapted from ref. 115.

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Spectrophotometric analysis and complexometric titrations have long been used to determine calcium, with much effort devoted to developing reagents and indicators that permit the... 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

Figure 14 Some examples of endpoint determination in titrations using chemiluminescent indicators. (A) Acid-base titration the endpoint is detected by the emission of light (B) complexometric titration the endpoint is detected by disappearance of light. M, metal acting as a catalyst X, excited state from the CL precursor acting as indicator.

---