EDTA titrations

The nickel ion freed may then be determined by an EDTA titration. Note that two moles of silver are equivalent to one mole of nickel and thus to one mole of EDTA. 

The equivalent amount of cadmium ion exchanged for the silver ion can readily be determined by EDTA titration procedures. 

Now that we know something about EDTA s chemical properties, we are ready to evaluate its utility as a titrant for the analysis of metal ions. To do so we need to know the shape of a complexometric EDTA titration curve. In Section 9B we saw that an acid-base titration curve shows the change in pH following the addition of titrant. The analogous result for a titration with EDTA shows the change in pM, where M is the metal ion, as a function of the volume of EDTA. In this section we learn how to calculate the titration curve. We then show how to quickly sketch the titration curve using a minimum number of calculations. 

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

A partial list of metallochromic indicators, and the metal ions and pH conditions for which they are useful, is given in Table 9.16. Even when a suitable indicator does not exist, it is often possible to conduct an EDTA titration by introducing a small amount of a secondary metal-EDTA complex, provided that the secondary metal ion forms a stronger complex with the indicator and a weaker complex with EDTA than the analyte. For example, calmagite can be used in the determination of... 

Zirconium is often deterniined gravimetrically. The most common procedure utilizes mandelic acid (81) which is fairly specific for zirconium plus hafnium. Other precipitants, including nine inorganic and 42 organic reagents, are Hsted in Reference 82. Volumetric procedures for zirconium, which also include hafnium as zirconium, are limited to either EDTA titrations (83) or indirect procedures (84). X-ray fluorescence spectroscopy gives quantitative results for zirconium, without including hafnium, for concentrations from 0.1 to 50% (85). Atomic absorption determines zirconium in aluminum in the presence of hafnium at concentrations of 0.1—3% (86). 

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. 

Procedures have also been devised for the determination of metallic constituents. Thus, mercury is absorbed in nitric acid and titrated with sodium diethyldithiocarbamate, whilst zinc is absorbed in hydrochloric acid and determined by an EDTA titration (see Section 10.65). 

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. 

General properties. The success of an EDTA titration depends upon the precise determination of the end point. The most common procedure utilises metal ion indicators. The requisites of a metal ion indicator for use in the visual detection of end points include ... 

Theory of the visual use of metal ion indicators. Discussion will be confined to the more common 1 1-complexes. The use of a metal ion indicator in an EDTA titration may be written as ... 

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... 

Murexide may be employed for the direct EDTA titration of calcium at pH =11 the colour change at the end-point is from red to blue-violet, but is far from ideal. The colour change in the direct titration of nickel at pH 10-11 is from yellow to blue-violet. 

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

This is the basis of the indicator action in the EDTA titration. The pH of 10 is attained by the use of an aqueous ammonia-ammonium chloride buffer mixture. 

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... 

Direct EDTA titrations of Bi, Th, Zn, Cd, Pb, Co, etc., are readily carried out and the colour change is sharp. Iron(III) and, to a lesser extent, aluminium interfere. By appropriate pH adjustment certain pairs of metals may be titrated successfully in a single sample solution. Thus bismuth may be titrated at pH =1-2, and zinc or lead after adjustment to pH = 5 by addition of hexamine. 

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 almost colourless leuco form of the base passes upon oxidation into the strongly coloured indamine. When titrating iron(III) at a pH of about 3 and the colourless hydrochloride of the leuco base is added, oxidation to the violet-blue indamine occurs with the formation of an equivalent amount of iron(II). At the end point of the EDTA titration, the small amount of iron(II) formed when the indicator was introduced is also transformed into the Fe(III)-EDTA complex FeY-, whereupon the blue indamine is reduced back to the leuco base. 

A. Adjustment of pH. For many EDTA titrations the pH of the solution is extremely critical often limits of + 1 unit of pH, and frequently limits of +0.5 unit of pH must be achieved for a successful titration to be carried out. To achieve such narrow limits of control it is necessary to make use of a pH meter while adjusting the pH value of the solution, and even for those cases where the latitude is such that a pH test-paper can be used to control the adjustment of pH, only a paper of the narrow range variety should be used. 

D. Attainment of the end point. In many EDTA titrations the colour change in the neighbourhood of the end point may be slow. In such cases, cautious addition of the titrant coupled with continuous stirring of the solution is advisable the use of a magnetic stirrer is recommended. Frequently, a sharper end point may be achieved if the solution is warmed to about 40 °C. Titrations with CDTA (see Section 2.26) are always slower in the region of the end point than the corresponding EDTA titrations. 

F. Alternative methods of detecting the end point. In addition to the visual and spectrophotometric detection of end points in EDTA titrations with the aid of metal ion indicators, the following methods are also available for end point detection. 

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 ... 

POTENTIOMETRIG EDTA TITRATIONS WITH THE MERCURY ELECTRODE... 

Buffer action 46 Buffer capacity 48 Buffer mixture universal, (T) 831 Buffer solutions 46, (T) 831 acetic acid-sodium acetate, 49 for EDTA titrations, 329 preparation of IUPAC standards, 569 Bumping of solutions 101 Buoyancy of air in weighing 77 Burette 84, 257 piston, 87 reader, 85 weight, 86... 

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... 

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... 

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