Metal ion indicator

Metal ion indicators (Table 12-3) are compounds whose color changes when they bind to a metal ion. Useful indicators must bind metal less strongly than EDTA does. 

A typical titration is illustrated by the reaction of Mg2+ with EDTA at pH 10. using Calmagite as the indicator. 

At the start of the experiment, a small amount of indicator (In) is added to the colorless solution of Mg2+ to form a red complex. As EDTA is added, it reacts first with free, colorless Mg2+. When free Mg2+ is used up, the last EDTA added before the equivalence point displaces indicator from the red Mgln complex. The change from the red Mgln to blue unbound In signals the end point of the titration (Demonstration 12-1). 

This demonstration illustrates the color change associated with Reaction 12-19 and shows how a second dye can be added to produce a more easily detected color change. 

Buffer (pH 10.0) Add 142 mL of concentrated (14.5 M) aqueous ammonia to 17.5 g of ammonium chloride and dilute to 250 mL with water. 

(a) Write the reaction whose equilibrium constant is the formation constant for EDTA complex formation and write the algebraic form of Kf. 

A typical analysis is illustrated by the titration of Mg with EDTA, with Calmagite as the indicator  

Most metal ion indicators are also acid-base indicators. Because the color of free indicator is pH dependent, most indicators can be used only in certain pH ranges. For example, xylenol orange (pronounced ZY-leen-ol) in Table 13-2 changes from yellow to red when it binds to a metal ion at pH 5.5. This color change is easy to observe. At pH 7.5, the change from violet to red is difficult to see. 

An important factor in the application of EDTA titration methods has been the development of suitable metal ion indicators, which permit visual titrations to be carried out in dilute solutions. A metal ion indicator is usually a dyestuff that forms metal ion complexes of a color different from that of the uncomplexed indicator. The complex forms over some characteristic range of values of pM, exactly as an add-base indicator forms a hydrogen ion complex over a characteristic range of pH 

Eriochrome black T was one of the first and most widely used of the metal indicators. Unfortunately, it is unstable in solution, probably because the molecule has both an oxidizing (nitro) and a reducing (azo) group. Lindstrom and Diehl developed as a replacement the indicator l-(l-hydroxy-4-methyl-2-phenylazo)-2-naphthol-4-sulfonic acid, called calmagite, with the structure 

It is stable in aqueous solution, has a sharper color change than eriochrome black T, and can be substituted for it without requiring changes in procedure. For the sake of brevity we shall consider in detail only this indicator the principles involved are similar for most other indicators. 

In the pH range 9 to 11, in which the dye itself exhibits a blue color, many metal ions form red 1 1 complexes as a result of the o, o -dihydroxyazo grouping. Calmagite, having a high molar absorptivity (about 20,000 at pH 10), is a sensitive detector for metals with which it reacts for example, 10 to 10 M solutions of magnesium ion give a distinct red color with this indicator. 

With magnesium ions the color-change reaction is represented by 

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  

Dyestuffs which form complexes with specific metal cations can serve as indicators of pM values 1 1-complexes (metal dyestuff = 1 1) are common, but l 2-complexes and 2 1-complexes also occur. The metal ion indicators, like EDTA itself, are chelating agents this implies that the dyestuff molecule possesses several ligand atoms suitably disposed for coordination with a metal atom. They can, of course, equally take up protons, which also produces a colour change metal ion indicators are therefore not only pM but also pH indicators. 

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  

This reaction will proceed if the metal indicator complex M-In is less stable than the metal-EDTA complex M EDTA. The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In). The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant) Ku  

The indicator colour change is affected by the hydrogen ion concentration of the solution, and no account of this has been taken in the above expression for the formation constant. Thus solochrome black, which may be written as H2In , exhibits the following acid-base behaviour  

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TABLE 11.32 Properties and Applications of Selected Metal Ion Indicators... 

Ethylenediaminetetra-acetic acid, largely as the disodium salt of EDTA, is a very important reagent for complex formation titrations and has become one of the most important reagents used in titrimetric analysis. Equivalence point detection by the use of metal-ion indicators has greatly enhanced its value in titrimetry. 

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. 

Some examples of metal ion indicators. Numerous compounds have been proposed for use as pM indicators a selected few of these will be described. Where applicable, Colour Index (C.I.) references are given.12 It has been pointed out by West,11 that apart from a few miscellaneous compounds, the important visual metallochromic indicators fall into three main groups (a) hydroxyazo compounds (b) phenolic compounds and hydroxy-substituted triphenylmethane compounds (c) compounds containing an aminomethyldicarboxymethyl group many of these are also triphenylmethane compounds. 

Bromopyrogallol red. This metal ion indicator is dibromopyrogallol sulphon-phthalein and is resistant to oxidation it also possesses acid-base indicator properties. The indicator is coloured orange-yellow in strongly acidic solution, claret red in nearly neutral solution, and violet to blue in basic solution. The dyestuff forms coloured complexes with many cations. It is valuable for the determination, for example, of bismuth (pH = 2-3. nitric acid solution endpoint blue to claret red). 

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... 

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. 

Murkovic Steinberg I., Lobnik A., Wolfbeis O.S., Characterisation of an optical sensor membrane based on the metal ion indicator Pyrocatechol Violet, Sensors Actuators B. 2003 90 (1-3) 230-235. 

The now familiar alternatives of visual and potentiometric detection are available. A number of organic dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the eye at concentrations of 10 6-10 7 mol dm 3 and can function as visual indicators. Most metal ion indicators will also undergo parallel reactions with protons bringing about similar colour changes. Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator function. 

Table 5.9 Some metal ion indicators for EDTA titrations...

More recently the introduction of an analytical reagent disodium ethylene-diaminetetraacetate, invariably termed as EDTA, an altogether latest titrimetric method has been used exclusively for the estimation of metals using metal-ion indicators. 

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. 

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. 

Pyrocatechol violet (Table 12-3) is to be used as a metal ion indicator in an EDTA titration. The procedure is as follows ... 

The metal ion indicator xylenol orange (Table 12-3) is yellow at pH 6 ( max = 439 nm). The spectral changes that occur as VOz+ is added to the indicator at pH 6 are shown below. The mole ratio V02+/xylenol orange at each point is... 

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