Complexometric titrations indicators

Another sensor, with calcein immobilized on cellulose, has been designed [79]. It is based on the observation that Co ", Cu " and Ni "> form nonfluorescent complexes at neutral pH (5-7), whereas calcein fluoresces strongly by itself. However, this sensor is not reversible in the pH zones analyzed. The addition of a metallic ion (non-quenching) such as Zn ", which displaces the metal from the fluorescent complex, has also been proposed, and also the use of immobilized calcein as a complexometric titration indicator [83]. 

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. 

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. 

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... 

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.

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

The method is particularly suited to complexometric titrations of mixtures or where there is no suitable visual indicator. 

During most titrations, the solution or suspensions are mixed sometimes continuously. Considering this, why might it be a good idea to use an indicator during a complexometric titration ... 

The equivalence point in complexometric titrations is invariably observed by the help of pM indicators. The relationship amongst pM, concentrations of ligand, chelate complex and stability constant may be established by the following equations ... 

The Cd ISE is used primarily in complexometric titrations with EDTA and Cd as an indicator [327,386] this approach is also the basis of stability constant determination [268,268a]. 

The most important applications of Cu ISEs are in the direct determination of Cu " in water [169, 372,410], complexometric titration of various metal ions using Cu " as an indicator [30, 143,269, 385] and complexometric titrations of Cu " [409]. This ISE has also been used in the determination of the equilibrium activity of Cu in various Cu complexes in order to determine the stability constants (see [46, 285, 317, 318,427, 445]), in the determination of the solubility of poorly soluble salts [122] and in the determination of the standard Gibbs transfer energies [58]. It can also be used in concentrated electrolytes [170]. 

Historically, pH sensitive dyes have been extensively used as indicators in acid-base titrations and in simple spot test papers, even leading to a common phrase in our everyday language, when people or topics are described as having passed the litmus test . The use of complexometric titrations for metal ions was a later but widely... 

The mono- or disodium salt of EDTA may also be used in complexometric titrations. The formula weights of these salts in their dehydrated form are 372.3 and 349.3, respectively. EDTA, or its sodium salt, is standardized against a standard calcium solution. The indicators that are commonly used for such titrations are organic dyes. The structures of two common indicators are as follows ... 

C.I. Mordant Black 11 is also used as an indicator in the complexometric titration of various bivalent metals with which it forms complexes [8],... 

Analytical Reagents. Various chelators give specific color changes on combining with different metal salts, such as vanadium, iron, cobalt, nickel, copper, and palladium salts and thus identify the corresponding metal ions [74], Other chelators, such as C.I. Mordant Black 11 (Eriochrome Black T) (2 see Section 3.11.1), are employed as indicators for complexometric titrations. 

The drug has been determined by complexometric titration of the Cu" 1" liberated by passing the solution through a column of cation exchange resin in the cupper form. A solution of 0.005 or 0.0025 M of disodium EDTA is the titrant and using murexide as an indicator (88). 

The solubility product constant (Ksp) of EDTA was determined by adjusting the pH of an aqueous solution to a low value using nitric acid, and leaving the system to reach equilibrium overnight at room temperature. The precipitate was filtered off, dried at 105°C, and weighed to determine the amount of solubilized material. Alternatively, the precipitate was analyzed by complexometric titration, using standardized 0.05 M Zn(II) solution and xylenol orange as indicator [12]. The estimated value of the solubility product is 10 24 66 (pKsp = 24.66). 

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. 

The use of metal-ion indicators to indicate the end-point of complexometric titrations is based on a specific colour change. Some individuals may find it difficult to detect a particular colour change (e.g. those with colour blindness). Alternative approaches for end-point detection are available based on a colorimeter/spectrophotometer (devices for measuring colour, see Chapter 26) or electrochemical detection (see Chapter 34). 

Cyanide ion may be determined by a complexometric titration with silver nitrate that uses a color indicator to detect the endpoint. If 20.00 mL of a 0.100 M solution of silver nitrate is required to titrate a 5.00 mL aliquot of a CN solution, the concentration of the original CN solution is... 

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