Indicators, acid-base spectrophotometric method

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. 

Here we shall confine ourselves to the solvents benzene and 1,2-dichloroethane (class 8). Considering benzene, many investigators have demonstrated since the 1930s the feasibility of titrations in this solvent using both potentiometric and spectrophotometric methods, paying much attention to acid-base indicator reactions under the influence of primary, secondary and tertiary amines. Association of carboxylic acids in benzene was studied at a later stage, mainly on the basis of colligative properties, IR spectroscopy and solvent extraction. ... 

Using weak bases (B) as indicators, which are partly converted in acidic solvents to the conjugated acids BH+, the Ho value is measured. Two typical indicators are the o-nitroanilinium ion (pK in water —0.29) and the 2,4-dinitroanilinium ion (pK in water —4.53)12. By means of spectrophotometric methods it is possible to determine the ratio of [BH+]/[B] for each indicator, therefore Ho can be calculated for any solvent system, provided that the pK value of this indicator in water (pA". i) is known. If Hq is known for a particular solvent system, the pKa values can be calculated for any acid-base pair. 

Acid dyes are used less frequently in spectrophotometric methods than the basic ones. Acid dyes form extractable ion-associates with hydrophobic cationic metal complexes. The dyes used are generally acid-base indicators. 

Useful exploratory studies of acid-base behaviour in solvents of low dielectric constant have been made by conductance " and potentio-metric " titrations. Association constants are usually obtained from spectrophotometric measurements. The strengths of various bases can be compared by means of their association with an indicator acid like 2,4-dinitrophenol. If both acid and base are colourless, a competition for the base can be established between the acid and an indicator acid like bromophthalein magenta In solvents like benzene, other reactions than simple 1 1 association between B and RX may occur. Self-association of the acid or base is one such auxiliary reaction. A classic example is the dimerisation of carboxylic acids in benzene. If allowance is not made for this, constant values of the quotient [BRX]j[B][RX] will not be obtained. (Variations in the quotient cannot be attributed to interionic forces or other nonideal behaviour BRX is scarcely dissociated into ions at all and in spectrophotometric work very low concentrations of B and RX can be used.) Evidence for association ratios other than 1 1 can be obtained from indicator studies. The method developed by Kolthoff and Bruckenstein for studying reactions in anhydrous acetic acid fails for reactions in benzene and similar solvents because more than one acid molecule reacts with the indicator to give complexes of the form/w J r"(HX)yi. In such studies it is generally a good approximation... 

Nitrite Nitrite is an important indicator of fecal pollution in natural waters as well as a potential precursor of carcinogenic species. A rush of flow and sequential injection spectrophotometric method based on Griess-type reactions has been proposed, also coupled to online sorbent enrichment schemes. The catalytic effect of nitrite on the oxidation of various organic species constitutes the basis of fairly sensitive spectrophotometric methods. Fluorometric methods based on the formation of aromatic azoic acid salts, quenching of Rhodamine 6G fluorescence, and direct reaction with substituted tetramine or naphthalene species have been also reported. Indirect CL methods usually involve conversion into nitric oxide and gas-phase detection as mentioned in the foregoing section. The redox reaction between nitrite and iodide in acidic media is the fundamental of a plethora of flow injection methodologies with spectrophotometric, CL, or biamperometric detection. New electrochemical sensors with chemically modified carbon paste electrodes containing ruthenium sites, or platinum electrodes with cellulose or naphthalene films, have recently attracted special attention for amperometric detection. 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

For the quantitative determination of Solarium alkaloids in plant materials some new methods have been developed for instance, gravimetric procedures using the afore-mentioned cholesterol precipitation (15) for tetraosides, especially with saturated aglycones volumetric methods with titration of the bases in water-free solvents using aromatic sulfonic acids (26c, 47, 48) a number of simplified spectrophotometric methods by applying the Clarke reaction (49), that is, the hlue color obtained with paraformaldehyde-phosphoric acid in the case of J -unsaturated steroid alkamines and their glycosides (12, 50-52a, 525), or with the aid of amphi-indicators of the tropaeolin type (52,53-57,57a). The well-known Liebermann-Burchard reaction is not applicable to nitrogenous 3 -hydroxy-J -steroids (55). ... 

It is well known that outside the pH range almost all predeterminations for unexcited molecules have been based on the Hammett indicator method (Hammett and Deyrup, 1932). Even in concentrated acid (or alkaline) solutions, where uncertainties in the value of b//bh become very serious, it is easy to measure the ratio of protonated to unprotonated indicator concentrations spectrophotometrically when the absorption peaks are sufficiently resolved. The difficulty arises in trying to extrapolate beyond the measurable [BH+]/[B] range to zero electrolyte concentration. Hammett and Deyrup assumed that the activity coefficient ratios of the type /b//bh+ were very similar for different indicators in the same acid solution. For the equilibrium (51) between two indicators A and B, a comparison of the concentration ratios [BH+]/[B] and [AH+]/[A] over an acidity range in which both could be measured would lead to a direct estimate of the difference in p-K-values using (52) and (53). Beginning with 4-nitroaniline, which is about half-... 

The concept of pH, however, is not applicable in such nonaqueous systems or in concentrated acid solutions. A new quantitative scale, therefore, was needed (5,9-11). The most useful and widely accepted method was proposed by Hammett and Deyrup in 1932 (12). They defined ho by equation 24, which can be determined experimentally by adding the neutral base B in low concentrations to an acid solution (BH+ is the acidic form of the indicator, Ksa+ is the thermodynamic equilibrium constant for BH+, and [BH+]/[B] is the ionization ratio generally determined spectrophotometrically). Equation 24 is usually written in the logarithmic form (eq. 25), where the quantity Hq is termed the Hammett acidity function. Since in dilute solutions of acids Abh+ is expressed as in equation 26, the Hammett acidity function becomes equal to pH. In concentrated solutions, however, Hq differs considerably from pH and this can be formally expressed by inserting activity coefficients in equation 24 (eq. 27). 

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