Acid/base indicators spectrophotometric determination

Discussion. The dissociation of an acid-base indicator is well suited to spectrophotometric study the procedure involved will be illustrated by the determination of the acid dissociation constant of methyl red (MR). The acidic (HMR) and basic (MR-) forms of methyl red are shown below. 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

The most frequently used detector in FI systems with gas-diffusion separation is the spectrophotometer. Quite often the gas-diffusion process offers sufficient selectivity to allow relatively non-specific chemical reactions in the acceptor stream to detect the analyte. Thus, carbon dioxide, sulfur dioxide, hydrogen sulfide, ammonia may all be determined using suitable acid-base indicators in appropriate buffer solutions used as the acceptor streams. The concentration of the buffer solutions may be adjusted to suit a certain concentration range for the analyte. In order to further enhance the selectivity and/or sensitivity more specific reagents may be introduced in the acceptor streams. In the previously mentioned example on the determination of cyanide [20] a modified pyrazolone-isonicotinic acid reaction was used for such purposes. Interferences due to Schlieren effects seem not to have been reported in gas diffusion spectrophotometric systems. This is understandable, since the matrix composition of acceptor streams is usually quite uniform, and the refractive index is little affected after absorbing the gaseous analytes. 

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. 

The rccison for the increased activity of protic acid-Lewis acid systems is ascribed to the enhancement of the protic acid acidity by the Lewis acid, (Table 2) 31). Whereas 100% HF has a value of —10.2, the acid system of HF +7% BF3 has an Ho value of —16.8, six orders of magnitude greater. The Hammett acidity function. Ho, is evaluated by the use of indicators, B, which are very weak bases and for which the acid ionization constants, Ka, of the conjugate acids, BH+, have been determined (usually spectrophotometrically), Eq. (9) ... 

H2O + Si-O-Al M(OH) + Si-0(H )-Al where Si-O-Al represents a part of the zeolite firework (10). It has also been shown that Bronsted acid sites can be present in low concentrations in alkali-metal zeolites. For example, Ramamurthy and coworkers have established the presence of low levels of Bronsted acidity in NaY and NaX zeolites using the color change of a base indicator (10, 11, 19). The acidic forms of retinyl acetate, retinol and retinyl Schiff bases are colored blue and are easily identified by both visual inspection and spectrophotometrically. Using these bases, they were able to determine whether or not Bronsted acids were present in low concentrations in various zeolite samples. Importantly, these studies showed that the presence of small quantities of acid sites... 

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... 

Kolthoff and Bruckenstein59, considering the determination of Ki and KA of acids and bases, applied a spectrophotometric measurement of the interaction of the acid or base of interest with a well chosen colour indicator base I referring for details to the original papers, we must confine ourselves to a concise explanation for the cases of an acid HX and a base B. 

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). 

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). ... 

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