Colorimetric acid-base indicators

All integrated sensors based on an interaction between the analyte and reagent (neither of which is used in a retained form) and regeneration of the latter rely on chemiluminescent reactions involving electroregeneration of the reagent or a quenching phenomenon. On the other hand, absorptiometric and reflectometric sensors of this type use colorimetric acid-base indicators supported on a suitable material. 

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 Clark and Tubs (1916) series of buffers are convenient for colorimetric pH measurement between pH 1.0 and 10.2 using acid-base indicators. This series, with more recently... 

In nitric acid medium, the precipitation of uranyl phosphate is incomplete. Uranyl monohydrogen phosphate is insoluble in water and in an acetic acid solution. It is soluble in mineral acid medium. These points explain the use of the acetic buffer to carry out the precipitation. One may also use uranyl acetate instead of nitrate, but the former is less stable than the latter. The titration is performed at about t = 80°C. The indicators are either cochineal tincture as internal indicator or potassium ferrocyanide as an external one. Cochineal tincture is an acid-base indicator. It is obtained by the extraction of cochineal by alcohol. The principal application of this precipitation reaction occurs in the field of analytical biochemistry. More precisely, it is the determination of phosphates in urine. Although this procedure is superseded by colorimetric methods, it remains an interesting replacement method. 

Alkalinity is measured by acid-base titration with methylorange or phe-nolphthalein as indicator. Phenolphthalein changes color at pH 8.3, whereas methylorange changes color at pH 4.3. At pH 8 the neutralization of the strong alkali ingredients like NaOH is essentially complete. Further reduction of the pH to 4 will also measure carbonates and bicarbonates. Colorimetric tests and glass electrode systems are used to determine pH. 

Ammonium chloride is analyzed by treatment with formaldehyde (neutralized with NaOH) and the product HCl formed is analyzed by titration using an acid-base color indicator such as phenolphthalein. Alternatively, it may be mixed with caustic soda solution and distdled. The distillate may be analyzed for NH3 by titration with H2SO4 or by colorimetric Nesslerization or with an ammonia-selective electrode (APHA, AWWA, WEF. 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC, American Pubhc Health Association). The presence of ammonia or any other ammonium compound would interfere in the test. The moisture content in NH4CI may be determined by Karl—Fischer method. 

Litmus (azolitmin) is the classical indicator for acid-base titrations, although by now it has been supplanted by much better indicators. Neither litmus nor azolitmin should be used in colorimetric determinations of pH because of their salt and protein errors. Litmus is of value only in the form of indicator paper (cf. Chapter Eleven). 

E. E. Chandler, J. Am. Chem. Soe., 30, 707 (1908), L. Rosenstein, ibid, 34. 1117 (1912) and E. Q. Adams and L. Rosenstein, ibid., 36, 1452 (1914), all report results for the ionization of dibasic organic acids in which the ratio Ki/Ka is very nearly four. However, Chandler s results have not been verified by later, and presumably more accurate work, and tthe other measurements are based on colorimetric determinations on Indicator solutions, involving many difficulties of interpretation. 

T raditionally, titration curve calculations are described in terms of equations that are valid only for parts of the titration. Equations will be developed here that reliably describe the entire curve. This will be done first for acid-base titration curves. In following chapters, titration curves for other reaction systems (metal complexation, redox, precipitation) will be developed and characterized in a similar fashion. For all, graphical and algebraic means of locating the endpoints will be described, colorimetric indicators and how they function will be explained, and the application of these considerations to (1) calculation of titration errors, (2) buffo design and evaluation, (3) sharpness of titrations, and finally, (4) in Chapter 18, the use of titration curve data to the determination of equilibrium constants will be presented. 

Many chemiluminescent compounds emit light only in neutral or, better in alkaline solution, e.g. luminol and lucigenin. Thus these can be used as neutralization indicators in acid-base titrations. Used in an end-point titration, light can be detected from strongly coloured or turbid or opaque solutions where colorimetric titrations are difficult. For example in the determination of the acidity of milk, of red wine, or mustard [4] or of dark coloured fats and oils [5,6]. It seems that luminol-fluorescein mixtures are better than luminol alone, because a hemin catalyst is not required here [7]. The system luminol - hemin leads to an irreversible destruction of hemin and is, therefore, not reversible with respect to the catalyst - in contrast to the luminol-fluorescein system [7]. 

Analytical and Test Methods. o-Nitrotoluene can be analyzed for purity and isomer content by infrared spectroscopy with an accuracy of about 1%. -Nitrotoluene content can be estimated by the decomposition of the isomeric toluene diazonium chlorides because the ortho and meta isomers decompose more readily than the para isomer. A colorimetric method for determining the content of the various isomers is based on the color which forms when the mononitrotoluenes are dissolved in sulfuric acid (45). From the absorption of the sulfuric acid solution at 436 and 305 nm, the ortho and para isomer content can be deterrnined, and the meta isomer can be obtained by difference. However, this and other colorimetric methods are subject to possible interferences from other aromatic nitro compounds. A titrimetric method, based on the reduction of the nitro group with titanium(III) sulfate or chloride, can be used to determine mononitrotoluenes (32). Chromatographic methods, eg, gas chromatography or high pressure Hquid chromatography, are well suited for the deterrnination of mononitrotoluenes as well as its individual isomers. Freezing points are used commonly as indicators of purity of the various isomers. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

This is distilled into a receiving flask containing boric acid indicator mixture and titrated against 0.001 M HCI. The colorimetric method using the autoanalyser is based on that used for plant materials (see below), but care should be taken that any precipitate formed does not collect in the flowcell, which must be occasionally inverted or cleared by passing a bubble of air through it. Any nitrate (and nitrite, which is usually insignificant) should be reduced... 

Titratable Acidity. The possibility of carboxylic acid group generation from excessive oxidation of corn starch was monitored by titratable acidity (TA). A 0.01 N NaOH solution was used to titrate a dilute aqueous starch suspension (20 mL of a 5% w/v sample) for the presence of acidic functional groups, using phenolphthalein as the indicator dye. An unreacted starch sample was also titrated to yield a sample blank value. TA values were expressed as mL of base required to reach the colorimetric phenolphthalein and end-point. 

It is based on the addition of Mn2+ solution, followed by die addition of a strong alkali to die sample in a glass-stoppered bottle. Dissolved 02 rapidly oxidizes an equivalent amount of the dispersed divalent manganous hydroxide precipitate to hydroxides of higher valence states. In die presence of iodide ions in an acidic solution, the oxidized manganese reverts to die divalent state, with die liberation of a quantity of iodine equivalent to die original dissolved 02 content. The iodine is then titrated with a standard solution of thiosulfate. The titration end point can be detected visually with a starch indicator, or by potentiometric techniques. The liberated iodine can be determined colorimetrically. 

In 1959, Harrison and Gilroy introduced a method based on the detection of the metal-containing components of FDR.124 The metallic components involved, namely, lead, antimony, and barium, originate from the primer and the bullet (lead and antimony). The method is based on colorimetric spot tests and involves swabbing the suspect s hands with cotton cloth damped with 0.1 M hydrochloric acid. The swab is allowed to dry and is then tested with one or two drops of a 10% alcohol solution of triphenylmethylarsonium iodide. The appearance of an orange ring indicates the presence of antimony. 

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