Organic acids spectrophotometric methods

Another assay that is very similar to the ABTS assay is the AGV-dimethyl-p-phenylenediamine (DMPD assay). In the presence of a suitable oxidant solution at an acidic pH, DMPD is converted to a stable and colored DMPD radical cation (DMPD +). Antioxidants capable of transferring a hydrogen atom to the radical cause the decol-orization of the solution, which is spectrophotometrically measured at 505 nm. The reaction is stable, and the endpoint is taken to be the measure of antioxidant efficiency. Antioxidant ability is expressed as Trolox equivalents using a calibration curve plotted with different amounts of Trolox (Fogliano and others 1999). This method is used to measure hydrophilic compounds. The presence of organic acids, especially citric acid, in some extracts may interfere with the DMPD assay, and so this assay should be used with caution in those extracts rich in organic acids (Gil and others 2000). 

In the Nile blue spectrophotometric method, 10 ml 2% aqueous hydrofluoric acid is added to a 10 ml sample contained in a polyethylene bottle. The mixture is shaken for about 2h. Aqueous ferrous sulfate 10% 10 ml and 1ml 0.1% aqueous Nile blue A are added, then extracted with o-dichlorobenzene (10 ml and 3x5 ml). The combined organic extracts are diluted to 50 ml with the solvent and the extinction measured at 647 nm. Interference from chloride ions up to 100 mg/1 can be eliminated by precipitation as silver chloride. 

The pKa of the protonation of the nitrogen in position 8 has been reported as 6.02 and the pKa for the carboxylate anion formulation has been reported as -0.94. These were determined by Staroscik and Sulkowska by a spectrophotometric method.(14) Further study by the same workers on the partition equilibria of nalidixic acid between water and various organic solvents led to calculations of the pKa values of 5.99 + 0.03 for N-protonation and -0.86 4- 0.07 for carboxylate anion formation.(12) Takasugi and co-workers reported the apparent pKa of nalidixic acid to be 5.9 at 28° by a spectrophotometric method.(13)... 

An extractive spectrophotometric procedure based on the complexation of reduced Iron(II) with 5-Chloro-7-iodo-8-hydroxyquinoline (CIHQ) for the estimation of micro amounts of vitamin C. The resulting brown colored complex was extracted into chloroform to give a reddish brown extract which shows an absorption band at 485 nm. This chelate was formed immediately and the apparent molar absorptivity and Sandell s sensitivity for vitamin C was found to be 8.5 x 105 dm3 mol"1 cm 1 and 2.072xl0 4g cm 2. Linear relationship between absorbance and concentration of ascorbic acid is observed up to 0.8 pg ml"1. Interference studies of different substances including sugars, vitamins and amino acids, metal ions and organic acids were carried out. The utility of the method was tested by analysing some of the marketed products of vitamin C... 

Extraction can be nsed for separation or isolation of the analyte from the sample matrix or vice versa as well as a preconcentration method. Extraction of metal ions is based on the reaction of weak organic acids with metal ions that give nncharged complexes that are highly solnble in organic solvents as ethers, hydrocarbons, ketones and polychlorinated species (generally chloroform and carbon tetrachloride). The efficacy of the extraction is mainly dependent on the extent to which solntes distribnte themselves between two immiscible solvents. The amonnts of analyte can be determined spectrophotometrically as well as with other available analytical methods. 

The BF2 chelates are hydrolytically stable (the majority of them may be recrystallized from water) whereas the BC12 chelates are easily hydrolyzed. The boric acid ester complex triptych boroxazolidines are also stable towards hydrolysis as are other aminoalcohol esters.83 A great number of the boron chelates are colored several spectrophotometric methods are based on chelate formation81 84 for the analytical determination of boric acid, organoboric acids and chelating organic compounds. The boron chelates are remarkable for their pharmacological properties as well.84 Various aspects of the boron chelates have been reviewed.81,83-86... 

Of the methods listed above, HPLC is the best for analyzing all organic acids in a sample, as it is able to quantitatively identify several different acids in one run. HPLC provides a method that is fast, sensitive, and reliable, which is an advantage over spectrophotometric and volumetric methods. HPLC methods are also very reproducible the retention times of the compounds are fairly constant. However, there will be differences in retention times between columns and there will invariably be... 

Many useful compounds such as amino acids, nucleic acids, alcohols, vitamins, antibiotics, foods, etc. are produced in fermentation industries. Furthermore, many organic and inorganic compounds are present in waste waters. The determination of these compounds is required for control of fermentation and environment. Analysis of these compounds can be done by spectrophotometric methods. However, complicated procedures and long reaction times are required. 

Often the enthalpy of reaction is obtained by measuring the equilibrium constant of an acid-base reaction over a range of temperatures. If In K is plotted versus l/T. the slope will be equal to -AH/R. Thus various experimental methods have been devised to measure the equilibrium constant by spectrophotometric methods. Any absorption which differs between one of the reactants (either acid or base) and the acid-base adduct is a potential source of information on the magnitude of the equilibrium constant since it gives the concentration of two of the three species involved in the equilibrium directly and the third indirectly from a knowledge of the stoichiometry of the reaction. For example, consider the extensively studied reaction between organic carbonyl compounds and iodine. The infrared carbonyl absorption is shifted in frequency in the adduct with respect to the free carbonyl compound. Thus the equilibrium mixture exhibits two absorption bands in the carbonyl region of the spectrum (Fig. 9.1) and the relative concentrations of the free carbonyl and the adduct can be obtained.3- Alternatively, one can observe the absorption of the iodine molecule, U, in... 

Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds. 

Most spectrophotometric methods for determining nitrate are based either on I), nitration or oxidation of appropriate organic reagents to form coloured compounds or on, 2), reduction of NO.i to NO2 or NH3 with subsequent determination of this species. Prevalent among the methods belonging to the first group are those using phenoldisulphonic acid, and xylenols. 

The colour reaction is carried out in 0.1 M HCl, and the time necessary for colour development in the aqueous pseudo-solution is 50 min. In the extractive spectrophotometric method [36], the time for reaction at pH 2-3 (in the presence of formic acid) is 30 min, after which the solution is neutralized to pH 6-7, and the piazselenol is extracted into toluene. The colour reaction may be accelerated by heating the solution. Within the pH range 5-10, the distribution coefficient of piazselenol between toluene and water is high, and one portion of toluene extracts practically all the selenium complex into the organic phase. The free reagent (DAB) is also extracted. Related solvents such as benzene and xylene may be substituted for toluene. 

Surface acidity and basicity were measured by adsorption of organic bases such as pyridine (PY, pk,=5.3), morpholine (MP, pk, = 8.33), piperidine (PP, pk, = 11.1) and acidic substrates like acrylic acid (AA, pk = 4.2), phenol (PH, pk, = 9.9), respectively by spectrophotometric method[15,16]. Redox properties (one electron donor and one electron acceptor) were determined by the same method using meta dinitrobenzene (DNB, electron affinity, EA = 2.21eV) and phenothiazine (PNTZ, ionisation energy, IE = 7.13eV) as the adsorbates. 

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. 

Ammonia (distillation, titrimetric, ammonia-selective electrode, phenate method) nitrite (ion chromatography, colorimetric) nitrate (ultraviolet spectrophotometric screening, nitrate electrode, reduction) organic (Kjeldahl, persulfate method) lodimetric and modifications, membrane electrode Acid digestion, colorimetric methods Gravimetric, atomic absorption spectrometry, colorimetric Colorimetry, iodometric, ion-selective electrode Gravimetric, turbidimetric, colorimetric... 

Some organic compounds form coloured products if they are oxidized by nitrate ions in acid solution. Examples are brucine (Jenkins and Medsker, 1964), diphenylbenzidine Atkins, 1954), reduced strychnidine Dal Pont, 1962), diphenylamine Isaeva, 1958) and resorcinol Costa, 1951). However, no reliable trouble-free method has been developed based on this technique. Another spectrophotometric method based on the formation of nitrosyl chloride in seawater containing 50 % concentrated sulphuric acid has been developed by Armstrong (1963), but this method cannot be recommended because of the difficulties connected with handling concentrated sulphuric acid solutions at sea, and because it is relatively insensitive. 

High performance liquid chromatographic methods (HPLC) were applied to analyse carotenoid [3], organic acid [4] and tocopherols [5]. The activity of lipoxygenase was spectrophotometrically determined by measuring A absorbance at 234 nm using linoleic acid as the substrate [6]. 

Comprehensive accounts of the various gravimetric, polarographic, spectrophotometric, and neutron activation analytical methods have been pubHshed (1,2,5,17,19,65—67). Sampling and analysis of biological materials and organic compounds is treated in References 60 and 68. Many analytical methods depend on the conversion of selenium in the sample to selenous acid, H2Se02, and reduction to elemental selenium when a gravimetric deterrnination is desired. 

In cases where it proves impossible to find a suitable indicator (and this will occur when dealing with strongly coloured solutions) then titration may be possible by an electrometric method such as conductimetric, potentiometric or amperometric titration see Chapters 13-16. In some instances, spectrophotometric titration (Chapter 17) may be feasible. It should also be noted that ifit is possible to work in a non-aqueous solution rather than in water, then acidic and basic properties may be altered according to the solvent chosen, and titrations which are difficult in aqueous solution may then become easy to perform. This procedure is widely used for the analysis of organic materials but is of very limited application with inorganic substances and is discussed in Sections 10.19-10.21. 

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