Spectrophotometric determination reactions

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

A reaction of mixed molybdenum polyoxometalates (POMs) with cyanine dye has been used for highly selective and sensitive spectrophotometric determination of phosphorus(V) and arsenic(V). Color of the solution is considerably changed by reaction of Keggin POMs with styrene cyanine dyes. Derivatives of l,3,3-threemethyl-3//-indol - astrazone violet (AV 3R), astrazone rose, astrazone yellow, astrazone red were investigated. 

Clearly the accurate measurement of the final (infinity time) instrument reading is necessary for the application of the preceding methods, as exemplified by Eq. (2-52) for the spectrophotometric determination of a first-order rate constant. It sometimes happens, however, that this final value cannot be accurately measured. Among the reasons for this inability to determine are the occurrence of a slow secondary reaction, the precipitation of a product, an unsteady instrumental baseline, or simply a reaction so slow that it is inconvenient to wait for its completion. Methods have been devised to allow the rate constant to be evaluated without a known value of in the process, of course, an estimate of A is also obtainable. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

Discussion. This section is concerned with the simultaneous spectrophotometric determination of two solutes in a solution. The absorbances are additive, provided there is no reaction between the two solutes. We may write ... 

Azo coupling reactions are often used for quantitative determination of trace compounds, e. g., for nitrous acid and for compounds that are either diazo or coupling compounds. Spectrophotometric determination of azo dyes formed from these compounds is possible down to concentrations of about 0.01 pg/mL. (An example is the determination of 4-aminophenazone by azo coupling with 4-nitrobenzenediazonium ions see Alwehaid, 1990.)... 

Phenolic antioxidants in rubber extracts were determined indirectly photometrically after reaction with Fe(III) salts which form a red Fe(II)-dipyridyl compound. The method was applicable to Vulkanox BKF and Vulkanox KB [52]. Similarly, aromatic amines (Vulkanox PBN, 4020, DDA, 4010 NA) were determined photometrically after coupling with Echtrotsalz GG (4-nitrobenzdiazonium fluoroborate). For qualitative analysis of vulcanisation accelerators in extracts of rubbers and elastomers colour reactions with dithio-carbamates (for Vulkacit P, ZP, L, LDA, LDB, WL), thiuram derivatives (for Vulkacit I), zinc 2-mercaptobenzthiazol (for Vulkacit ZM, DM, F, AZ, CZ, MOZ, DZ) and hexamethylene tetramine (for Vulkacit H30), were mentioned as well as PC and TLC analyses (according to DIN 53622) followed by IR identification [52]. 8-Hydroquinoline extraction of interference ions and alizarin-La3+ complexation were utilised for the spectrophotometric determination of fluorine in silica used as an antistatic agent in PE [74], Also Polygard (trisnonylphenylphosphite) in styrene-butadienes has been determined by colorimetric methods [75,76], Most procedures are fairly dated for more detailed descriptions see references [25,42,44],... 

Diphenylcarbazone and diphenylcarbazide have been widely used for the spectrophotometric determination of chromium [ 190]. Crm reacts with diphenylcarbazone whereas CrVI reacts (probably via a redox reaction combined with complexation) with diphenylcarbazide [ 191 ]. Although speciation would seem a likely prospect with such reactions, commercial diphenylcarbazone is a complex mixture of several components, including diphenylcarbazide, diphenylcarbazone, phenylsemicarbazide, and diphenylcarbadiazone, with no stoichiometric relationship between the diphenylcarbazone and diphenylcarbazide [192]. As a consequence, use of diphenylcarbazone to chelate Crm selectively also results in the sequestration of some CrVI. Total chromium can be determined with diphenylcarbazone following reduction of all chromium to Crm. 

Hydrolysis products of neomycin may be an amino-sugar, a pentose or furfural depending on the reaction conditions chosen. Each of these entities has been utilised for indirect spectrophotometric determination of neomycin. 

Cyanide in biological tissue and fluids can be measured spectrophotometrically after reaction with methemoglobin (Tomoda and Hashimoto 1991). The detection limit is 0.4 pg/mL. Other performance data were not reported (Tomoda and Hashimoto 1991). Cyanide in urine has been determined using microdiffusion separation and colorimetric determination (Brimer and Rosling 1991). Detection limits are in the ng/L range other performance data were not reported (Brimer and Rosling 1991). 

This is a spectrophotometric assay based on the reaction of diphenylamine with the deoxyribose moiety of DNA to produce a complex that absorbs at 600 nm. The reaction is specific for deoxyribose and RNA does not interfere. It can be used on relatively crude extracts where direct spectrophotometric determinations of DNA concentration are not possible. 

For first-order reactions then, there is no compressibility term in the expression for In k, no matter what concentration scale is used. For higher order reactions involving molar concentrations, Eq. (22) could be applied when accurate rate data are available. Whether Eq. (27) should be applied depends on the method used for obtaining the data. If a spectrophotometric determination of the relative decrease in [A] is used, a relative measure of (d In k/dp)T is obtained from Eq. (27). If an absolute determination of [A] can be made at various times, Eq. (24) can be used directly, and k and (d In k/dp)T can be immediately obtained. The situation is easily generalized to higher order kinetics. In some cases, where AVf < 0 and the method of measurement detects [A] but not [X ], there may be a slight displacement of the quasi-equilibrium with pressure which leads to different initial concentrations of A. When AVf can be determined from Eq. (22), it may appear pressure-dependent, i.e.,... 

The oldest application of solvent extraction in spectrophotometric determinations uses extraction from the original aqueous solution and subsequent back-extraction into a second aqueous phase. Here the extractant provides only separation or concentration, as in the case of Np(IV) and Pu(IV) determination [16]. However, as only a few element species (e.g., Mn04, Cr04 ) are capable of absorbing light in the UV-VIS range, usually all spectrophotometric methods are based on reactions of analytes with... 

COMPOUNDING OF ERRORS. Data collected in an experiment seldom involves a single operation, a single adjustment, or a single experimental determination. For example, in studies of an enzyme-catalyzed reaction, one must separately prepare stock solutions of enzyme and substrate, one must then mix these and other components to arrive at desired assay concentrations, followed by spectrophotometric determinations of reaction rates. A Lowry determination of protein or enzyme concentration has its own error, as does the spectrophotometric determination of ATP that is based on a known molar absorptivity. All operations are subject to error, and the error for the entire set of operations performed in the course of an experiment is said to involve the compounding of errors. In some circumstances, the experimenter may want to conduct an error analysis to assess the contributions of statistical uncertainties arising in component operations to the error of the entire set of operations. Knowledge of standard deviations from component operations can also be utilized to estimate the overall experimental error. 

Analysis. The colorimetric method for In is capable of a detection limit of 20 ppb. Indium or an In compound in the flame gives an indigo blue color (451.1 nm). This photon line allows for the spectrophotometric determination ofinby AAS (atomic absorption flame spectroscopy). The method is sensitive to about 300 ppb. With ETAAS, this limit drops to 10 ppb, as it does with ICPAES. ICPMS drops the limit to 0.01 ppb. Alizarin detects In, as well as Al, but the reaction with Al can be masked by addition of F to a spot test. The limit of detection is about 1 ppm. 

Ionized calcium may be determined spectrophotometrically after reaction with murexide or using a Ca2 +-specific electrode. 

C. Ammonia can be determined spectrophotometrically by reaction with phenol in the presence of hypochlorite (OC1) ... 

Nitrite ion, N02, is a preservative for bacon and other foods, but it is potentially carcinogenic. A spectrophotometric determination of N02 makes use of the following reactions ... 

There are many studies of the analytical detection and quantitative determination of mercury based on reactions with N donors, e.g. the spectrophotometric determination of mercury(II)... 

Kerr el. al. [307] employed high performance liquid chromatography for the determination of uranium in groundwaters. The sample was passed through a small reversed phase enrichment cartridge, to separate the uranium from the bulk of the dissolved constituents. The uranium was then back flushed from the cartridge onto a reversed phase analytical column. The separated species were monitored spectrophotometrically after reaction with arsenazo(III). The detection limit was in the 1-2gg L 1 range with a precision of approximately 4%. 

To determine the effect of acid catalysed decomposition of NADH on the electrochemical response in our experiments, the decrease in oxidation current for NADH was recorded as a function of time. The results of this experiment were compared with the decrease in NADH concentration as spectrophotometrically determined. The rates of decrease of the current and the concentration of NADH are both first-order and occur on similar timescales (Fig. 2.14). Analysis of the data for the two experiments provide first-order rate constants of 1.68 and 1.16 x 10-4 s-1 for the electrochemical and spectrophotometric measurements, respectively. The small difference between these two constants can be explained by the additional consumption of NADH by reaction at the electrode during the electrochemical measurement. This electrochemical process is also a first-order rate process, and the extent of the effect can be determined by using the treatment of Hitchman and Albery [50] for electrolysis using a rotating disc electrode. The results are consistent with the observed difference in the two rate constants. 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

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