Spectrophotometric measurements, and

However, the situation for the latter acid was reconsidered in detail by van de Graaf, Hoefnagel and Wepster236. They obtained values of pK and pK2 (water, 25 °C) of 2.419 and 4.877 respectively by spectrophotometric measurements and calculated a value of pKd of 4.83 for the ionization of the neutral acid. (The ratio of zwitter-ion to neutral acid present is 10.5 89.5.) From this pKa value and 4.21 for the pKa value of benzoic acid, ap of NH2 is —0.62. This is the recommended value in the IUPAC report79. 

Under conditions equivalent to spectrophotometric measurements and for a solution of a light-absorbing compound of known ex and concentration, the thickness of the system traversed by light (/) is given by the Beer-Lambert law (Eq. 41) ... 

Acetic acid measurement involves conversion of acetate to acetyl-CoA by acetyl-CoA synthetase with the consumption of ATP (Boehringer Mannheim, 1986). Acetyl-CoA reacts with oxaloacetate and water in the presence of citrate synthetase to form citrate and CoA. Oxaloacetate for this reaction is obtained from malate by the action of malate dehydrogenase with concomitant conversion of NADH from NAD+. NADH is spectrophotometrically measured and correlated to acetic acid concentration. 

The ratio /spectrophotometric measurement, and the value of a is then calculated from eq. (3.5) to yield the desired absorption constant. The numerous absorption constants found in the literature arise from the choice of quantities incorporated in the constant b. Some of the terms most commonly used to express absorption in minerals are summarized in table 3.2. Note that optical densities (O.D.), representing the direct output from many spectrophotometers, lack specificity about sample thickness and element concentrations. Absorption coefficients (a) indicate that sample thicknesses have been measured or estimated. Molar extinction coefficients (e) require chemical analytical data as well as knowledge of sample thicknesses. 

The diffusion of P into thermally grown films from P vapor was investigated using secondary ion mass spectrometry, wet-chemical spectrophotometric measurements and X-ray photo-electron spectroscopy. The diffusion coefficients of P in films at 1273 to 1373K were given by ... 

The data in the first two lines were obtained from spectrophotometric measurements, and those in the last line by direct titration. 

ECD is basically a spectrophotometric measurement and as such is subjected to the same precautions as those applied for the collection of an ordinary UV-Vis unpolarized absorption spectrum. First, the total sample absorption must not exceed 1.5 absorbance units, with 0.8 being a recommended value. This may be achieved by adjusting either the path length (O.l-lOOmm) of the sample cuvette... 

An illustrative example generates a 2 x 2 calibration matrix from which we can determine the concentrations xi and X2 of dichromate and permanganate ions simultaneously by making spectrophotometric measurements yi and j2 at different wavelengths on an aqueous mixture of the unknowns. The advantage of this simple two-component analytical problem in 3-space is that one can envision the plane representing absorbance A as a linear function of two concentration variables A =f xuX2). 

Silver diethyldithiocarbamate [1470-61-7] is a reagent commonly used for the spectrophotometric measurement of arsenic in aqueous samples (51) and for the analysis of antimony (52). Silver iodate is used in the determination of chloride in biological samples such as blood (53). 

The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

O Connor and Cox and Yates have reviewed the many acidity function scales. A major use of acidity functions is for the measurement of the strengths of very weak bases. The procedure utilizes spectrophotometric measurements of the concentration ratio Cb/cbh+ in solutions of known acidity function and application of Eq. (8-89). One problem is the estimation of the spectra of the pure forms (protonated and unprotonated) of the base, for the spectra are subject to the medium effect, and corrections must be applied. Another problem is that the base... 

Discussion. The procedure is based on the formation of yellow tetraiodo-antimonate(III) acid (HSbI4) when antimony(III) in sulphuric acid solution is treated with excess of potassium iodide solution. Spectrophotometric measurements may be made at 425 nm in the visible region or, more precisely, at 330 nm in the ultraviolet region. Appreciable amounts of bismuth, copper, lead, nickel, tin, tungstate, and molybdate interfere. 

When reporting spectrophotometric measurements, details should be given of the concentration used, the solvent employed, the make and model of the instrument, as well as the slit widths employed, together with any other pertinent information. 

In flame spectrophotometric measurements we are concerned with solutions having very small concentrations of the element to be determined. It follows that the standard solutions which will be required for the analyses must also contain very small concentrations of the relevant elements, and it is rarely practicable to prepare the standard solutions by weighing out directly the required reference substance. The usual practice therefore is to prepare stock solutions which contain about 1000 ig mL 1 of the required element, and then the working standard solutions are prepared by suitable dilution of the stock solutions. Solutions which contain less than 10 igmL 1 are often found to deteriorate on standing owing to adsorption of the solute on to the walls of glass vessels. Consequently, standard solutions in which the solute concentration is of this order should not be stored for more than 1 to 2 days. 

The indirect methods discussed thus far have dealt with measurement of color only as it can be correlated with physical characteristics of materials and the effect of these materials on radiant energy. As has been pointed out, the reflectance spectro-photometric curve describes a property of the material. A change in the reflectance spectrophotometric properties may not always result in a change in visual color. The reason is that color of the object is not an unchangeable characteristic of the object itself, dependent only upon these reflectance properties, but is also dependent upon the quality of the illuminating light and the sensitivity of the observer s eye. Thus the measurement and description of visual color are psychophysical problems... 

Kilpatrick et al.111 measured the rate of desulphonation of mesitylenesulphonic acid in excess of 12-13.5 M sulphuric acid. The rate coefficients are given in Table 43 (p. 73), and these were determined by spectrophotometrically measuring the concentration of sulphonic acid present at equilibrium, after reacting the sulphonic acid with the sulphuric acid of appropriate concentration again the increase in rate of desulphonation with increase of acid strength is much less than the increase in the rate of sulphonation. The increase in rate of desulphonation of mesitylenesulphonic acid with increasing acid concentration was less than that for 1,2,4,5-tetra-, 1,2,3,5-tetra-, and penta-methylbenzenesulphonic acids703 (Table 244) the... 

Other detection methods are based on optical transmittance [228-231], Alcohol sulfates have been determined by spectrophotometric titration with barium chloride in aqueous acetone at pH 3 and an indicator [232] or by titration with Septonex (carbethoxypentadecyltrimethylammonium bromide) and neutral red as indicator at pH 8.2-8.4 and 540 nm [233]. In a modified two-phase back-titration method, the anionic surfactant solution is treated with hyamine solution, methylene blue, and chloroform and then titrated with standard sodium dodecyl sulfate. The chloroform passing through a porous PTFE membrane is circulated through a spectrometer and the surfactant is analyzed by determining the absorbance at 655 nm [234]. The use of a stirred titration vessel combined with spectrophotometric measurement has also been suggested [235]. Alternative endpoint detections are based on physical methods, such as stalag-mometry [236] and nonfaradaic potentiometry [237]. 

Ratzlaff, K. L., and bin Darus, H., Optimization of Precision in Dual Wavelength Spectrophotometric Measurement, Anal. Chem. 51, 1979, 256-261. 

From spectrophotometric measurements (at 425 mfi) on the appearance and disappearance of dimer, values for the rate coefficients ki and k were found to be (at 0 °C and ionic strength 1.0 M) 6.7x10 and 3.3x10 l.mole . sec , respectively. Under the same conditions k, was calculated to be 3.9x10 1. mole . sec. ... 

Kinetic studies on the reactions (in the absence of oxygen, in perchlorate media) have been made " for the ions CrX where X = Cl", F", NJ and CN" the rate data was obtained using spectrophotometric measurements for the... 

In the absence of oxygen the rate law obtained from spectrophotometric measurements at wavelengths 382 m/x (trans-isomers) and 510 or 512 m/x (cis-isomers) was found to be of the form... 

The reduction of Co(lll) by Fe(II) in perchloric acid solution proceeds at a rate which is just accessible to conventional spectrophotometric measurements. At 2 °C in 1 M acid with [Co(IlI)] = [Fe(II)] 5 x 10 M the half-life is of the order of 4 sec. Kinetic data were obtained by sampling the reactant solution for unreacted Fe(Il) at various times. To achieve this, aliquots of the reaction mixture were run into a quenching solution made up of ammoniacal 2,2 -bipyridine, and the absorbance of the Fe(bipy)3 complex measured at 522 m/i. Absorbancies of Fe(III) and Co(lll) hydroxides and Co(bipy)3 are negligible at this wavelength. With the reactant concentrations equal, plots of l/[Fe(Il)] versus time are accurately linear (over a sixty-fold range of concentrations), showing the reaction to be second order, viz. 

Equation (41.11) represents the (deterministic) system equation which describes how the concentrations vary in time. In order to estimate the concentrations of the two compounds as a function of time during the reaction, the absorbance of the mixture is measured as a function of wavelength and time. Let us suppose that the pure spectra (absorptivities) of the compounds A and B are known and that at a time t the spectrometer is set at a wavelength giving the absorptivities h (0- The system and measurement equations can now be solved by the Kalman filter given in Table 41.10. By way of illustration we work out a simplified example of a reaction with a true reaction rate constant equal to A , = 0.1 min and an initial concentration a , (0) = 1. The concentrations are spectrophotometrically measured every 5 minutes and at the start of the reaction after 1 minute. Each time a new measurement is performed, the last estimate of the concentration A is updated. By substituting that concentration in the system equation xff) = JC (0)exp(-A i/) we obtain an update of the reaction rate k. With this new value the concentration of A is extrapolated to the point in time that a new measurement is made. The results for three cycles of the Kalman filter are given in Table 41.11 and in Fig. 41.7. The... 

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