Spectrophotometric methods

In the early work of Hammett and Deyrup8 the measurement of the ionization ratio was based on the color change of the indicator. The solutions containing the indicator were compared at 25°C in a colorimeter with a standard reference. This reference was water, when the indicator was colorless in its acid form, and 96% sulfuric acid (or 70% perchloric acid), when the indicator was colorless in the basic form. 

For example, when the indicator was colored in water the authors define a stoichiometric color intensity relative to water 7W = Cw/Ca, where Ca and Cw are the stoichiometric concentrations of indicator in solution A and in water. On the other hand, the specific color intensity of the colored form relative to water is defined as Sw = [B]w/[B]a, where [B]w is the concentration of the colored base in water and [B]ais concentration in solution A. Because the indicator exists only in its basic form in water, [B]w = Cw and in solution A, Ca = [B]a+ [BH+]a. The ionization ratio is given by Eq. (1.22). 

Despite seven decades of technical and scientific progress, the original Hammett method has not become obsolete. The colorimeter has been replaced by modern spectrophotometers that can be operated at selected wavelengths extending the spectra beyond visible into the ultraviolet region of the electromagnetic spectrum. The experimental variable, which is wavelength-dependent, is the optical density D. D is related to the concentration by the Beer-Lambert law [Eq. (1.23)]. 

C is the concentration of the absorbing species, l is the length of the cell, andis the molar absorptivity (or extinction coefficient). If at a given wavelength A, eBH +, eB, and sK are the extinction coefficients, respectively, of acid form of the indicator, its basic form, and of the unknown solution, the ionization ratio is given by Eq. (1.24). 

For a greater precision in this determination, A should be chosen so as to have the maximum difference between eBH+ and B. For this reason, the areas between the absorption line and the baseline of both acidic and basic forms of the indicator should be measured and compared. 

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A2.6540-g sample of an iron ore known to contain 53.51% w/w Fe is dissolved in a small portion of concentrated HCl and diluted to volume in a 250-mL volumetric flask. A spectrophotometric method is used to determine the concentration of Fe in this solution, yielding results of 5840, 5770, 5650, and 5660 ppm. Determine whether there is a significant difference between the experimental mean and the expected value at a = 0.05. 

A spectrophotometric method for the quantitative determination of Pb + levels in blood yields an Sjtand of 0.474 for a standard whose concentration of lead is 1.75 ppb. How many parts per billion of Pb + occur in a sample of blood if Ssamp is 0.361 ... 

A second spectrophotometric method for the quantitative determination of Pb + levels in blood gives a linear normal calibration curve for which... 

A third spectrophotometric method for the quantitative determination of the concentration of in blood yields an Sjamp of 0.193 for a 1.00-mL sample of blood that has been diluted to 5.00 mb. A second 1.00-mL sample is spiked with 1.00 )J,L of a 1560-ppb Pb + standard and diluted to 5.00 mb, yielding an Sspike of 0.419. Determine the concentration of Pb + in the original sample of blood. 

A fifth spectrophotometric method for the quantitative determination of the concentration of Pb + in blood uses a multiple-point standard addition based on equation 5.6. The original blood sample has a volume of 1.00 mb, and the standard used for spiking the sample has a concentration of 1560 ppb Pb +. All samples were diluted to 5.00 mb before measuring the signal. A calibration curve of Sjpike versus Vj is described by... 

A sixth spectrophotometric method for the quantitative determination of Pb + levels in blood uses CQ+ as an internal standard. A standard containing 1.75 ppb Pb + and 2.25 ppb CQ+ yields a ratio of Sa/Sis of 2.37. A sample of blood is spiked with the same concentration of Cu +, giving a signal ratio of 1.80. Determine the concentration of Pb + in the sample of blood. 

EDTA forms colored complexes with a variety of metal ions that may serve as the basis for a quantitative spectrophotometric method of analysis. The molar absorptivities of the EDTA complexes of Cu +, Co +, and Ni + at three wavelengths are summarized in the following table (all values of e are in cm )... 

As noted, this procedure is adapted from a standard spectrophotometric method. The instructions for the spectrophotometric method indicate that the absorbance should be measured 5-10 min after adding the ascorbic acid. Why is this waiting period necessary in the spectrophotometric method, but not necessary in the FIA method ... 

The reduction of the yellow-colored Mo(VI) complex to the blue-colored Mo(V) complex is a slow reaction. In the standard spectrophotometric method, it is difficult to reprodudbly control the amount of time that reagents are allowed to react before measuring the absorbance. To achieve good precision, therefore, the reaction is allowed sufficient time to proceed to completion before measuring the absorbance. In the FIA method, the flow rate and the dimensions of the reaction coil determine the elapsed time between sample introduction and the measurement of absorbance (about 30 s in this configuration). Since this time is precisely controlled, the reaction time is the same for all standards and samples. 

The spectrophotometric method recommends using standard solutions of phosphate in the range of 2-10 ppm, whereas the FIA method recommends standards in the range of 10-60 ppm. Explain why the methods use a different range of standards. 

Pandey, S. McHale, M. E. R. Horton, A. M. et al. Kinetics-Based Indirect Spectrophotometric Method for the Simultaneous Determination of Mn04 and Gt207 ,... 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

In most analytical procedures for determining the total phosphoms content (normally expressed in terms of P20 ), the phosphates are converted to the orthophosphate form. Typically, condensed phosphates are hydrolyzed to orthophosphate by boiling in dilute mineral acid (0.1 N). The orthophosphate is then deterrnined by gravimetric or spectrophotometric methods. For gravimetric deterrnination, insoluble phosphomolybdates (or magnesium ammonium orthophosphate) is formed. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

Volumetric, gravimetric, and spectrophotometric methods for the analysis of rhenium are mainly of historical interest. These are described elsewhere... 

Infrared Spectrophotometry. The isotope effect on the vibrational spectmm of D2O makes infrared spectrophotometry the method of choice for deuterium analysis. It is as rapid as mass spectrometry, does not suffer from memory effects, and requites less expensive laboratory equipment. Measurement at either the O—H fundamental vibration at 2.94 p.m (O—H) or 3.82 p.m (O—D) can be used. This method is equally appticable to low concentrations of D2O in H2O, or the reverse (86,87). Absorption in the near infrared can also be used (88,89) and this procedure is particularly useful (see Infrared and raman spectroscopy Spectroscopy). The D/H ratio in the nonexchangeable positions in organic compounds can be determined by a combination of exchange and spectrophotometric methods (90). 

Kinetic mles of oxidation of MDASA and TPASA by periodate ions in the weak-acidic medium at the presence of mthenium (VI), iridium (IV), rhodium (III) and their mixtures are investigated by spectrophotometric method. The influence of high temperature treatment with mineral acids of catalysts, concentration of reactants, interfering ions, temperature and ionic strength of solutions on the rate of reactions was investigated. Optimal conditions of indicator reactions, rate constants and energy of activation for arylamine oxidation reactions at the presence of individual catalysts are determined. 

Table. Extraction-spectrophotometric methods of metals determination...

It is established, that the natural and synthetic polymers influence on spectrophotometrical, protolytical and complex-formating properties of azodyes in different degree. The result of interaction between anions of organic dyes and polymers is formation of specifical hydrophobic-hydrated adducts. Express spectrophotometrical methods of polymer content determination in water solutions with the help of polymer adducts have been elaborated. 

Spectrophotometric methods based on an enhancement of the blue color produced on reduction of 12-molybdophosphate (arsenate) in the presence of antimony(III) are widely used for the determination of phosphoms(V) or arsenic(V). However, nature of heteropoly blue, their spectra, mechanism of the reaction are obscure. In addition, mixed POMs were shown as very efficient analytical forms for the determination of P(V) and As(V). 

Concentration limits of the diphosphate-ion, admissible to determination of magnesium and cobalt, manganese and cobalt, zinc and cobalt by spectrophotometric method with application of the l-(2-pyridylazo)-resorcinol (PAR) are presented. Exceeding maintenance of the diphosphate-ion higher admissible supposes a preliminary its separation on the anionite in the H+-form. The optimum conditions of cobalt determination and amount of the PAR, necessary for its full fastening are established on foundation of dependence of optical density of the cobalt complex with PAR from concentration Co + and pH (buffer solutions citrate-ammoniac and acetate-ammoniac). 

There are a number of the optimal conditions for sorption pointed below volume of extract - 4 ml ratio of aqueous and organic phases is 1 1 cyllindric foampolyurethan (marc - T 25-3,8) with diameter 8 mm, height - 4,5 mm, mass - 0.007 g. Time of full soi ption is 20 min. Completeness of soi ption is determined by spectrophotometric method. 

Uranium and thorium are the first members of natural radioactive chain which makes their determination in natural materials interesting from geochemical and radioecological aspect. They are quantitatively determined as elements by spectrophotometric method and/or their radioisotopes by alpha spectrometry. It is necessary to develop inexpensive, rapid and sensitive methods for the routine researches because of continuous monitoring of the radioactivity level. 

SOLID-PHASE SPECTROPHOTOMETRIC METHOD FOR THE DETERMINATION OF ANIONIC SURFACTANTS WITHOUT LIQUID-LIQUID EXTRACTION... 

PVSA-SG film was used for determination of Fe(Phen) + and Zn + as ternary complex Zn +-Phen-bengal rose by spectrophotometric method. The calibration graph was linear in the concentration 5T0 -5T0 mol/lfor Fe(II) and FlO - 5T0 mol/1 for Zn(II). The film can be regenerated and reused. LG-PDMDA-SG film was shown to be perspective modificator of the PG electrode surface and used for voltammetric detection of Mo(VI) at ppb level. 

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