Colorimetric analyses

For many years the use of dithizone as a colorimetric reagent for lead was almost standard practice. Nowadays it is rather rarely used because of a range of disadvantages. These include the rather modest sensitivity and the possibility of interference from other metals [1]. In particular, the time-consuming nature of the method and the considerable experience necessary to achieve reliable results discourage many potential users. The method may still however be of use in laboratories lacking instrumentation for the other analytical methods. 

The formation of colored derivatives often results from one or several redox reactions. 

An acute examination of this reaction shows that it proceeds through two oxidization steps. The first one involves the oxidization of mbth, which gives a strongly electrophilic diazonium salt  

21 Some Applications of Redox Reactions in Qualitative Analysis 

The actual coupling reaction (that occurring between the diazonium salt and phenol) is the second step. It is characterized by a loss of two electrons and by a loss of one or two protons according to the acido-basic form phenol was under before the reaction. At least formally, one may imagine a pathway through the intermediary product 

It is rational to think that in a first step, a first mbth molecule is condensing with the aldehyde molecule to give the corresponding hydrazone. 

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Total sulfur M 07-052 Combustion in furnace and colorimetric analysis... 

Aminothiazole present in urine or blood plasma forms a colored Schiff base when 5-nitrofurfural is added the colorimetric analysis of the Schiff base allows the quantitative determination of this thiazole (1571). The Schiff base may also be dosed by polarographic of spectro-photometric methods (1572). 

Mercury vapour in air Diffusive samplers with qualitative onsite colorimetric analysis and quantitative cold vapour atomic absorption spectrometry in the laboratory 59... 

A number of different methods to monitor the amount of methylimidazole left in a final ionic liquid are known. NMR spectroscopy is used by most academic groups, but may have a detection limit of about 1 mol%. The photometric analysis described by Holbrey, Seddon, and Wareing has the advantage of being a relatively quick method that can be performed with standard laboratory equipment [13]. This makes it particularly suitable for monitoring of the methylimidazole content during commercial ionic liquid synthesis. The method is based on the formation and colorimetric analysis of the intensely colored complex of l-methylimidazole with cop-per(II) chloride. 

Colorimetric analysis based on visual estimation is not very exact. Some more accurate data on the H2, I2, HI system at equilibrium are shown in Table 9-1. The reaction is... 

The variation of the colour of a system with change in concentration of some component forms the basis of what the chemist commonly terms colorimetric analysis. The colour is usually due to the formation of a coloured compound by the addition of an appropriate reagent, or it may be inherent in the desired constituent itself. The intensity of the colour may then be compared with that obtained by treating a known amount of the substance in the same manner. 

Cobalt, sepn. of from nickel, (cm) 532 Codeine and morphine, D. of 740 Coefficient of variation 135 Colloidal state 418 See also Lyophilic, Lyophobic Colorimeters light filters for, 661 photoelectric, 645, 666 Colorimetric analysis 645 criteria for, 672 general remarks on, 645, 672 procedure, 675 solvent selection, 674 titration, 652... 

In each of the preceding studies, one major advantage of the near-IR technique is the rapid evaluation of the sample without the need for traditional extraction techniques and subsequent chromatographic or colorimetric analysis. For this reason, near-IR is an ideal form of analysis in a quality control, or on-line, process analysis environment. In a complementary mid- and near-IR... 

In addition to the chemical effects of varying salinity, there are optical interferences in colorimetric analysis which are peculiar to estuarine samples. Saline waters and river waters have, in the absence of colorimetric reagents, an apparent absorbance arising from ... 

GC and HPLC allow for ready quantification of the components exiting the column in that the area under the peak in the chromatogram is proportional to the amount of component present. However, to make a quantitative analysis, it is essential to have a calibration curve (see Chapter 14) for each of the components of interest. This means making solutions of differing but known concentrations, injecting them, and finding the relationship between peak area and amount of material present in a manner similar to that described in Section 14.9, for colorimetric analysis. In many cases, the software that controls the chromatograph can be set up to automatically do this analysis (see also Chapter 15). 

When using a spectrophotometer for a colorimetric analysis, both the 0% and 100% transmittance (oo and 0 absorbance) readings must be set. Once the instrument has warmed up, with the light beam blocked and with nothing in the sample compartment, the readout is set to 0% transmittance (oo abs.). Again, this measurement is done to set / in the absorbance equation shown earlier. A blank, a solution containing all the components used in the analysis except the analyte being measured, is placed in a cuvette, placed in the sample... 

Acid-base (neutralization) reactions are only one type of many that are applicable to titrimetric analysis. There are reactions that involve the formation of a precipitate. There are reactions that involve the transfer of electrons. There are reactions, among still others, that involve the formation of a complex ion. This latter type typically involves transition metals and is often used for the qualitative and quantitative colorimetric analysis (Chapters 8 and 9) of transition metal ions, since the complex ion that forms can be analyzed according to the depth of a color that it imparts to a solution. In this section, however, we are concerned with a titrimetric analysis method in which a complex ion-forming reaction is used. 

A good example of a bidentate ligand is the 1,10-phenanthroline molecule, which, since it forms a stable complex ion with Fe2+ ions that is deep orange color, is used in the colorimetric analysis of iron(II) ions (see Experiment 19 in Chapter 7). This ligand is shown in Figure 5.17(a). The two bonding sites are... 

Experiment 19 Colorimetric Analysis of Prepared and Real Water Samples for Iron... 

IUPACC Commission on Spectrochemical and other Optical Procedures for Analysis Spectrophotometric Data for Colorimetric Analysis, London, Butterworth Publishers Ltd., 1963. 

However, near-stoichiometric Fe " ion binding to NifU-1 or NifU was observable only in experiments conducted at 2°C in anaerobic samples that had been pretreated with dithiothreitol to ensure reduction of any intrasubunit or intersubunit disulfides. At room temperature, <10% of the NifU-1 or NifU was in a Fe bound form, and colorimetric analysis indicates that the remainder of the Fe is in solution was in the form of free Fe " ion. Hence this mononuclear Fe -bound species is more likely to be an intermediate in the reduction of Fe ion by NifU or NifU-1 rather than an initial step in cluster assembly on the NifU-1 domain of NifU. In this connection, it is important to note that Fe is rapidly reduced to Fe by cysteine in aqueous solution (Schubert, 1932). The physiological significance (if any) of the apparent ferric reductase activity associated with the NifU-1 domain of NifU remains to be established. 

In Figure 5.1, the fraction of iron whose concentration is being reported is identified as the total dissolved iron concentration. In practice, this fraction is operationally defined by the analytical method used in its measurement. For the data in Figure 5.1, the total dissolved iron concentration was determined by filtration to remove the solid iron, followed by colorimetric analysis to quantify the solutes. Another analytical technique, such as filtration followed by atomic absorption spectrophotometry, might yield a different total dissolved concentration, so it is important to be aware of the analytical methods used. To address this issue, marine chemists engage in intercalibration experiments to assess differences in results from various analytical methods. 

Colorimetric analyzers spectrophotometrically measure the increase in color (absorbance) of a solution resulting from contact with a measured volume of air. The absorbance is linearly proportional to the concentration of the colored species, within known limits. Continuous colorimetric analysis of total oxidants is carried out with a solution of neutral buffered potassium iodide (KI). In 1953, Littman and BenolieF developed the first colorimetric oxidant recorder to come into general use. Instruments of this design, using a 20% neutral buffered potassium iodide solution, later changed to 10%, were incorporated into the Los... 

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