Chemical analysis methods colorimetry

COLORIMETRY. A method of chemical analysis thal deals with the measurement of the light absorption by colored solutions. Since light absorption depends upon the concentraiion of a specific constituent in solution, colorimetry is frequently used by geologists to determine qualitatively the trace quantities of many elements. 

The chemical characterization of soils and soil components include a number of methods, ranging from simple measurements such as pH or electrical conductivity to elemental analysis after total dissolution by digestion with HNO3/HCI/HF (Sparks 1996). Table 7.8 summarizes the most important analysis methods for elements of interest in soils. Many of these methods have been covered in previous sections. Some of them (e.g., chromatography, colorimetry) are common, well-known analytical techniques (Skoog et al. 2004 Harris 2010). 

Wet chemical methods involve sophisticated sample preparation and standardization with National Bureau of Standards reference materials but are not difficult for the analytical chemist nor necessarily time consuming (Figure 1). The time from sample preparation to final results for various analytical methods, such as GFAA (graphite furnace atomic absorption), ICP (inductively coupled plasma spectroscopy), ICP-MS (ICP-mass spectrometry), and colorimetry, ranges from 0.5 to 5.0 h, depending on the technique used. Colorimetry is the method of choice because of its extreme accuracy. Typical results of the colorimetric analysis of doped oxides are shown in Tables I and II, which show the accuracy and precision of the measurements. 

Absorption spectrometry is a traditional method used for the measurement of various chemical substances and makes it possible to carry out visual colorimetry allowing easy measurements. Conventional absorption spectrophotometry is the measurement of numerical values such as that of absorbance to carry out qualitative and quantitative analysis. In such cases, if the spectra obtained are complicated, the determination often becomes difficult. However, even if the spectral changes are quite complicated, our eyes recognize them simply as color changes. Determination utilizing the colors themselves is a perceptual method instead of simple absorption spectrophotometry. 

Experimentally, the investigation of adsorption from solution is much simpler than that of gas adsorption. A known mass of adsorbent solid is shaken with a known volume of solution at a given temperature until there is no further change in the concentration of the supernatant solution. This concentration can be determined by a variety of methods involving chemical or radiochemical analysis, colorimetry, refractive index, etc. The experimental data are usually expressed in terms of an apparent adsorption isotherm in which the amount of solute adsorbed at a given temperature per unit mass of adsorbent - as calculated from the decrease (or increase) of solution concentration - is plotted against the equilibrium concentration. 

Ion chromatography has become an indispensable tool for the analytical chemist in the area of anion analysis. In many cases this method has superseded conventional wet chemical methods such as titration, photometry, gravimetry, turbidimetry, and colorimetry, all of which are labor-intensive, time-consuming, and occasionally susceptible to interferences. Publications by Darimont [1] and Schwedt [2] have shown, that ion chromatographic methods yield results comparable to conventional analytical methods, thus dissolving the scepticism with which this analytical method was initially met. In the field of cation analysis, ion chromatography is attractive because of its simultaneous detection and sensitivity. It provides a welcome complement to atomic spectroscopic methods such as AAS and ICP. 

Colorimetric methods, whereby chemical species are determined by their ability to alter the colour intensity of a dye, have limited application in palaeolimnology, because there are more suitable alternative methods for most elements. However, colorimetry remains the method of choice for P (e.g., APHA, 1980). A flow injection method can be used to automate the analysis (e.g.. Mas et al., 1990). If the total P concentration of a sample is required, wavelength dispersive XRF is a good alternative. 

The first quantitative analytical fields to be developed were for quantitative elemental analysis, which revealed how much of each element was present in a sample. These early techniques were not instrumental methods, for the most part, but relied on chemical reactions, physical separations, and weighing of products (gravimetry), titrations (titrimetry or volumetric analysis), or production of colored products with visual estimation of the amount of color produced (colorimetry). Using these methods, it was found, for example, that dry sodium chloride, NaCl, always contained 39.33% Na and 60.67% Cl. The atomic theory was founded on early quantitative results such as this, as were the concept of valence and the determination of atomic weights. Today, quantitative inorganic elemental analysis is performed by atomic absorption spectrometry (AAS), AES of many sorts, inorganic MS (snch as ICP-MS), XRF, ion chromatography (1C), and other techniques discussed in detail in later chapters. 

A variety of methods have been used for the analysis of inorganic and organic cations traditional spectroscopic techniques such as colorimetry, wet chemical methods such as gravimetric analysis, turbidimetry, and titrime-try, and electrochemical techniques such as use of an ion-selective electrode and amperometric titrations. Some of these methods suffer from interferences and limited sensitivity they can be labor-intensive and are often difficult to automate. 

Various analytical techniques such as spectrophotometry/colorimetry, FL and infrared spectrometry, voltammetry, thin-layer chromatography (TLC), gas chromatography (GC), and HPLC based on ultraviolet, diode array, or fluorometric detectors have been reported in the literature for analysis of vitamin E. Various critical and comprehensive reviews are available on vitamin E quantification in food and clinical samples (Ball, 1988, 1998 Lumley, 1993). The AOAC International Official Methods of Analysis (1995) provides several methods based on older, chemical approaches. The applications of these analytical techniques are briefly summarized below. 

Analytical chemistry is that branch of science that deals with the determination of the composition of matter, its elements, ions, radicals and compounds, by chemical or physical methods. It is, therefore, one of the bases on which the whole structure of chemistry is erected. The methods employed are very numerous and include the following chromatography, electro-analysis, elementary analysis, gas analysis, gas chromatography, gravimetric analysis, colorimetry, mass analysis, micro-analysis, polarography, potentiometry, qualitative and quantitative analysis, spectral analysis, thermal analysis, spot analysis and many others. 

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