Colorimetry, elemental analysis

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. 

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). 

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. 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Elemental composition A1 65.82%, N 34.18%, the metal is determined by wet analysis or AA spectroscopy. NH3 liberated on hydrolysis may be determined by titration or colorimetry (see under Ammonia). 

Elemental composition Be 49.11%, N 50.89%. Analysis may be performed by treatment with HCl. The soluble BeCL solution is then measured for Be by AA or ICP techniques. The ammonia liberated is determined by titrimetry, colorimetry or by ammonia-selective electrode (see Ammonia). 

Elemental composition Ca 29.46%, H 0.74%, P 22.77%, O 47.04%. The compound may be identified by x-ray analysis. Calcium may be analyzed by AA or ICP spectrometry in aqueous matrix following acid digestion. Phosphorus in the aqueous solution may be determined by colorimetry (see Phosphorus). 

Elemental composition Ca 38.76%, P 19.97%, 0 41.26%. Calcium may he analyzed by AA, and ICP, or x-ray methods (see Calcium). The orthophosphate anion may be analyzed by colorimetry (see Phosphorus). For colorimetric analysis the insoluble tribasic phosphate must be brought into aqueous phase hy dissolving in dilute sulfuric acid. 

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 focus of this research and other mass balance studies has been on trace elements (1,2,3). However, in future studies on speciation it will be necessary to know the concentrations of the elements present in amounts above 1%. Therefore, analyses of the oil shale and spent shale samples were performed for these elements. Atomic absorption and colorimetry were used for many of these analyses. Some major element results also were obtained by the broad-range instrumental analysis surveys. The comparison of the results obtained by the different techniques shows large discrepancies. 

Historically, analysis for selenium has been difficult, partly because environmental concentrations are naturally low. Indeed, selenium analysis still remains problematic for many laboratories at concentrations below 0.01 mg a relatively high concentration in many environments (Steinhoff et al., 1999). Hence, selenium has often been omitted from multi-element geochemical surveys despite its importance (Darnley et al., 1995). Analytical methods with limits of detection of <0.01 mgL include colorimetry, total reflectance-XRF, HG-AFS, gas chromatography... 

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. 

---