Soil color intensity

The principle behind the test method(s) is that antibodies are made of proteins that recognize and bind with foreign substances (antigens) that invade host animals. Synthetic antibodies have been developed to complex with petroleum constituents. The antibodies are immobilized on the walls of a special ceU or filter membrane. Water samples are added directly to the cell, while soils must be extracted before analysis. A known amount of labeled analyte (typically, an enzyme with an affinity for the antibody) is added after the sample. The sample analytes compete with the enzyme-labeled analytes for sites on the antibodies. After equilibrium is established, the cell is washed to remove any um-eacted sample or labeled enzyme. Color development reagents that react with the labeled enzyme are added. A solution that stops color development is added at a specified time, and the optical density (color intensity) is measured. Because the coloring agent reacts with the labeled enzyme, samples with high optical density contain low concentrations of analytes. Concentration is inversely proportional to optical density. 

The systematic development of spot test methods of analysis occupied Fritz Feigl in Vienna and Rio de Janeiro for half a century up to 1970. Although in the past few decades chemical analysis has undergone a formidable process of sophistication with the development of advanced instrumental tools, there has been at the same time a contrary trend toward simplification in selected areas in the form of simple, rapid, and inexpensive spot and screening tests. Commercial companies are selling large numbers of compact spot test systems for the rapid establishment of the presence or absence of particular substances in clinical, food, water, soil, and forensic samples. The tests are essentially qualitative, but often can be semiquantitative if procedures as simple as visual comparison of color intensity are used. 

Fe is relatively soluble in weakly acidic water but will be quickly oxidized under neutral or weakly alkaline conditions, and precipitated as iron oxi-hydroxides (goethite, a-FeOOH) that cause soils in temperate-humid climates to be yellow-brown (siallitic type of weathering). Under tropical-humid climatic conditions, silicon will be removed completely and iron ions fixed in hematite, thus coloring tropical soils (laterites) intensively red (allitic type of weathering). 

The intensity and color of the poppies in Monet s painting The Poppyfield, near Giverny are influenced by the acidity or basicity of the soil. Of course artistic license could also have something to do with the intensity and color of the poppies in this painting. 

The resultant magenta color is remarkably intense per unit of parathion this, and other characteristics, indicated the ready adaptability of this method to the quantitative estimation of parathion in and on fruits, vegetables, foliage, and woody tissues, and in soil. The present paper reports upon endeavors to define or elaborate upon the parameters... 

An excellent example of this type of analysis involves the determination of phosphate in soil extracts. Soil is extracted with an appropriate extractant and added to a solution of acid molybdate, with which the phosphate reacts to produce a purple- or blue-colored solution of phosphomolybdate. Standard phosphate solutions are prepared, reacted with acid molybdate, and the intensity of the phosphomolybdate color produced is measured. A standard curve (also called a calibration curve) is prepared (see Section 14.10) from which the intensity of the color is directly related to the concentration of phosphate in the extract. 

If a water sample contains both soluble and insoluble manganese (Mn) compounds and ions, and it is filtered to separate the dissolved and insoluble fractions, and the filtrate and insoluble residue are analyzed separately, the results can be expressed as total dissolved Mn and total suspended or insoluble Mn. Phosphorus (P) can be determined colorimet-rically as the ortho-phosphate ion, P043, in aqueous samples after a reaction that forms an intensely blue-colored derivative. However polyphosphate ions and other ions and compounds containing P do not form this derivative. Total P in a sample can be determined with the same colorimetric procedure after acid hydrolysis and oxidation of all ions and compounds containing P to P04 3. In some elemental analyses the sample is treated with reagents designed to make available for measurement some fraction of an element or elements but not the total amount. For example, a soil sample may be treated with water at pH 3 to simulate the leaching process of acid rain. A total elemental analysis of the filtrate provides information about just those elements solubilized by the mild acid treatment. This can be called the determination of total mild acid leachable elements. 

Stream humic substances were isolated from water by vacuum concentration (Shapiro, 1957 Barth and Acheson, 1962 Black and Christman, 1963a Midwood and Felbeck, 1968), freeze concentration (Black and Christman, 1963a Shapiro, 1967), and solvent extraction (Shapiro, 1957 Barth and Acheson, 1962 Black and Christman, 1963a). Almost all humic substances studied were obtained from dilute organically colored waters, because sufficient quantities of stream humic substances for intensive studies could be isolated with the least effort, and because many uncolored waters were not believed to contain humic substances. Because the isolation of stream humic substances was so time consuming it was considered acceptable to use soil humic substances obtained by extraction of soils as representative of stream humic substances in water quality studies. 

One colorimetric technique still in use in archaeology is a field test for soil phosphate. Phosphate in soils and sediments is an indicator of past human activity. Phosphate testing of soil samples is used to look for such indications (Fig. 4.4). A small sample of soil is mixed with hydrochloric and ascorbic acid to release soluble phosphates. The reaction produces a distinctive blue color in the presence of phosphate and the intensity of the color reflects its concentration. Results can be determined by eye or with the use of an instrument. 

Suppose that in a particular soil solution all indicators with EX values above + 200 millivolts converted to their reduced forms (which tend to be colorless), but all indicators with EX below -f 200 millivolts remained in the oxidized (colored) form. The Eh of the solution must then have been very near +200 millivolts. Methylene blue, which is intensely colored in the oxidized state, has been used as an indicator of the onset of strongly reducing conditions in soil solutions. It becomes colorless at an Eh of about +11 millivolts, assuming a pH of 7 (see Table 7.2). 

---