Analytical techniques - precipitation reactions

Many qualitative and quantitative methods for soluble antigens are based on their precipitation by antibodies. It is important that the conditions for each assay are carefully optimized in order to achieve the desired end. 

Gels are used in immunoprecipitation techniques to stabilize the precipitate, enabling both the position and the area of the precipitate to be measured. The point has already been made that maximum precipitation occurs when the equivalent proportions of both antigen and antibody are available. Hence, if a high concentration of antigen is permitted to diffuse into a gel that contains a uniform concentration of antibody, at some point in the concentration gradient of antigen that is 

Procedure 7.1 Quantitation of albumin by single radial immunodiffusion 

Agar gel containing 10% (v/v) of a polyclonal antibody to human albumin. 

Standard solutions of human albumin with concentrations ranging from 10 to 100 mg I 1. 

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Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

Techniques for the analysis of trace amounts of stable cesium include neutron activation analysis (NAA), and optical emission and atomic absorption spectroscopy (Iyengar et al., 1980). Often, the analysis of cesium in biological samples was only developed as a by-product of multielement NAA. Older analytical techniques include precipitation with potassiumiodobis mutha-te(III) or with K3[Fe(CN)6] in the presence of acetic acid. A more modem approach is that of precipitation with NH4Fe[Fe(CN)6] to the CsFe[Fe(CN)6], a reaction which is mainly used to minimize the intestinal absorption of radiocesium in farmed or wild animals (Jander and Blasius, 1989). 

Polymeric supports of variable solubility have been investigated as an alternative to insoluble supports used in solid-phase synthesis. Reactions are performed in homogeneous media by choosing an appropriate solvent that solubilizes the polymer, and purification is performed by precipitation. This methodology benefits both solution-phase and solid-phase syntheses. Moreover compound characterization can be easily undertaken at any stage of the synthesis, since the support is soluble in standard spectroscopic solvents. A direct real-time control is possible, whereas a solid-phase protocol relies on a cleave and analyze strategy that consumes compound, imparts delay, and thus can only be accomplished at the end of synthesis. For these reasons soluble polymeric supports are preferred to conventional insoluble supports (resins, plastic pins), and they are compatible with analytical techniques such as NMR and mass spectrometry. 

In this most widely used type of simple radio-reagent method, an excess of radioactive reagent solution of known analytical concentration is usually used. However, it is necessary to separate the active product from the excess of radioactive reagent. For this reason, this procedure makes use of various separation techniques, such as precipitation, formation of extractable chelates, or sorption. The majority of determinations are based on classical precipitation reactions. 

With high-quality salts, the quantities of treat chemicals needed to provide the desired excesses often are more than the amounts consumed in the precipitation reactions. The process is monitored by analysis for the reagent concentrations in the treated brine. The analytical techniques are simple, and the usual practice is to have these checks run by the plant operators. Laboratory personnel perform more exhaustive analyses as required. 

Subsequent to any decomposition, but also in the case of liquid samples such as water and urine, the analytes of interest are generally present in dilute solution together with a large excess of foreign ions (e.g.. alkali-metal and alkaline-earth cations). Separation and concentration of the analytes may be necessary to improve the limit of detection and exclude interference. Useful techniques in this regard include liquid-liquid extraction. solid-phase extraction, special precipitation reactions, and electrolytic deposition. 

Analytical techniques used in qualitative analysis include flame tests (Chapter 2) and precipitation reactions (Chapters 3 and 13). Analytical techniques used in quantitative analysis include titrations (Chapter 1), inductively coupled plasma (ICP) spectroscopy (Chapter 22 on the accompanying website), ultraviolet—visible spectroscopy (Chapter 23 on the accompanying website), infrared spectroscopy and various chromatographic techniques (Chapter 23). Analytical techniques used in structural analysis include NMR, IR spectroscopy, mass spectrometry and visible—ultraviolet spectroscopy. Important areas that employ analytical techniques include ... 

Another way to view the behavior may be related to another old analytical technique of pH titrations to determine the concentration of orthophosphate in a solution by causing a precipitation of Ag3P04 by the reaction by the Gerber Miles titration. 

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

Subject areas for the Series include solutions of electrolytes, liquid mixtures, chemical equilibria in solution, acid-base equilibria, vapour-liquid equilibria, liquid-liquid equilibria, solid-liquid equilibria, equilibria in analytical chemistry, dissolution of gases in liquids, dissolution and precipitation, solubility in cryogenic solvents, molten salt systems, solubility measurement techniques, solid solutions, reactions within the solid phase, ion transport reactions away from the interface (i.e. in homogeneous, bulk systems), liquid crystalline systems, solutions of macrocyclic compounds (including macrocyclic electrolytes), polymer systems, molecular dynamic simulations, structural chemistry of liquids and solutions, predictive techniques for properties of solutions, complex and multi-component solutions applications, of solution chemistry to materials and metallurgy (oxide solutions, alloys, mattes etc.), medical aspects of solubility, and environmental issues involving solution phenomena and homogeneous component phenomena. 

II.l INTRODUCTION Before the student attempts to carry out the analytical reactions of the various cations and anions detailed in Chapters III and IV, he should be familiar with the operations commonly employed in qualitative analysis, that is with the laboratory technique involved. It is assumed that the student has had some training in elementary practical chemistry he should be familiar with such operations as solution, evaporation, crystallization, distillation, precipitation, filtration, decantation, bending of glass tubes, preparation of ignition tubes, boring of corks, and construction of a wash bottle. These will therefore be either very briefly discussed or not described at all in the following pages. 

Extraction techniques that involve a chemical reaction can be classified as nonselective extraction or concentration, when more than one analyte is extracted from the solution by using the organic collectors (e.g., 8-hydroxyquinoline and dithizone derivatives) and selective extraction or separation. The first step in such an extraction technique is the formation of the complex by adding the reagent(s) to the solution of analyte, and after the extraction of the complex in an organic solvent. The problem that can arise in a SIA system with these types of extraction is the precipitate that is formed, and this can contaminate the other sample and also can block the tubing. To avoid these problems, it is necessary either to dilute the sample in such a way that the precipitation equihbria will not be reached and that all the complex will remain dissolved in the solution, or by derivatization of the hgands to... 

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