Volumetric analysis standards

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. 

Arsenic trioxide may be made by burning arsenic in air or by the hydrolysis of an arsenic trihaUde. Commercially, it is obtained by roasting arsenopyrite [1303-18-0] FeAsS. It dissolves in water to a slight extent (1.7 g/100 g water at 25°C) to form a weaMy acidic solution which probably contains the species H AsO, orthoarsenous acid [36465-76-6]. The oxide is amphoteric and hence soluble in acids and bases. It is frequendy used as a primary analytical standard in oxidimetry because it is readily attainable in a high state of purity and is quantitatively oxidized by many reagents commonly used in volumetric analysis, eg, dichromate, nitric acid, hypochlorite, and inon(III). 

Potassium hydrogen phthalate [877-24-7] M 204.2. Crystd first from a dilute aqueous solution of K2CO3, then H20(3mL/g) between 100° and 0°. Before being used as a standard in volumetric analysis, analytical grade potassium hydrogen phthalate should be dried at 120° for 2h, then allowed to cool in a desiccator. 

In titrimetric analysis (often termed volumetric analysis in certain books), the substance to be determined is allowed to react with an appropriate reagent added as a standard solution, and the volume of solution needed for complete reaction is determined. The common types of reaction which are used in titrimetry are (a) neutralisation (acid-base) reactions (b) complex-forming reactions (c) precipitation reactions (d) oxidation-reduction reactions. 

The history of reference materials is closely linked with the development of analytical chemistry. In the 19th Century all chemicals were, in comparison with those of today, of poor purity. Thus, for volumetric analysis suitable purified materials as primary standards had to be specified. One of the first examples was the recommendation of As(III) oxide by Gay-Lussac (1824) for this purpose. Somewhat later, Sorensen (1887) proposed criteria for the selection of primary chemical standards. These were further elaborated by Wagner (1903) at the turn of the last century. It is worthwhile mentioning that their criteria were quite similar to those used today. 

As we have seen that the volumetric analysis essentially requires the precise and accurate measurement of weights and volumes of interacting solutions. However, the weights are measured upto the fourth place of decimal by using a manually operated good analytical balance or a single-pan electrical balance that need to be calibrated periodically with the help of a standard weight box. 

Part—I has three chapters that exclusively deal with General Aspects of pharmaceutical analysis. Chapter 1 focuses on the pharmaceutical chemicals and their respective purity and management. Critical information with regard to description of the finished product, sampling procedures, bioavailability, identification tests, physical constants and miscellaneous characteristics, such as ash values, loss on drying, clarity and color of solution, specific tests, limit tests of metallic and non-metallic impurities, limits of moisture content, volatile and non-volatile matter and lastly residue on ignition have also been dealt with. Each section provides adequate procedural details supported by ample typical examples from the Official Compendia. Chapter 2 embraces the theory and technique of quantitative analysis with specific emphasis on volumetric analysis, volumetric apparatus, their specifications, standardization and utility. It also includes biomedical analytical chemistry, colorimetric assays, theory and assay of biochemicals, such as urea, bilirubin, cholesterol and enzymatic assays, such as alkaline phosphatase, lactate dehydrogenase, salient features of radioimmunoassay and automated methods of chemical analysis. Chapter 3 provides special emphasis on errors in pharmaceutical analysis and their statistical validation. The first aspect is related to errors in pharmaceutical analysis and embodies classification of errors, accuracy, precision and makes... 

Volumetric analysis involves using a solution of accurately known concentration, a standard solution, in a quantitative reaction to determine the concentration of the other reactant. The procedure is known as titration. One solution is measured quantitatively into a conical flask using a pipette. The other solution is dispensed from a burette until a permanent colour change appears in the solution in the conical flask. 

Before being used as a standard in volumetric analysis, analytical reagent grade AgNO3 should be finely powdered, dried at 120 for 2h, then cooled in a desiccator. 

Amidosulphonie acid reacts quantitatively with the alkalis, and with carbonates and borates, and its use as a primary standard in volumetric analysis has been suggested.3... 

The volume of reagent (titrant) required for stoichiometric reaction of analyte is measured in volumetric analysis. The stoichiometric point of the reaction is called the equivalence point. What we measure by an abrupt change in a physical property (such as the color of an indicator or the potential of an electrode) is the end point. The difference between the end point and the equivalence point is a titration error. This error can be reduced by subtracting results of a blank titration, in which the same procedure is carried out in the absence of analyte, or by standardizing the titrant. using the same reaction and a similar volume as that used for analyte. 

The validity of an analytical result depends on knowing the amount of a primary standard. A solution with an approximately desired concentration can be standardized by titrating a primary standard. In a direct titration, titrant is added to analyte until the reaction is complete. In a back titration, a known excess of reagent is added to analyte, and the excess is titrated with a second standard reagent. Calculations of volumetric analysis relate the known moles of titrant to the unknown moles of analyte. 

The reaction is then quantitative and important use of it is made in volumetric analysis, not only as a method for estimating arsenic, but a standard solution of pure arsenious oxide containing sodium bicarbonate is used as a standard in iodimetry. 

The peroxide value (PV) of an oil or fat is defined as the quantity of peroxide oxygen present in the sample. This classical iodometric method is a volumetric analysis based on the titration of iodine released from potassium iodide by peroxides in a biphasic system using a standardized thiosulfate solution as the titrant and a starch solution as the indicator (see Background Information, discussion of peroxide value). This method will detect all substances that oxidize potassium iodide under the acidic conditions of the test, therefore the purity of the reagents is critical. 

Although consumption of the hydroperoxide is normally complete, the absence of this peroxide in the reaction mixture should be established by testing with moist starch-iodide paper or by iodometric titration.2 The amine oxide content may be determined by titration with standard aqueous hydrochloric acid after any amine present has been consumed by reaction with methyl iodide for 1 hour at room temperature.3 From this volumetric analysis the submitters determined the yield of amine oxide to be 86%. The checkers found that the reaction could be followed by measuring the n.m.r. spectra in -butyl alcohol solution where the n.m.r. N-methyl signals of the amine (at S 2.03) and the amine oxide (at S 2.98) are readily observed. 

Furan-2-carboxylic (2-furoic) acid [88-14-2] M 112.1, m 133-134 , b 141-144 /20mm, 230-232 /760mm, pKj -7.3 (O-protonation), pKj 3.32. Crystd from hot water (charcoal), dried at 120° for 2h, then recrystd from CHCI3, and again dried at 120° for 2h. For use as a standard in volumetric analysis, good quality commercial acid should be crystd from CHCI3 and dried as above or sublimed at 130-140° at 50-60mm or less. 

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