Other gravimetric methods

Helical spring balances first used by McBain and Bakr have been extensively used for adsorption measurements. The spring is suspended inside a glass tube by attachment to a hook at the top. A bucket containing the adsorbent is connected to the bottom of the spring. The bottom of the tube, containing the sample, is immersed in the coolant. The upper portion of the tube is connected to a vacuum pump, source of adsorbate, and manometer. 

Many improvements in design and material have been made since McBain and Bakr s original paper. Gregg and Sing, as well as Young and Crowell, review some of the more recent developments of spring as well as beam balances. 

Other gravimetric techniques include flowing a mixture of carrier gas and adsorbate through a tube packed with adsorbent. Stopcocks at each end of the tube permit isolation of the sample. The tube is then removed from the flow stream and weighed. Two successive equal weights indicate the equilibrium is achieved. The quantity adsorbed must be corrected for the gas contained in the void volume. 

The following comparisons of the three most common adsorption techniques (volumetric, continuous flow and gravimetric) are based on the assumption that routine measurements are to be made. Special requirements may oblige the experimenter to choose one method in preference to another. 

Volumetric measurements require that adsorbed volumes be corrected for ideality. The continuous flow method does not require this correction because the detector senses the gas concentration change at ambient temperature and pressure. Gravimetric apparatus measure the adsorbed weight directly and needs no correction for deviations from ideality. 

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Elemental composition (in anhydrous salt) Th 48.33%, N 11.67%, O 40.00%. The aqueous solution may be analyzed for thorium (See Thorium) and for nitrate ion by ion chromatography, nitrate ion-specific electrode, and colorimetric methods. The water of crystaUization can be determined by DTA, TGA, and other gravimetric methods. 

Other gravimetric methods involve precipitation with tungstosilic acid... 

The relative insolubility of some of the salts of silver is in the order chloride, cyanide, thiocyanate, bromide, iodide, and sulphide. The metal is usually estimated gravimetrically as chloride, or by electrolytic deposition. It can also be weighed as chromate.1 Other gravimetric methods are reduction to metal by hypophosphorous acid,2 and by alkaline glycerol and other reagents.3... 

Before we look more closely at specific gravimetric methods and their applications, let s take a moment to develop a broad survey of gravimetry. Later, as you read through the sections of this chapter discussing different gravimetric methods, this survey will help you focus on their similarities. It is usually easier to understand a new method of analysis when you can see its relationship to other similar methods. 

Gravimetric methods based on precipitation or volatilization reactions require that the analyte, or some other species in the sample, participate in a chemical reaction producing a change in physical state. For example, in direct precipitation gravimetry, a soluble analyte is converted to an insoluble form that precipitates from solution. In some situations, however, the analyte is already present in a form that may be readily separated from its liquid, gas, or solid matrix. When such a separation is possible, the analyte s mass can be directly determined with an appropriate balance. In this section the application of particulate gravimetry is briefly considered. 

The fermentation-derived food-grade product is sold in 50, 80, and 88% concentrations the other grades are available in 50 and 88% concentrations. The food-grade product meets the Vood Chemicals Codex III and the pharmaceutical grade meets the FCC and the United States Pharmacopoeia XK specifications (7). Other lactic acid derivatives such as salts and esters are also available in weU-estabhshed product specifications. Standard analytical methods such as titration and Hquid chromatography can be used to determine lactic acid, and other gravimetric and specific tests are used to detect impurities for the product specifications. A standard titration method neutralizes the acid with sodium hydroxide and then back-titrates the acid. An older standard quantitative method for determination of lactic acid was based on oxidation by potassium permanganate to acetaldehyde, which is absorbed in sodium bisulfite and titrated iodometricaHy. 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Enzymatic Gravimetric Methods for TDF, SDF, and IDF. These methods use an a-amylase and protease to remove starch and reduce protein. They differ from each other in the conditions for gelatinization of starch. Elimination of detergent permits recovery of soluble fiber, which is not possible with the detergent methods. 

Encycl 6, E173-L details a literature method for detg w in NC and proplnts thru the use of dry K carbonate pumice to absorb w in a gravimetric procedure, Encycl 6, E176-R describes a differential d measurement of the solvents used in proplnt manuf for the detn of ale, eth w (Ref 2). Other older gravimetric methods are given in Refs 1, 3 4... 

Other methods have involved compleximetric titration, nephelom-etry, potentiometric titration, and gravimetric methods. In 1981, a paper by Mayanna and Jayaram11 outlined the determination of caffeine in a wide variety of products including pharmaceuticals and food products using sodium N-chloro-p-toluene-sulphonamide (chloramine-T) in a titri-metric procedure. 

A good number of pharmaceutical substances can be assayed gravimetrically. The gravimetric methods adopted vary according to the nature of the substance under determination. However, most of the substances being estimated gravimetrically fall into one or the other categories stated below, which would be discussed briefly with suitable examples ... 

It is important to note that a number of different techniques are based on the Wilhelmy method. Here, we will describe only the capillary rise method (based on the Wilhelmy gravimetric plate technique) presented by Lim and Wang [101] and Wang [192]. For more detail on the other Wilhelmy methods, please refer to Mathias et al. [9]. 

The application of SFE for the preparation of samples in the analytical laboratory has received serious attention as a sample preparation step for extracting analytes of interest from a bulk matrix prior to their determination by other analytical methods including chromatographic, spectro-metric, radiochemical, and gravimetric techniques. 

Tellurium and its compounds can be analysed by AA, ICP-AES and other spectrophotometric methods. Also, the metal can be identified by volumetric, gravimetric, and simple colorimetric measurements. 

Water of crystaUization in hydrated salts can be measured by thermo-gravimetric analysis. Zinc can be analyzed in an aqueous solution by AA or ICP. Sulfate can be identified by precipitation with barium chloride solution or by ion chromatography. The zinc content in the heptahydrate is determined by AA, ICP and other instrumental methods. 

Zalkowska and Piotrowska have reported a gravimetric method for the determination of procaine and other organic compounds by the Buerger method [92]. The accuracy of the gravimetric results for carbon and hydrogen were reported to depend on variation in the balance readings. 

Total dietary fiber (TDF) content of potato dry matter is determined according to the AACC (2000) method 32-05 following the total dietary fiber assay procedure (Megazyme k-TDFR 01/05). This is a gravimetric method that is simpler and faster than other analysis methods. In addition to total dietary fiber content, both soluble and insoluble dietary fiber content can be determined by this method. 

Other characterization methods that are of value are dynamic scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). A sample DSC is shown in the middle of Figure 15.2. Most cure reactions are exothermic, and the heat generated by cure can cause excessive heat to build up in the polymer if control is not exercised. DSC measures the generation of heat as a function of time and temperature. This can be used to predict the temperature at which the laminate will begin to cure (the onset of the peak in Fig. 15.2) and the temperature or time at which cure will be complete, further improving the selection of cure cycles to try. 

The empirical tests described below determine the chalking and weathering differences between two pigments, but do not always give the true differences. Other test methods have therefore been developed, e.g., determination of mass losses on weathering (gravimetric test) [1.51]. 

Gravimetric Methods.—The vanadium compound is converted into sodium vanadate by fusion or other method, and after separation from other salts (e.g. arsenate, molybdate, phosphate, chromate, tungstate) is precipitated from nearly neutral solution either as (a) mercurous vanadate or (b) basic lead vanadate. In (a), mercurous... 

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