Potassium gravimetric analysis

Polyimide surface modification by a wet chemical process is described. Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) and poly(bisphenyl dianhydride-para-phenylenediamine) (BPDA-PDA) polyimide film surfaces are initially modified with KOH aqueous solution. These modified surfaces are further treated with aqueous HC1 solution to protonate the ionic molecules. Modified surfaces are identified with X-ray photoelectron spectroscopy (XPS), external reflectance infrared (ER IR) spectroscopy, gravimetric analysis, contact angle and thickness measurement. Initial reaction with KOH transforms the polyimide surface to a potassium polyamate surface. The reaction of the polyamate surface with HC1 yields a polyamic acid surface. Upon curing the modified surface, the starting polyimide surface is produced. The depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and ER IR, is controlled by the KOH reaction temperature and the reaction time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. Relationship between surface structure and adhesion is discussed. 

The surface characteristics of modified whiskers can be analyzed by detecting the thermal weight loss curve. Figure 3.21 shows the thermal gravimetric analysis (TGA) curve of potassium titanate whiskers modified with PMMA. 

If the nonionic surfactant is extracted from water into an organic solvent as its potassium tetrathiocyanatozincate(II) complex, its original concentration can be related to the concentration of zinc in the extract, as determined by atomic absorption spectrometry (117) or visible spectrophotometry (118). The gravimetric barium chloride/molybdophosphoric acid method for determination of nonionics has also been adapted to an atomic absorption finish, with the residual molybdenum being determined in the supernate after centrifugation (45). Similarly, the bismuth in the barium/ethoxylated surfactant/tetraiodobismuthate precipitate can be determined by AAS (52). This procedure is discussed with gravimetric analysis. 

Analytical methods employed in soil chemistry include the standard quantitative methods for the analysis of gases, solutions, and solids, including colorimetric, titrimetric, gravimetric, and instrumental methods. The flame emission spectrophotometric method is widely employed for potassium, sodium, calcium, and magnesium barium, copper and other elements are determined in cation exchange studies. Occasionally arc and spark spectrographic methods are employed. 

A standard wet chemical analysis (ASTM D-811) is available for determination of aluminum, barium, calcium, magnesium, potassium, silicon, sodium, tin, and zinc. The procedure involves a series of chemical separations with specific elemental analysis performed by using appropriate gravimetric or volumetric analyses. 

For the analysis of molybdenum, the sample is decomposed by fuming with a few drops of nitric acid and sulfuric acid in a platinum crucible and the molybdenum is determined gravimetrically7 as the 8-quinolinol complex. From the filtrate, potassium is determined gravimetrically as K2S04. Fluoride is determined by titration with a standard solution of thorium nitrate using sodium alizarinsulfonate as indicator, after steam distillation of fluorosilicic acid.8 The determination of the oxidation state of molybdenum is carried out by oxidizing a known amount of the compound with a known amount of potassium dichromate in hot 2 N sulfuric acid and titrating the excess dichromate with standard Fe2+ solution. 

The test is based on the opalescence formed when potassium precipitates with the anionic reagent tetraphenylborate (Figure 6.11.1). This reagent has found widespread use in the determination of potassium by gravimetric, titrimetric, and as in the case of the pharmacopoeial limit test, turbidimetric analysis. 

Classical noninstrumental methods are used in routine analysis due to their easily accessible laboratory equipment required for implementation. In addition, they are often the official methods proposed by the corresponding government commissions. Acid-base titrations are used for ingredients that present acid-base properties (e.g., potassium and/or sodium hydroxides), iodometric titrations for the determination of oxidizing agents (e.g., hydrogen peroxide in hair-care products) even gravimetric determinations are carried out (e.g., oxalic acid and/or its alkaline salts by precipitation with calcium or zinc by 8-hydroxyquinoline). 

The gravimetric determination of potassium as its barely soluble tetraphenylborate is recommended and described here. This method has also been applied to seawater for an ocean-wide survey of the element (see Table 11-1). The procedure is preferred because of its high accuracy and precision and the minimum amount of equipment required to perform the analysis. Basically, the method outlined here is that developed by Wittig (1950) and Raff and Brotz (1951). A brief description of a potentiometric method is presented in Section 11.2.3.6. 

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