Molybdenum gravimetric

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. 

Al, Si, P Digestion with diethyl ether-concentrated sulphuric acid-potassium persulphate. Silicon determined gravimetrically. Aluminium determined by titrimetric EDTA method. Phosphorus determined by spectrophotometric molybdenum blue method. 

Gravimetric methods and chemical methods, such as colorimetric measurements based on the arsenic-molybdenum blue complex (1,2,3) and arsine generation in combination with silver diethyldithiocarbamate (4, 5, 6,7), have been used to measure arsenic in aqueous media. Various instrumental methods such as differential pulse polarography (8), heated vaporization atomic absorption (9), arsine generation in combination with atomic absorption spectroscopy (10, 11, 12) or non-dispersive atomic fluorescence spectroscopy (13), and optical emission spectroscopy (14) can be used to determine arsenic in aqueous solutions. 

An alternative approach to these gravimetric methods is determination procedme based on the formation of colored complexes. Orthophosphate in acidic solution reacts with molybdic acid and vanadic acid to form yellow-orange vanadomolybdophosphoric acid which has maximum absorption at 330 nm. Another approach is a two-stage reaction in which yellow phosphomolybdate is produced from the addition of ammonium molybdate to the orthophosphate in acidic solution, and the phosphomolybdate is then reduced by ascorbic acid to a molybdenum blue, which is measured at 890 nm. This last method is the basis for an AOAC-approved spectrophoto-metric method. 

Smith SL, Unsworth A. A comparison between gravimetric and volumetric techniques of wear measurement of UHMWPE acetabular cups against zirconia and cobalt-chromium-molybdenum femoral heads in a hip simulator. Proc InstMech Eng 1999 213(6) 475-83. 

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. 

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