Chromium gravimetric

Cadmium and inorganic compounds of cadmium in air (X-ray fluorescence spectroscopy) Chromium and inorganic compounds of chromium in air (atomic absorption spectrometry) Chromium and inorganic compounds of chromium in air (X-ray fluorescence spectroscopy) General methods for samphng and gravimetric analysis of respirable and inhalable dust Carbon disulphide in air... 

Methods. Polyamic acid in NMP was spin-coated onto a Si or Quartz wafer (diameter = 2.25 inches) coated with Cr, and then cured to polyimide at 400 °C. The purpose of the 500-750-A-thick layer of chromium is to enhance wettability and to give good reflectance to the Quartz wafer. Kapton H (PMDA-ODA) and Upilex S (BPDA-PDA) films were employed for gravimetric analysis. Around 5-um thick layers were used to measure the thickness change. The 100-1000-A-thick layers were employed to obtain XPS and ER IR spectra. The samples for contact angle measurement, XPS and ER IR were dried under vacuum at ambient temperature for 12-24 h and the samples for gravimetric analysis were dried at 85 °C for 12 h. The samples for film thickness measurement were fully re-cured to polyimide. 

Estimation of Chromium.—In the analysis of chromites or of other substances containing chromium, such as leather ashes, pigments, etc., it is first necessary to obtain a solution. This is done by finely powdering the substance and heating it with a suitable flux in a crucible, preferably of nickel. Many fluxes have been employed, usually caustic alkali or alkali carbonates, but the one in most common use at present is sodium peroxide, whereby the chromium compound is rapidly converted to a chromate. - Excess of alkali is removed by boiling with ammonium carbonate, which also precipitates any iron present. The filtrate is then acidified with dilute sulphuric acid and the chromium estimated either by gravimetric or volumetric methods. 

Gravimetric Methods.—When the chromium is in solution as a chromic salt, it can be precipitated completely from the boiling solution by the addition of ammonium hydroxide in the presence of ammonium salts. The precipitate may be dried at 100° C. and converted by gentle ignition to the dark green sesquioxide and weighed as such. Ammonium sulphide or hydrazine sulphate may also be used as the precipitant. 

More important is the nature of the product prepared. Although the previous procedures for the synthesis of chromium(II) acetate imply that the product is anhydrous, other sources indicate that it is hydrated. Thermo-gravimetric experiments performed on moist fresh products show that the deep red material is the 1-hydrate and that there are no higher hydrates of appreciable stability. The 1-hydrate can be obtained by heating the moist reaction product for about 45 minutes at 58° in a stream of nitrogen flowing at 400 ml./hour or by pumping on the... 

If there is no available salt of constant composition, such as copper(II) sulphate or iron(III), aluminium or chromium alums, it is advisable to prepare first a stock solution of an approximate concentration, slightly higher than required and to determine the concentration by a gravimetric or volumetric method. After suitable calculations the solution is diluted with pure solvent to obtain a solution containing exactly, e.g., 1 mg/ml of the given element. In some cases, standard solutions are obtained by dissolving a precisely weighed amount of the element in its pure form. 

In gravimetric analysis, the solution-preparation step has its own special significance. The analyte must be separated from interfering species, or the interferents must be masked. For example, if iron(III) is to be estimated as its hydrated oxide in the presence of chromium(III), then the mixture is initially treated with perchloric acid so as to oxidize chromium(III) to chromate (chromium(VI), Cr04 ), followed by addition of ammonia to precipitate the hydrated iron oxide. Sometimes it is necessary to remove interferents, for example, when calcium is to be estimated as calcium sulfate in the presence of barium. The barium is removed as its chromate and the calcium is precipitated quantitatively as its sulfate. 

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. 

Series P of activated carbons was prepared by carbonization in Nj (1123K) of chromium-tanned leather waste followed by activation in CO (1098K) for different periods of time to cover the 7-70% burn-off range (burn-off is included in the nomenclature of the samples). Adsorption of Nj (77K), CO2 (273K) and n-butane (273K) and preadsorption of n-nonane were determined in conventional gravimetric systems in order to characterize the carbons. 

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