Abstract
Five collaborative analyses of two samples of magnesia-chrome refractory were carried out by thirteen analysts. The following is the procedure they recommend. Fuse 0.5 g of the powdered sample (about 200 mesh) with 2 g of boric acid and 3 g of anhydrous sodium carbonate in a platinum crucible. Cool, dissolve in warm water and add 40 ml of perchloric acid. Evaporate until white fume appears. Boil gently, and add several drops of hydrochloric acid intermittently and then two portions of 0.1 g dried sodium chloride, until the supernatant liquid becomes clear, to remove the chromium completely. Cool, dilute with warm water and filter through No. 5B filter. Evaporate the filtrate again to fumes to recover the silica, and dilute it with water to 250 ml. Pipet a 25 ml aliquot, add a slight excess of 0.1 M EDTA, adjust the pH to 3.5, boil for several min and back-titrate with a standard copper(II) solution at hot against PAN to obtain the amount of aluminum and iron(III) oxides. Determine the latter in another aliquot by Zimmermann-Reinhardt method after addition of a small amount of hydrochloric acid. For determination of calcium and magnesium oxides, remove the hydroxides after precipitation with ammonia water from a 100 ml aliquot, evaporate the filtrate to about 70 ml and add sodium hydroxide to make pH 12.312.5. Add 5 g of sodium carbonate and 10 ml of ethanol, and dilute to 100 ml. After filtration, dissolve the precipitate in dilute hydrochloric acid and adjust the volume to 250 ml. Determine calcium by the atomic absorption spectrometry or EDTA titration, and the sum of calcium and magnesium by EDTA titration. Because a large excess of magnesium is usually present, the pH readjustment method or the addition of a slightly defficient amount of EDTA before the titration is needed. Chromium is determined with a separate sample by the ordinary redox titration.
