ISO 439:2020 pdf download – Steel and cast iron — Determination of silicon content — Gravimetric method.
Heat the uncovered beaker slightly until the attack is complete and then, increase the rate of heating. As soon as the first white perchloric acid fumes appear, cover the beaker with the watch glass and continue fuming for about 20 min. If the alloy contains chromium at contents greater than 1 %, it is recommended to wait for the appearance of the characteristic red colour of the chromic acid. At this stage, chromium can be removed as chromyl chloride: slowly pour down along the sides of the beaker 1 ml to 2 ml of hydrochloric acid (5.1). Continue heating, until abundant perchloric fumes appear and chromium re- oxidises. Repeat as many times as necessary until the perchloric fumes are no longer orange-coloured, when hydrochloric acid is added again. Allow to cool, carefully moisten with 5 ml of hydrochloric acid (5.1), heat slightly, dilute with 100 ml of water at 70 °C to 80 °C and heat again until the salts are dissolved (taking care not to allow the solution to boil). 8.3.2 Filtration and washing With a rubber-tipped glass rod, detach any hydrated silicon dioxide or acid-insoluble silicon compounds that may be adhering to the beaker and filter immediately through a filter paper (6.3) containing a little filter-paper pulp of the same quality. Wash the beaker and the filter with hot hydrochloric acid (5.3), transferring the hydrated silicon dioxide and acid-insoluble silicon compounds to the filter, and complete the washing, first with hot hydrochloric acid (5.2) and then with cold water until the iron salts are completely eliminated. NOTE The filter is washed thoroughly in order to avoid popping and loss of residue due to perchloric acid during ignition.
8.3.3? Recovery? of? silicon? compounds? in? the? filtrate Transfer the filtrate and the washings to the beaker previously used for the dissolution, evaporate them by heating until dense white fumes of perchloric acid are evolved, and maintain a steady refluxing of acid on the walls of the beaker for about 20 min. Moisten with water and dilute according to the procedure specified in 8.3.1, then filter through a second filter paper (6.3) and wash according to the procedure specified in 8.3.2. 8.3.4 Ignition, volatilization of silicon dioxide and weighing Put the two filters and their contents together in a platinum crucible (6.1). Heat at between 500 °C and 600 °C until the filters are completely incinerated, then cover the crucible partially with a platinum cover and ignite in the muffle furnace (6.2) at 1 100 °C for 30 min to 45 min, depending on the quantity of silicon dioxide and, for samples containing molybdenum, until a constant mass is obtained. Allow to cool, add approximately 2 ml of sulfuric acid (5.7) to the crucible, heat carefully and continue heating until the sulfuric acid fumes are completely eliminated. Then ignite in the muffle furnace at 800 °C to constant mass.