392 



On a Quantitative Blowpijje Assay of Chromium. 



2. 



and tlien set up for evaporation. Filtration and evaporation are 

 to be carried on simultaneously. The platinum vessel is set on 

 the grate of the lamp ; the glass cover is clamped into the holder 

 (fig. 2, full size), which in its turn is clamped 

 into the same arm which carries the filtering 

 support; the two parts are then joined and 

 adjusted to the proper height. 



A two-inch filter is about the proper size 

 in this case. Filters as small as these may be 

 made so as to form an angle 5-8° larger than 

 that of the fannel, so that while closely ad- 

 justed at the upper edge^ their lower part 

 hangs free within. The weight of fluid is 

 insufficient to break down the paper, while 

 filtration is essentially accelerated. So soon 

 as tlie principal solution has passed through, 

 it is poured into the evaporator from above 

 (without soiling the neck) and evaporation 

 begun. Previously, however, oil of vitriol 

 is added until the solution becomes dark and 

 free from chromic acid ; then five to eight 

 drops more, and finally three to eight spoon- 

 fuls of bisulphate of potash, the quantity va- 

 rying inversely as the amount of nitre pre- 

 viously used. An excess does no harm. 

 From time to time the filtrate is transferred 

 to the evaporator, except the last one and a 

 half or two fluiddrachms, which are reserved. 

 The evaporation usually goes ahead of the 

 filtration, and when finally the fluid in the 

 evaporator is reduced so low as to evolve 

 white fumes of nitric acid, the lamp is re- 

 moved, the arm carrying the glass cover , 

 slightly raised, then moved sideways, and the glass unclanipea. 

 The latter is thea rinsed into the platinum vessel with the re- 

 served part of the filtrate, and finally and vcr}r economically, 

 with the washing bottle. The whole of the rinsings ought not 

 to fill more than one-third of the vessel, which is then coverea 

 ^ith a watch-glass-shaped platinum cover, sliglitly smaller than 

 the circumference of the rim, and not fitting too closely an 

 around- Thus the fluid may be kept briskly boiling without 

 loss; in a short time a frothing noise is heard, and shortl} 

 after fumes of oil of vitriol appear. At this period the cle^ 

 green liquid suddenly changes color, and deposits a peach-bJos- 

 som-colored powder, which on cooling turns light green. ^^ 

 heating is continued until dense fumes cease to issue; the nr 

 being then removed, and the cover carefully laid aside, upsia 



\M 



