JUNE. 1951
e
QUICKER STEAM-BATH EVAPORATIONS GEORGE E. ROUSE Ottawa University, Ottawa, Kansas
ONEof the big headaches of the analytical chemist has the only ones which would pass through the steam bath always been the slowness of evaporations when they must he accomplished quantitatively on the steam bath. Where, by boiling, a solution may be evaporated to dryness with splattering in a relatively few minutes, its careful evaporation on the steam bath requires several hours. The main reason for this slowness is the fact that the heat of the steam bath is applied only to the bottom of the solution. If steam could also he applied to the sides of the solution, it is reasonable to expect that such evaporation would be much more rapid. Therefore, "surrounded evaporations" were checked against the ordinary type of steam bath evaporation. Since steam as ordinarily confined in a bath is approximately as hot as boiling water, two types of evaporation were checked against the conventional method. In one, all hut the top half inch of outer surface of the beaker was surrounded by steam only, while in the other, the bottom of the beaker was allowed to dip into the surface of the boiling water and the remaining outer surface of the beaker except the top half inch was surrounded by steam. Various volumes of natural salt water4ensity 1.0051-were timed for evaporation. The steam bath was of the constant level type and the water was heated to near boiling before the solutions were placed in position. For the water-steam surrounded method 800-ml. beakers were used, while for the steam surrounded method 400-ml. beakers sufficed. In each case similarsized beakers containing like amounts of solution were evaporated conventionally. AU beakers were covered with watch glasses supported by glass hooks. Only beakers of the size mentioned were used as they were
rings and form a nearly steam-tight fit. The accompanying table gives some results of such tests. Times for Evaporation
---800 ml. beakerSurrounded by boiling Surrounded ConvaH.0 and Cmwenby stearn, liaal, steam, tional, mzn. mzn. min. min.
400ml. beaker
7
ML.
3,,1,
From the above results we see that surrounding by steam only reduces the time of evaporation by approximately 29 per cent. When the bottom of the beaker is in the boiling water and the remainder surrounded by steam the time is reduced approximately 51 per cent. It was also found that the air currents in the room had considerable effect on the rate of evaporation, especially that of the conventionally evaporated solutions. More tendency for the salt to crystallize on the sides of the beaker was noted than in the conventional method. To make this method generally applicable, new covers, designed to fit the various sized beakers, would have t o be made. The beakers may he supported by hanging by the lip, or, better, a perforated false bottom of stairstep construction may he built inside the bath. Such steps should he so adjusted that each will allow a given size of beaker to set on.it, the beaker's lip just clearing the cover. For convenience in cleaning, the false hottom should be removable.
