ERDMAN’S FLOAT

This useful little instrument, invented as its name implies, by Erdman, is used to ensure accuracy in the readings of Mohr’s burette. It is in the form of an elongated gla bulb, loaded with a globule of mercury at the bottom, the same as a hydrometer, and with a gla hook at the top, by means of which it can be placed in or removed from the liquid in the burette at pleasure. The float has a circular mark scratched by a diamond, running round the middle, which, when the instrument is placed in the fluid in the burette, should correspond with the graduation or degree on the burette at which the fluid stands. The actual height of the fluid in the burette is of no consequence, since, if the operation be commenced with the line on the float opposite the 0 gradation on the burette, the same proportional division is always maintained. It is most e ential that, when the fluid is being drawn off, the float should accompany it in its descent without wavering, and that the circular mark upon it should always be parallel to the graduations of the burette. Another condition is, that when the float has been pre ed down in the fluid of the closed burette it should slowly rise again. A correspondent in Liebig’s ‘Annalen der Chemie und Pharmacie’ for April, 1875, states that Erdman’s floats generally become lined internally with a green or yellow layer, from the oxidation of the mercury, and are thus rendered opaque and consequently usele . He proposes to place the mercury in a distinct cell, hermetically sealed from the upper part of the float which carries the circular mark. He has had floats of this construction in use for years. [278] See Burette . [278] ‘Chemical News.’