Art. LXII.—On a new Form of Burette.
[Read before the Philosophical Institute of Canterbury, 13th October, 1881.]
Plate XXXVII., fig. 2.
There being now so many forms of burettes from which to make a choice when about to conduct a volumetric analysis, it may appear wholly unnecessary to add another to the list. It will be well, however, to remember that one or two are practically obsolete, their form allowing of but rough and uncertain results. Gay Lussac's burette, which gives far better indications, is of too fragile a nature to be practically useful. Binks's also, though less liable to breakage, is not much handier in use than the previous one, and with it is open to another objection, viz., special means have to be taken to ensure the correct reading of the level of the solution. Mohr's burette, of which several modifications have been introduced, is the one now generally employed; but when dealing with potassium permanganate it cannot be used. To remedy this defect a glass stopcock has been substituted for the caoutchouc tubing, and this form has now mostly superseded the others previously mentioned, as it also allows of the use of Erdmann's float. The glass stopcock is somewhat liable to fracture, and this form is not so easily cleaned as the original.
Some years back I devised a burette, an exact description of which I came upon while searching in the “Journal of Science,”* from which I make extract:—
“A new burette has been lately used in Paris. It consists of an upright tube drawn out to a fine aperture below, like that of Mohr, and supported in the same manner. The opening at the top is fitted with a perforated
[Footnote] * “Quarterly Journal of Science,” vol. iii., n.s. (x.,o.s.), 1873, p. 182.
cork, through which plays a glass rod, reaching down to the bottom and ground conically, so as to fit watertight into the tapering delivery-end of the burette. A lateral aperture at the top serves to charge the instrument. This form is useful in working with solutions of permanganate of potash or other reagents which attack the indiarubber which in Mohr's pattern connects the delivery-tube to the body of the burette.” (Pl. XXXVII., left-hand figure).
This pattern, owing to the central rod, does not admit of the insertion of an Erdmann's float. If the glass rod is broken it is next to impossible to obtain one exactly similar in gauge, and unless such an one is obtained the burette is useless, the graduations having been made when the original rod was in the tube. This is a serious objection which cannot be overlooked.
The burette which I now purpose to describe is readily cleansed, may be used with potassium permanganate, and allows of Erdmann's float being employed. This burette consists of a graduated tube, drawn out at the lower extremity to a moderately small aperture. The top of the tube is closed by a cork fitting airtight, through which passes a small glass tube. This tube, an inch or two above the cork, is bent at right angles into a horizontal direction and then again vertically but downwards. A short piece of caoutchouc tubing is slipped over the end of the small tube, which terminates a few inches above the delivery aperture of the larger one. A screw clamp fastened on to the burette-stand serves to pinch the caoutchouc tubing, and by varying, as required, the pressure on the latter the flow of liquid is regulated, or arrested entirely. By inserting a short length of glass tube into the free end of the caoutchouc (the pressure on which has been released for the time being) and applying suction the burette may be filled with ease, in a very short time, and without the production of a single bubble (pl. XXXVII., right hand fig.).
This form cannot be used in cases where the solution is required to stand for two or three days in the burette, to be used for a fresh analysis, as the air confined above the liquid is subject to the variations of temperature, which when rising would cause the solution to be slowly forced out of the delivery aperture.
It may not be out of place to remark that Mohr's burettes, the stopcocks of which have been broken off, may easily be converted into the new form and thus be rendered serviceable again.