A Two-Liter Pipet tor Bomb Calorimetry In b m b calorimetry, a large quantity of water is used to absorb the heat produced by combustion of the sample. Since the specific heat of water depends on mass and, to ensure precision among combustions, the same mass of water must be used in each trial. Paar, for example, recommends that 2000.0 f 0.5 g of water be used in their instrument. Because of the inconvenience of weighing this amount of liquid, several laboratory texts' recommend usinz 2 1 of water a t 25'C rather than 2000 g. Parr in fact sells a bucket filling system that drll\.rrs n rmswnt wlumeolwater at cmstnnt temperature. In many renrh~ngIshoratorier 2-1wlumrtrir flaxk+are wed, even though I I is difficult tcr p u r out of thrm and the) are rather hem\, and unwwldv when full. To rliminatr these ~mhlem*.we haw drviwd n ?-I " ~ i w l "
is a four-step process. 1) Blow out the bottom of the flask t o a uniform, slightly convex shape. 2) Shorten the neck. (optional) 3) Attach a large-bore stopcock t o the lowest point of the bottom. (For rapid draining, the stopcock's bore should be a t least 3 mm.) 4) Attach a leveling tube through the side of the neck, positioning its inlet near the volume mark on the flask (optional).
The pipet is clamped to s sturdy ring stand such that the calorimeter's bucket may he positioned just under the tip. The pipet is filled, with a beaker and funnel, until the leveling tube overflows and then drained through the stopcock into the bucket. (If the leveline tube is omitted.. the oioet should be filled to the mark.) This pipet does ""1 deliver exactly 2 1, but will, if kept clean, deliver a constant volume. In 10 rrialsovrr two months wing water thermostattrd at 25.0"(.', our pipet delivered 1999.5 ~ 0 . 2 1g, or 2005.4 ml ul water. All thr trials were within 0.5 g of the mean, ie., within the limit suggested by Parr
..
'
For example. Shoemaker. David P., Garland. Carl W.. Stainfeld,Jeffrey I., and Nibler. Joseph W.. "Experiments in Physlcal Chemishy." 4th ed., McOtaw-Hill, New Y a k , 1981, p. 135. Roland P. Stout Drake University Oes Moines. IA 50311
R3d
.lournal of Chemical Education
