Illustrated Guide to Home Chemistry Experiments

80 DIY Science: Illustrated Guide to Home Chemistry Experiments

TBSTINGE URETTE ACCURACy

Some burettes, particularly inexpensive “student” models,

are not linear, because the interior walls are not straight

and parallel. That is, the accuracy may differ according

to which section of the burette you use. For example, the

burette may give accurate readings near the top of the

tube, inaccurate readings in the middle of the tube, and

accurate readings near the bottom of the tube.

Pure water weighs 1.00 g/mL, so you can use your balance

to calibrate your burette to determine its accuracy over its

full range. To do so, fill the burette as closely as possible

to the 0.00 mL (top) graduation line with distilled water,

using an eyedropper or Beral pipette to add the final drops

necessary. Weigh a beaker that has at least the capacity

of your burette, and record the mass to 0.01 g. Transfer

5.00 mL of water to the beaker, reweigh it, and record the

mass. Transfer 5.0 mL of water and reweigh the beaker

repeatedly until you have emptied the burette. (You can

speed this process up by leaving the beaker on the balance

pan and setting up the burette above it.) When you finish,

subtract each of the mass readings from the next mass

reading to determine how much mass corresponded to

each 5.00 mL transfer. Ideally, each 5.00 mL transfer

should increase the mass by 5.00 g, but if the burette isn’t

linear, the mass increase may not always correspond to

the volume added.

FIGURE 5-11:

Titration setup using a 10 mL Mohr pipette and a pipette pump

6.ransfer the remaining titrant from the Beral pipette T

back into the graduated cylinder. Read the final volume

on the graduated cylinder and determine the volume

used by subtraction.

TITRATE USING A moHR pIpETTE
You can use a Mohr pipette with a pipette pump or pipette bulb,
as shown in Figure 5-11, to perform titrations with high accuracy.
There are a few tricks to getting good results when titrating with
a pipette:

• Use a high-capacity Mohr pipette. Most Mohr pipettes
  contain from 0.2 mL to 2.0 mL, but 10.0 mL models are readily
  available. I use a 10.0 mL model with 0.1 mL graduations that
  allow interpolation to 0.02 mL or better.


• Estimate as closely as possible the volume of titrant that will
  be needed, and make sure it falls well within the capacity of
  the pipette.