Chapter 5: Mastering Laboratory Skills 81ASUTEB oL ERRoR vERSUS RELATIvE ERRoR
Typical graduated cylinders are accurate to about 1% at
full scale, which means that a 100 mL graduated cylinder
provides results accurate to about 1.0 mL and a 10 mL
graduated cylinder provides results accurate to about 0.1
mL. These errors are absolute, so if your titration uses only
10.0 mL from the 100 mL graduated cylinder, for example,
the absolute error is still ± 1.0 mL, which translates to a
10% error of the smaller volume.
You can minimize this source of error by using a graduated
cylinder with a maximum capacity that is near the volume
of titrant you expect to require. For example, if you expect
to need about 8 mL of titrant, start with a 10 mL graduated
cylinder filled to capacity. A typical 10 mL graduated
cylinder is accurate to within 0.1 mL, so your results are an
order of magnitude more accurate if you use the smaller
graduated cylinder.- ractice using the pipette pump or pipette bulb until you get P
the hang of releasing one drop at a time. - For finer dispensing control, you can use a short length of
flexible tubing, a pinchcock clamp, and a short piece of glass
tubing drawn to a point to convert the pipette to an actual
burette. If you use this method, fill the pipette/burette by
releasing the pinchcock clamp and using a pipette bulb or
pipette pump to draw titrant up into the pipette/burette
through the tip. When the pipette/burette is full, clamp the
tubing and remove the pipette bulb or pump.
Perform titrations using this setup just as you would if you
were using an actual burette.Titration by mass difference takes
advantage of the fact that a laboratory balance can measure
masses very accurately. If your balance has at least centigram
(0.01 g) resolution and you have a volumetric flask, your results
with this method will be at least as accurate as those you obtain
with a burette.
Determining the mass of an accurately known volume of titrant
allows us to calculate the mass per unit volume (or density) of
the titrant with high accuracy. If we subsequently run a titration
starting with a known initial mass of titrant, we can determine
the mass of the titrant remaining when the titration is complete.
By subtracting the final mass of titrant from the initial mass, we
can determine what mass of titrant was required to complete
the titration. Knowing the density of the titrant, we can convert
that mass to volume by dividing the mass of titrant required by
the density of the titrant. Table 5-2 shows example values for a
titration I completed using the mass difference method.
To titrate by mass difference, take the following steps, modifying
them as necessary to account for the capacity of your balance
and the volumetric glassware you have available.
- Weigh an empty 100 mL graduated cylinder or (better) a
100 mL volumetric flask. Record the mass as accurately
as possible. - Carefully fill the graduated cylinder or volumetric flask to
the 100.0 mL line. Reweigh the container and record the
mass as accurately as possible. - Subtract the mass of the container when empty from the
mass of the container when filled and record that value as
the mass of 100.0 mL of the titrant. - Transfer sufficient titrant to complete the titration to a
small beaker or similar container. Add a Beral pipette to
the beaker, determine the combined mass of the beaker,
titrant, and Beral pipette as shown in Figure 5-12, and
record the combined mass as accurately as possible.
5. Complete the titration and return the Beral pipette to
the beaker.
6. Again determine the combined mass of the beaker,
remaining titrant, and Beral pipette, and record that
combined mass as accurately as possible.
7. Subtract the final combined mass from the initial
combined mass to determine how many grams of titrant
were required to complete the titration.
8. Convert grams of titrant to mL of titrant by multiplying
the number of grams of titrant required to complete the
titration by 100 and then dividing that result by mass of
100.0 mL of titrant to determine the number of mL of
titrant that were needed to complete the titration.