Illustrated Guide to Home Chemistry Experiments

Chapter 5: Mastering Laboratory Skills 79

CoSERN vING TITRANT

If you are certain that the titration will require much

less than the full capacity of the burette, you can conserve

your titrating solution by partially filling the burette. Make

sure that you fill it far enough to allow the tip to be filled

and air bubbles to be eliminated. For example, if you

are using a 50 mL burette and have calculated that the

titration should require about 20 mL of titrant, fill the

burette to about 30 mL and run a few mL through the tip

to eliminate air bubbles. Leave yourself some slack. If you

run out of titrant before you’ve neutralized the analyte,

you’ll either have to repeat the titration from scratch

or add more titrant to the burette, which reduces the

accuracy of your titration.

waste solutions) and remove the burette from the clamp. Rinse the
burette thoroughly inside and out with tap water, making sure to
run the rinse water through the tip. If the burette uses a pinchcock,
remove the pinchcock and rubber tubing. If it uses a ground
glass stopcock, remove the stopcock. If it uses a Teflon stopcock,
release the tension on the stopcock and/or remove the stopcock,
depending on the design of the burette. Rinse the burette, tip, and
stopcock assemblies thoroughly with tap water and then with
distilled water and place them aside to dry thoroughly.

PRFORMIE nG TITRATIOnS WITHOUT
A BURETTE
If you don’t have a burette, you can use one of the methods
described in the following sections to perform titrations with
differing levels of convenience and accuracy.

TITRATE USING A GRAdUATEd CyLINdER
If your only volumetric glassware is a graduated cylinder, you can
use it to perform quick-and-dirty titrations with fair accuracy. To
titrate using your graduated cylinder, take the following steps:

1. If possible, calculate the approximate amount of titrant
   you expect to be required to complete the titration.


2. Fill the graduated cylinder with titrant until the titrant
   reaches the top graduation mark as closely as possible.
   Use a Beral pipette to add the final few drops to bring
   the level of the meniscus as close as possible to the line.


3. Use the Beral pipette to withdraw about 2 mL of titrant,
   and set the pipette aside. Invert the pipette to make
   sure that none of the titrant is lost.


4. Slowly trickle titrant from the graduated cylinder into
   the titration vessel with swirling until you have nearly
   reached the equivalence point. You’re approaching the
   equivalence point when the indicator changes color
   where the titrant is being added, but that color change
   disappears as you swirl the titration vessel.

FIGURE 5-10: Reading the meniscus on a burette

AddNGI pARTIAL dRopS

As you near the end point of a titration, you can add

partial drops by manipulating the stopcock carefully. As

you release each partial drop, use a wash bottle filled with

distilled water to flush the partial drop from the tip of the

burette directly into the solution being titrated.

pRELINARmI y TITRATIoNS

If you have no idea where the end point of a titration will

occur, you can save a lot of time by doing a fast trial run

instead of slowly adding titrant from the beginning of the

titration. To do so, run titrant quickly into the titration

vessel until the indicator begins to change color. If you’re

lucky, the titration will not have reached its end point, and

you can begin adding titrant more slowly until you reach

the end point. If you overshoot the mark while adding titrant

quickly, note the reading on the burette and use something

less than that as the starting point for the real titration.

For example, if adding 22.0 mL of titrant was just a bit too

much, when you start the real titration you can quickly run

20.0 mL of titrant into the titrating vessel and then add

titrant drop-by-drop until you reach the end point.

5.hen you are near the equivalence point, stop adding W

titrant from the graduated cylinder, and begin adding

titrant drop-by-drop from the Beral pipette. Continue

adding titrant dropwise from the Beral pipette until you

reach the equivalence point, which is indicated by a

color change that persists for at least 10 or 15 seconds

as you continue to swirl the titration vessel.