Chapter 20 Laboratory: Quantitative Analysis 367
1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.
- Weigh the clean, dry, empty 100 mL volumetric flask and
record its mass to 0.01 g on line A of Table 20-3.
- Fill the 100 mL volumetric flask to the index line with
seawater. Weigh the filled flask, and record its mass to
0.01 g on line B of Table 20-3.
- Subtract the mass of the empty flask from the mass
of the filled flask, and record the mass of 100.0 mL of
seawater to 0.01 g on line C of Table 20-3.
- Calculate the density of your seawater sample in g/mL,
and enter that value on line D of Table 20-3.
- Weigh a clean, dry 150 mL beaker and record its mass to
0.01 g on line E of Table 20-3.
- Transfer the contents of the 100 mL volumetric flask
to the weighed beaker. To make sure you’ve done a
quantitative transfer, rinse the volumetric flask several
times with a few mL of distilled water, and add the rinse
water to the beaker.
- Place the beaker on the hotplate and boil the solution
gently to evaporate the water. Keep a close eye on the
beaker. When the liquid is nearly boiled off, use the
stirring rod to break up any solid masses, reduce the heat
and continue heating until the contents of the beaker
appear to be completely dry.
dR. mARy CHERvENAk CommENTS:
If you have access to an oven, use it to dry the sample.
Although the temperature inside the oven is lower
than the temperature of the hotplate, extended drying
even at a lower temperature is the most effective way
to eliminate all moisture from the sample because the
heat is uniform across the sample, instead of being
concentrated on the bottom.
9.w the beaker to cool completely and reweigh it. Jot Allo
down the mass to 0.01 g and return the beaker to the
hotplate. Heat the beaker strongly for several minutes,
allow it to cool completely, and reweigh it. If the mass
is the same as the mass you jotted down earlier, you
can assume the sample is completely dry. If the second
reweighing yields a smaller mass, the sample is not
yet completely dry. Heat the beaker again and reweigh
it until two sequential weighings yield the same mass.
Record that mass to 0.01 g on line F of Table 20-3.
Subtract the mass of the empty beaker from the
combined mass of the beaker and solid residue to
determine the mass of the total dissolved solids. Record
that mass to 0.01 g on line G of Table 20-3. Calculate the
mass percentage of total dissolved solids and enter that
value on line H of Table 20-3.
PRTII: A ETER d mINE CHLoRIdE IoN qUANTITATIvELy
In Part II, we’ll quantitatively analyze the chloride ion content
of a seawater sample by using silver nitrate as a titrant to
precipitate chloride as silver chloride. Because the endpoint
of this titration is difficult or impossible to determine directly,
we’ll use potassium chromate as a titration indicator. A change
in color from bright yellow to orange indicates that all of the
chloride ion has been precipitated and that silver chromate
is beginning to form. At its endpoint, the titrated solution
resembles orange juice.
- If you have not already done so, put on your splash
goggles, gloves, and protective clothing.
- Use the pipette to transfer 5.00 mL of seawater to the
125 mL Erlenmeyer flask. Note the volume of the aliquot
as accurately as possible, interpolating between the
graduation marks on the pipette. Record the volume on
line I of Table 20-3.
- Use the graduated cylinder to transfer about 45 mL of
distilled water to the 125 mL Erlenmeyer flask and swirl
the flask to mix the contents. Place a stirring rod in the
flask, and leave it there until the titration is complete.
- Add sufficient 0.1 M potassium chromate solution to the
flask to give the solution a bright lemon-yellow color.
(The exact amount is not critical. The yellow chromate
ion serves as a titration indicator, so if in doubt, use
more rather than less.)
- Rinse the 50 mL burette with a few mL of 0.1 M silver
nitrate solution, allow it to drain into a waste container.
- Transfer at least 40 mL of 0.1 M silver nitrate solution to
the burette. Drain a couple mL into the waste container
and make sure there are no bubbles in the burette,
including the tip. Record the initial volume as accurately
as possible, interpolating between the graduation marks
on the burette, and record that volume on line J of
Table 20-3.
- We calculated that we should need about 28 mL of silver
nitrate titrant, so begin by running 20 mL or so of titrant
into the receiving flask with constant swirling. As the
silver nitrate is added, it reacts with the chloride ions
in the seawater, forming a milky precipitate of silver
chloride, which is tinted bright yellow by the chromate
ions present in the indicator.
- Continue adding titrant more slowly, and with constant
swirling and stirring. (The stirring helps to break up the
curds of silver chloride that otherwise prevent complete
mixing of the solutions.) As you approach the endpoint
of the titration, you’ll see a orange tint begin to appear in
the yellow liquid at the point where the titrant comes into
contact with it (Figure 20-2). That signals that the endpoint
of the titration is near. Continue adding titrant dropwise
with constant swirling and stirring until a noticeable color
change occurs, from yellow to a distinct orange.
10.
