368 DIY Science: Illustrated Guide to Home Chemistry Experiments
10.
11.
12.
13.
14.
- Continue stirring vigorously for at least 30 seconds
to make sure that no unreacted chloride ions remain
trapped within the silver chloride curd. As you stir, the
contents of the flask will probably return to the original
bright yellow color as additional chloride ions are
released from the silver chloride curd.
Continue adding titrant dropwise with vigorous stirring
until the orange coloration remains for at least 30
seconds before fading. At that point, you’re about one
drop short of the end point. Add that drop of titrant
with stirring and verify that the orange tint persists
for at least a full minute. Record the final volume of
silver nitrate on line K of Table 20-3. Subtract the initial
volume reading from the final volume reading, and
enter the volume of titrant required on line L of
Table 20-3.
Using the actual molarity of the nominally 0.1 M silver
nitrate titrant and the actual volume used, calculate
the number of moles of silver ions required to reach the
endpoint. Enter that value on line M of Table 20-3.
One mole of silver ions react with one mole of chloride
ions, so the number of moles of chloride ions in the
5.00 mL aliquot is identical to the number of moles of
silver ions in the volume of titrant required to reach
the endpoint. On that basis, calculate the mass of the
chloride ions present in the aliquot, and enter that
value on line N of Table 20-3.
Calculate the mass of chloride ions per liter of
seawater, and enter that value on line O of Table 20-3.
Dispose of the contents of the flask as noted in the
Disposal section. Wash and dry all of the glassware
you used. The silver chloride precipitate can be quite
tenacious, covering the inside of the flask with a milky
white film even after you’ve washed it. To remove this
film, rinse the inside of the flask with a 6 M or higher
solution of aqueous ammonia solution.
FIGURE 20-2: The first appearance of silver
chromate tints the solution orangePRTIII: A dETERNEmI SULfATE IoN qUANTITATIvELy
In Part III, we’ll quantitatively analyze the sulfate ion content
of a seawater sample by using barium nitrate as a titrant to
precipitate sulfate as barium sulfate. After we add sufficient
barium nitrate solution to the aliquot of seawater to ensure that
all of sulfate ions present in the sample have been precipitated,
we’ll filter that precipitate, rinse it thoroughly with water to
remove any traces of soluble salts, dry the precipitate, and
determine its mass. With the mass of barium sulfate known,
it requires only a simple calculation to determine the mass of
sulfate ions present in the seawater sample.
- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Transfer a 100.0 mL aliquot (or a 95.0 mL aliquot; see
the introduction to this section) of seawater from the
100 mL volumetric flask to the 250 mL beaker.
Record the volume of the aliquot on line P of
Table 20-3.- Rinse the flask two or three times with a few mL of
distilled water to make sure you’ve transferred all of the
sample to the beaker. - Measure about 40 mL of 0.1 M barium nitrate in the 100
mL graduated cylinder, and pour it slowly, with stirring,
into the 250 mL beaker that contains the seawater
sample. The solution immediately assumes a white,
milky appearance as insoluble barium sulfate is formed
by the reaction of the barium nitrate with the sulfate
ions present in the seawater sample. - Continue stirring the solution for 30 seconds or so to
make sure that the solutions are completely mixed and
that the reaction is complete. When you remove the
stirring rod from the beaker, use your wash bottle to
rinse any solids on the stirring rod into the beaker. - Fold a piece of filter paper to prepare it for use, weigh it,
and record its mass to 0.01 g on line Q of Table 20-3. - Insert the folded filter paper in the funnel, place
a receiving vessel under the funnel, and pour the
contents of the beaker into the funnel. Rinse the
beaker thoroughly several times with a few mL of
distilled water and transfer the rinse to the filter funnel
to make sure that all of the solids have been transferred
to the filter paper.