366 DIY Science: Illustrated Guide to Home Chemistry Experiments
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute any suitable containers of
similar size for the 150 mL beaker and/or the 125 mL
Erlenmeyer flask. - You may substitute a 100 mL graduated cylinder for the
100 mL volumetric flask, with some loss of accuracy. - You may substitute a 150 mL beaker for the evaporating
dish. - You may substitute a 10 mL pipette for the 5 mL pipette.
- If you use actual seawater, allow it to settle completely or
filter it to remove any algae or other suspended solids. - If you do not have access to seawater, you can make 100
mL of artificial seawater by adding about 3.5 g of sea
salt (sold in grocery stores) to a 100 mL volumetric flask
and making up the solution to 100 mL. Record the actual
mass of the salt to 0.01 g, because you’ll need that mass
later to determine the mass percentages of chloride and
sulfate ions. Dr. Mary Chervenak adds that there’s also a
product called “Instant Ocean” for salt water fish tanks
available in most pet stores that works well.
CUTIOA nS
Silver nitrate is toxic, corrosive, and stains skin, clothing,
and other organic materials. Potassium chromate is
extremely toxic and a known carcinogen. Soluble barium
salts are toxic. Wear splash goggles, gloves, and protective
clothing. Wear a disposable respirator mask if you work
with solid potassium chromate or barium nitrate. (An
N95 or N100 disposable mask from the hardware store or
home center is fine.)z
to be easily analyzed quantitatively in a home lab. We’ll analyze
the sulfate ion by precipitating it with a slight excess of barium
nitrate solution. Barium ions react with sulfate ions to produce
a precipitate of insoluble barium sulfate, which we’ll separate
by filtration. After determining the mass of the barium sulfate,
we can easily calculate the concentration of sulfate ions in the
seawater sample.
POCEDURER
Before we begin the analysis, we need to decide what sample
sizes are appropriate for our quantitative tests.
We know that a seawater sample should contain roughly 20 g of
chloride ions per liter. The atomic mass of chlorine is about
35.45 g/mol, so the molarity of seawater with respect to chloride
ion is about 20/35.45 = 0.56+ M. If we use a 5 mL aliquot, that
sample should contain about 0.0028 moles of chloride ion. One
mole of chloride ion reacts stoichiometrically with one mole of
silver nitrate, so we’d need about 0.0028 moles of silver nitrate
to react with that 5 mL aliquot of seawater. Our 0.1 M silver
nitrate titrant contains 0.1 mol/L or 0.0001 mol/mL, so we
should need about 28 mL of titrant, well within the range of our
50 mL burette.We know that a seawater sample should contain roughly 3 g
of sulfate ions per liter. The formula weight of the sulfate ion is
about 96.06 g/mol, so the molarity of seawater with respect
to sulfate ion is about 3/96.06 = 0.03+ M. If we use another
5 mL aliquot of seawater, that sample should contain about
0.00016 moles of sulfate ion. One mole of sulfate ion reacts
stoichiometrically with one mole of barium nitrate, so about
0.00016 moles of barium nitrate would react with that 5 mL
aliquot of seawater to produce about 0.00016 moles of barium
sulfate. The formula weight of barium sulfate is 233.43 g/mol, so
this reaction would produce only about 0.04 g of barium sulfate.Such a small amount of product is likely to introduce significant
errors, so we’ll use a much larger sample. It’s convenient to use
the 100 mL volumetric flask to obtain that sample. If we start
with 100.00 mL of seawater in the volumetric flask and remove a
5.00 mL aliquot for chloride ion testing, that leaves 95.00 mL of
seawater in the flask. If a 5 mL aliquot would yield about 0.04 g of
barium sulfate, a 95 mL aliquot will yield about 0.76 g of barium
sulfate, an amount that is much easier to weigh accurately. (Of
course, if you have plenty of seawater available, you can simply
use a 100.0 mL aliquot. Use the 95 mL aliquot only if you’ve
made up an artificial seawater sample.)We can also calculate about how much 0.1 M barium nitrate
titrant should be needed. If about 0.00016 moles of barium
nitrate are needed to react completely with a 5 mL aliquot of
seawater, the amount needed to react with a 95 mL aliquot of
seawater is (95 mL/5 mL) · 0.00016 mol = 0.0030 mol. Our 0.1 M
barium nitrate titrant contains 0.1 mol/L or 0.0001 mol/mL,
so we’ll need about 30 mL to react completely with the 95 mL
aliquot of seawater. To ensure that all of the sulfate ions are
precipitated, we’ll add a slight excess of barium nitrate.PRTI: A ETERd mINE SEAwATER dENSITy
ANd ToTAL dISSoLvEd SoLIdS
In Part I, we’ll determine the mass of a 100.00 mL sample of
seawater, which allows us to calculate its density. We’ll then
evaporate the water to determine the mass of the dissolved
solids. (If you make your own artificial seawater from purchased
sea salt, you can skip the evaporation step and simply enter the
mass of sea salt you dissolved to make your artificial seawater on
line G of Table 20-3.)