Chapter 20 Laboratory: Quantitative Analysis 365LABORATORY 2 0.3:
qUANTITATIvE ANIoN ANALySIS of SEAwATER
Seawater is a complex solution of dissolved
solid salts. Although salinity varies with location,
depth, time of year, and other factors, one
kilogram of standard seawater as defined for
analytical purposes contains about 964.83 g
of water and about 35.17 g of dissolved salts.
That 35.17 g is made up of about 19.35 g
(55.0%) chloride ions, 10.78 g (30.7%) sodium
ions, 2.71 g (7.71%) sulfate ions, 1.28 g (3.64%)
magnesium ions, 0.41 g (1.17%) calcium ions,
0.40 g (1.13%) potassium ions, and 0.24 g
(0.68%) other ions, including strontium,
bromide, fluoride, bicarbonate, carbonate,
and others.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance and weighing papers£ hotplate£ beaker, 150 mL£ Erlenmeyer flask, 125 mL£ volumetric flask, 100 mL£ graduated cylinder, 100 mL£ evaporating dish£ burette, 50 mL£ serological or mohr pipette, 5 mL£ stirring rod£ funnel£ ring stand£ support ring£ wire gauze£ burette clamp£ filter paper£ seawater (200 mL)£ silver nitrate, 0.1000 m (~50 mL)£ potassium chromate, 0.1 m (a few mL)£ barium nitrate, 0.1000 m (~40 mL)In this lab, we do a quantitative analysis for chloride and
sulfate ions, the only two anions that are present in seawater
in sufficient quantity to be realistic targets for quantitative
analysis in a home chemistry lab. For illustrative purposes, we’ll
use different methods to analyze chloride ions and sulfate ions.
We’ll determine chloride ion content by titrimetric (volumetric)
analysis, and sulfate ion content by gravimetric (mass) analysis.
As we learned in the preceding chapter, chloride ions can be
precipitated by silver ions in the form of insoluble silver chloride.
For a qualitative analysis, the simple fact that adding silver nitrate
to a solution of chloride ions causes a precipitation is sufficient
to establish the presence of those two ions. But a quantitative
analysis requires that we add just sufficient silver ions to
precipitate all of the chloride ions, and no more.
Unfortunately, it’s almost impossible to determine that endpoint
directly. Silver chloride precipitates as a fluffy whitish solid
that forms clumps and takes a long time to settle. Visually,
it’s impossible to tell if you’ve added insufficient silver nitrate,
just the right amount, or too much. Fortunately, there’s an
elegant solution to this problem, based on the differential molar
solubilities of silver chloride and silver chromate. (See Lab 13.3
for more details.)
We’ll add a small amount of lemon-yellow potassium chromate
solution to our seawater sample as an indicator. Although silver
chromate is considered insoluble, it is slightly more soluble than
silver chloride. As we titrate the seawater sample with standard
silver nitrate solution, the less-soluble silver chloride precipitates
first. (That precipitate appears yellow rather than white because
of the chromate ion present in the solution.) Only after all of the
silver chloride has precipitated does the silver nitrate titrant
begin to react with the chromate ion, forming insoluble, brick-red
silver chromate. The end point is readily visible as an abrupt color
change from lemon yellow to a distinct orange that resembles
orange juice.The sulfate ion is present in seawater at a much lower
concentration than the chloride ion, but at a level high enough