Quantitative Chemical Analysis (CHEM 318)
Lab #2
Determination of Chloride Ion in Sea Water Via
Selective Precipitation
Introduction:
Seawater is predominantly comprised of water (of course!) but only to a certain extent. About 3.5%
of its weight is actually derived from dissolved salts. Almost all elements can be found dissolved in
ocean water, but only seven elements contribute to 99.8% of the weight of dissolved elements in the
ocean (Table 1).
Constituents
Chloride
Sodium
Sulfate
Magnesium
Calcium
Potassium
Carbon
Bromide
Common Concentration
Form
(ppt)
Cl
19.35
Na+
10.77
SO4
2.71
Mg+
1.29
2+
Ca
0.41
+
K
0.40
HCO30.12
Br
0.067
% weight
55.04
30.64
7.71
3.68
1.17
1.14
0.33
0.19
As early as the late 19th Century, it was found that, regardless of the absolute concentration of the
total solids, the ratios between the major elements (the seven in the table above) are virtually
constant. In 1884, during the famous H.M.S. Challenger’s voyage around the world, William
Dittmar made careful determinations on 77 water samples, representative of all oceans and
determined that there were no significant regional differences in the relative composition of
seawater. Therefore, measurement of one of the major constituents will consequently determine the
amounts of all the other ions. This measure of the total amount of salt is called salinity and is
usually reported in parts per thousand (ppt, which is equivalent to g/kg).
Salinity determinations are now most frequently done using a salinometer. It is an instrument that
measures water conductivity and temperature, from which salinity can be computed. However,
salinity can be measured by a variety of other techniques. The classical procedure makes use of the
fact that the most abundant constituent in seawater is the chloride ion. This measurement is made
through the titration of chloride with silver nitrate from a water sample. The result is reported as
chlorinity in parts per thousands (ppt). The salinity of the sample can then be calculated using the
following equation:
S (ppt) = 1.80655 Cl ppt
Here we will determine the chloride concentration in our seawater sample, and its salinity, through
titration using silver nitrate following the equation below:
+
Ag + Cl - ⇔ AgCl
Ksp = 1.7.10-10
We will know that the titration of Cl- reached its end point because we will use an indicator solution
(K2CrO4), which turns orange when Ag2CrO4 precipitates.
2−
2−
+
2Ag + CrO4 ⇔ AgCrO4
Ksp = 1.1.10-12
Part 1: Stoichiometric determination of volume needed
Note: You will need to answer this section prior to titration
Question 1: If you know that the salinity of the sample (estimated using the refractometer) is ~37
ppt, then calculate the chlorinity in grams of Cl- per kilogram of seawater. (use 1.05 g/kg as the
density for seawater).
Question 2: Predict ahead of the xperiment what volume of 0.4M AgNO3 solution you will need to
precipitate all Cl- ions form the 2 ml seawater sample.
Part 2: Titration and calculations
Question 3: Calculate the chlorinity and salinity based on the titration. How different/similar are
they to the predicted values you obtained from question 2?
Question 4: a) Calculate the molar solubility of Ag+ in AgCl and Ag2CrO4. b) Does this explain the
precipitation of AgCl before Ag2CrO4 (why)?
Question 5: a) Demontrate quantitatively that all (or almost all) Cl- has precipitated out of solution
prior to the precipitation of Ag2CrO4.
Summary of the Method:
Material checklist:
1) Titrant and indicator solutions
2) 25 ml burets
3) 50 ml beaker or conical flask
4) Utility clamps & Ring stand
6) Magnetic stirrer and bar
Caution! Please wear gloves and safety goggles to perform this experiment. Avoid spilling it on
your skin or clothing. The chromate solution needs to be used with care as chromate is a known
carcinogen. Silver nitrate solution causes staining of skin and fabric (chemical burns). Any spills
should be rinsed with water immediately
Note: Please make sure you transfer all the measurements (volume) in your notebook to be able to
calculate the salinity later on.
1)
2)
3)
4)
5)
6)
7)
8)
Pour 2 ml of seawater into a beaker and add 10 ml of distilled water.
Add 5 ml of indicator solution.
Place the burette (previously filled with 0.1 M AgNO3 solution) over the beaker so that the
solution can drip slowly into the beaker. Ensure that you have sufficient room to turn the tap
of the burette freely.
Start the stirring bar.
You are now ready to perform the titration.
Add a small quantity of the titrant and watch as the solution’s color stabilizes.
Under vigorous stirring (magnetic stirrer) and with burette tip very near the surface of the
solution, titrate until a faintly reddish-brown color (silver chromate) is observed and stays
permanent
Record the current burette reading to (the nearest 0.1 ml).
It is important to stir vigorously and proceed very slowly during the titration in order to keep the
AgCl precipitate from coagulating. Flocs of AgCl can trap chloride ions and thus prevent from
being reached. Coagulation seems to increase just before reaching the endpoint.
The molarity of the Cl- solution is then determined using the volume-molarity equality.
(V titrant ) x ( M titrant ) = (V seawater ) x ( M Cl ) (1)
where Vtitrant is the volume of added AgNO3 and the Vseawater and MCl are the volume and molarity,
[Cl-], of the seawater sample. Note that if the volumes used are in milliliters (ml) then the
concentrations are in millimolar (mM).