LABORATORY ACTIVITY 2:
STUDENT VERSION
Activity 2: Determination of the Salinity of Seawater
Introduction
This method involves chemical volumetric analysis. A silver nitrate (AgNO 3 ) solution
of known concentration is used to precipitate out the chlorides in a seawater sample.
The procedure is complicated by the fact that the dense white precipitate, silver
chloride (AgCl), does not settle rapidly. It is impossible to tell when all the chlorides
have been removed from the seawater unless an indicator is used to signal complete
precipitation by a visual color change. The indicator in the analysis is chromate ion.
When all of the chloride ion is exhausted, the chromate ion reacts with silver ions and
produces silver chromate, which is red. The instant a permanent orange tinge
appears in the solution (one that doesn’t vanish with mixing), the addition of silver
nitrate is stopped. The final solution color should look like that of orange juice.
The chemical reactions are:
The titration is carried out in a 150-mL Erlenmeyer flask, using a white background
and good lighting, but away from direct sunlight. Constant but gentle stirring is
essential, since the silver chloride tends to form clots, which trap some of the
reagents. A magnetic stirrer is very helpful. The silver nitrate is added until the faint
reddish or orange tinge remains after stirring for 45 s. At this point one drop should
give the desired end point. Observable flocs of silver chloride will obscure the end
point, since the solutions that they contain in their interstices will leak out slowly
and cause the end point to be impermanent. A stirring rod left permanently in the
flask, especially if a magnetic stirrer is not used, will help to break up these curds.
Purpose
1. To determine the chlorinity (and thus the salinity, since they are directly
related) of a sample of seawater.
2. To use the titration technique with a reaction different from an acid/base
reaction.
Safety
Wear protective goggles throughout the laboratory activity. Avoid getting AgNO 3 on
hands since it stains the skin.
Procedure
1. With a pipet deliver 5.0mL seawater into a 150-mL Erlenmeyer flask.
2. Using a 10-mL graduated cylinder, add 5.0mL potassium chromate indicator
solution, and begin to add the silver nitrate solution from a buret while
constantly stirring.
3. When the solution turns orange, stop adding silver nitrate. The color should
persist for less than 20 sec. Continue adding silver nitrate dropwise, mixing all
the while, until orange color persists. Record the volume of silver nitrate added.
4. Thoroughly wash your hands before leaving the laboratory.
Data Analysis and Concept Development
From a simple algebraic proportion one can determine the chlorinity of the seawater
because it has been shown experimentally that the salinity is a constant 1.80655
times higher than the chloride level. Thus, the salinity (S) can be calculated (using
the equation S ppt = 1.80655 Cl ppt; the chlorinity (Cl) of the artificial seawater
should be about 19.05 ppt).
1. What is the number of moles of silver used in the titration (AgNO 3 is 0.2184 M)?
2. Determine the number of moles of chloride in the sample.
3. Determine the grams of chloride present in the sample.
4. What is the chlorinity of 10g of sample? (The density of seawater is about
1.05 g / mL 1 ).
5. What is the salinity of the sample in ppt?
LABORATORY ACTIVITY 2:
TEACHER NOTES
Activity 2: Determination of the Salinity of Seawater
Major Chemical Concept
Quantitative determination of the chloride concentration in a sample of seawater.
Level
General high school chemistry
Expected Student Background
Students should have done titrations and titration calculations.
Time
55 min
Safety
Read the Safety Considerations in the Student Version.
Materials (For 24 students working in pairs)
Nonconsumables
12 Burets
12 Buret stands and clamps
12 Graduated cylinders, 10-mL
12 Erlenmeyer flasks, 150-mL
12 Magnetic stirrers (optional)
12 Pipets, transfer, 10-mL
2 Storage bottles, 1-L
1 Volumetric flask, 1-L
Consumables
Distilled water (in wash bottle)
Indicator solution (3.5 g, potassium chromate, K 2 CrO 4 , reagent grade, per
liter solution)
0.2184 M Silver nitrate, AgNO 3 (37.11 g AgNO 3 , reagent grade, to 1.000 L in a
1-L volumetric flask. Store solution in a tightly capped brown glass bottle.)
Advance Preparation
Prepare:
Potassium chromate (observe safety precautions)
Silver nitrate (observe safety precautions)
Pre-Laboratory Discussion
1. Demonstrate the proper titration techniques and remind students that
silver nitrate will stain their hands.
2. Discuss the reaction that gives the red-brown color
 
and demonstrate a titration showing the students the final end point color.
3. Review the proper use of a transfer pipet.
Teacher-Student Interaction
Circulate in the laboratory. Ask questions comparing this titration with previously
done acid-base titrations. Ask why it is not important to use an exact amount of
indicator solution.
Post-Laboratory Discussion
Part A
Discuss differential solubility. Since the Ag 2 CrO 4 (s) is much more soluble than
AgCl(s), it is only after almost all of the AgCl(s) is precipitated that the red-brown
end point be reached with the precipitation of Ag 2 CrO 4 (s).
Anticipated Student Results
 
 
Assessing Laboratory Learning
1. If 24.0 mL of 0.250 M AgNO 3 is required to reach the end point of the
seawater solution (CrO 4 2 (aq) reaction with Ag + (aq) to give a red-brown
precipitate), determine the number of moles of Cl  ions present.
2. Determine the grams of chloride present in the sample.
3. What is the chlorinity of a 10.0 mL sample? (The density of the seawater is
determined to be 1.05 g/mL.)
4. What is the salinity of the sample? (1.80655) Chlorinity (ppt) = Salinity (ppt)
CAUTION: Use appropriate safety guidelines in performing demonstrations.
 
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