SCC 201 Experiment 5- Titration of a Newtown Creek Environmental Water
Sample to Determine the Amount of Chloride Ions
Competency-Inquiry/Problem Solving and writing communication
Learning objectives:
To analyze and calculate the amount of chloride ions in an environmental sample by precipitation
titration
To determine the equivalence point by visual observation
To become familiar with using pipettes and burettes for quantitative measurements
Introduction:
The chloride ion is an inorganic anion that is naturally occu
ing in environmental waters. Seawater
contains 1.94% chloride and can be commonly found in varying forms such as potassium chloride (KCl),
sodium chloride (NaCl) and magnesium chloride (MgCl2) with these salts all being water soluble (1). The
Environmental Protection Agency has set a standard for the chloride concentration in drinking water to
e 250 mg/L (2). Although chloride is non-toxic to humans, high levels of chloride can affect plants and
co
ode away infrastructure such as roads and pipes. Possible sources that increase the chloride
concentration include industrial processes, sewage, road salts, and fertilizers.
Titration is a technique whereby adding a measured quantity of known concentration (titrant) is reacted
with an unknown substance (analyte). The goal of this process is to determine the concentration of the
unknown using the stoichiometry of the chemical reaction between analyte and titrant. In this
experiment, a precipitation titration will be conducted where the reaction of the analyte and titrant will
give ionic compounds with limited solubility.
In today’s experiment, the amount of chloride in water from Newtown Creek will be determined by
titrating the chloride analyte with silver nitrate solution and using potassium chromate as an indicator.
The titrant is slowly added to the environmental water sample whereby the silver ions react with
chloride ions in a 1:1 ratio to form a silver chloride precipitate:
XXXXXXXXXXAg+ (aq XXXXXXXXXXCl- (aq) → AgCl (s)
The end point of the titration occurs when all of the chloride ions has precipitated out of solution (as
solid AgCl) and the free silver ions begin to react with the chromate ions (CrO4 2-) to give a
own-red
precipitate. Knowing the volume/concentration of the titrant used to attain the equivalence point and
stoichiometry of the chemical reaction, the amount of chloride ions can be calculated. Ideally, the
titration should be ca
ied out at a pH between 7 and 9 as a low pH environment will cause the
chromate anion to protonate and lower the chromate concentration where an endpoint cannot be
detected. Conversely, a high pH will lead to the formation of a
own silver hydroxide compound which
again inhibits the observation of the endpoint.
Pre-Laboratory Exercise
1. Define the terms equivalence point, endpoint, indicator, analyte and titrant?
2. What are the chloride ion limits for drinking water secondary standards set-forth by the
Environmental Protection Agency?
Experimental Procedure
1. Using a 10 mL volumetric flask, place 10 mL of filtered Newtown Creek water sample in an
Erlenmeyer flask and measure the pH. The pH value should be in the range of 7-9. If your
environmental sample has a pH value outside this range, adjust the acidity with nitric acid or
sodium hydroxide.
2. Add 3-4 drops of potassium chromate indicator to the Erlenmeyer flask. A light yellow color
solution should appear.
3. Set up a 50 mL burette:
a) Obtain a burette, burette tip, burette clamp, accompanying ring stand and assemble. A
magnetic sti
er device will be used as a base for the apparatus set-up.
) Clean the burette and burette tip with distilled water then discard. Afterwards, pre-rinse the
urette with a few mL portion of standard AgNO3 solution to remove any remaining water
droplets.
c) Place a funnel at the top of the burette and fill the burette with standard AgNO3 solution to
the 0.00 mL mark. Allow a few mL to run (to remove air bu
les) and refill up to the 0.00 mL
mark. (NOTE: You should only need to fill up the burette once)
4. Place a stir bar in the Erlenmeyer flask, turn on the magnetic sti
er so that a gentle swirl of
solution is reached.
5. Record the concentration of AgNO3
6. Titrate with AgNO3 solution. A white precipitate will initially form. The endpoint will be
identified by the first appearance of a permanent
ownish-red precipitate (with the
accompanying white precipitate). Record the volume of silver nitrate used. (Note: The first trial
should be used as a “rough” titration)
7. Repeat the titration until the volume of titrant used for different trials are in agreement.
8. Calculate the concentration chloride ion concentration (Molarity, mg/L and parts per million)
Data Sheet: Experiment 5 - Titration of a Newtown Creek Environmental Water
Sample to Determine the Amount of Chloride Ions
Student: XXXXXXXXXX_________________________ XXXXXXXXXXLab Partner: __________________
Instructor and Section: _________________________ XXXXXXXXXXDate: XXXXXXXXXX__________________
Instructor’s Signature: _________________________
Data:
pH of water sample:________________ XXXXXXXXXXMolarity of AgNO3 standard: __________
Trial 1 Trial 2 Trial 3
Volume of water sample
added (mL)
Initial volume of AgNO3
(mL)
Final volume of AgNO3
Volume of AgNO3 used
(mL)
Moles of AgNO3 used: XXXXXXXXXX_________________________
Moles of Cl- ions added to Erlenmeyer flask: XXXXXXXXXX_________________________
Volume of analyte sample added (L): XXXXXXXXXX_________________________
Concentration of Chloride Ion (M): XXXXXXXXXX_________________________
Concentration of Chloride Ion (mg/L): XXXXXXXXXX_________________________
Concentration of Chloride Ion (ppm): XXXXXXXXXX_________________________
Average volume of AgNO3
used (mL)
Questions to Address in Lab Report:
Show all calculations to find moles and concentrations of chloride?
Write the relevant chemical equation(s) associated with silver ions? What is the purpose of chromate
ions in chloride determination?
What are some possible sources of e
or in this lab? (Note- this is not a pure water sample)
What are some of the concerns with high levels of chloride ions in environmental waters? Does the
measured chloride concentration from Newtown Creek exceed the Secondary Standard limits set-forth
y the Environmental Protection Agency?
In theory, if the chloride concentration in the titrated water sample is too high, what method(s) could
e employed to reduce the amount of chloride ions?
What are some factors (natural or artificial) that could influence the amount of chloride in Newtown
Creek?
Reference
1. Balasu
amanian, A. (November 2016) Properties of Seawater. Retrieved from URL:
https:
www.researchgate.net/profile/A_Balasu
amanian/publication/309785723_Properties_
of_Seawater-Documentary/links/582363e208ae7ea5be71fa4b.pdf?origin=publication_detail
2. Environmental Protection Agency (Fe
uary XXXXXXXXXXRetrieved from URL:
https:
www.epa.gov/dwstandardsregulations/secondary-drinking-water-standards-guidance-
nuisance-chemicals
https:
www.researchgate.net/profile/A_Balasu
amanian/publication/309785723_Properties_of_Seawater-Documentary/links/582363e208ae7ea5be71fa4b.pdf?origin=publication_detail
https:
www.researchgate.net/profile/A_Balasu
amanian/publication/309785723_Properties_of_Seawater-Documentary/links/582363e208ae7ea5be71fa4b.pdf?origin=publication_detail