CVEN 3454/5404 Water Chemistry
Lab 1: Titration of Unknown
Acids
Purpose
The main purpose of Lab 1 is to determine the identity of an
unknown acid by measuring the effect of strong base addition on the acid solution (i.e., a
titration). A plot of pH vs. the volume of base added will yield data that can be used to
determine the acid dissociation constant(s) (pKa values) of the unknown acid. A
second purpose is to measure the pH of some local waters affected by acid mine
drainage to start assembling a water
chemistry data base for future laboratories.
Materials
- pH meter (Orion, model 250A or 290A)
- pH electrode (Orion Triode® model
9107BN) and electrode stand
- pH buffers (4.01 and 7.00)
- stir plate, stir bars, beakers, 50 mL burette,
stand
- sodium hydroxide (NaOH) solution, unknown
concentration, about 0.1 M (exact concentration to be determined)
- potassium biphthalate (C8H5KO4),
about 0.1 M (exact concentration provided by TA).
- water samples (see number 2)
Procedure
- Calibrate your group's pH electrode and meter with the pH 4 and 7
buffer solutions. Be sure to rinse the electrode with deionized water (dW) before
immersing it into the next solution. Remember to
make sure that the auto-shutoff feature has been turned off (press 2nd+setup,
press yes
down to setting 1-4, press down arrow
to change the setting to "no", press yes
to go to setting 2-1, press measure
to return to measurements).
- Measure the pH of the the water samples collected today from water
within reach of the banks of the water bodies. The waters were collected from
the following locations:
1. Big Five Tunnel drainage (temporary settling basin)
2. Big Five Tunnel drainage (bottom of waste rock pile, about 40 m
from tunnel)
3. Big Five Tunnel drainage (sedimentation pond, about 150 m from tunnel)
4. Big Five Tunnel drainage (discharge to Lefthand Creek, about 250 m
from tunnel)
5. Lefthand Creek (about 40 m upstream of Big Five Tunnel drainage)
6. Lefthand Creek (about 40 m downstream of Big Five Tunnel
drainage)
- A NaOH solution of unknown concentration (approximately 4 g NaOH
per liter, or about 0.1 M) will be prepared by the TA and provided in a plastic
bottle (because high pH solutions dissolve glass!). We will determine the exact
concentration of this NaOH solution so that we can use it to titrate the unknown acid.
To do this, we will titrate a primary standard solution of potassium biphthalate,
which the TA has very carefully prepared using a sensitive balance.
a. add about 50 mL of the NaOH solution to a burette.
b. transfer 40 mL (does not have to be exactly 40 mL, but know the volume exactly) of the
potassium biphthalate
solution to a 125 mL beaker.
c. place the beaker on a stir plate, add a magnetic stir bar, and stir the solution.
d. place the pH electrode in the solution and record the initial pH.
e. add small amounts (0.5 to 1.0 mL) of NaOH solution to the biphthalate
solution while stirring. If you know that you are on a plateau in the
titration curve (pH increasing slowly as a function of NaOH added), you can
increase the volume added to perhaps 2 mL.
f. record the volume of NaOH added and the resulting pH in your lab notebook in a
table that looks like this:
|
burette volume
(mL) |
volume added
(mL) |
cumulative volume
(mL) |
pH |
| xx.x |
xx.x |
xx.x |
x.xx |
| ... |
... |
... |
... |
The "burette volume" is the volume
remaining in the burette.
The "volume added" is the difference
between burette volumes for each addition.
The "cumulative
volume" is the total volume dispensed from the burette up to the current
addition.
You may have a burette from which you can read the cumulative volume
directly.
While you are titrating, plot the pH versus the volume of base added to produce
a titration curve. The
titration curves for this standardization of the NaOH solution and for the ensuing unknown
acid should be included in your lab reports in the Results section.
- Refill your burette with NaOH solution and titrate ~40 mL (know
the exact volume) of one of the unknown organic or inorganic acid solutions, which will be
provided, with your NaOH solution of known concentration.
Possible Unknown Acids
|
acid |
pKa1 |
pKa2 |
pKa3 |
| acetic acid |
4.76 |
-- |
-- |
| tartaric acid |
3.03 |
4.37 |
-- |
| boric acid |
9.24 |
-- |
-- |
| phosphoric acid |
2.14 |
7.21 |
12.38 |
Lab Technique Note
NaOH solutions should never be stored in glass vessels, especially
those with ground-glass joints (like a volumetric flask or reagent bottle). Such solutions cause the glass
to dissolve and, when the solution evaporates from the joint, the dissolved silica
precipitates. The silica precipitation "freezes" the joint shut.
What should you do if you encounter a ground-glass joint frozen in
such a manner? Don't try to twist and yank the stopper -- most likely it or the top of the
flask will break in your hand. A technique recommended in the Corning Glassware catalog
suggests immersing the frozen joint in a freshly-opened Coke -- why?
Questions to Address in Lab Report
- (methods -- lab notes) How long did you wait before you read the pH
during the titration? Why?
- (results) What was the exact concentration (molar) of the NaOH stock
solution provided?
- (results) What were the pH values of the water samples you measured?
- (discussion) What was the unknown acid? What was its concentration
(molar)? Explain your reasoning.
- (discussion) Were you able to detect the loss of the first proton
from your unknown acid? Why or why not?
- (discussion) To the best of your current ability, comment on the
change in pH in the Big Five Tunnel and Lefthand Creek waters as they move downstream.
Explain the the role that dilution of the acid mine drainage in Lefthand Creek
might play.
Last updated on
September 10, 2007 at
10:17 AM by Joe Ryan