Experiment 10
Acid – Base Titration
Definitions:
Titration
• A technique to accurately and precisely measure something, most
often acids or bases
Acids
• Most acids are molecules that behave as if they were ionic compounds
in which the cation is the hydrogen cation, H+ (which happens to be a
proton).
• When acids dissolve in water, they release an H+ cation (hydrogen
ion), which binds to water.
Reminder: Acids sometimes undergo a name change when they are
dissolved in water, e.g. Hydrogen chloride becomes hydrochloric acid.
Bases
• A soluble compound containing the OH- anion (hydroxide) is called a
base.
Note: NH3 is a special case. NH3 dissolves in water, but is also thought to
react slightly with water to also produce some hydroxide anion: NH3 + H2O
→ NH4+ + OH- .
Concentration: Molarity
• The ratio of the number of moles to a fixed volume, the liter, is given
the name: Molarity (M)
• Molarity (M) = moles of solute / liters of solution
M = mol / L
Neutralization Reactions: Acid – Base Titration
• Acid-Base reactions (H+ + OH- → H2O) are also called Neutralization
Reactions
• We use neutralization reactions to determine the concentration of an
unknown acid or base
• The procedure of neutralization is called a titration. With this
procedure we carefully add just enough acid solution to neutralize a
known volume of a base solution
- Or visa versa
• A dye such as phenolphthalein, that changes color when the [H+] ≈ 1 x
10-7 M, is employed in the solution to indicate when neutrality has
been reached.
• The dye, phenolphthalein changes from colorless to a pink color as
soon as any excess OH- occurs.
• The change may require only a single drop (50 µL) of your acid
solution
Reading a Buret:
Must read with proper line of sight to avoid “parallax” error
Example Calculation:
What volume of a 0.100 M HCl solution is required to neutralize 25.0 mL of
a 0.350 M NaOH solution?
Solution:
1. Label the knowns and unknowns, e.g.: What volume (V1) of 0.100 M
HCl (M1) solution is required to neutralize 25.0 mL (V2) of a 0.350 M
NaOH (M2) solution?
2. Substitute into the following equation and solve for V1:
M1 x V 1 = M2 x V 2
(0.100 M HCl) (V1) = (0.350 M NaOH) ( 25 mL)
V1 = [(0.350 M NaOH) (25 mL)] / 0.100 M HCl
V1 = 87.5 mL
Today’s experiment:
• HCl concentration = 0.094872 moles / liter = Moles / L
• Careful technique is critical to good results
• Grading will be based on accuracy in determining an unknown base
solution
Equipment:
Two separate check out steps:
1. Check out two burets and a buret clamp
You will also need a spare “waste” beaker
Must work with great care at all times to avoid breaking off the glass tips
during:
• Cleaning
• Handling
• Titration
Check burets for cleanliness
• Rinse with water. No water droplets on inside
• Use small beakers for adding water for rinsing
• Wash with soap and buret brush if water beads up
• Note: Measurements are to hundredths of a mL, and a single drop of
water ≈ 50 µL or 5 hundredths of a mL. Extra water droplets adhering
to the walls of the cylinder can significantly affect accuracy.
Rinse burets
• Rinse with tap water several times including the stopcock and tip
• Final rinse with deionized water
• Check to be sure stopcocks are secure
2. Check out reagents:
• Exchange a clean 250 mL Florence flask and a 250 mL beaker for the
reagents in comparable containers
• Note: The Florence flask contains 250 mL of the unknown NaOH.
The same solution will be employed next week, so use it sparingly
and appropriately.
• Write your name on the Florence flask with a Sharpie. When done,
seal the flask with Parafilm and store in a designated cupboard.
Burets: Final preparation
• Complete two careful 5 mL rinses with the reagent prior to filling
each buret
• Red buret for acids
• Blue buret for bases
Titration Measurements
• Measures volume to one hundredths of a milliliter
• Use small funnels for adding reagents to burets
• Add reagents to just below the 0 mL mark
• Run some reagent out to fill the stopcock and tip
• Trick to reading the meniscus: hold something behind the cylinder
• Accurately read / record the initial volume reading. Starting at an
exact number is not important. Accurately reading the number is.
Part I
• Add approximately 20 mL of HCl and then accurately read the
volume reading. This will be recorded at the initial HCl reading.
• Add two drops of phenolphthalein
• Begin titrating with your unknown NaOH solution, swirling the flask
as needed
• As the volume of NaOH approaches the HCl volume, slow the
addition to about a drop every 5 seconds while continuing to swirl the
flask
• Neutralization is completed with the clear solution turns permanently
slightly pink with the addition of a single drop.
• Record your final volume as the final reading
• Complete your calculations of the NaOH concentration and bring to
the instructor for review before completing your remaining runs
Part II
Check burets for cleanliness
• Rinse with water. No water droplets on inside
• Use small beakers for adding water for rinsing
• Wash with soap and buret brush if water beads up
• Note: Measurements are to hundredths of a mL, and a single drop of
water ≈ 50 µL or 5 hundredths of a mL. Extra water droplets adhering
to the walls of the cylinder can significantly affect accuracy.
Rinse burets
• Rinse with tap water several times including the stopcock and tip
• Final rinse with deionized water
• Check to be sure stopcocks are secure
Unknown Acid Concentration
Today, you will be given an unknown acetic acid (HC2H3O2) solution.
• There are several different possible unknown solutions. You will be
graded on how close your experimental value comes to the true value
of your unknown.
• Record your locker number on your Part II worksheet.
• Note that the data sheet for Part II is the reverse of that for Part I (acid
and base parts have been reversed)
• The calculations are also reversed: MaVa = MbVb. This time you are
solving for Ma.
Note that this week, your unknown will require anywhere from 10 – 35 mL
of your standard NaOH solution, depending on which unknown you have.
• You may wish to make a “rough cut” of how much NaOH you will
need by taking a couple of mL of unknown and titrating it to get into
the range of NaOH you will need. Then you can make adjustments as
to how much acid and base you will need for your accurate titration.
• You will use the NaOH value you determined from Part I. Check
your returned Part I for any corrections you may need to incorporate.
• Be watchful of how much of your NaOH you use. You should have
enough for approximately 4 titrations of the unknown acid
concentration.
• MaVa = MbVb
Calculation of mass percent:
• Mass % = g solute / g solution
• Molarity = moles HC2H3O2/ L solution
• Molar mass of HC2H3O2 = 60.053 g.
• Mass % acetic acid = g HC2H3O2 / g solution (Assume the density
of the solution is 1.00g / mL)
• Assume the density of the solution is 1.00g / mL (this will work up to
three sig figs).
• Mass % acetic acid = (moles HC2H3O2/ L) x (conversion moles to g)
x (conversion L solution to g).