Transcript Burets

The Care and
Feeding of Burets
• A buret is a long cylindrical tube with a valve
at the bottom end, which is used for making
very precise volumetric measurements.
• When liquid is dispensed through the valve at
the bottom of the buret, the difference
between the final volume reading and the
initial volume reading measures the volume of
liquid which has come out of the buret.
What Does a Buret Look Like?
Top of the buret,
showing the liquid’s
A typical buret setup
Buret stopcock
(note that the buret tip is
completely full of liquid, with
no air bubbles in the liquid)
Reading a Buret
• The burets that we will be using are marked at 0.0
mL on the top line, and 50.0 mL on the bottom line,
with 0.1 mL increments.
– This means that the line on the meniscus must be
read to the nearest 0.01 mL to ensure that the
correct number of significant figures have been
Setting Up a Buret
Rinsing the Buret
• A buret must be scrupulously clean in order to
obtain precise and reproducible results.
• Rinse the buret well with tap water, and then rinse it
twice with 5 mL portions of deionized water.
• Rinse the buret twice with 5 mL portions of the
solution with which the buret will be filled.
– If the buret is rinsed with water, and then filled with the
solution immediately, the solution will be diluted slightly
by the residual water in the buret. Rinsing the buret with
the solution will ensure that the buret is wet with whatever
solution it is being filled with, so the concentration of the
solution will not change when the buret is filled.
– Make sure that some of the water and the wash solution
go through the buret tip.
Rinsing the Buret
Filling the Buret
• After the buret has been rinsed, attach it to a buret
clamp on a ring stand.
• Fill the buret with the solution which will be
measured (use a funnel if necessary). Fill the buret
above the 0.0 mL line.
• Check to make sure that the buret is not leaking.
Filling the Buret
Getting Rid of Air Bubbles
• If the buret tip contains air bubbles, this will affect
the volume measurement. Open the valve to allow
some of the solution to run out of the buret, until
there are no more bubbles of air in the buret tip.
Acid-Base Titrations
• A titration is a procedure that is often used for
determining the concentration of a solution. Most
commonly, a standard solution of known
concentration is reacted with a solution of unknown
• By measuring the volume of standard solution that
reacts with a known volume of the unknown
solution, the concentration can be calculated from
the reaction stoichiometry.
• A common example of this process is an acid-base
titration, in which an acid or base of unknown
concentration reacts with a base or acid of known
concentration in a neutralization reaction:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Acid-Base Titrations
• The solution of unknown concentration which is
being titrated is placed in an Erlenmeyer flask,
which can be easily swirled to ensure complete
• The titrant, which will react with the unknown
solution, is placed in the buret. This is a solution of
known concentration (or with a concentration that is
determined by another series of titrations known as
• The valve at the bottom of the buret is opened, and
the titrant is added to the unknown solution, causing
a chemical reaction to take place. The addition is
stopped when the number of moles of titrant which
has been added is equal to the number of moles of
reactant in the solution which is being titrated.
Acid-Base Titrations Illustrated
B a se of kn o w n
co ncen tra tio n
H 2O
A cid of u n kno w n
co n cen tratio n
H 2O
b eg inn in g o f titration
H 2O
H 2O
H 2O
n o O H add ed
so m e O H - added ;
so m e H h as b eco m e H 2 O
H 2O
H 2O
eq u iv alen ce p oin t
all H + has beco m e H 2 O
The Equivalence Point
• In the preceding example, at the equivalence point,
the number of moles of NaOH added from the buret
equals the number of moles of HCl which were
initially present.
• The concentration of the unknown HCl solution can
be calculated from the reaction stoichiometry:
L NaOH 
mol NaOH
mol HCl
L HCl soln.
1 mol HCl
1 mol NaOH
 M HCl
 mol HCl
End Points and Indicators
• The end point of the titration, where we
experimentally estimate the equivalence point to be,
is usually signaled by the color change of an acidbase indicator, such as phenolphthalein.
• Phenolphthalein is colorless in an acidic
environment, but pink in a basic environment.
– Once the end point of the titration is reached, one excess
drop of the NaOH solution turns the phenolphthalein pink.
Titration Video
Titration Video
Calculations: Initial and Final Readings
• In this example, 15.00 mL of HCl of unknown
concentration is being titrated with 0.5115 M NaOH.
• Shown below are the initial and final readings on the
buret. The difference between the initial and final
readings is the volume of NaOH titrant which has
been dispensed from the buret.
Initial reading
2.38 mL
Final reading
18.51 mL
Volume of titrant = final reading – initial reading
= 18.51 mL – 2.38 mL
= 16.13 mL
Calculations: Moles of Titrant
• Once the volume of the titrant and its concentration
are known, the number of moles of titrant which
have been used can be determined.
0.01613 L × 0.511 mol/L NaOH = 0.008251 mol NaOH
Calculations: Moles of Unknown
• The reaction stoichiometry is then used to determine
the number of moles of HCl initially present:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
0.008251 mol NaOH 
1 mol HCl
1 mol NaOH
 0.008251 mol HCl
Calculations: Molarity of the Unknown
• Once the number of moles of HCl are known,
dividing by the initial volume of HCl (in this
example, 15.00 mL) gives us the concentration of
the solution:
0.008251 mol HCl
0.01500 L HCl soln.
 0.5500 M HCl
Titration of Vinegar
• In this experiment, the concentration of acetic acid,
HC2H3O2, in commercial vinegar will be determined
by titration against a standard solution of NaOH.
The equation for this neutralization reaction is:
HC2H3O2(aq) + NaOH(aq)  NaC2H3O2(aq) + H2O(l)
• From the known concentration of the sodium
hydroxide, and the volume of the solution that
emerges from the buret, the number of moles of
NaOH can be determined, which allows the number
of moles of HC2H3O2 originally present and the
concentration and weight percent of the acid to be