For best results please view this as a slide show.
You can hit the F5 key or go to the Slide Show tab
on the menu bar and click on From Beginning.
Page Down and Page Up will move you through
the presentation.
If you have a Mac do whatever you have to do to
play it as a slide show – I don’t know Macs well.
Dr. Buckley
e-mail: [email protected]
Experiment #10 – Determination of Acetic Acid in Vinegar
Laboratory Overview
CHEM 1361
August 2010
Gary S. Buckley, Ph.D.
Department of Physical Sciences
Cameron University
Table of Contents
(you may click on any of the topics below to go directly to that topic)
•Chemical Background
•Experimental Scheme
•Standardization of Sodium Hydroxide
•Determination of Acetic Acid in Vinegar
•Experimental Notes
•Refresher on Accuracy, Precision, Average, and Standard Deviation
•Accuracy and Precision
•Measures of Accuracy and Precision
•Accuracy and Precision – An Example
Chemical Background
Vinegar, by law, is required to have at least 4%, or 4 grams acetic acid per 100 mL
of vinegar. In this experiment you will use an analytical method called titration to
determine the % acetic acid in a vinegar sample.
Acetic acid, written as HC2H3O2 or CH3COOH, reacts with NaOH according to the
following chemical equation:
HC2H3O2 (aq) + NaOH (aq) → HOH (ℓ) + NaC2H3O2 (aq)
A known volume of vinegar may be reacted with a measured quantity of sodium
hydroxide. Since the acetic acid and sodium hydroxide react in a 1:1 mole ratio,
knowledge of the number of moles of NaOH used leads directly to the number of
moles of acetic acid in the sample of vinegar. The following slides work this out in
more detail.
Return to Table of Contents ↑
Experimental Scheme
NaOH
Sodium hydroxide of a known concentration will be delivered
from a buret as indicated in the figure to the right. The
Erlenmeyer flask will contain the vinegar sample. A few
important points:
1. The concentration of the sodium hydroxide must be
known. This concentration is determined through a
process called standardization.
2. Reaction of the sodium hydroxide and the acetic acid
leads to a clear solution – there is no way of knowing
when the number of moles of sodium hydroxide
matches the moles of acetic acid – unless:
3. You add an indicator that will change color at that
point, called the end point. In this case,
phenolphthalein will be added as the indicator. It will
change from clear to pink at the end point .
Vinegar +
phenolphthalein
Return to Table of Contents ↑
Standardization of Sodium Hydroxide
Making an accurately known concentration of sodium hydroxide from
starting reagents is virtually impossible. Solid sodium hydroxide is
hygroscopic – absorbs water readily from the air – so obtaining a mass that
is purely sodium hydroxide is not feasible. It is also available as a 50 %
solution but the concentration is not known to high accuracy.
Thus, to know accurately (say, four significant figures), the concentration of
a sodium hydroxide solution it must be reacted with a known number of
moles of some other substance. A variety of primary standards –
substances whose purity is well known – is available to carry out such a
reaction. In this experiment, the primary standard potassium hydrogen
phthalate, KHC8H4O4,will be used to determine the concentration of the
sodium hydroxide. Potassium hydrogen phthalate is also referred to
affectionately as KHP. Be alert – it is not potassium, hydrogen, and
phosphorous. Its formula is KHC8H4O4 and its molar mass is 204.2 g/mol.
Return to Table of Contents ↑
Standardization of Sodium Hydroxide (continued)
The reaction between KHP and sodium hydroxide is as follows (in net ionic form):
HC8H4O4- (aq) + OH- (aq) → C8H4O4 2- (aq) + HOH (ℓ)
The KHP mass will be accurately recorded, from which the number of moles may be
determined using its molar mass (204.2 g/mol). Using phenolphthalein as the
indicator, sodium hydroxide solution will be added until a faint pink end point is
reached. At that point, the moles of sodium hydroxide added will equal the number
of moles of potassium hydrogen phthalate added. By determining the volume of
sodium hydroxide solution added, its molarity may be determined by :
M NaOH =
mol NaOH delivered to endpoint (equal to mol KHP measured out)
L NaOH solution delivered to endpoint
Return to Table of Contents ↑
Determination of Acetic Acid in Vinegar
Once the sodium hydroxide solution has been standardized, it may be used to
determine the quantity of acid in other solutions, in this case acetic acid in
vinegar. Acetic acid and sodium hydroxide react according to the equation (net
ionic form):
HC2H3O2 (aq) + OH- (aq) → C2H3O2 2- (aq) + HOH (ℓ)
Since the molarity of the sodium hydroxide solution is known after the
standardization, the moles of sodium hydroxide delivered in the titration may
be determined. The moles of sodium hydroxide will match the moles of acetic
acid at the end point.
The molarity of the acetic acid in the vinegar may be determined by dividing the
number of moles of acetic acid by the volume (in L) of the vinegar titrated. To
get to the % acetic acid in vinegar (g/100 mL) from there, the moles of acetic
acid in one liter may be converted to grams and the result divided by 10 to
reflect that 100 mL is one-tenth of one liter.
Return to Table of Contents ↑
Experimental Notes
The primary piece of lab apparatus used in this experiment is called
a buret (or burette), pictured to the right. Its function is to provide
a means of accurately determining the volume of a solution
delivered . At the bottom of the buret is a stopcock that allows
solution to flow or blocks its flow depending on its orientation.
The positions of the stopcock are illustrated below.
Flow is blocked if the stopcock
is turned horizontally.
Solution flows if the stopcock:
If the stopcock is turned
is turned vertically.
Filling the buret is a simple matter of using a funnel to pour your
solution through the top WHILE THE STOPCOCK IS CLOSED. After
filling, allow some solution to pass through the stopcock to a waste
container to remove air bubbles from the tip.
Experimental Notes (continued)
Reading the Buret
To attain the full benefit of using a buret, it must be read
correctly. Notice the volumes increase as you go down the
buret. When making a reading, you are simply recording
the volume as indicated on the buret – don’t try to make it
harder by subtracting from 50 or anything. Furthermore,
notice how the buret is marked to the 0.1-mL place. This
suggests that EVERY reading must be to the .01-mL place –
we always estimate one digit on an analog device.
Aqueous solutions inside of the buret show the meniscus
discussed in Experiment #2. The reading is always taken at
the bottom of the meniscus. In this case the reading would
be 1.40 mL – notice the 0 is included to indicate the
meniscus was right at the line.
You will notice the table in the lab book is set up with the
final volume first followed by the initial volume. This helps
see that the determination of the volume delivered is made
by subtracting the initial volume reading from the final
volume reading.
Meniscus
Return to Table of Contents ↑
Refresher on Accuracy, Precision, Average, and Standard Deviation
The following few slides are an encore presentation of the discussion of
some topics discussed in Experiment #2. In both your standardization
process and your determination of the % acetic acid in vinegar, you will
be able to find an average and a standard deviation since you will do
multiple trials of each.
The average is linked to the accuracy of the results – how close your
results are to the “real” value which, by the way, we do not really know.
We are simply trying to determine it experimentally.
The standard deviation is linked to the precision of the results – how
close your repetitions are to each other. A small standard deviation
indicates a consistent experimental technique, though it does not say
anything about the accuracy.
Return to Table of Contents ↑
Accuracy and Precision
Measurements in the laboratory are an attempt to find the “real” value
of a physical quantity. Two terms used in relation to measurement are:
•Accuracy – the nearness of the measured value to the “real” value
•Precision – the nearness of repeated measurements to each other
Note that a measured result may be accurate, but not precise; not
accurate, but precise; both accurate and precise; or neither accurate nor
precise.
The next couple of slides give one method for considering accuracy and
precision.
Return to Table of Contents ↑
Measures of Accuracy and Precision
Average or mean – the average, sometimes called the mean,
is simply the sum of repeated attempts to measure the same
quantity divided by the total number of attempts.
Standard deviation – the standard deviation is an indication
of the precision of repeated measurements. If one takes N
measurements of the same physical quantity , the standard
deviation, s, is given by:
N
( xi
s
x) 2
i 1
N 1
Where xi represents the ith measurement and x represents
the average of all of the measurements.
Return to Table of Contents ↑
Accuracy and Precision – An Example
Suppose you had measured the molarity of a solution (doesn’t
matter if you know what molarity is yet or not) and got the three
values of 0.1042 M, 0.1004 M, and 0.1033 M. The average is given
by:
0.1042M 0.1004M 0.1033M
x
3
0.1026M
The standard deviation would be calculated as:
s
(0.1042 0.1026) 2 (0.1004 0.1026) 2 (0.1033 0.1026)2
3 1
0.0020M
The result would be reported as 0.1026 ± 0.0020 M, or more
properly as 0.103 ± 0.002 M as we try to keep one significant figure
in the standard deviation and round the average to match that
number of decimal places.
Return to Table of Contents ↑