Lab 5: Acids, Bases and Operational Definitions
Short, written answers are required to the questions below and are designed to
enhance the students' awareness of some of the procedural details of the laboratory.
They will be checked as the students enter the laboratory and count for 1 of 5 points
for the laboratory exercise.
1.
Reproduce the diagram of the well plate below and label it as you plan to use it.
Interpretation of the experiment is simplified if the known solutions are arranged
in the order found in Data Table 4.1.
2.
A correlation between pH of a solution and its color in the presence of Yamada's
universal indicator is given in the procedure section of the lab. What color would
you expect strongly basic solution?
3.
What is the purpose in Part II of the experiment, in which the known solutions are
spotted on DY4 paper?
4.
Why, after dyeing the multi fiber strip, it is rinsed first with distilled water and
then with tap water?
Introduction
Acids and Bases:
The Development of an Operational Definition
The concept of acids and bases originated in Greece in the fifth century BC and
represents one of the longest, continually used ideas by which materials are classified
according to certain properties. For acids, these properties include a sour taste (NOT an
endorsed mode of observation in a contemporary undergraduate laboratory!) and the
ability to react with bases in a reaction which negates the acid and base properties of the
reactants, commonly called a neutralization reaction. Bases may be defined by similar
properties. In ancient Greece, these ideas were well received because of their consistency
with the reigning philosophy of warring opposites, a philosophy which remains powerful
to this day: yin and yang, good and evil, etc.
An operational definition is one which defines a concept in terms of how it is measured.
For example, temperature is a measure of the heat energy in a system. However, the
measurement of temperature most frequently involves observing the volume of a small
capsule of mercury at the bottom of a thin, calibrated column. The volume of mercury is
sensitive to temperature; it expands when heated and contracts when cooled. If the
mercury is maintained at a constant pressure, its volume is linearly related to its
temperature. Importantly, although temperature may be used to evaluate the heat
energy of a sample, it tells us very little about the nature of heat energy.
The classification of a material as acid or base, based upon the color of an indicator
substance in the presence of that material, is another example of an operational definition.
For example, most students are familiar with litmus paper, small strips of pink paper
which turn blue when dipped into basic solution, or red when dipped into acid. The
indicator material in litmus paper is a colored substance derived from lichens. Again, be
mindful that the color exhibited in the presence of litmus doesn't tell us anything about
the nature of acids or bases, but it does provide a very quick method of evaluating
whether a particular sample is an acid or base. More generally useful indicators may be
formulated by mixing two or more indicators together. A particularly useful indicator
mixture widely used in modern laboratories is called Yamada’s universal indicator. The
first part of this lab involves constructing an operation definition of acids and bases based
on Yamada’s universal indicator.
Although the properties and behavior of acids and bases are central to the study of
chemistry, there are many contexts in which these ideas find applications. Many of the
dyes used in fabrics are highly colored acidic or basic compounds capable of acid/base
indicator color changes. Consequently, new dyes must be tested for color stability.
Additionally, dyes often bind only to certain fabrics. When a dye binds to a fabric under
all conditions the fabric is likely to encounter (hot water, detergent), the color is
described as fast; if it comes out on washing, the color is said to have run. Finally,
because the binding of a dye to fabric is often an acid base type interaction, the indicator
behavior of a dye may change when bonded to a fabric. In the second part of the
experiment, the acid base indicator behavior and color fastness of DY4 on 13 types of
fabric will be investigated.
Procedural Overview
Part I
In this part of the experiment, water solutions of six substances (aluminum sulfate,
ammonium chloride, boric acid, ammonium acetate, sodium phosphate and sodium
carbonate) will be tested with Yamada’s universal indicator and litmus indicator. The
acid or base properties of these six substances will be known to the students and included
on the data table for Part I. Although color judgment is not entirely objective, it should
be possible to classify the colors for the Yamada indicator results as red, orange, yellow,
green, blue and violet. Since the solutions will have been previously identified as acid,
base or neutral, a new component of the operational definition for acids and bases will
now include color in the presence of universal indicator. This component of the
operational definition may now be used to measure the acid base characteristics of six
unknown solutions. Litmus indicator is also tested both to provide a reference to a more
common indicator and to invite a critical comparison of the usefulness of these two
indicators.
A common and convenient technique for testing a series of small scale reactions involves
use of a well plate, a technique encountered in the second lab. In this experiment, a 6×4
well plate will be used. This configuration enables the two indicator solutions to be
tested with 6 known solutions and 6 unknown solutions as shown in Figure 4.1 below.
6 x 4 24 Depression Well Plate
Universal
Indicator
Known
Solutions
Litmus
Indicator
Unknown
Solutions
Figure 4.1
Part II
In this part of the experiment, the fabric dye Direct Yellow 4 (DY4) will be studied. Its
indicator behavior will be evaluated using the six solutions of known acid/base
characteristics and its binding properties will be investigated using a multifiber strip.
Finally, the dyed fabric will be tested for color stability in a basic solution.
____________________________________________________________________
Materials
6x4 well plate, dropping bottles, Direct Yellow 4 dyed paper, multifiber strip, 150 mL
beaker
Chemicals
Yamada’s universal indicator solution, litmus indicator solution, six known solutions, six
unknown solutions (see Data Table I)
Procedure
Part I
1.
Rinse the well plate thoroughly with distilled water and dry it out with a paper
towel. Place the well plate on a paper towel (a white surface) to facilitate
observing the results.
2.
Add about 10 drops (~1/8 inch) of Yamada’a universal indicator solution to the
six wells in the first row and to the six wells in the third row of the well plate. See
Figure 4.1.
3.
Add about 10 drops (~1/8 inch) of litmus indicator solution to the 6 wells in both
rows two and four. See Figure 4.1.
4.
Add three drops of each of the six known solutions to each of the six wells in row
1 and to the six wells in row 2.
5.
Record the color observations in Data Table 4.1.
6.
Add three drops of each of the six unknown solutions to each of the six wells in
rows 3 and 4.
7.
Record the color observations in Data Table 4.1.
8.
Ask the lab instructor to check the well plate and data table entries. Based on this
check, the instructor may request that some parts of the experiment be repeated.
9.
Use your observations to answer the questions in the conclusion and discussion
part of the lab
Data Table 4.1
Solution or
Substance Tested
Yamada
Indicator
Color
Litmus
Indicator
Color
Acid, Base
or Neutral
aluminum sulfate in
water
ammonium chloride
in water
boric acid in water
acid
ammonium acetate
in water
sodium phosphate in
water
sodium carbonate in
water
carbon dioxide in
water
distilled water
neutral
pH
acid
acid
base
base
unknown solution A
unknown solution B
unknown solution C
unknown solution D
The pH scale is a quantitative measure of acidity or basicity; it runs from 0 (very acidic)
to 14 (very basic) with a value of 7 representing neutral, neither acidic nor basic. The
correlation of color for Yamada's universal indicator and pH value is shown below:
pH =
2
red
4
6
8
10
12
orange
yellow
green
blue
violet
Use this correlation to estimate the pH for the six known and unknown solutions tested.
Part II
10.
Neatly rip a strip (~1/2 inch wide) from the piece of DY4 dyed paper. Place a
drop of each of the known solutions along the strip so that there are six spots on
the strip.
11.
Enter your observations on Data Table 4.2. Estimate the pH at which the DY4
dye exhibits indicator behavior. The pH values for the known solutions are taken
from Date Table 4.1. KEEP IN MIND that the color observed on the DY4
paper is for the DY4 indicator and NOT Yamada indicator.
Data Table 4.2
Solution
Color on DY4
Dyed Paper
aluminum sulfate
ammonium chloride
boric acid
ammonium acetate
sodium phosphate
sodium carbonate
pH @ which DY4 changes
12.
13.
14.
Rip the remainder of the DY4 dyed paper in small pieces (~1/2 in2) and place
them in the 150 mL beaker with 50 mL of distilled water and the multifiber strip.
Heat the beaker on the hot plate (the lab instructor will explain the necessary
setting on the hot plate) just to the point of boiling and then turn off the hot plate.
Allow the beaker to sit on the hot plate for 5 minutes.
Remove the multifiber strip from the beaker and rinse it well with distilled water.
This is best accomplished by laying the strip in your hand and directing a heavy
stream of water from the wash bottle at the strip. Be sure to rinse each segment
thoroughly!!
Record the results of the experiment in Data Table 4.3. Now repeat the rinsing
process with tap water. Recall from the lab demonstaration that tap water is basic.
Note any change in color in the fabrics on which the dye is fast. Record your
observations in Data Table 4.3.
Data Table 4.3.
Fiber
Color
Fast?
Color pH
Sensitive
Acetate
Self Extinguishing Fiber
Arnel
Bleached Cotton
Creslan 61
Dacron 54
Dacron 64
Nylon 66
Orlon 75
Spun Silk
Polypropylene
Rayon
Wool
Conclusion and Discussion
When answering the questions below, limit the color references to red, yellow, green,
blue, orange and violet.
1.
What universal indicator colors are associated with acid solutions?
2.
What universal indicator colors are associated with base solutions?
3.
What universal indicator color is associated with neutral solutions?
4.
Recall that the philosophical origins of acids and bases may be traced to the
warring opposites philosophy of Empedocles; such a philosophy tends to
emphasize extremes (good vs. evil, black vs. white). Litmus is a two color
indicator whereas universal indicator is a multicolor acid base indicator. This
multicolor behavior of the universal indicator represents chemical reality and not
chemical philosophy. Do the results suggest that there are only two extreme
measurements: acid or base?