Lab 1: Acids and Bases

Background:
Acids and bases are found in household substances that are used every day. Acids are commonly found
in foods like citrus fruits, soda and vinegar, while bases are commonly found in cleaning products such
as bleach, drain cleaner and ammonia. Acids tend to have sour taste and smell associated with them;
whereas, bases tend to have a slick, soapy feeling when touched. Both can be nearly harmless or
extremely caustic depending upon their concentration.

What defines a substance as acidic or basic? An early definition of acids and bases was developed by
the Swedish chemist, Svante Arrhenius in 1884. He determined that when substances were dissolved in
water certain ions were released forming the solution. If H+ were released, the solution was defined as
Arrhenius acid; however, if -OH were released, then the solution is defined as an Arrhenius acid. The
problem with Arrhenius’ definition is that it did not explain why other negative ions instead of hydroxide
(such as F-) can make a basic solution in water (Broderick et al., 2020). Protons (H+) cannot exist freely in
water and instead combine with water molecules to form the hydronium ion (H3O+).
−¿ ¿
+ ¿+ OH( aq) ¿
+¿+ H2 O(l) H3 O( aq) ¿
H (aq)

Research by chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923 developed a
definition of acids and bases that accounts for the discrepancy in Arrhenius’ definition. They
determined that an acid is defined as anything that can donate a proton (H+) and bases are anything that
can accept a proton (H+). In the Brønsted-Lowry definition, when an acid dissociates, the concentration
of protons increases in the solution.
+ ¿¿
−¿+H 3 O(aq )¿
HA (aq) + H 2 O(l) A(aq)

K a =¿ ¿

When a base dissociates, it takes the proton from the water producing a higher concentration of
hydroxide (-OH).
−¿¿
+¿+OH( aq) ¿
B(aq )+ H 2 O(l ) HB (aq )

K b =¿ ¿

Additionally, when dissociation of acids or bases occurs, a conjugate acid and conjugate base are
formed, resulting in the final solution reaching a dissociation equilibrium.

+ ¿¿
−¿+H 3 O(aq )¿
HA (aq) + H 2 O(l) A(aq)
 Acid Base Conjugate Conjugate
Base Acid

For any acid or base, a water dissociation also occurs (Broderick et al., 2020). The Kw value at 25°C is
1.00 x 10-14.
−¿ ¿
+¿+OH (aq) ¿
H 2 O(l) H 3 O(aq)

K w =¿ ¿

Ka and Kb values can be used to determine the strength of an acid or a base and predict equilibrium in an
acid base reaction. See table below (Lower, 2020).

HA Ka A- Kb

HI
Acid Strength
2.0 x 109 I- 5.5 x 10-24

H2SO4 1.0 x 102 HSO4- 1.0 x 10-16

HNO3 2.3 x 101 NO3- 4.3 x 10-16

H3O+ 1.0 x 100 H2O 1.0 x 10-14

HF 6.3 x 10-4 F- 1.6 x 10-11

CH3COOH 1.7 x 10-5 CH3COO- 5.8 x 10-10

NH4+ 5.6 x 10-10 NH3 1.8 x 10-5

H2O 1.0 x 10-14 -

OH 1.0 x 100

NH3
−¿¿
1.0 x 10-35 NH 2 1.0 x 1021
Another theory of acids and bases is the Lewis Theory, which was developed to explain the behavior of
solids and gases. A Lewis Acid is defined as species that accepts a pair of electrons and is also known as
an electrophile; whereas a Lewis Base is a species that donates an electron pair and is also known as a
nucleophile. A Lewis Acid will have a vacant orbital while a Lewis base with have lone pair of electrons.
The reaction between a Lewis Acid and a Lewis Base results in a coordinate covalent bond (Abudra &
Badial, 2020).
−¿ A−B¿

A+¿+ B ¿

Lewis Lewis Coordinate

Acid Base Covalent Bond

All three definitions share the idea of the donation and acceptance from one species to the other;
however, we will focus on the Brønsted-Lowry definition to discuss the physical and chemical properties
in this lab. Acids and bases can be determined through qualitative and quantitative observations using
the pH scale. The pH scale was developed in 1909 by Søren Sørensen and is a negative logarithmic scale
based on the concentration of the hydrogen ion (hydronium ion).

pH=−log ¿ ¿ ¿

To calculate the concentration of hydronium in molarity (M), the inverse equation is below.

The pH scale ranges from 0 to 14 with the lower the number indicating the higher the acidity and the
higher the number indicated the higher the basicity. Substances with a pH less than seven are
considered acidic and those greater than seven are basic. A substance that has a pH of seven is
considered neutral.

Similarly, the pOH scale is also a negative logarithmic scale but instead is based on the concentration of
hydroxide in the solution. pOH of seven is still neutral; however, solution with a pOH less than seven is
basic while one that is greater than seven is acidic.

pOH =−log ¿

To calculate the concentration of hydroxide ion in molarity (M), the inverse equation is below.

¿
pH and pOH are related to each other by the following equation.

pH + pOH=14.00

In this lab, you will be testing a variety of household chemicals to determine their pH and identify if the
substance is acid, basic or neutral. One method for detecting pH is to use a pH meter, which can detect
small changes in voltage caused by the loss of electrons that occurs with the donation of H +, which
corresponds to known measurements on a calibrated pH meter.

Another way to determine pH is with a reaction that causes a color change. One such substance is red
cabbage, which contains anthocyanins from which it gets its purple-red color. As protons (H +) are added
or removed from the anthocyanins, varying colors form that correspond to the pH of the solution
(Compound Interest, 2017). Litmus paper also contains a chemical that reacts similarly to the chemical
found in red cabbage.

Objectives:
 Understand the pH of various household chemicals
 Quantitatively and qualitatively value the pH of various household chemicals.
 Practice calculating the concentration of [H3O+] and [-OH].
 Compare and contrast the values of pH and pOH with the concentration values of [H3O+] and [-
OH] in solution.
 Apply the definitions of acids and bases in a laboratory setting.
 Research and assess the clinical implications of changes in the pH of the body on health and
disease.

Materials:
 Red Cabbage
 Distilled/Deionized water
 Variety of household chemicals: Vinegar, baking soda solution, spray cleaner, bleach, dish
soap, juice, milk, soda, shampoo, conditioner, milk of magnesia, Tums, mouthwash,
toothpaste, body wash, water, etc.
 Test tubes or disposable plastic cups
 Beakers
 pH meters
 Calibration buffers pH 4, 7 and 10
 Plastic transfer pipets
 Stir rods
 Paper towels
 Gloves
 Safety glasses/goggles
  Disposable aprons or lab coats

Procedure:

1. Preparing the red cabbage indicator

a. Chop cabbage into medium sized pieces and place into a large, heat proof beaker (1000
mL)
b. Cover with distilled/deionized water and bring to a boil for a few minutes until a deep
purple color forms.
c. Set aside to cool; once cool, strain the solid pieces of cabbage out of the solution and
use the liquid as an indicator.
d. Cabbage juice can be stored in a refrigerator for a few days before the experiment.
Remember to set it out in before the lab order for it to reach room temperature.

2. Prior to the start of the experiments:

a. Students will make a prediction if the substance to be tested will be an acid, a base or
neutral.
b. Prepare solutions of the selected household substances in 150- or 250-mL beakers.
Some substances can be used as is, but here are some suggestions for others.
i. Dissolve a small amount of baking soda in distilled/deionized water.
ii. Bleach or spray cleaners containing bleach – dilute at least 50%.
iii. Shampoo, condition, shower gels, toothpaste – dilute with enough water to thin
out the product and make it more of a liquid.

3. Red Cabbage Indicator

a. At each station, provide a beaker with the red cabbage indicator and plastic transfer
pipets dedicated to each solution and indicator.
b. Transfer a few mL of red cabbage indicator to a plastic cup (enough to just cover the
bottom) or into a test tube (~ 1/3 full).
i. Add a few mL of the household solution and flick the bottom of the test tube to
mix.
ii. Indicate the color change in the column Red Cabbage Indicator on the
Measuring pH student lab handout Color/pH and compare it to the chart from
Compoundchem.com (see instructor resources); indicate the pH value based on
the chart in this column also.
iii. Repeat for each solution at the station.
c. Once each group finishes recording their results, the groups will move to the next
station and repeat until all samples have been tested and results recorded.

Experimental Results and Calculations:

1. Using the results from the pH meter readings, determine if the substance measured is an acid,
base or neutral solution and indicated the results in the column Actual: Acid/Base, Neutral? Of
the Measuring pH student lab handout.
a. Compare the prediction column and the actual column and circle any substance where
the actual results differed from the prediction.
b. Discuss why you predicted the result you did and why your results were different with
your group and with the class.
2. Compare the pH meter reading with the Red Cabbage indicator pH results.
a. Underline any substances where the pH values differed significantly between the two
results.
b. Discuss why this might happen with your group and with the class.
3. Using the pH meter readings, calculate the [H3O+] and [-OH] and indicate your results in the
respective columns of the Measuring pH student lab handout.