Lab 22: Acid/Base Indicators Lab
Objective: to predict color changes to acid-base indicators and then test those predictions.
Background:
Acids are defined as substances that give off H+ ions as the only positive ion in solution. These H+ ions
combine with the water they dissolve in to make H3O+, or hydronium. A base is defined as a substance that gives off
OH- as the only negative ion in solution.
How do we know if a solution is acidic or basic? The measure of acidity or alkalinity of a solution is the pH
scale.
Solution
Acid
Base
Neutral
pH
Below 7
Above 7
Exactly 7
There are many substances whose color is sensitive to the acidity or basicity of a solution. Substances that
change color when the pH changes are called indicators. There are many indicators, but the most commonly used
ones are found on Reference Table M.
The color of an indicator changes within a certain range of pH’s. For example, take the following indicators
from Reference Table M:
Indicator
methyl orange
bromthymol blue
Approximate pH Range for
Color Change
3.2 – 4.4
6.0 – 7.6
Color Change
red to yellow
yellow to blue
Methyl orange is red from a pH of 3.2 or less and yellow at a pH at or above 4.4. So what is happening between the
numbers on the table? It changes color from one to another. Methyl orange is different shades of orange as it goes
from 3.2-4.4. Methyl orange is red in strong acids, but yellow in weak acids, neutral solutions and bases. Not a very
good indicator for telling if you have an acid or a base.
Bromthymol blue, on the other hand, works very well at telling if you have an acid or a base. Since the color
change takes place around a neutral pH, bromthymol blue will be yellow in an acid and blue in a base. It’s actually
different shades of green between the pH’s listed on the table, but they don’t tell you that, so that’s information you
won’t be responsible for.
These indicators are great, but the best they can do is to tell you, if you choose the right one, if you have an
acidic solution or a basic solution. This is called a qualitative process. It gives some vague idea of what the range of
pH is, but not what the actual pH is.
Another way of telling if you have an acid or a base is to measure its pH directly. This can be done using an
electronic pH probe or by using paper soaked in a mixture of different indicators. The paper will turn a very specific
color at a specific pH. You place the paper into the solution and then compare the color to the pH color key on the
bottle. This is more useful than using indicators, because you now know the actual value for pH. This is a quantitative
process, and is more useful in a greater variety of applications. These include testing the pH of swimming pools and
aquariums!!!
In this lab, you will first test solutions with known pH’s with indicators after making predictions about what you
think the colors will be. You will then test common household materials to see if they are acidic or basic. What fun! 
Lab 22: Acid/Base Indicators
© 2013, Mark Rosengarten
A/E Chemistry
1
Materials
Common Materials for Parts 1 and 2:
Set of indicators
pH paper vial
2 overhead acetate sheets
Reference Table M
mixing grids for Parts 1 and 2
Part 1 Materials:
Dropper bottle of HCl
Dropper bottle of NaOH
Part 2 Materials:
Sample bottle of detergent
dropper bottle of vinegar
seltzer
dropper bottle of ammonia water
Experimental Procedure
Part 1: Known Substances (use data table 1)
1) Get your goggles on your face then get equipment to your place!
2) Place one overhead acetate sheet on the Part 1 Mixing Grid. Place one drop of HCl in the first spot (labeled 1)
and one drop on the NaOH box right next to it, labeled 2.
3) Test the HCl and NaOH individually with pH paper and place the numerical values for the pH on Data Table: Part
1.
4) Using this tested pH of the HCl and the NaOH, go down the left column of the data table and use Reference Table
M to predict what color you should get with each indicator if you put them in solutions what that pH. Record your
predictions under the LEFT HCl and NaOH columns (Predicted Color).
5) Place one drop of HCl on each box for the rest of the spaces under the HCl column and do the same for NaOH.
6) Test each drop of acid and base with the appropriate indicators. Be very careful not to touch the tips of the
dropper bottles to the acid or base on the acetate sheet, or you will contaminate them.
7) Record the colors in the RIGHT columns for HCl and NaOH, under your recorded pH.
8) Flush the acetate sheet with plenty of water and dispose of in the red disposal can.
9) Complete the analysis section under the Part 1 Data Table.
Part 2: Common Substances (use data table 2)
1) At the bottom of the acetate sheet you have room to test the acidity/alkalinity of two common household
substances. Choose two indicators from the ones you did above that can tell definitively if your substance is an
acid or a base. On your predictions table, write which indicators you will use, and what your predicted outcome is.
2) Add one drop of each of the household substances onto the acetate mixing grid. Test the drops with your chosen
indicators.
3) Record your results on the data table.
4) Flush the acetate sheet with plenty of water and dispose of in the red disposal can.
5) Return all materials to where they originally came from.
6) Complete the analysis section under the Part 2 Data Table.
Lab 22: Acid/Base Indicators
© 2013, Mark Rosengarten
A/E Chemistry
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Name:
Group:
Minutes:
Grade:
Lab 22: Acid/Bases Indicators
Objective: to predict color changes to acid-base indicators and then test those predictions.
This page will be turned in with your graph and completed questions on a separate sheet of paper stapled to the back.
pH paper (record
pH number)
Indicator
Data Table - Part 1: Known Substances
HCl
NaOH
pH value:_________ pH Value:_________
Predicted Color:
Predicted Color:
Actual Color:
HCl
NaOH
HCl
Actual Color:
NaOH
Methyl Orange
Bromthymol Blue
Phenophthalein
Litmus
Bromcresol Green
Thymol Blue
Analysis: How well did your predictions for the indicator colors match your actual results? Use your data to support
your answer, this will require more than one sentence.
Data Table - Part 2: Predicting Acidity/Alkalinity Of Common Substances
Which indicators will you use to test each
Predicted color if the
Predicted color if the
of the household substances?
substance is an acid?
substance is a base?
Lab 22: Acid/Base Indicators
© 2013, Mark Rosengarten
A/E Chemistry
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Analysis: Determine if each common substances that was tested are acidic, basic or neutral and explain how you
figured it out. Use data to support your analysis!!
Common
Substance
Results of
Indicator 1
Results of
Indicator 2
Acid, Base,
or Neutral?
Identity of
Substance
Chemical Name
Of Substance
Formula of
Substance
1
2
Conclusions: (Answer in complete sentences below)
1) For each of the indicators used, use Reference Table M to create a chart to show what color each indicator would
be at a pH of
Indicator
Predicted Color:
pH = 2
Predicted Color:
pH = 12
Methyl Orange
Bromthymol Blue
Phenophthalein
Litmus
Bromcresol Green
Thymol Blue
2) For the following indicators, what is the lowest pH that each will be BLUE?
a) bromthymol blue:___________
b) thymol blue:________
c) litmus:__________
3) For the indicators in 2), above, what is the LOWEST pH at which ALL THREE of these indicators would be BLUE?
Cite evidence from question 2 to support your statement.
4) A student tested a solution with colorless phenolphthalein and found that the solution did not turn magenta pink
(red). This student concluded that the substance must be an acid. Is this a valid conclusion? Why or why not?
Lab 22: Acid/Base Indicators
© 2013, Mark Rosengarten
A/E Chemistry
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5) Which two indicators on Reference Table M would be the most useful at determining if a solution is acidic or
basic? Cite evidence to support your answer.
6) For substance 1:
WHY is it an acid or a base, based on the formula?
7) For substance 2:
WHY is it an acid or a base, based on the formula?
8) Each pH change of 1 is a tenfold change in the acidity or alkalinity of a solution. For example, a pH of 1 is 10
times more acidic than a pH of 2 and 100 times more acidic than a pH of 3. A pH of 13 is 10 times more alkaline
than a pH of 12, and 1000 times more alkaline than a pH of 10.
a) Apple juice has a pH of 3, lemon juice has a pH of 2. How many times more acidic is lemon juice than apple
juice?
b) Milk of Magnesia (used for neutralizing excess stomach acid when you have heartburn) has a pH of 10. Drain
cleaner (used to unclog grease buildup in sink drains) has a pH of 14. How many times more alkaline is drain
cleaner than milk of magnesia?
Lab 22: Acid/Base Indicators
© 2013, Mark Rosengarten
A/E Chemistry
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MIXING GRID
Indicator
pH paper
HCl
1
NaOH
2
Methyl Orange
Bromthymol
Blue
Phenophthalein
Litmus
Bromcresol
Green
Thymol Blue
Substance 1
Substance 2
Indicator 1
Indicator 2
DO NOT CLEAN UP UNTIL ALL PARTS OF THIS LAB
HAVE BEEN COMPLETED
Lab 22: Acid/Base Indicators
© 2013, Mark Rosengarten
A/E Chemistry
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