Lab pH Measurements and Indicator Lab

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					Lab: pH Measurements and Indicator Lab
                                                           Name: ____________________________

Acids and bases are chemical groups which have been know to man for some time. Acids cause foods to
have a sour taste and turn litmus paper red. Bases have a smooth feel between the fingers and turn litmus
paper blue. Acids are solutions containing more hydrogen (H+) ions than hydroxide (OH -) ions while bases
contain an excess of hydroxide ions rather than hydrogen ions. Pure water, which is neutral, exists mostly
as H20 molecules not having an excess of either hydrogen or hydroxide ions. When water breaks up it
forms an equal number of H+ and OH- ions and has neutral rating.

The pH scale is used to describe hydrogen ion concentration in acids and bases. This scale compares
acid/base values ranging from 1 to 14. Solutions with concentrations of less than pH 7 are acids while
concentrations of more than pH 7 are basic. Acid rain, which is acid water, has an excess of H+ ions and a
pH of less than 7.

The pH scale was established using logarithms and is based on the power of 10. A complete understanding
of the pH scale requires knowledge of logarithms, but a basic understanding can be obtained by
remembering each step of the pH scale is a multiple of the power of 10. Neutral solutions have a pH of 7
and the concentration of H+ ions is 1 x 10 –7 moles per liter. If a solution with the pH 5 has a concentration
of H+ ions of 1 x 10-5 moles per liter. The difference between pH 5 and pH 7 is two steps or 10 2/100x more
concentrated than the pH 7 solutions. In the same way a solution with the pH 10 is three steps or 10 3/1000x
less concentrated than pH 7.

An important method of determining pH values involves using indicators with the property of changing
color in solution. Other than litmus paper various indicator dyes change colors according to the strength of
the acid or base applied to it. These natural and artificial indicators are used as standards to identify pH
levels in solutions. Some indicators identify only acids and bases while others are pH sensitive and
determine the pH value of a solution. i.e. Bromo thymol Blue turns blue in a base, yellow in acids, and
green in neutral solutions.

PURPOSE: The purpose of the lab is to standardize indicators for testing acids and bases.

MATERIALS: standardized pH solutions of acids and bases, Methyl Orange, Bromo thymol Blue,
Phenolphthalein, Universal Indicator, litmus paper, testing plates

Part One: Effect of pH on Indicator Dyes
Place one drop of acid solution (vinegar) in cavities 1, 5, and 9. Place one drop of water in cavities 2, 6, and
10. Place one drop of basic solution in cavities 3, 7, and 11. To cavities 1, 2, and 3 add one drop of
Methyl Orange, to cavities 5,6, and 7 add one drop of Bromo thymol Blue, and to cavities 9, 10, and 11 add
one drop of Phenolphthalein. Record results and observations – Clean your chemplate

Part Two: Determination of pH with Indicator Dyes
Place one drop of each respective pH solutions in each of their respective cavities. i.e. pH 1 in cavity one,
pH 3 in cavity three, pH 5 in cavity 5, etc. * Note: place solution pH 13 in cavity 12. Add one drop of
Bromo thymol Blue to each cavity and record your results.

Repeat this process using the same pH solutions with Methyl Orange, Phenolphthalein, and Universal
Indicator. Record your results and observations each time. Clean plate between each use.

Part Three: Determination of pH of unknowns
Place one drop of Unknown #1 in cavity 1, Unknown #2 in cavity 5, and Unknown #3 in cavity #9. Add
one drop of Universal Indicator to each cavity to determine the pH of each solution. Record and report
Lab Results/Records
A. Record Results for Part One on Bold printed lines. Record Results for Part Two with
the appropriate pH number.
       a. Methyl Orange: acid solution:             water:           base solution:

pH:1                  pH3:                   pH5:                     pH7:
       pH9:                pH11:                              pH13:
       b. Phenolphthalein: acid solution:            water:                  base solution:

pH:1                  pH3:                   pH5:                     pH7:
       pH9:               pH11:                            pH13:
       c. Bromo thymol Blue: acid solution:            water:                 base solution:

pH:1                  pH3:                   pH5:                     pH7:
       pH9:                   pH11:                           pH13:
       d. Universal Indicator

pH:1                  pH3:                   pH5:                     pH7:
       pH9:                   pH11:                           pH13:

B. What is the control in this experiment?                 ____________________

C. How is Universal Indicator unique as a test solution?

D. For what purpose would each of the tested solutions be best suited?
       a. Methyl Orange:

       b. Phenolphthalein:

       c. Bromo thymol Blue

       d. Universal Indicator:

E. Identify the Unknown solutions.
       a. Unknown ___ – acid/base/neutral           pH - ______
       b. Unknown ___ - acid/base/neutral           pH - ______
       c. Unknown ___ - acid/base/neutral           pH - ______

Lab Extension
Design a test for a household solution. Once the test has been approved, bring the
solution from home and test the pH of the solution in class.
        Substance - ___________________
                Predicted results - ___________________ pH - ________
                Test results - ______________________ pH - ______
                                                                                Updated – 9/01/06

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