© pH Tricks: Magic Colors
Materials
1000-mL Erlenmeyer, 350-mL clear plastic cups (7)
Chemicals ethanol (EtOH); p-nitrophenol solution (1% w/v) [e.g. 1 g p-nitrophenol in 100 mL EtOH solution]; phenolphthalein solution (1% w/v) [e.g. 1 g phenolphthalein in 100 mL EtOH solution]; thymolphthalein solution (1% w/v) [e.g. 1 g thymolphthalein in 100 mL EtOH solution]; potassium hydroxide solution (2 M)
[e.g. 28 g KOH in 250 mL solution]; DI water
Abstract
In what follows, PP=phenolphthalein solution, TP=thymolphthapein solution, and NP= p-nitrophenol solution.
The 1000-mL flask is charged with 1000 mL of water and 5 mL of 2 M potassium hydroxide solution.
The contents of this flask are mixed well to form a dilute KOH solution. Add 5 mL of ethanol to each plastic cup, and then charge each as follows: RED = 10 drops PP; ORANGE = 9 drops NP + 4 drops PP; YELLOW
= 9 drops NP; GREEN = 9 drops NP + 2 drops TP; BLUE = 4 drops TP; INDIGO = 4 drops PP + 2 drop TP;
VIOLET = 4 drops PP + 6 drops TP. Setup the cups reading ROY G BIV from left to right.
Develop the colors by adding some dilute potassium hydroxide solution from the flask to each cup in order - ROYGBIV.
Obligatory but Very Important Note
Please check the demonstration video for details on the above abstract. Are there possible hazards and risks in this demonstration? Yes, absolutely. Potassium hydroxide is caustic on skin contact. Ethanol and all of the indicator solutions are flammable. All of the indicators are toxic by ingestion to some extent. Solid and solution p-nitrophenol can be absorbed through skin contact. We have not experienced specific problems in our use of the demonstration, but potential problems are there. This video demonstration manual is distributed to chemists and chemistry teachers, and the assumption is made that professionals using the manual are knowledgeable about materials, chemicals, demonstration procedure, and demonstration risks. If there is any doubt about risk, then please show your students the video rather than doing the demonstration.
Demonstration Note
The indicators used in this demonstration are the same indicators that are used in the magic bottle traffic signal demo. These three indicators have two characteristics that are particularly useful in chemical magic. First, they are each colorless in acidic solution. Second, their basic solution colors constitute the primary subtractive colors. These two characteristics combine to allow all types of colorless to color transformations.

Indicators, of course, play a big role in chemical magic shows. A particularly useful indicator that also allows for the development of any of the seven basic spectral colors is Yamada’s universal acid/base indicator. One approach to chemical magic involving universal indicator exploits the hydrolysis of cations and anions to give acidic or basic solutions:
(1) NH
4
+
+ H
2
O → NH
3
+ H
3
O
+
(2) M(H
2
O) x y+
+ H
2
O → M(H
2
O) x-1
(OH)
(y-1)+
+ H
3
O
+
(3) A
-
+ H
2
O → HA + OH -
For a discussion of this approach, as well as recipes for universal indicators, see Foster, S.F. and
Gruntfest, J.Chem.Educ.
, 14
, 274(1937). Yamada’s universal indicator is a fairly well known indicator. The pH color ranges for Yamada’s Universal Indicator is interesting and fairly easy to remember. The color changes follow the standard seven color spectrum mnemonic – ROYGBIV. The approximate pH transitions for these color changes are 4,5,6,7,8,9,10. The recipe for the indicator is listed below:
Mix 0.005 g thymol blue + 0.012 g methyl red + 0.06 g bromothymol blue + 0.1 g phenolphthalein; add to 100 mL ethanol; titrate this solution to pH 7 (green) with 0.1 M KOH; dilute to 200 mL with water.
A popular formulation of universal indicator is sold by Fisher Scientific.
For a nice demonstration of an acidic salt reacting with a basic salt, consider a mixture of baking soda
(NaHCO
3
) and common alum (AlK(SO
4
)
2
•12H
2
O). The dry solids do not react. When this mixture is moistened, however, carbon dioxide is liberated because of the acidic hydrolysis of the aluminum ion. Some types of baking powder are mixtures of sodium bicarbonate and aluminum sodium sulfate.
The chemical demonstration described above is suggested for use by chemical educators and other chemical professionals interested in the instructional use of chemical magic. It is assumed that qualified chemical professionals using this manual are familiar with the properties of the chemicals and with the characteristics of the materials involved in all of the demonstrations. Any attempts to repeat the demonstrations in this manual MUST be carried out under the supervision of a qualified chemical professional.
CheMagic Demonstration Notes
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