Brother Gregory Investigates End
End pH and Indicators
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Research Assistant Wanted


Brother Gregory wants you to help him find answers to various questions about the physical structure of matter, atoms, and related subjects. In these investigations he will ask you a question and give you the tools to find the answer. You then become his research assistant. You must carry out the experiment, gather data, analyze your results and give Brother Gregory the answer he seeks.

B Dyes

"Come and look at this," Herr Otto Grunewald ordered his guest, waving him into the oak lined study. "They just arrived yesterday". He pointed to a row of clear glass bottles lined up on his desk. Each was filled with a colored liquid that caught and reflected the light. "Aren't they beautiful?"

Brother Gregory took a few steps towards the desk, took off his thin spectacles and wiped the glass in them carefully with a soft cloth. He glanced, at his host hoping for some clue, then, receiving none, adjusted the corrective lenses back on his nose and peered closely at the small bottles.

"What are they?" he asked at last, standing up and straightening his back. All he received was a frustrated snort from this host, the well known Brno industrialist and textile manufacturer, Herr Otto Grunewald. Sweeping the first bottle into his large hand, the German replied, "This is the new cloth dye invented by William Henry Perkin," he announced proudly, throwing out his chest and waving the bottle under Mendel's startled nose. "It is the first sample to arrive in Brno."

"And this," he went on, grabbing the next container, "is the only specimen of Magenta, or fuchsin, anywhere in Bohemia, why, it was only was discovered by Verguin just four years ago (1859). I'm the first textile producer around here to own this much. It cost a fortune, but I'm going to use it to produce a whole new line of dyed cloth. I feel it is much better than Perkin's Mauve."

"Impressive," said Mendel diplomatically, taking a short step back from his German host, who was a large man with a tendency to explosive bursts of enthusiasm. "But I'm not sure why you asked me to come and see them."

"It's simple, Mendel," Grunewald answered him, carefully returning the valuable bottles of dye to his desk. "I have just paid a lot of money for these new, synthetic cloth dyes. I think they are going to revolutionize the textile industry. In a few years everyone will be using them instead of the older, natural plant dyes. But I want to be first."

Cloth "Very admirable," murmured Mendel, his interest finally aroused, "you mean that these are new chemicals, made by chemists and they ..."

"Exactly," shouted Grunewald, who never liked anyone to finish his sentences for him. "They are completely artificial, but no one understands much about their properties. Before I ruin a large amount of cloth trying to color them with these dyes, I want to know more about their properties."

"Just any of their properties, or have you something in mind?" Brother Gregory asked, knowing well his host from previous encounters.

"Mendel, you really surprise me some times," grunted Grunewald, "there is only one property I'm interested in - color! I want you to find out how the colors of these dyes change in different kinds of solutions, so we will know the best conditions to use in the dyeing vats. Understand?"

"I think so," said Mendel slowly, "I'll get my research assistants on the problem right away."


The Questions

Brother Gregory wants you to investigate the properties of various colored dyes. He has discovered that all of the samples given to him by Herr Grunewald change color when exposed to different hydrogen ion concentrations. He wants you to -

  1. find out the relationship between hydrogen ion concentration and pH,


  2. find out what colors the dyes turn in different hydrogen ion concentrations.


Tools of the Trade

To carry out this investigation you must first determine the relationship between hydrogen ion concentration and the pH of the liquid or solution.



B Hydrogen ion - the atomic center of an atom of hydrogen; one proton; a tiny particle of great density and a positive charge; written H+ - a proton is not a stable entity on its own.
B Hydronium ion - in solutions in water, hydrogen ions combine with water molecules to form a stable, hydronium ion.
H+ + H2O --> H3O+ (hydronium ion)
B Hydronium ion concentration - this value (also know as the hydrogen ion concentration), is written [H3O+], or sometimes [H+].

Typical hydronium ion concentrations are; pure water 1.0 x 10-7, strong acid 1.0 x 10-2, strong base 1.0 x 10-13. (Note: writing concentrations like this - 10-2 - is called exponential notation).

B Hydroxide ion concentration - this value is written [OH-].
B Acid - is a compound that furnishes hydrogen ions (H+), to a solution in water.
B Base - is a compound that furnishes hydroxide ions (OH-), to a solution in water.
B Indicator - a chemical dye that changes color depending on the hydrogen ion or hydroxide ion concentration in the same solution. Litmus, for example, is blue in a basic solution (high hydroxide ion concentration) and red in acidic solution (high hydrogen ion concentration).
B pH - a numerical value on a scale between 0 and 14 (typically) that expresses the hydronium (hydrogen) ion concentration without using exponential notation.


The Investigation - part one

Use the pH Calculator to determine the pH values for the following common substances:

Substance [H3O+]
Stomach acid 3.1 x 10-2
Vinegar 1.26 x 10-3
Tomato juice 6.3 x 10-5
Blood 5.0 x 10-8
Baking soda solution 3.0 x 10-9
Ammonia 3.16 x 10-12

Start Here pH Calculator


The Investigation - part two

Brother Gregory wants you to add samples of each dye, called an indicator, to the beaker containing a solution of sodium hydroxide (a base). Each dye, or indicator, will turn a characteristic color in the alkaline solution where it is interacting with hydroxide ions.

Make a note of the color of each indicator in the presence of hydroxide ions.

Hydrochloric acid is put into the long, thin burette positioned over the beaker while the tap at the bottom is closed. Drop by drop, acid is dripped out of the burette into the beaker containing the base and the indicator dye. The hydrogen ions (really, hydronium ions) from the acid neutralize the hydroxide ions in the base to produce more water:

H3O+ + OH- --> H2O + H2O

Also some of the water molecules will spontaneously break up into hydronium and hydroxide ions.

H2O + H2O --> H3O+ + OH-

This happens all the time, and both processes are going on at once. Normally, however, the amount of water in the beaker is so huge that it is safe to assume that the amount of water is constant and the only thing that changes is the amount (better known as the concentration) of each ion.

B There are hydronium ions and hydroxide ions present in every solution. Sometimes there are more hydroxide ions, and the solution is basic, sometimes there are equal amounts of hydronium and hydroxide ions and the solution is neutral, and sometimes there are more hydronium ions and the solution is acidic.

As you add more and more acid, you reach the point where all the hydroxide ions have been effectively neutralized by the added hydronium ions. This is the point of neutrality.

Then, as you add more and more hydronium ions, the solution becomes acidic. At some point the indicator dye will change color. Make a note of the new color and make a note of the hydronium ion concentration at which this color change occurs.

B For each of the indicator dyes, you should titrate (that is the term used) the base with the acid and find out both the color change and the range of the hydronium (hydrogen) ion concentration over which the change occurs.

B NOTE: You have a choice of how much acid is added in each drop, and the strength the acid in the burette. In the beginning, you will need a lot of acid to make any change to the hydronium ion concentration in the beaker, but, as you approach neutrality, be careful! The hydronium ion concentration will change very rapidly and you might miss the point of color change.

Experiment with changing the strength of the acid at different points in the titration and try to find the exact point where the indicator dye changes from one color to another.

Start Here Indicators
and Hydrogen Ions

Science at a Distance
© 1999 Professor John Blamire