How ph indicators work
The objective of this chemistry science fair project is to measure the change in pH of two salt solutions, connected by a salt bridge, as a current is passed through them.
The processes involved in the making of a breaking of chemical bonds all come down to the movement of electrons. When the electrons move from one molecule to another, the processes involved are called oxidation/reduction (redox) reactions. In this science fair project, the chemical reactions are driven by an external applied voltage: a battery is connected to a salt solution, and the voltage from the battery causes chemical reactions in the salt water. Reactions that are caused by the flow of electrons from a battery are called electrochemical reactions. Electrochemistry deals with situations where oxidation and reduction reactions are separated, so that the electrons flow between the redox reactions as a current. In the process, water molecules are split, creating hydrogen and oxygen gas. One of the goals of the clean energy movement is to find ways split water molecules more efficiently, so that the hydrogen gas produced can be used as a fuel.
Water molecules are made up of two hydrogen atoms and one oxygen atom, H2 O. The hydrogen atoms are covalently bound to the oxygen atom. But the hydrogen atoms are not so tightly attached that they can't occasionally drift apart from the oxyen atom. When this happens, it creates two ions, or charged particles, which can cause a change in the pH of the solution. You will measure the change in pH of two salt solutions, connected by a salt bridge, as a current is passed through them.
The following equation represents a water molecule dissociating, into a hydrogen ion and a hydroxide ion.
H2 O → H + + OH -
- H2 O = water
- H + = hydrogen ion
- OH - = hydroxide ion
Note that the number of atoms is conserved—there are two hydrogens and one oxygen on both sides of the equation. Also note that the net charge is also conserved—the neutral water molecule gives rise to one positive and one negative charge, which add up to zero charge. In pure water at room temperature, the rate of dissociation is low. One water molecule in 10 million is split into hydrogen and hydroxide ions. One in 10 million is represented in scientific notation as 1.0 X10 -7. As you will see later, this corresponds to a pH of 7.0.
Acids are solutions that have a higher concentration of hydrogen ions than hydroxide ions. For example, when hydrochloric acid is added to water, it gives rise to hydrogen ions:
HCl → H + + Cl -
- HCl = hydrochloric acid
- H + = hydrogen ion
- Cl - = chloride ion
The hydrochloric acid molecule dissociates into a hydrogen ion and a chloride ion. There are more hydrogen ions in the hydrochloric acid solution than there are hydroxide ions, so the solution is acidic.
Bases are solutions that have a higher concentration of hydroxide ions than hydrogen ions. For example, when sodium hydroxide is added to water, it gives rise to hydroxide ions:
NaOH → Na + + OH -
- NaOH = sodium hydroxide
- Na + = sodium ion
- OH - = hydroxide ion
The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.
Figure 1. The pH scale. (Wikipedia, 2009.)
The pH scale is logarithmic. Each whole number value of pH below 7 is 10 times more acidic than the next higher value. For example, a solution with a pH of 4.0 is 10 times more acidic than a solution with a pH of 5.0. A solution with a pH of 3.0 is 1,000 times more acidic than a solution with a pH of 6.0. The same holds true for pH values above 7, each of which is 10 times more alkaline (another way to say basic ) than the next lower whole value. For example, pH 9.0 is 10 times more alkaline than pH 8.0 and 100 times (10 times 10) more alkaline than pH 7.0.
In addition to adding an acid or a base to water, the pH can be changed by electrolysis. In this chemistry science fair project, you will use a 9-volt (V) battery to cause the electrolysis of water. You will track the changes in the pH values over time.
Water can be decomposed by passing an electric current through it. At the negative electrode, electrons from a battery are added to the water molecules. The negative terminal of the battery is also called
the cathode (cathodes attract cations). Adding an electron results in a reduction reaction. The reduction reaction that takes place at the cathode produces hydrogen gas and hydroxide ions.
This is the equation for the reduction of water at the cathode (negative):
2H2 O + 2 e - → H2 (gas) + 2OH -
This says that the two water molecules react with two electrons supplied by the negative pole of the battery (the cathode) to produce hydrogen gas and 2 hydroxide ions. This solution will be basic because of the hydroxide ions.
At the other electrode, attached to the positive terminal of the battery, electrons are removed from the solution by the electrode. This completes the circuit so current can flow. At this electrode, called the anode (anodes attract anions), water is oxidized to produce oxygen gas and hydrogen ions.
The equation for the oxidation of water at the anode (positive) is:
H2 O → 1/2 O2 (gas) + 2H + + 2e -
This equation indicates that water reacts at the anode to form oxygen gas, hydrogen ions, and electrons.
To summarize, at the cathode (negative terminal), electrons pass into the solution and cause a reduction reaction. At the anode (positive terminal), electrons leave the solution, completing the circuit and causing an oxidation reaction.
The oxidation reaction cannot occur without the reduction reaction, so these two reactions are coupled and occur at the same time. If the equations are added together, similar terms cancel out and the sum yields the net overall reaction:
3 H2 O + 2 e - → H2 (gas) + 2OH - + 1/2 O2 (gas) + 2H + + 2e -
Equation 6 is formed by adding Equations 4 and 5.
First, cancel out the electrons:
3 H2 O → H2 (gas) + 2OH - + 1/2 O2 (gas) + 2H +
Then combine the hydrogen ions and hydroxide ions to form water:
3 H2 O → H2 (gas) + 2H2 O - + 1/2 O2 (gas)
Then arrange the water molecules on either side of the equation:
H2 O → H2 (gas) + 1/2 O2
Which is equivalent to Equation 10, if the 1/2 O2 (gas) term looks untidy:
2 H2 O → 2 H2 (gas) + O2
In order to carry out electrolysis, a current has to flow from the anode to the cathode. In other words, the solution has to conduct electricity. Since pure water is a poor conductor, the reaction can be facilitated by adding a salt that readily forms ions in solution. The salt functions as an electrolyte, allowing current to flow through the solution. Table salt (sodium chloride) will work, but it has the drawback that the chloride ions react with the electrode. Magnesium sulfate, is a good choice for the electrolyte because it dissolves readily in water and the ions it forms (positive magnesium ions and negative sulfate ions) do not react with the electrodes.
In order to close the circuit but keep the hydrogen ions and the hydroxide ions separated, the electrodes will be immersed in two solutions that are in separate containers and connected with a salt bridge. The salt bridge allows ions to flow (current to pass), but keeps the solutions from mixing. In this case, the salt bridge is a piece of paper towel immersed in both of the solutions.
The current flowing through the circuit made by the salt bridge can be measured using a multimeter. See the Science Buddies page, Multimeter Tutorial for more information. The value of the current measures the amount of charge passing through a point in the circuit over a given time period. To approximate the total charge that has passed through the circuit, you can multiply the average current by the amount of time elapsed. That is, the charge, Q, that has passed through the solutions over a given time period, T, equals the time multiplied by the average current, I.
Q = IT
- Q = charge
- I = current
- T = time
In order to track the changes in pH as the reaction proceeds, you will use an inexpensive pH pen meter. As an option, you can use pH paper, and also add pH-sensitive dyes to the solutions to watch the pH changes visually.
Terms and Concepts
- Oxidation/reduction reactions
- Electrochemical reaction
- Hydrogen atom
- Oxygen atom
- Hydrogen ion
- Hydroxide ion
- Hydrochloric acid
- Sodium hydroxide
- pH scale
- Reduction rate
- Magnesium sulfate
- Salt bridge