Salt Bridge Over Electrified Waters: How Electricity Changes pH
AbstractYou have probably heard the saying that "water and electricity don't mix." Well, in this chemistry science fair project you will mix them, to create two solutions, one basic and one acidic. The apparatus is very simple, but the chemistry is complex and offers many avenues for exploration.
David Whyte, PhD, Science Buddies
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, H2O. 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 oxygen 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.
H2O → H+ + OH-
- H2O = 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.
Diagram of the pH scale lists common items along the scale as examples for basic and acidic items. The scale is red at the top (0 pH) and slowly changes to blue at the bottom of the scale (14 pH). The most acidic item on the scale is battery acid with a pH value of -0.1 and the most basic item is household lye with a pH of 13.5. Water has a pH level of 5.5.
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):
2H2O + 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:
H2O → 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 H2O + 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 H2O → H2 (gas) + 2OH- + 1/2 O2 (gas) + 2H+
Then combine the hydrogen ions and hydroxide ions to form water:
3 H2O → H2 (gas) + 2H2O- + 1/2 O2 (gas)
Then arrange the water molecules on either side of the equation:
H2O → H2 (gas) + 1/2 O2
Which is equivalent to Equation 10, if the 1/2 O2 (gas) term looks untidy:
2 H2O → 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 reference How to Use a Multimeter 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
- What gas will form if you use sodium chloride instead of magnesium sulfate as an electrolyte?
- What is the ratio of the volume of hydrogen gas to the volume of oxygen gas formed in the electrolysis of water? Hint: See Equation 9 or 10 in the Introduction.
- How does the pH of the solution near the positive terminal (anode) of the battery change as electrolysis proceeds?
- What happens to a chemical when it is oxidized?
- American Chemical Society. (1999). Electrolysis of Water. Retrieved April 14, 2009.
- Wikipedia Contributors. (2009). Electrolysis of Water. Wikipedia: The Free Encyclopedia. Retrieved April 14, 2009.
- CR Scientific. (n.d.). Electrolysis experiments. Retrieved April 14, 2009.
- The Department of Chemistry at the University of Illinois. (n.d.). Electrolysis of water using an electrical current. Retrieved April 14, 2009.
- Arizona Energy. (2008). Electrolysis: Obtaining hydrogen from water: The Basis for a Solar-Hydrogen Economy. Retrieved April 15, 2009.
Materials and Equipment
- Measuring cup
- Graduated cylinder, 100-mL; available from Carolina Biological, item #: 721603
- Scale, accurate to 1 gram (g); such as these pocket scales available from Carolina Biological
- Epsom salts (magnesium sulfate, 7H2O), 1 pint; available at grocery and drug stores
- Paper towel
- Plastic spoons (2)
- Small glass, ceramic, or plastic bowls (2)
Pen-type pH meter; available online from suppliers such as Carolina Biological, item # 186000
- Note: This might also be available for use in your school laboratory.
- As another option, you can use pH paper; available from Carolina Biological, item # 894726
Universal Indicator Solution; available from Carolina Biological, item #: 848263. Note: If you are ordering this chemical through Carolina Biological Supply Company, the chemical must be ordered by a teacher and shipped to a school or business address, so plan accordingly.
- Alternatively, you could use cabbage extract as a liquid pH indicator (see the Science Buddies project Cabbage Chemistry to learn how to make a pH indicator from cabbage leaves.)
- Lab notebook
- Battery, 9-V
- 9 V battery snap connector, available from Jameco Electronics
Two small pencils, sharpened on both ends. The graphite will be used as an electrode.
- Cut or break off the end with the eraser before sharpening.
- Alternatively, you can use pencil lead from a mechanical pencil.
- Alligator clip test leads, available from Jameco Electronics
- Insulated wire, 22-gauge, available from Jameco Electronics
- Wire strippers, available from Jameco Electronics
- Glue gun or clear plastic tape
- Masking tape
- Permanent marker
- Electrical tape
- Graph paper
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Preparing the Solutions
- Add 200 mL of water to the measuring cup.
- Place a paper towel on the scale and measure out 45 g of magnesium sulfate-7H2O.
- Dissolve the 45 g of magnesium sulfate-7H2O in the water with one of the plastic spoons.
- Pour 75 mL of the magnesium sulfate solution into each of the two small bowls. Use your graduated cylinder for precise measurements.
- Cut a square of paper towel about 7 cm on a side.
Place a single paper towel in both solutions, so that it forms a "salt bridge" between them.
- The salt bridge should be soaked with the solution. If there is a dry spot, add some magnesium sulfate solution to wet it.
- It is not critical how the paper towel is placed in the solutions, as long as it makes good contact with both solutions.
- Add a few drops of the universal pH indicator to each bowl. This will give you visual cues about the pH of the solution.
Use the pH meter to check the starting pH of the magnesium sulfate solutions.
- Use the instructions that came with the instrument.
- It is a good idea to calibrate the pH meter. If standard solutions came with your pH meter, use them as explained in the instrument's instructions.
- Record the time and the pH in your lab notebook.
Setting up the Battery and Electrodes
- Attach the 9-V snap connector to the battery.
- Cut two pieces of wire, about 30 cm long.
- Strip about 4 cm of the insulation from the ends of the wire. See the Science Buddies wire stripping tutorial if you do not know how to strip wire.
- Wrap the bare wire from one of the wires around the graphite on one of the pencils. Repeat with the other pencil.
- Affix the wire onto the graphite with a glue gun or with clear plastic tape.
Attach the other end of the wires from the pencil lead to the battery terminals.
- Use masking tape and a permanent marker to label the pencils with the polarity of the battery terminals to which they are attached.
- Use electrical tape to secure the connections between wires.
- You could also use the wires with alligator clips to make connections, if you choose.
Tracking the pH Changes
- Place one of the pencils in one of the solutions, and the other pencil in the other solution. Immerse the lead that is not attached to the wire.
- Start the stopwatch or timer when the second electrode is immersed.
- Note if there are any bubbles forming near the pencil lead. What is forming the bubbles?
- Measure the pH of each solution every 10 minutes.
- Note the color of the solutions in your lab notebook (along with the time).
Keep recording the pH of both solutions every 10 minutes for 90 minutes.
- Feel free to take measurements at other time points, if you choose.
- Graph the pH vs. time for each solution.
- Make a graph showing the difference in pH between the two solutions vs time. How does this "translate" into the actual concentrations of hydrogen and hydroxide ions in each solution?
- Make a note on the graph of the color of the solution that corresponds to the recorded pH, if you added an indicator.
- Repeat the entire procedure at least two more times so that you have data for a total of three or more trials.
Ask an Expert
- Try adding a few drops of other liquid pH indicators, such as bromothymol blue or methyl red. Make a data table showing the polarity of the electrode, the pH, and the time at which the indicator changed color.
- Vary the voltage of the battery. How does this affect the pH change?
- Use a multimeter to measure current (I) through the salt bridge. Graph "charge" vs. "pH change" over a time period of your choosing. See Equation 1. (To measure current, the multimeter has to be in the circuit, so that current flows through the meter. And make sure the probe wires are in the correct sockets).
- Use a multimeter to measure the resistance across the salt bridge. Does it vary with the voltage, current, or pH? Most multimeter brands from a hardware or auto supply store will work. One example is the Equus 3320 Auto-Ranging Digital Multimeter, available from Amazon.com.
- Vary the amount of magnesium sulfate. How does the concentration of salt affect the current and the rate of pH changes?
If you like this project, you might enjoy exploring these related careers:
- Science Fair Project Guide
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- How to Use a Multimeter