An Investigation to Find Out the Changes in pH of Two Salt Solutions Connected Using a Salt Bridge

The mechanism of breaking and forming a chemical bond can be linked to the relative motion of electrons. As electrons are transferred from one product, molecule, or atom to another, oxidation-reduction reactions are needed. The word “redox reaction” is widely used to describe this phenomenon. An external voltage source is used in this experiment to power the chemical reactions (Gelles et al., 2016). A DC supply (battery) is moved into a salt solution. The voltage from the battery then starts a chemical reaction in the salt solution. The resultant process is known as an electrochemical reaction. It refers to a chemical reaction where the electrons flow from a DC supply. Electrochemistry is a branch of science which deals with scenarios in which the reduction and oxidation processes are separated such that flow of electrons in the reactions takes a pattern of a current. In this process, dissociation of water molecules occurs resulting in the formation of oxygen and hydrogen gas (Gelles et al., 2016). Clean energy movements have made attempts to come up with ways of splitting water molecules in the most efficient way to extract hydrogen gas for use as an alternative fuel supply.

From the chemical formula of water, H2O, Covalent bonds hold the oxygen and hydrogen atoms together. Hydrogen atoms are held together by hydrogen bonds which are not strong enough. As a result, the hydrogen atom can detach itself from the molecule. Two ions or charged particles are created when this occurs. This results in a change of the solution pH. The change in pH of the two salt solutions connected by the salt bridge is measured after passing the current through them (Min et al., 2005).

The research question

The purpose of this study is to find out the change in pH of two salt solutions connected by the salt bridge after passing a current through them.

When undergoes an electrolysis process, hydroxyl ions and hydrogen ions are formed as shown in the equation below:

H2O → H+ + OH-

Which can be represented by the word equation as:

Water → Hydrogen ions + Hydroxyl ions

This means that water decomposes when an electric current is passed through it. Electrons from the battery move into the water molecules at the cathode. The Cathode is the electrode connected to the negative terminal of the battery (Min et al., 2005). Addition of electrons is a reduction reaction. The reaction leads to the production of hydroxyl ions and hydrogen gas at the cathode. The following equation represents the process at the cathode:

2H2O + 2 e- → H2 (gas) + 2OH-

At the positive electrode, known as the anode, an oxidation reaction occurs in which electrons are removed from the water molecules. Therefore, a complete circuit is formed which allows the flow of current. At this stage, water molecules are oxidized resulting in the formation of hydrogen ions and oxygen gas (Lu et al., 2014). Below is an equation illustrating the oxidation process.

H2O → 1/2 O2 (gas) + 2H+ + 2e-

A current flow from the positive electrode (anode) to the negative electrode (cathode) is required for electrolysis process to be achieved. The solution (electrolyte) has to be a good conductor of electricity. Salt is added to water to provide the necessary ionization and improve on the electrical conductivity of water. In this case, the salt is an electrolyte which allows current flow in this process. Sodium chloride can serve the purpose, but the anions (chloride ions) produced as a result will react with the anode. Magnesium sulfate readily forms a solution with water and the ions formed don’t react with electrodes (Lu et al., 2014). Therefore, it is the best choice for this application.

A salt bridge is used to complete the circuit and separate hydroxide and hydrogen ions. The electrodes are immersed in two different solutions, and the electrical connection between them is achieved by the bridge which is a piece of cloth to allow electron flow (Cheng et al., 2011).

Hypothesis

In the reduction process, the presence of hydroxyl ions results in a basic solution while for the oxidation process, the solution is expected to have a low pH (acidic) value since it contains hydrogen ions. This statement is just a hypothesis which is subject to confirmation at the end of the investigation.

A pH pen meter can be used to find out how the solution pH is changing with time. Similarly, a pH paper can also be utilized for the same role. These are inexpensive methods that can be used for performing this task. Sensitive dyes can be used to improve the visual appearance as the process proceeds.

Variables

Time (independent and controllable)

Current (controllable)

pH values (dependent)

Materials and Setup (ASPECT 2)

Measuring cup

100-mL Graduated cylinder,

1 gram accuracy scale

Epsom salts

Permanent marker

Piece of cloth

Two Plastic spoons

Pair of scissors

Stopwatch

Two small ceramic glasses or plastic cups.

9-V battery snap connector

Two small pencils (sharpened at both ends)

Alligator clip test leads

22-gauge insulated wire

Wire strippers

transparent plastic tape or glue gun

Masking tape

pH pen meter

Electrical tape

Graph paper

9-V Battery

Universal pH Indicator Solution

Variable resistor

Lab notebook

Experimental setup

Figure 1 shows experimental set up used

Steps that were taken to manage controllable variables

In this case, though time is an independent factor, it is also a controllable parameter. The length of time for which the experiment runs will affect the results. Therefore, to get better results, a significant length of time can be allowed for this process. The current was also varied using a variable resistor.

Procedure

Step 1: preparation of solutions

Using the measuring cup, water was added to the 200mL mark.

Using the scale provided, 50g of Epsom salt was measured.

Using a plastic spoon, 50g of Epsom salt was dissolved in water contained in the measuring cup.

An equal amount (75mL) of the solution was poured into each of the two ceramic glasses. Graduated cylinder was used to get the precise measurements.

A square piece of cloth of approximately 70mm on the sides was cut out.

A single piece of cloth was suspended in both solutions to form a salt bridge.

Into each glass, a universal indicator solution was added.

The initial pH of the solution was measured using the pH pen meter.

The change in pH and time was recorded in the notebook.

Step 2: setting the electrodes and the battery

The snap connector was attached to the battery.

Two pieces of wire of about 300mm long were cut out.

A length of about 40mm from both ends of the wires was stripped off.

One of the stripped ends of the wires were wrapped around a graphite pencil. The same step was taken in the other pencil.

The plastic tape was used to fix the wire onto the pencil.

The opposite ends of the wires were attached to the terminals of the battery.

Step 3: monitoring the changes in pH

One pencil (electrode) was positioned in a glass of the solution. The other pencil was also put in the other solution. The pencil lead not attached to the wires were immersed in the solutions.

The stopwatch was started when both electrodes had been immersed in the electrolyte.

The pH of the solutions was measured at an interval of 10 minutes for a total of 90 minutes.

The color changes in the solution were recorded.

The pH was graphed against time.

All the steps above were repeated twice to get three sets of data.

DATA COLLECTION AND PROCESSING

Raw data (ASPECT 1)

Table 1 showing the pH values in the first experiment

Time (minutes)

pH values (cathode solution)

pH values( anode solution)

0

7.0

7.0

10

7.2

6.9

20

7.35

6.8

30

7.6

6.5

40

7.7

6.3

50

7.9

6.2

60

8.2

6.0

70

8.3

5.9

80

8.6

5.8

90

8.8

5.7

Table 2 showing the pH values in the second experiment

Time (minutes)

pH values (cathode solution)

pH values( anode solution)

0

7.0

7.0

10

7.1

6.8

20

7.2

6.8

30

7.5

6.6

40

7.6

6.4

50

7.9

6.3

60

8.1

6.2

70

8.3

5.9

80

8.6

5.8

90

8.8

5.6

Table 3 showing the pH values in the third experiment

Time (minutes)

pH values (cathode solution)

pH values( anode solution)

0

7.0

7.1

10

7.2

6.9

20

7.3

6.8

30

7.5

6.6

40

7.7

6.4

50

7.9

6.2

60

8.1

6.1

70

8.3

5.8

80

8.5

5.7

90

8.7

5.6

Data processing (ASPECT 2)

The above three sets of data were collected to enhance accuracy. Their average values were calculated as shown below:

Sample calculation

We shall use two data sets recorded after 10 minutes of the experiment.

Cathode solution

= 7.17

Anode solution

= 6.87

The rest of the values were calculated and fed in the table below:

Data presentation (ASPECT 3)

time (minutes)

pH values (cathode solution)

pH values (anode solution)

0

7.00

7.00

10

7.17

6.87

20

7.28

6.80

30

7.53

6.57

40

7.67

6.37

50

7.90

6.23

60

8.13

6.10

70

8.30

5.87

80

8.57

5.77

90

8.77

5.63

The above values have been represented in a graph for better interpretation.

Figure 2 shows a graph of pH against time

CONCLUSION AND EVALUATION

The aim of this experiment was to find out how pH of two electrolytes changed when a varying current was passed through it. From the initially stated hypothesis, there is a close relation to the obtained results. The solution connected to an anode was acidic whereas the cathode solution was basic. This agrees with the hypothesis made. There were bubbles seen around the electrodes which were as a result of the production of oxygen and hydrogen gases.

Possible sources of errors:

Capillary action can result in mixing of the electrolyte in the cause of experiment. This can lead to ions combining and interfere with the pH values.

All the three sets of data were collected using the same battery. This may result in inconsistency in the performance.

Impurities in the starting solution can alter the pH values. For instance, one of the solutions had a pH of 7.1 yet experiment had not yet started.

Recommendations

The battery should be charged or recharged to improve the consistency of results.

Distilled water should be used to eliminate possible sources of impurities.

Works Cited

Min, Booki, Shaoan Cheng, and Bruce E. Logan. “Electricity generation using membrane and salt bridge microbial fuel cells.” Water research 39.9 (2005): 1675-1686.

Lu, Jianming, David Dreisinger, and Thomas Glück. “Manganese electrodeposition—a literature review.” Hydrometallurgy 141 (2014): 105-116.

Cheng, Li-Jing, and Hsueh-Chia Chang. “Microscale p H regulation by splitting water.” Biomicrofluidics 5.4 (2011): 046502.

Gelles, Jeff, David F. Blair, and Sunney I. Chan. “The proton-pumping site of cytochrome c oxidase: a model of its structure and mechanism.” Biochimica et Biophysica Acta (BBA)-Reviews on Bioenergetics 853.3-4 (2016): 205-236.