EXPERIMENTS ON ELECTROLYSIS OF SODIUM CHLORIDE

Mike Clark

Toowoomba, Australia

 

 

Electrolysis of sodium chloride solution with inert electrodes can be done in several different ways, producing different sets of products.

* Solid sodium chloride melts at just over 800oC, and electrolysis of molten sodium chloride yields sodium metal at the cathode and chlorine gas at the anode.

* Concentrated aqueous sodium chloride solution with a mercury cathode produces a solution of sodium metal in mercury ("sodium amalgam") and chlorine at the anode.

* Dilute aqueous sodium chloride solution produces hydrogen at the cathode and oxygen at the anode.

* Concentrated aqueous sodium chloride solution produces hydrogen at the cathode and chlorine at the anode.

* Aqueous solutions of intermediate concentrations may produce both chlorine and oxygen at the anode.

 

Simple and useful gas electrodes can be constructed as follows:

Materials needed: disposable plastic 5 mL graduated pipette, surgical rubber or silicone tubing, plastic 5 mL disposable syringes, fine copper wire, 2 cm pieces of platinum wire or graphite pencil leads, small polypropylene transfer pipettes, (optional: silicone putty), glue.

 

Instructions:

Cut a plastic graduated pipette into lengths of 5 to 6 centimetres.

 

Cut pieces of surgical rubber tubing about 4 centimetres long.

Prepare graphite or platinum electrodes as shown in the following diagram.

 

 

Assemble the electrodes as shown in the following diagram. Graphite or platinum should be used for anodes, fine copper wire may be used for cathodes. Fine copper wire is passed under the rubber tubing. On the anode side, a small dab of silicone putty is used to insulate the copper where it passes from the top of the polypropylene tubing. Where the surgical rubber to the bottom of the syringe, it is recommended that the rubber be folded back on itself, and that a suitable glue be used to create a securely sealed joint.

 

An alternative and more durable method involves warming the top end of the polypropylene tube and pressing it flat between the fingers. It can then be bent over the top of the plastic tube, and the rubber sleeve pushed over both the plastic tube and the flattened polypropylene tube encasing the copper wire.

 

If the plunger of the syringe requires lubrication, a couple of drops of glycerol should be placed in the top of the syringe.

 

 

EXPERIMENT ELECTROLYSIS OF DILUTE SODIUM CHLORIDE SOLUTION

 

AIM To observe the effect of electrolysis on dilute sodium chloride solution.

 

MATERIALS REQUIRED

Sodium chloride solution 0.10M, 25 mL beaker, watch glass, cathode and anode assemblies as described above, stand with two clamps, low voltage DC source, leads, solid bromothymol blue indicator.

 

PREDICTIONS AND PLANNING

At the cathode, sodium and hydrogen ions are available for reduction. Hydrogen ions are reduced far more easily than sodium ions, so they will react to form hydrogen gas, which can be collected in the syringe.

At the anode, hydroxide and chloride ions are available for oxidation. Hydroxide ions are oxidised more easily than chloride ions in dilute solution, so they will react to form oxygen gas, which can be collected in the syringe.

 

 

Hydrogen and oxygen gas should be expected in a 2:1 ratio by volume, since the net reaction is electrolysis of water.

While there should be no net change in pH, there may be localised change. Loss of hydrogen ions at the cathode should leave the region of the cathode with a net excess of hydroxide ions, that is, basic. Loss of hydroxide at the anode should leave an excess of hydrogen ions, that is, an acidic solution. These changes should be detectable with bromothymol blue, which is green at the neutral pH of sodium chloride solution, but turns blue in basic solution, yellow in acidic solution.

 

 

 

 

 

To test the gases produced:

Both hydrogen and oxygen gases have very low solubilities in water, and appear as bubbles. Chlorine gas, if it forms, is more soluble, and will not form bubbles until the water is saturated. It causes a solution of potassium iodide to turn brown.

 

Hydrogen and oxygen gases are odourless; whereas chlorine has a strong unpleasant odour.

 

Hydrogen gas is flammable; it can be tested by bubbling it into a solution of detergent, then holding a burning match to the bubble. The gas ignites with a soft sound.

 

Oxygen gas will cause a glowing splinter of wood to ignite.

 

INSTRUCTIONS

a. Place about 10 mL of 0.10M sodium chloride solution in a 25 mL beaker, and add a couple of grains of solid bromothymol blue, and stir until the indicator dissolves. (Even a tiny amount may dissolve slowly.) Stir until the indicator has dissolved. If the solution is yellow, dip a glass rod or spatula in very dilute sodium hydrogencarbonate solution, transfer it to the indicator solution and stir; repeat until the solution turns green. If the solution is blue, carry out the same procedure with very dilute acetic acid or with diluted vinegar, until the solution turns green.

b. Use the syringes of both a cathode and an anode assembly to fill the plastic tubes and about 1 mL of the syringe with the salt/indicator solution, leaving the assemblies free of air-bubbles.

c. Place the open ends of the tubes in the beaker and clamp the syringes.

d. Connect the electrodes to a five to six volt DC source. (A lower voltage provides a rather slow reaction.)

e. If gas accumulates in the lower tube and does not bubble through into the syringe, use the plunger of the syringe to draw the gas through into the tube of the syringe.

f. If the electrolysis is allowed to continue long enough, testable volumes of gas can be collected. A sample of water from the anode tube can be tested for chlorine simply by careful smelling: there should be no smell, indicating an absence of chlorine. The anode solution can be isolated easily for testing by using the syringe to expel the contents of the tube into a watch-glass.

 

OBSERVATIONS

a. How quickly do bubbles start to appear around the electrodes? At which electrode are bubbles forming more quickly?

b. Describe and explain the colour changes around the electrodes.

c. Is the ratio of gas volumes at cathode and anode two to one?

 

INTERPRETATION AND EVALUATION

a. Which observations agree with predictions-

i) gases are formed at electrodes?

ii) colour changes around electrodes?

iii) absence of any odour of chlorine from the solution around the anode?

iv) the ratio of cathode to anode gases is 2:1 by volume?

 

b. If the volume of gas at the anode is too small, and there is no evidence that chlorine has been forming in the solution, then an explanation is needed. Suppose that hydrogen peroxide has been forming: its presence can be supported by adding a drop of potassium iodide/starch mixture (or using a KI/starch indicator paper). A dark blue colour supports (but does not prove) the presence of hydrogen peroxide.

 

NOTE: if only hydrogen and oxygen only have been formed, then the "average pH" of the contents of the beaker should be the same at the end of the electrolysis as at the beginning. If all solutions are discharged from the syringes, without loss, and mixed, then the final indicator colour should be the same as the original green colour.

 

EXPERIMENT

ELECTROLYSIS OF SATURATED SODIUM CHLORIDE SOLUTION

 

AIM To observe the effect of electrolysis on saturated sodium chloride solution.

 

MATERIALS REQUIRED

Saturated sodium chloride solution, 25 mL beaker, cathode and anode assemblies (see pages 13-14), stand with two clamps, low voltage DC source, leads, watch glass, universal indicator paper or solution.

 

PREDICTIONS AND PLANNING

At the cathode, sodium and hydrogen ions are available for reduction. Hydrogen ions are reduced far more easily than sodium ions, so they will react to form hydrogen gas, which can be collected in the syringe.

 

At the anode, hydroxide and chloride ions are available for oxidation. Chloride ions are oxidised more easily than hydroxide ions in concentrated solution, so they will react to form chlorine gas, some of which will dissolve in the anode solution. Some will form bubbles which may collect in the syringe.

 

To test the gases produced:

Both hydrogen and oxygen gases have very low solubilities in water, and appear as bubbles. Chlorine gas, if it forms, is more soluble, and will not form bubbles until the water is saturated. It causes a solution of potassium iodide to turn brown.

 

Hydrogen and oxygen gases are colourless and odourless; whereas chlorine is greenish-yellow in colour, and has a strong unpleasant odour.

 

 

Hydrogen gas is flammable; it can be tested by bubbling the gas into a solution of detergent, then holding a burning match to the bubble. The gas will ignite with a soft sound.

 

INSTRUCTIONS

a. Place about 10 mL of saturated sodium chloride solution in a 25 mL beaker.

b. Use the syringes of both a cathode and an anode assembly to fill the plastic tubes and about 1 mL of the syringe with the salt solution.

c. Place the open ends of the tubes in the beaker and clamp the syring es.

d. Connect the electrodes to about 4 volts DC.

e. If gas accumulates in the lower tube and does not bubble through into the syringe, use the plunger of the syringe to draw the gas through into the tube of the syringe.

f. A sample of water recovered into a watchglass from the anode tube can be tested for chlorine simply by careful smelling. It will also turn KI/starch solution or paper dark blue, and bleach litmus paper.

g. Recover some solution from the cathode tube and use universal indicator to determine its pH.

 

OBSERVATIONS

a. How quickly do bubbles start to appear around the electrodes? At which electrode are bubbles forming more quickly?

b. Describe and explain the changes observed around the anode. Is there any evidence of formation of oxygen gas?

c. Is chlorine formed around the anode?

d. How alkaline is the solution from the cathode region?

e. Can the gas collected at the cathode be identified as hydrogen?

 

INTERPRETATION AND EVALUATION

a. Which observations agree with predictions-

i) hydrogen gas is formed at the cathode?

ii) an alkaline solution is formed at the cathode?

iii) a strong odour of chlorine from the solution around the anode?

iv) an absence of oxygen from the anode?

 

NOTE: A very faint smell of chlorine may be detected over the solution in the beaker during the electrolysis. Accumulation of gas in each electrode tube pushes solution out of the tubes, so both chlorine solution and sodium hydroxide solution are being mixed together in the beaker. Chlorine reacts reversibly with hydroxide ions in solution, forming chloride and hypochlorite ions; most of the chlorine is absorbed, leaving only a very small amount in solution to cause a very faint odour.