ELECTROLYSIS OF SILVER NITRATE SOLUTION

Mike Clark

Toowoomba, Australia

This demonstration is very pretty to watch, and can be manipulated to show how small changes in the physical conditions of an electrolysis can affect the physical properties of the products of the reactions. It is conducted under a microscope, so is useful only where microscopes are available. Alternatively, if a video hook-up from a microscope is available, the reactions could be displayed to good effect on video.

It can be quite fiddly to set up, and may require some practice before good observations can be obtained.

A solution of silver nitrate contains silver ions and nitrate ions, as well as hydrogen and hydroxide ions from the aqueous solvent. Silver is a rather unreactive metal, so it might be expected that silver ions will be reduced and precipitated as metallic silver at the cathode. What cannot be predicted without experiment is the appearance of the crystals of silver metal that forms. At an inert platinum anode, two oxidations are possible: hydroxide to oxygen, and/or silver(I) to silver(III). Both oxidations can be observed. Spectacular results occur when conditions favour production of silver(III).

• • • AIM 1. To observe the effect of electrolysis of aqueous silver nitrate solution using an inert (platinum) anode and silver cathode.

      2. To observe how varying the conditions of the electrolysis affect the physical appearance of the products.

       

      The two parts of the AIM may be investigated separately, at different times, or only the first part may be investigated. The instructions given below are quite narrowly prescriptive, but some possible variations are suggested that could be used if the second part of the AIM is to be investigated.

       

MATERIALS REQUIRED

• • • Microscope with low power magnification (40X), microscope slide (cavity type preferred), pasteur pipette, 2M solution of silver nitrate (340mg dissolved to 1.00 mL of solution), piece of narrow silver ribbon (2 mm wide, 2 to 2.5 cm long), platinum wire electrode, adhesive tape, electric leads with clips, small piece of thick aluminium foil (approx 5 cm x 3 cm), low voltage power source (preferably <2 volts, and not exceeding 3 volts), voltmeter, small variable potentiometer.

       

      The voltage source could be one or two dry cells, or a 240 volt AC/DC adaptor with a 1.5 or 3.0 volt setting. A small potentiometer serves well for controlling the voltage. Fine silver ribbon can be obtained at modest cost from jewellery trade suppliers, and can be cut into narrower strips as required.

       

      (Two major variations that could be used for the second part of the AIM are in the concentration of silver nitrate solution, and in the shape of the silver cathode. Whether a piece of silver ribbon is cut to a blunt tip, as will be specified below, or to a pointed tip resembling a fine wire, affects the outcome of this demonstration.)

       

SETTING UP THE DEMONSTRATION

• • • Tape the electrodes to the middle of the microscope slide so that their tips are about 1.5 mm apart. The tips of both electrodes should be visible at the edges of the low-power field of the microscope. The tip of the platinum wire should point to the middle of the blunt (2 mm wide) tip of the silver cathode, or to the pointed tip if a pointed silver electrode is being used..

       

      Connect the electrodes to the power source as shown in the diagram above. Connect the voltmeter across the electrodes so that the voltage of the electrolysis can be monitored, and controlled as necessary. Before the electrolysis starts, the voltage should not be exceeding about 3 volts.

       

      

      One clip is attached to the piece of aluminium foil, the other is left loose. To close and open the circuit easily when the electrolysis is being carried out, the loose clip can be touched to, and removed from, contact with the aluminium.

       

      When all equipment is ready, use the pasteur pipette to place one or two droplets of 2M silver nitrate solution between the electrodes. (Be careful to avoid tiny splashes; this solution stains!) It will be convenient to rotate the nose-piece of the microscope to one side while the solution is being added.

       

      Touch the loose clip to the aluminium to close the circuit, and adjust the potentiometer so that the voltage does not exceed 1.3 volts. An even lower voltage, around 1.1 to 1.2, is better.

       

OBSERVATIONS

• • • The regions around both anode and cathode should be watched.

       

      Crystals of silver form around the cathode. The exact shape of these, and the rate and manner at which they grow into the gap between the electrodes, depends upon the voltage and especially upon the shape of the cathode. The appearance of the anode products depends also upon the shape of the cathode. A sharp-tipped cathode at a low voltage gives the most attractive-looking growth of metallic silver, but a blunt-tipped cathode causes more attractive crystals of black Ag(I)Ag(III)O₂ to form at the anode.

       

      Some bubbles appear around the anode, especially if the voltage is high (above about 1.3 volts). Crystals of Ag(I)Ag(III)O₂ also form around the anode from oxidation of silver(I) to silver(III). At a voltage between 1.1 and 1.2, when the cathode is blunt-tipped, the crystals will grow into large spear-shapes. At higher voltages, or opposite a sharp-tipped cathode, the crystal will tend to be small. Even after the current is disconnected, bubbles will continue to form around the anode.

       

      After the electrolysis has continued for a short time, it may be interesting to disconnect the power source and connect the electrodes with a short-circuit. The original anode becomes a cathode, and it is possible to watch partial reversal of the original electrolytic reactions.

       

INTERPRETATION

• • • It is clear that the oxidation of Ag(I) to Ag(III) occurs more easily than oxidation of hydroxide to oxygen under the conditions provided.

       

      The effect of cathode-shape on the formations of crystals at both cathode and anode suggests that the shape of the electric field in the solution affects ion current densities, and thus the manner in which crystals form.

       

      It is thought that the bubbles that form around the anode after the power has been disconnected may be oxygen, but they have not been tested to confirm this.

       

      Removal of the same kind of ions from the solution at both cathode and anode, by two different reactions, is an unusual chemical effect.

       

      A major outcome of this demonstration is aesthetic: the reaction is simply pretty, and fascinating to watch!