The most important question regarding electroplating is the following: how is the amount of gold being deposited on the substrate (cathode) controlled?

The answer to this question lies in Faraday’s First Law of Electrolysis which states that ‘The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electric charge at that electrode’

The electrochemical equivalent (Z), with S.I units kgC^-1 or kgA^-1s^-1, is a constant for a particular substance, that tells us the mass of the substance deposited at an electrode when a certain amount of current is applied in a given period of time.

Mathematically:

(where: m = mass of material deposited; Q= Charge; I= Current; t= time; Z= electrochemical equivalent)

m ∞ Q
m ∞ I * t

m = Z * I * t

Z = m/ I * t

The electrochemical equivalent for Gold is 2.4496 gA^-1hr^-1, which means that if 1 Ampere of current is passed through the electrodes for 1 hour, 2.4496 grams of Gold will be deposited at the cathode. Using this, the current and time for electroplating can be fixed depending on the mass of gold that needs to be deposited.

The thickness of the gold layer deposited on the substrate can also be calculated using the density of gold:

(Let: Density = d; mass of gold deposited = m; volume of gold deposited = v; surface area of substrate = a; thickness of gold deposited = t)

d = m/ v

d= m/ a * t

t = m/ a * d

Using these principles, substances can be electroplated with a high degree of precision.

For copper plating, the substrate is attached to the cathode (negative electrode) in the centre of the container and pieces of pure copper form the two anodes (positive electrode). The electrolyte (solution in which electrodes are inserted in) is made up of copper sulphate.

At the anode:

Cu —–> Cu²⁺ + 2e^–

The copper piece attached to the anode oxidises (loses electrons) due to the potential difference applied. The Cu ions then move into the solution, and the electrons lost flow from the anode towards the cathode.

At the cathode:

Cu²⁺ + 2e^– —–> Cu

The copper ions get deposited at the cathode where they gain electrons (which have travelled from the cathode), to form Cu atoms.

The image below shows the process in action. As you can see, the electrolyte is continuously stirred to ensure the concentration of copper ions throughout the solution remains constant, which in turn helps ensure that copper is evenly electroplated over the substrate.

Electroplating with gold involves an inert anode, i.e., one that does not take part in the reaction. As a result, a non-reactive metal needs to be used for the anode, which in this case is either Platinum or Graphite. The electrolyte contains the gold ions that move toward the cathode, i.e., toward the substrate. In this case, the electrolyte used is complex salt ion gold cyanide {Au (CN)₂}^-1 dissolved in water. The process can be represented as follows:

At the anode:  

 4H₂0 —-> 4 H⁺¹ + 2 O₂ + 4 e^–

The water molecules move towards the inert anode, where the oxygen oxidises (loses electrons) to form oxygen molecules and the hydrogen becomes ions. The 4 electron lost then flow towards the anode.

At the cathode:   

4(Au (CN)₂)^-1 + 4 e^–  ——-> 4 Au + 8 CN^-1

The negatively charged gold cyanide molecules move towards the cathode where the gain 4 electrons (which have travelled from the cathode), to form gold atoms and cyanide ions. The gold atoms get deposited onto the surface and the cyanide ions go back into solution.

I visited a gold plating factory, which coated various objects (from jewellery to statues) with gold. This was a very interesting application of Faraday’s Laws of Electrolysis (see my next post for more details on the theory).

There are three main reasons for electroplating:

1. Electroplating with a relatively inert metal like gold ensures that the substrate lasts for a longer period of time, as the gold will not react with the air and wear away in the way that Iron, for instance, would.
2. Electroplating with gold improves the conductivity of the substrate. This is much more cost effective than using the conducting material for the entire material, as gold is extremely expensive.
3. Electroplating also improves the appearance of the product.

Step 1: Obtaining the substrate

The “substrate” refers to the raw material, or object that needs to be electroplated. These are made out of a range of materials, from brass to steel.

Step 2- Surface Preparation:

a) Water Rinse:

The substrate is first immersed in de-ionised water in order to remove any soluble impurities on the surface of the substrate. De-ionised water is, as its name indicates, water with its ions (such as Calcium) removed, i.e., it is pure H2O.

De-ionised water is more effective than normal tap water to clean the substrate as it is a better solvent. The charge in water molecules is separated, due to polarisation, to form a slight positive and negative pole in pure water. This dipole nature makes it easy for ions to dissolve in de-ionised water. In normal tap water on the other hand, previously dissolved ions reduce the polarity, making it less effective in dissolving more ions.

 

 b) Ultrasonic Cleaning:

Ultrasonic Waves are waves with frequencies greater than 20,000 Hz (20 KHz). During Ultrasonic Cleaning, the substrate is immersed in water and ultrasonic waves are passed through the water, using the machine shown in the image.

Ultrasonic waves strike the surface of the substrate with high energy, making it an effective tool for displacing any microscopic dirt particles on the surface of the substrate. “Cavitation bubbles” are induced by the high frequency, which agitate the solvent. This then produces extremely high forces on any contaminants on the surface of the substrate. It also penetrates any cracks to remove dirt, oil, rust, grease, bacteria, and even fingerprints. The substrate is not allowed to rest on the bottom of the device during the process, because then part of the object will not have sufficient exposure to the solvent.

c) Acid Bath

Next, the substrate is immersed in extremely diluted sulphuric acid. This dissolves any surface impurities that are not soluble in water. The acid is diluted to ensure that it does not react with the substrate itself (to prevent corrosion).

d) Copper Plating

The last step of the surface preparation is copper/ nickel plating. Please see the theory post for details on reactions at the cathode and anode that enable this to occur.

The substrate is plated with copper for the following reasons:

• Copper increase the conductivity of the surface allowing for more efficient electroplating.
• Copper Plating makes the surface smooth, allowing for an even coating when it is finally electroplated.
• Copper Plating also makes the surface bright, as the image below shows, making the final product have a greater aesthetic value.

Step 3- Gold Electroplating:

In a similar process to which the substrate was electroplated with copper, it is now electroplated with gold to form the final product. Please see the theory post for details of the reactions.

The substrate is continuously rotated to ensure that an even coating of gold results. This, and the final product, are shown below: