Gold Electroplating is a method of depositing a thin layer of gold onto the surface of another metal.
Gold plating finds its uses in jewelry, semiconductor industry. It often employed in electronics to provide a corrosion resistant surface.
In this instructable, I am making alkaline gold plating solution which can deposit gold by electrolysis.
Traditionally, such a solutions prepared using extremely poisonous cyanide compounds which are reasonably restricted.
My goal is to make the solution with the publically available chemicals, which are not as toxic to handle. In any case, the final product will be just as poisonous as the traditionally prepared solution.
For this reason, I must warn you. This experiment is extremely dangerous, and I don't recommend anyone without experience to repeat it.
Step 1: Nitric Acid.
If you already have nitric acid at the concentration 50% or above, you may skip this step.
In order do dissolve gold, we will need to prepare aqua regia. It is a mixture of nitric and hydrochloric acid at molar ratio 1/3. Hydrochloric acid sold commercially under the name of muriatic acid. However, nitric acid is restricted for public use. Therefore it has to be prepared from available chemicals.
There are quite a few ways to make nitric acid at laboratory conditions. On my youtube channel, I am covering two methods.
First, by reacting sodium nitrate with the sulphuric acid followed by distillation. This process produces acid at the concentration of above 90%. At this concentrations, nitric acid is highly corrosive and fuming a toxic nitrogen dioxide gas.
And the second method, which less dangerous, is a concentrating very dilute nitric acid that is commercially available for the public. I am using fractional distillation to concentrate nitric acid from 30% to 50% concentration. However, I could achieve the azeotropic concentration of 68% if I used proper fractionating column.
For the gold plating solution, this method of nitric acid preparation is favorable as it is relatively safer.
For more details, please watch the video.
Step 2: Making Gold(III)Chloride
To make gold chloride, we need to dissolve gold in aqua regia. (mixture of nitric acid and hydrochloric acid at molar ratio 1/3)
In my case, I have 18K gold scrap, but the purity of gold makes no difference as gold will be purified to 24K during the process.
The reaction of gold and aqua regia produces toxic fumes so the process must be conducted outside or in the fume hood.
It takes around 20 minutes for gold to dissolve at room temperature. At the end of the reaction, we have the solution of dissolved gold and copper compounds and solid crystals of silver salts.
Now this solution needs to be filtered, to separate solids from the liquid.
Next, the gold is precipitated out of the solution in its elemental form by the addition of sodium metabisulfite solution.
During the addition, the toxic sulfur dioxide gas produced, so this process can only be conducted in the fume hood or outside.
Once there is no more visible reaction happens during the addition of the sodium metabisulfite, that would mean all of the gold has precipitated.
And the last step is to separate the solid gold and dissolve it in aqua regia once again. This time there will be no solids and the solution will be the red color. Now the acid needs to be evaporated at 120 degrees centigrade to obtain red crystals of gold(III)chloride.
For more details, please watch the video.
Step 3: Final Step, Making Gold Plating Solution.
In the final step, we need to make a strongly alkaline solution of gold chloride and potassium ferrocyanide.
Using this solution and electrical current, we can deposit gold over the surface of other metals.
This solution contains gold cyanide ions, which are extremly poisonous.
Under no circumstances, this solution can be mixed with acids, as it will release hydrogen cyanide gas. (I can't even find the correct word to describe the toxicity of this gas)
Neither it can be disposed of down the drain.
For the details and demonstrations, please watch the video.
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Thank you for the attention.