I was doing an experiment in my chemistry class and found something odd. We prepared a colloid of gold with the following procedure. 1: Measure out 20ml of Hydrogen Tetrachloroaurate (iii) in grad cylinder. 2: Add to 250ml Erlenmeyer flask and fill to 200ml with distilled water. 3: Put on a hot plate and heat to a gentle boil 4: Add 2 ml of sodium citrate with pipette to solution 5: Continue boiling gently for approximately 10 minutes, ie until the color stops changing. 6: Remove from heat and let cool to room temperature.

Then I asked my teacher if I could do a side experiment with the colloid and boil off the water. My teacher was fine with me coming in to do this and I did the following procedure.

1: Added 22ml of previous solution to small beaker. (I just added an amount that seemed reasonable and measured after) 2: Put on hot plate with a small glass stir rod to create a nucleation site so it doesn't boil uncontrollably. 3: Heat to boil and let boil until solution is dry.

My question has to do with towards the end: As the solution got down to the last few mls the solution was quite dark almost blood colored, but then very quickly turned almost black and the solution turned more viscous. What I want to know is why the solution turned viscous and black, the only hypothesis I could think of was a decomposition of any left over sodium citrate but that seemed unlikely, it would decompose into sodium carbonate and CO2 since I was boiling it with the hot plate set to high, which is over the decomposition temperature, but that wouldn't increase the viscosity. My other question is about what happened when I re-added distilled water. When I re added the water and heated a small amount since the residue didn't initially dissolve, there was a very small amount of color as you would expect from the gold, but there was also a small amount of black solid that floated to the top of the solution. I am curious if anyone has an idea what this solid was.

If anyone wanted to know the dry solid that was left over was grey with a faint purplish hue, with streaks of white throughout it (pretty sure the white is either sodium citrate or sodium carbonate).


2 Answers 2


Your original hypothesis is right. The remaining citrate salt decomposed and charred. Note that citrate ion is also oxidized while reducing Au(IV) to elemental gold. So your solution does not contain citrate ions only. Do you recall childhood "invisible" ink experiments? Write something with lemon juice on a white paper, let it dry and when the paper was ironed, brown colored writing appeared on the paper. Although the charring reactions are very complex, I am just reminding that citrate would not directly decompose to sodium carbonate and carbon dioxide by simply heating the solution, organic compounds go through a complex pathway of decomposition. The black mass that you saw floating was most likely charred organic compounds coming from thermal decomposition. Gold would still be there, but no longer in colloidal state.


The solution thickened because the gold particles had less volume to move around in as the water boiled off and thus were closer together, causing the colloid to go from a liquid with gold particles to a paste of gold particles and water, so to speak. The darkening happened for the same reason; as the water evaporated, the density of particles per mL increased, meaning that any photons passing through the solution had a higher chance of being blocked by a particle as time passed, until there were so many particles per mL no light could pass through without being stopped.


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