Monday, June 23, 2014

Crystal growing: Sodium Citrate

Sodium citrate is a sodium salt of the citric acid, white crystalline solid with formula: \(C_3H_5O(COONa)_3\). It has multiple uses in chemistry and biology, but I like it because it produces very nice crystals (see also gallery with more photos).

Single crystals of sodium citrate.
Same crystals on my palm, for scale.

Preparing chemicals

Sodium citrate is not usually sold in supermarkets, but it is easy to prepare from the commonly available products:

  • citric acid: \(C_3H_5O(COOH)_3\) , and
  • baking soda: \(Na H CO_3\).

To prepare sodium citrate, dissolve some citric acid in water and gradually add small portions of soda. Every time you put new portion of soda, intensive reaction will start, producing lots of carbon dioxide gas. It can be roughly described by the equation:

$$ 3 NaHCO_3 + C_3H_5O(COOH)_3 \rightarrow C_3H_5O(COONa)_3 + 3CO_2 \uparrow + 3H_2O $$
Reaction of citric acid with baking soda.

Continue adding soda until the reaction stops (you'll need quite a lot of it). The process looks simple, but it took several hours for me, because adding big portions soda makes reaction too intense, producing lots of foam. To grow the crystals on the top photo I used only 50g of citric acid, so you don't need really much of it.

Growing

There are 2 main methods of growing crystals at home:

  • Cooling method.
  • Evaporation method.

Cooling method

I used the following variation of this method:

  1. Prepare saturated solution of sodium citrate at room temperature,
  2. put some additional solid sodium citrate in it, and
  3. Heat it up, stirring. Additional salt dissolve, because its solubility increases with temperature. Don't worry if it has not dissolved completely.
  4. Cool the solution down. I used a bowl with cold water to do it quickly. You now have supersaturated solution.
  5. Now put seed crystal, attached to a thin thread (thin fishing lane) in it: the crystal will start growing quickly (several mm per hour).
  6. After several hours, extract the crystal, put it into some temporary container (don't let it dry), re-saturate the solution again and repeat the process.
The biggest crystal on the following photo was grown by this method. Note that with sodium acetate it is important to put seed crystal into cold supersaturated solution. It you pot it into hot solution, it will crack.

The biggest crystal was grown by the cooling method

This method allows to grow big crystals very fast (several days), and crystal size is only limited by the size of the container and available chemicals. Unfortunately, fast growth makes crystal less perfect and less transparent. To make transparent crystals, you may choose much slower evaporation method

Evaporation method

It's another classic method of crystal growing. There are no tricks here. Prepare saturated solution, put seed crystal into it and put the container to the corner of your table, waiting patiently until the crystal is growing. Cover the container from dust, but not tightly, to allow water slowly evaporate.

This method is much slower and requires several weeks, but the result worths it: nice, very transparent crystals

Crystals grown by the evaporation method have good transparency

Safety

Sodium citrate is not toxic, and is in fact used in culinary. So you can even eat your crystals. They are not tasty though.

Preservation

Unfortunately, these crystals do not store well on air. In my case, after a week or two on air, they started dehydrating, forming white opaque spots, growing with time and eventually consuming whole crystal. There are different solutions to this. Some people prefer covering finished crystals with nail polish. I am storing them in the tightly closed bottle with the saturated solution. Another alternative is to store them in oil or kerosene.

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