1. Prepare a 2% solution of agar.
   • Mix 20 g of agar with 1 L of distilled water.
2. Heat almost to a boil. Stir frequently until solution is clear.
3. Remove from heat and add 10 mL of 1% phenolphthalein solution
   • 1 g phenolphthalein in 100 mL 95% ethanol (If pink, titrate with HCl until clear)
4. Pour agar into a shallow tray to a depth of 3 cm and allow it to set (over night).

Silicone Ice-cube trays can be used. It is difficult to find them in the appropriate metric measurements. 3 cm cubes are approximately 1.2 inches and there are molds for this.