Preparation of Rochelle Salt (Potassium Sodium Tartrate)

Here you will learn how to prepare Rochelle salt from baking soda and cream of tartar, which are available from a grocery store.

Here's what you need

• 500 g (1 lb) of baking soda (sodium bicarbonate)[NaHCO3]
• 200 g (7 oz) of cream of tartar (potassium bitartrate)[KHC4H4O6] [see note below]
• Oven
• Pyrex container
• Jar with lid
• 500 mL (2 cup) glass beaker or Pyrex measuring cup
• Sauce pan with water
• 2 mL (1/2 tsp) measuring spoon
• Spoon for stirring
• Coffee Filter
• Filter paper or paper towelling

NOTE: Some people have gone to bulk food or health food stores where they found a less expensive cream of tartar. Unfortunately, what is sold there as "cream of tartar" frequently has been NOT potassium bitartrate, but rather a mixture of calcium sulfate (Plaster of Paris), monocalcium phosphate, fumaric acid, and corn starch. This mixture definitely WILL NOT WORK. If the sales person cannot guarantee that what they offer is potassium bitartrate, don't buy it, at least for this purpose.

Here's what you do for the first reaction

This involves the conversion of baking soda (sodium bicarbonate, NaHCO3) to sodium carbonate (washing soda, Na2CO3).

1. Place the contents of a 500 g box of baking soda into a suitable Pyrex container.
2. Heat in an oven at about 65 deg C for one hour.
3. Increase the temp to 120 deg C and hold there for about an hour.
4. Repeat this increase for 175 and 230 deg C, for an hour each.
5. Remove the container and allow to cool to room temperature.
6. Place the sodium carbonate into a sealed container until used further.

Here's what you do for the second reaction

This involves the reaction of cream of tartar (potassium bitartrate, KHC4H4O6) with sodium carbonate (Na2CO3) to produce Rochelle salt (potassium sodium tartrate, NaKC4H4O6).

1. Place a suspension of 200 g (7 oz) (maximum) of cream of tartar in 250 mL (one cup) of water into a beaker of at least 500 mL (2 cups) capacity.
2. Heat the beaker by placing it into a saucepan containing water.
3. Heat the saucepan (e.g. on a stove or laboratory hot plate) until the outer water is just simmering.
4. Add about half a teaspoon (2.5 mL) of sodium carbonate to the beaker and stir the contents. The solution will bubble.
5. Add more sodium carbonate stepwise until no more bubbles form.
6. Filter the hot solution by using filter paper of a coffee filter.
7. Concentrate the solution (by evaporation) to about 400 mL or a little less by heating.
8. Allow the filtrate to cool and then store in a cool place for several days.
9. Collect the resulting crystals by decantation (pouring the excess liquid into another container) or by filtration.
10. Dry the crystals by blotting with clean filter paper or paper towelling.
11. For a better yield, concentrate again this solution left over after step 9 by heating and repeat steps 7 to 10 above.

This should yield about 210 g of Rochelle salt.