How to Make Piezoelectric Rochelle Salt Crystals



Rochelle salts are easy to make crystals, and are a interesting because they are piezoelectric, which means that when they are mechanically stressed they produce electricity. The formal name is potassium sodium tartrate tetrahydrate, with a chemical formula of KNaC4H4O6·4H2O.

This tutorial will show you how to make rochelle salts, with a bit of chemistry theory.

Equipment

  • Kitchen scale.
  • Kitchen stove or hot plate.
  • Pirex container - 2 to 4 cups capacity. The container should be twice the size of the batch that you intend to make, because it will bubble over as you add ingredients.
  • Pan that is big enough for the Pirex container to fit.
  • Funnel
  • Coffee filters. These can be purchased at any grocery store. They are found in the coffee isle.
  • Stirring spoon
  • Glass container with lid

Ingredients

  • Sodium Carbonate (Soda Wash), Na2CO3. Note that this is different than sodium bicarbonate which is common baking soda. Soda wash can be purchased from most grocery stores such as walmart in the clothes detergent isle. Armand hamond is a common brand. It can also be purchased online from amazon. You can also make it out of baking soda by heating it for several hours.
  • Potassium bitartrate (Cream of Tarter), KC4H5O6. This can be purchased at any grocery store, and is found in the spice isle. Amazon is a cheaper source.
  • Distilled water. This can be found at any grocery store.

Directions

  • Fill a sauce pan half full of tap water, and bring to a gentle boil.
  • Pour 1 cups (about 250 grams) of distilled water into a Pyrex container, and place container in the boiling water bath of the sauce pan.
  • Set the scale to grams, and measure 188 grams of cream of tarter, making sure to tare the scale with the holding container. Paper cups are good for this purpose.
  • Pour the cream of tarter, into the distilled water of the Pirex container. It will not dissolve at this point, and will settle on the bottom of the container.
  • Measure 53 grams washing soda and set aside.
  • Slowly add the washing soda to the Pyrex container, only a half teaspoon at a time, as the mixture will bubble up releasing carbon dioxide. If you add to much it will spill over the container.
  • Continue adding washing soda and stirring until the mixture turns clear, and stops bubbling. It will take about a half hour, to complete this process.
  • Depending on the accuracy of your scale, and how carefully you measured, you may need to add a bit more washing soda. If you start to see undissolved soda wash at the bottom of the container, then you've put too much in, and you are done. This excess will be filtered out.
  • Place a coffee filter in a funnel, and pour the solution into another glass bowl. Place on a lid, and store it in a cool place.

Theory

We'll explain how we came up with the ratios. Each substance has a molar mass which is the number of grams per mole. For simplicity we want to make one mole of crystals. To compute molar mass just add up the atomic weights. For example: H20 is 2 hydrogens of mass 1 with 1 oxygen of mass 16. 1x2+16x1= 18g/mol.

There are some nifty molar mass calculators that you can use such the one here: molecular weight calculator.

You can also look at the wiki for each chemical to find the molar mass.
Chemical Chemical Formula Molar Mass (g/mol)
Sodium Carbonate Na2CO3 106
Potassium bitartrate KC4H5O6 188.18
Rochelle Salts KNaC4H4O6·4H2O 282.1


Notice that the rochelle salt only has a single sodium atom and that the sodium carbonate has 2 sodium atoms. Thus we need only half a mole of sodium carbonate to combine with one mole of Potassium bitartrate. So the mixing ration is 53/188.18= 0.2816, by weight. Since only the sodium is used the C03 is released as CO2.

2*KC4H5O6 + Na2CO3 => 2*KNC4H4O6 + C02

The rochelle salt retains water, approximately, 4 water molecules per molecule of salt, and so it is slightly heavier than you would expect, because of this extra water. If there was no water, then it would weigh 210.16/mol, but in actuality it weighs 282.1 because it contains 72g/mol of water.
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