Calcium Hydroxide Preparation
Calcium hydroxide is a strong base with a wide variety of uses in industry, ranging from mixing mortar and plaster to water treatment. In its native state, it's a white powder that dissolves poorly in water.
Hardware stores often sell calcium hydroxide as "pickling lime" or "slaked lime." However, it's possible to prepare your own by following the procedure outlined below.
NOTE: Read all warnings before beginning this procedure. Do not attempt this procedure unless you are already familiar with welding equipment, and are certain of your ability to perform these steps safely.
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Things You'll Need
- Calcium carbonate (crushed limestone or chalk)
- Safety goggles
- Hammer (or mortar and pestle)
- Distilled water
- Porcelain crucible with lid
- Ringstand and iron ring
- Clay triangle
- Crucible tongs
- Borosilicate glassware beaker
- Air-acetylene torch or Bunsen burner
- Gloves
- Welding gloves
- Polycarbonate face shield
- Fireproof lab bench
Instructions
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1
Find a source of pure calcium carbonate. Some brands of commercial chalk are mixed with other compounds, and hence are unsuitable for this purpose. If you are using limestone, choose the whitest shade you can find; darker limestones contain contaminants.
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2
Put on your safety goggles and prepare the calcium carbonate. If it is not already powdered, crush it to a fine powder using a hammer or a mortar and pestle.
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3
Add the powdered calcium carbonate to the porcelain crucible until it is approximately half-full. The powder level must be at least 1 centimeter below the rim.
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4
Make sure there are no children or animals nearby for the remainder of this procedure. Ensure there are no flammable materials close by. Perform the remainder of this procedure on a flameproof lab bench or outside in a suitable location where there is no danger of starting a fire.
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5
Put on the safety goggles, face shield and welding gloves. Mount the iron ring on the ringstand (the iron ring will have a clamp to secure it to the stand) and position the clay triangle on the iron ring. Put the porcelain crucible in the clay triangle and ensure that it fits snugly and will not fall through.
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6
Heat the porcelain crucible with the Bunsen burner or the air-acetylene torch. Do not use an oxy-acetylene torch since the temperature may be too high. For the first 30 seconds or so leave the lid off the crucible; afterwards, USING THE TONGS, position the lid on the crucible but put it on loosely so gases can escape from the crucible.
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7
Heat the crucible until it's red hot, then heat it for another five minutes.
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8
Turn off the Bunsen burner or torch and leave the crucible to cool until cool enough to handle; generally allow at least an hour. Do not remove the lid from the crucible while it is cooling. Remove the welding gloves & face shield.
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9
Add distilled water to the beaker. The beaker should be made of borosilicate heat-resistant glassware (e.g. Pyrex). Make sure you are wearing safety goggles and gloves for the next step.
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10
Add a little calcium oxide at a time to the water. Do NOT add the water to the calcium oxide. Note that when calcium oxide and calcium hydroxide react, the heat released by the reaction may be sufficient to crack ordinary glassware, which is why borosilicate is preferable. Take care to avoid splashing yourself or your surroundings while doing so.
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11
Watch the solution as you add the calcium oxide. A white precipitate will form at the bottom of the beaker. Once the bottom of the beaker is covered in white precipitate, cease adding calcium oxide.
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12
Decant the liquid from the top of the beaker while leaving the precipitate in the beaker. The liquid is a solution of calcium hydroxide in water. Keep it stored in an airtight container; if it's allowed to come in contact with air, the calcium hydroxide will slowly react with carbon dioxide to form calcium carbonate.
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Tips & Warnings
Calcium oxide is also available from many supply stores in the form of quicklime. If you want to prepare calcium hydroxide without first preparing calcium oxide, you can buy calcium oxide and use it to perform steps 9-12 while omitting the first part of this procedure.
Calcium oxide and calcium hydroxide are strong basic or alkaline chemicals that can cause serious burns or blindness if allowed to come in contact with eyes or skin. The dust from these chemicals can cause burns or irritation if inhaled or allowed to come in contact with your eyes. Make sure you store and handle these chemicals safely and wear eye protection at all times during this experiment.
Do not mix calcium oxide with acid, since this combination can cause a violent reaction.
Exercise special caution when adding calcium oxide to water, since the reaction releases a great deal of heat -- potentially enough to ignite flammable materials or vapors in the vicinity. Ensure there are no flammable materials anywhere nearby during this procedure.
A Bunsen burner or acetylene torch will heat the porcelain crucible to high temperatures -- temperature sufficient to cause serious burns or injury if mishandled. Only attempt this procedure if you are familiar and comfortable with the use of an acetylene torch. Do not attempt this procedure if you have any doubt about your ability to perform it safely.
References
- Photo Credit sandstone and mortar image by green308 from Fotolia.com
