Lye
Lye is the common name of a caustic hydroxide base, either sodium hydroxide (soda lye) or potassium hydroxide (potash lye). It's most commonly used for making soap by mixing lye with fats, but can also be used to cure and tenderize foods in small amounts. Sodium hydroxide may additionally be used in the production of paper, and water treatment. Potassium hydroxide can be used to produce saltpeter.
Prerequisites
From alkali carbonates
From alkali chlorides (e.g., salt)
- Salt
- Electricity
- Iron
- Carbon electrodes
- Mercury
Production
Production from Alkali Carbonates
Potash lye is generally easier to produce using primitive methods than soda lye, as potash (K2CO3) is much easier to procure than soda ash (Na2CO3), the latter of which requires the use of the ash from certain coastal plants, whereas former merely needs wood ash.
The general procedure is as follows, first prepare a solution of slaked lime by adding quicklime to water, 2 parts by weight of quicklime for potash, 1 part by weight for soda ash. Do this slowly as the reaction is very exothermic. Next, simply add your carbonate of choice into the solution, either 5 parts by weight of potash or 2 parts by weight of soda lime. Then the solution can be carefully boiled down in a non-metallic container (a solution of hot lye will easily eat through most metals, and will easily eat through flesh), leaving relatively pure lye. This can be recrystallized if higher purity is needed.
Two reactions have occurred in this process. First, the slaking of quicklime to produce slaked lime:
- CaO + H2O → Ca(OH)2
Then, the double displacement of the carbonate with the slaked lime to produce lye and limestone:
- K2CO3 + Ca(OH)2 → 2 KOH + CaCO3
or,
- Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3
Some lye can be extracted from potash by leaching, without the use of quicklime. This solution is very impure but sufficient for making low-quality soap.
Production from Alkali Chlorides
Once an abundant source of electricity has been procured, the alkali carbonate process becomes far too inefficient to be considered worthwhile. The next step in technology is therefore producing lye using the chloralkali process, which is to say, using electrolysis to turn salt into lye. Seeing as sodium chloride is by far the most abundant soluble salt on the planet, this means that rather than potash lye being the most efficient to produce, now soda lye is the most efficient.
The lowest tech version of the chloralkali process is the Castner-Kellner process, wherein a thin layer of mercury is used to separate the different sections of the cell (preventing the formation of sodium hypochlorite), while permitting the movement of sodium ions. This is a result of mercury's ability to form an alloy with sodium.
The adjacent image shows the cross-section of a Castner-Kellner cell.
The carbon anodes (A, A) pass into the outer compartments which contain brine, and the iron cathodes into the middle compartment which contains sodium hydroxide solution. When the current passes, chlorine is evolved at the anodes and flows out through pipes (not shown), and sodium is produced on the surface of the mercury (M) which covers the floor of the whole apparatus. The sodium forms an amalgam with the mercury, and by rocking the apparatus on the device, B, B, the sodium amalgam flows into the compartment, D, where the sodium is liberated by the action of the electric current, which passes between the cathode and the amalgam. The sodium reacts with the water forming hydrogen, which passes off through pipes (not shown) and sodium hydroxide, which flows into a special tank. Both the chlorine and sodium hydroxide are nearly pure.[1]
Uses
Lye is a powerful base, particularly useful in the production of soap. Soda lye in particular may also be used in the production of paper from wood, via soda pulping.
Potash lye can be used to neutralize nitric acid and create saltpeter.
Dependencies
- Lye
See also
- ↑ Newell, Lyman Churchill (1903). Descriptive Chemistry.