Sodium hydroxide

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Sodium hydroxide
Sodium hydroxide Sodium hydroxide
General
Systematic name Sodium hydroxide
Other names Lye, Caustic Soda
Molecular formula NaOH
Molar mass 39.9971 g/mol
Appearance White solid
CAS number [1310-73-2]
Properties
Density and phase 2.1 g/cm³, solid
Solubility in water 111 g/100 ml (20°C)
Melting point 318°C (591 K)
Boiling point 1390°C (1663 K)
Basicity (pKb) -2.43
Hazards
MSDS External MSDS
EU classification Corrosive (C)
R-phrases R35
S-phrases S1/2, S26, S37/39, S45
NFPA 704

NFPA 704.svg

0
3
1
 
Flash point Non-flammable.
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Other anions Sodium chloride
Sodium sulfate.
Other cations Potassium hydroxide
Calcium hydroxide
Related bases Ammonia, lime.
Related compounds
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)

Sodium hydroxide, also known as lye or caustic soda, is a caustic metallic base. Its chemical formula is NaOH. Forming a strongly alkaline solution when dissolved in a solvent such as water, caustic soda is widely used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps, and detergents. Worldwide production in 1998, was around 45 million tons. Sodium hydroxide is also the most common base used in chemical laboratories, and it is widely used as a drain cleaner.

Contents

General properties

Pure sodium hydroxide is a white solid; available in pellets, flakes, granules, and also as a 50-percent saturated solution. It is deliquescent and also readily absorbs carbon dioxide from the air, so it should be stored in an airtight container. It is very soluble in water, with liberation of heat. It also dissolves in ethanol and methanol, though it exhibits lower solubility in these solvents than does potassium hydroxide. It is insoluble in ether and other non-polar solvents. A sodium hydroxide solution will leave a yellow stain on fabric and paper.

Chemical properties

Sodium hydroxide is completely ionic, containing sodium ions and hydroxide ions. The hydroxide ion makes sodium hydroxide a strong base which reacts with acids to form water and the corresponding salts, for example, with hydrochloric acid, sodium chloride is formed:

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

In general, such neutralization reactions are represented by one simple net ionic equation:

OH(aq) + H+(aq) → H2O

This type of reaction releases heat when a strong acid is used. Such acid-base reactions can also be used for titrations, and indeed this is a common way for measuring the concentration of acids.

Related to this is the reaction of sodium hydroxide with acidic oxides. The reaction of carbon dioxide has already been mentioned, but other acidic oxides such as sulfur dioxide (SO2) also react completely. Such reactions are often used to "scrub" harmful acidic gases (like SO2 and H2S) and prevent their release into the atmosphere.

2NaOH + CO2Na2CO3 + H2O

Sodium hydroxide slowly reacts with glass to form sodium silicate, so glass joints and stopcocks exposed to NaOH have a tendency to "freeze." Flasks and glass-lined chemical reactors are damaged by long exposure to hot sodium hydroxide, and the glass becomes frosted. Sodium hydroxide does not attack iron or copper, but many other metals such as aluminium, zinc, and titanium are attacked rapidly. In 1986, an aluminum road tanker in the UK was mistakenly used to transport 25 percent sodium hydroxide solution, causing pressurization of the contents and damage to the tanker. For this same reason aluminum pans should never be cleaned with lye.

2Al(s) + 6NaOH(aq) → 3H2(g) + 2Na3AlO3(aq)

Many non-metals also react with sodium hydroxide, giving salts. For example, phosphorus forms sodium hypophosphite, while silicon gives sodium silicate.

Unlike NaOH, the hydroxides of most metals are insoluble, and therefore sodium hydroxide can be used to precipitate metal hydroxides. One such hydroxide is aluminium hydroxide, used as a gelatinous floc to filter out particulate matter in water treatment. Aluminum hydroxide is prepared at the treatment plant from aluminum sulfate by reaction with NaOH:

6NaOH(aq) + Al2(SO4)3(aq) → 2Al(OH)3(s) + 3Na2SO4(aq)

Sodium hydroxide reacts readily with carboxylic acids to form their salts, and it is even a strong enough base to form salts with phenols. NaOH can also be used for the base-driven hydrolysis of esters (as is saponification), amides and alkyl halides. However, the limited solubility of NaOH in organic solvents means that the more soluble KOH is often preferred.

Basic hydrolysis of an ester

Manufacture

In 1998, total world production was around 45 million tons. Of this, both North America and Asia contributed around 14 million metric tons, and Europe produced around 10 million metric tons.

Methods of production

Sodium hydroxide is produced (along with chlorine and hydrogen) via the chloralkali process. This involves the electrolysis of an aqueous solution of sodium chloride. The sodium hydroxide builds up at the cathode, where water is reduced to hydrogen gas and hydroxide ion:

2Na+ + 2H2O + 2e → H2 + 2NaOH

To produce NaOH, it is necessary to prevent reaction of the NaOH with the chlorine. This is typically done in one of three ways, of which the membrane cell process is economically the most viable.

  • Mercury cell process (also called the Castner-Kellner process)—sodium metal forms as an amalgam at a mercury cathode; this sodium is then reacted with water to produce NaOH. There have been concerns about mercury releases, although modern plants claim to be safe in this regard.[1]
  • Diaphragm cell process—uses a steel cathode, and reaction of NaOH with Cl2 is prevented using a porous diaphragm. In the diaphragm cell process, the anode area is separated from the cathode area by a permeable diaphragm. The brine is introduced into the anode compartment and flows through the diaphragm into the cathode compartment. A diluted caustic brine leaves the cell. The caustic soda must usually be concentrated to 50 percent and the salt removed. This is done using an evaporative process with about three metric tons of steam per metric ton of caustic soda. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and must often be purified by liquefaction and evaporation.[2]
  • Membrane cell process—similar to the diaphragm cell process, with a Nafion membrane to separate the cathode and anode reactions. Only sodium ions and a little water pass through the membrane. It produces a higher quality of NaOH. Of the three processes, the membrane cell process requires the lowest consumption of electric energy and the amount of steam needed for concentration of the caustic is relatively small (less than one metric ton per metric ton of caustic soda).[3]

An older method for sodium hydroxide production was the LeBlanc process, which produced sodium carbonate, followed by roasting, to create carbon dioxide and sodium oxide. This method is still occasionally used. It helped to establish sodium hydroxide as an important commodity chemical.

Major producers

In the United States, the major producer of sodium hydroxide is the Dow Chemical Company, which has annual production around 3.7 million tons from sites at Freeport, Texas, and Plaquemine, Louisiana. Other major U.S. producers include Oxychem, PPG, Olin, Pioneer Companies, Inc. (PIONA), and Formosa. All of these companies use the chloralkali process.[4]

Uses

General applications

Sodium hydroxide is the principal strong base used in the chemical industry. In bulk, it is most often handled as an aqueous solution, since solutions are cheaper and easier to handle. It is used to drive for chemical reactions and also for the neutralization of acidic materials. It can be used also as a neutralizing agent in petroleum refining.

Gold pennies

Sodium hydroxide has also been used in conjunction with zinc for creation of the famous "Gold pennies" experiment. When a penny is boiled in a solution of NaOH together with some granular zinc metal (galvanized nails are one source), the color of the penny will turn silver in about 45 seconds. The penny is then held in the flame of a burner for a few seconds and it turns golden. The reason this happens is that granular zinc dissolves in NaOH to form Zn(OH)42-. This zincate ion becomes reduced to metallic zinc on the surface of a copper penny. Zinc and copper when heated in a flame form brass.

Use in chemical analysis

In analytical chemistry, sodium hydroxide solutions are often used to measure the concentration of acids by titration. Since NaOH is not a primary standard, solutions must first be standardized by titration against a standard such as KHP. Burettes exposed to NaOH should be rinsed out immediately after use to prevent "freezing" of the stopcock. Sodium hydroxide was traditionally used to test for cations in Qualitative Inorganic Analysis, as well as to provide alkaline media for some reactions that need it, such as the Biuret test.

Soap making

Soap making (cold process soap, saponification) is the most traditional chemical process using sodium hydroxide. The Arabs began producing soap in this way in the seventh century, and the same basic process is still used today.

Biodiesel

For the manufacture of biodiesel, sodium hydroxide is used as a catalyst for the transesterification of methanol and triglycerides. This only works with anhydrous sodium hydroxide, because water and lye would turn the fat into soap which would be tainted with methanol.

It is used more often than potassium hydroxide because it costs less, and a smaller quantity is needed for the same results. Another alternative is sodium silicate.

Aluminum etching

Strong bases attack aluminum. This can be useful in etching through a resist or in converting a polished surface to a satin-like finish, but without further passivation such as anodizing or allodizing the surface may become corroded, either under normal use or in severe atmospheric conditions.

Food preparation

Food uses of lye include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel color production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in lye to soften them, while pretzels and German lye rolls are glazed with a lye solution before baking to make them crisp.

Specific foods processed with lye include:

  • The Scandinavian delicacy known as lutefisk (from lutfisk, "lye fish").
  • Hominy is dried maize (corn) kernels reconstituted by soaking in lye-water. These expand considerably in size and may be further processed by cooking in hot oil and salting to form corn nuts. Nixtamal is similar, but uses calcium hydroxide instead of sodium hydroxide.
  • Hominy is also known in some areas of the Southeastern United States, as the breakfast food grits, dried and ground into a coarse powder. They are prepared by boiling in water, with the addition of butter and other ingredient to suit the tastes of the preparer.
  • Sodium hydroxide is also the chemical that causes gelling of egg whites in the production of Century eggs.
  • German pretzels are poached in a boiling sodium hydroxide solution before baking, which contributes to their unique crust.

Delignification of cellulosic materials

Sodium Hydroxide, in addition to Sodium Sulfide, is a key component of the white liquor solution used to separate lignin from cellulose fibers in the Kraft process. It also plays a key role in several following stages of the process of bleaching the brown pulp resulting from the pulping process. These stages include oxygen delignification, oxidative extraction, and simple extraction, all of which require a strong alkaline environment with a pH > 10.5 at the end of the stages.

Domestic uses

Sodium hydroxide is used in the home as an agent for unblocking drains, provided as a dry crystal (for example, "Drāno") or as a thick liquid gel. The chemical mechanism employed is the conversion of grease to a form of soap, and so forming a water soluble form to be dissolved by flushing; also decomposing complex molecules such as the protein of hair. Such drain cleaners (and their acidic versions) are highly caustic and should be handled with care.

Beginning in the early 1900s, lye has been used to relax or straighten the hair of persons of African ethnicity. Among men, this treatment was often called a process. However, because of the high incidence and intensity of chemical burns, chemical relaxer manufacturers began switching to other alkaline chemicals (most commonly guanidine hydroxide) during the latter quarter of the twentieth century, although lye relaxers are still available, usually under use by professionals.

Tissue digestion

This is a process that was used with farm animals at one time. This process involves the placing of a carcass into a sealed chamber, which then puts the carcass in a mixture of lye and water, which breaks chemical bonds keeping the body intact. This eventually turns the body into a coffee-like liquid, and the only solid remains are bone hulls, which could be crushed between one's fingertips.

Illegal drugs

Sodium hydroxide is a key reagent in the process of making Methamphetamine and other illegal drugs. Contrary to popular media reports, it is not actually an "ingredient" in these drugs, but simply a strong base used to manipulate the pH at various points in a chemical synthesis.

Safety

Solid sodium hydroxide or solutions containing high concentrations of sodium hydroxide may cause chemical burns, permanent injury or scarring, and blindness.

Solvation of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables.

The combination of aluminium and sodium hydroxide results in a large production of hydrogen gas:
2Al(s) + 6NaOH(aq) → 3H2(g) + 2Na3AlO3(aq).
Mixing these two in a closed container is therefore dangerous.

Notes

  1. Euro Chlor, Chlorine Online Diagram of mercury cell process. Retrieved April 4, 2008.
  2. Euro Chlor, How is chlorine made? Retrieved April 4, 2008.
  3. Euro Chlor, Chlorine Online Diagram of membrane cell process. Retrieved April 4, 2008.
  4. Kirk-Othmer Encyclopedia of Chemical Technology.

References

  • Heaton, Alan, ed. 1996. An Introduction to Industrial Chemistry. Glasgow, UK: Blackie. ISBN 0-7514-0272-9
  • Moore, John T., Ed.D. 2004. Chemistry Made Simple. New York: Broadway Books. ISBN 0767917022
  • Seidel, Arza, ed. 2006. Kirk-Othmer Encyclopedia of Chemical Technology. Hoboken, NJ: John Wiley. ISBN 047148508X

External links

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