Sodium hydroxide

Sodium hydroxide
Preferred IUPAC name Sodium hydroxide
Systematic name Sodium oxidanide
Other names Caustic soda Lye
Identifiers
CAS number	1310-73-2 (anhydrate) Y, 14014-06-3 (2H),(anhydrate), 12179-02-1 (monohydrate)
PubChem	14798 (anhydrate) Y, 23675120 (17O),(anhydrate) Y, 23676750 (2H),(anhydrate) Y
ChemSpider	14114 (anhydrate) Y, 9346004 (17O),(anhydrate) Y, 9193979 (2H),(anhydrate) Y, 10732929 (monohydrate) Y, 392291 (dihydrate) Y
EC number	215-185-5
UN number	1823
KEGG	C12569
MeSH	Sodium+hydroxide
ChEBI	32145
RTECS number	WB4900000
Gmelin Reference	68430
SMILES [OH-].[Na+]
InChI InChI=1S/Na.H2O/h;1H2/q+1;/p-1 Key: HEMHJVSKTPXQMS-UHFFFAOYSA-M
Properties
Molecular formula	NaOH
Molar mass	39.99711 g/mol
Appearance	white solid, hygroscopic
Density	2.13 g/cm3
Melting point	318 °C, 591 K, 604 °F
Boiling point	1388 °C, 1661 K, 2530 °F
Solubility in water	1110 g/L (20 °C)
Solubility in ethanol	139 g/L
Solubility in methanol	238 g/L
Solubility in glycerol	soluble
Acidity (pKa)	~13
Refractive index (nD)	1.412
Hazards
MSDS	External MSDS
EU Index	011-002-00-6
EU classification	Corrosive (C)
R-phrases	R35
S-phrases	(S1/2), S26, S37/39, S45
NFPA 704	0 3 1
Flash point	Non-flammable
Related compounds
Other anions	Sodium hydrosulfide Sodium amide
Other cations	Lithium hydroxide Potassium hydroxide Rubidium hydroxide Caesium hydroxide
Related compounds	Sodium oxide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium hydroxide (NaOH), also known as lye and caustic soda, is a caustic metallic base. It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner. Worldwide production in 1998 was approximately 45 million tonnes. Sodium hydroxide is a common base in chemical laboratories.

Pure sodium hydroxide is a white solid; available in pellets, flakes, granules and as a 50% saturated solution. It is hygroscopic and readily absorbs water 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. Molten sodium hydroxide is also a strong base, but the high temperature required limits applications. It is insoluble in ether and other non-polar solvents. A sodium hydroxide solution will leave a yellow stain on fabric and paper.

Contents 1 Physical properties 2 Chemical properties 3 Retailing and production 4 Methods of production 4.1 Chloralkali electrolysis 4.2 Older methods 4.3 Nelson's process 4.4 Other 5 Uses 5.1 Alumina production 5.2 Soap production 5.3 Oil drilling 5.4 Fuel production 5.5 Pulp manufacturing for producing paper 5.6 Biodiesel 5.7 Aluminium etching 5.8 Cleaning agent 5.9 Leaded glass window restoration 5.10 Paint stripper 5.11 Tissue digestion 5.12 Food preparation 5.13 CD/DVD manufacturing 5.14 Domestic uses 5.15 Use in chemical analysis 6 Safety 7 See also 8 References 9 Bibliography 10 External links

Physical properties

Δ H° dissolution for aqueous dilution is –44.45 kJ / mol;

From aqueous solutions at 12.3–61.8 °C, it crystallizes in monohydrate, with a melting point 65.1 °C and density of 1.829 g/cm³;

The standard enthalpy change of formation (Δ H° form) is –734.95 kJ / mol.

Chemical properties

Sodium hydroxide is predominately ionic, containing sodium cations and hydroxide anions. The hydroxide anion makes sodium hydroxide a strong base which reacts with acids to form water and the corresponding salts. In the reaction 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(l)

This type of reaction with a strong acid releases heat, and hence is referred to as exothermic. Such acid-base reactions can also be used for titrations (along with a pH indicator, which is a common method to determine the concentration of acids.)

Another type of reaction that sodium hydroxide is involved in is with acidic oxides, such as sulfur dioxide. Such reactions are often used to "scrub" harmful acidic gases (like SO₂ and H₂S) produced in the burning of coal and prevent their release into the atmosphere. For example,

2 NaOH + CO₂ → Na₂CO₃ + H₂O

In the laboratory, with careful control of conditions, sodium metal can be isolated from the electrolysis of the molten monohydrate according to the following reaction:

4 NaOH·H₂O(l) → 4 Na(l) + O₂(g) + 6 H₂O(g)

The monohydrate does not need to be heated in order to melt, as the process produces enough heat due to ohmic heating. However, it must be initiated with a small quantity of liquid water to create an electrically conductive electrolyte. As the system's temperature increases, the monohydrate will start to melt at about 65 °C as stated above. Only when the temperature reaches about 100 °C can sodium be isolated. Below this temperature, the water produced will react with the sodium, above this point, any water formed will be driven off in the vapour phase, creating an essentially anhydrous reaction. While this process has some advantages over other electrolytic processes, it is not preferred by most chemists for several reasons: a marginal quantity of sodium produced boils at the electrode interface, the vapour thus given off consists primarely of fumed sodium oxide, which tends settle on any surface in close proximity with corrosive consequences.

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 since iron does not have amphoteric properties (i.e., it only dissolves in acid, not base). A few transition metals, however, may react vigorously with sodium hydroxide.

In 1986, an aluminium road tanker in the UK was mistakenly used to transport 25% sodium hydroxide solution , causing pressurization of the contents and damage to the tanker. The pressurization was due to the hydrogen gas which is produced in the reaction between sodium hydroxide and aluminium:

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

Unlike NaOH, the hydroxides of most transition metals are insoluble, and therefore sodium hydroxide can be used to precipitate transition metal hydroxides.

Aluminium hydroxide is used as a gelatinous flocculant to filter out particulate matter in water treatment. Aluminium hydroxide is prepared at the treatment plant from aluminium sulfate by reacting it with NaOH.

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

Sodium hydroxide reacts readily with carboxylic acids to form their salts and is even a strong enough base to form salts with phenols. NaOH can be used for the base-driven hydrolysis of esters (as in 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

Retailing and production

In 1998, total world production was around 45 million tonnes. North America and Asia collectively contributed around 14 million tonnes, while Europe produced around 10 million tonnes. In the United States, the major producer of sodium hydroxide is the Dow Chemical Company, which has annual production around 3.7 million tonnes from sites at Freeport, Texas, and Plaquemine, Louisiana. Other major US producers include Oxychem, PPG, Olin, Pioneer Companies (which was purchased by Olin), Inc. (PIONA), and Formosa. All of these companies use the chloralkali process^[1].

Methods of production

Chloralkali electrolysis

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:

2 Na⁺ + 2 H₂O + 2 e^– → H₂ + 2 NaOH

More accurately:

2 Na⁺Cl^– + 2 H₂O + 2 e^– → H₂ + 2 Cl^– + 2 NaOH

The Cl^– ions are oxidized to chlorine gas at the anode.

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 ions are reduced to sodium metal, which forms an amalgam with 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.^[2]
• Diaphragm cell process; uses a steel cathode, and the reaction of NaOH with Cl₂ is prevented using a porous diaphragm, often made of asbestos fibers. 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 sodium hydroxide must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of sodium hydroxide. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and is purified by liquefaction and evaporation.^[3][4]
• 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 tonne per tonne of sodium hydroxide).^[3][5]

Older methods

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, which readily absorbs water to create sodium hydroxide. This method is still occasionally used. It helped establish sodium hydroxide as an important commodity chemical.

The Leblanc process was superseded by the Solvay process in the late 19th century.

Nelson's process

A solution of sodium chloride in water contains Na⁺, Cl^–, H⁺, and OH^– ions because of the dissociation of sodium chloride and water:

NaCl → Na⁺ + Cl^–

H₂O → H⁺ + OH^–

During electrolysis, Na⁺ and H⁺ ions move toward the cathode, but H⁺ ions are discharged more easily than Na⁺ ions at the cathode, so the hydrogen gas is liberated. Similarly, Cl^– ions are oxidized at the anode and liberated as chlorine, so the solution becomes richer in Na⁺ and OH^– ions.

Process: A perforated U-shaped steel tube is suspended in a rectangular steel tank to act as a cathode. The steel tube is lined with asbestos inside to act as a diaphragm and prevent the reaction between the products formed during electrolysis. A saturated solution of sodium chloride in water (brine) is placed in the cell and a carbon rod is dipped in the solution to act as an anode.

The solution becomes richer in sodium hydroxide, which collects in the catch basin at the bottom of the tank. Chlorine gas is evacuated through an outlet at the top. Steam is passed into the tank through an inlet to keep the electrolyte warm and keep the perforations clear. Sodium hydroxide solution is then evaporated to form liquid solution. It is solidified in the form of pellets or flakes which are 98% sodium hydroxide. The pellets or flakes may be further purified by dissolution in alcohol. The impurities like NaCl and Na₂SO₄ are not soluble and are removed by filtration. The filtrate is evaporated to remove alcohol. The purification yields lye that ranges from 99–100% pure sodium hydroxide; however, prolonged contact with air allows the formation of sodium carbonate by reaction of sodium hydroxide with carbon dioxide and decreases the purity of the sodium hydroxide.

Other

Sodium hydroxide may be formed by the metathesis reaction between calcium hydroxide(also known as lime) and sodium carbonate (also known as soda ash):^[6]

Ca(OH)₂ + Na₂CO₃ → CaCO₃ + 2 NaOH

Uses

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 chemical reactions and also for the neutralization of acidic materials. It can be used also as a neutralizing agent in petroleum refining. It is also used for heavy duty and industrial cleaning.

Alumina production

In the Bayer process, sodium hydroxide is used in the refining of alumina containing ore (bauxite) to produce alumina (aluminium oxide) which is the raw material used to produce aluminium metal via the electrolytic Hall-Héroult process. Since the alumina is amphoteric, it dissolves in the sodium hydroxide, leaving impurities behind.

Soap production

Sodium hydroxide is traditionally used in soap making (cold process soap, saponification).^[7] It was made in the nineteenth century for a hard surface rather than liquid product because it was easier to store and transport.

Oil drilling

Sodium hydroxide is used as an additive in drilling mud to increase alkalinity. This serves two functions. Firstly raising alkalinity in bentonite mud systems increases the mud viscosity. A secondary function of raising alkalinity is to neutralise any acid gas (such as hydrogen sulfide and carbon dioxide) which may be encountered in the geological formation as drilling progresses.

Fuel production

Sodium hydroxide can be added to poor quality crude oil to remove sulfurous impurities in a process known as caustic washing. The process forms a highly toxic waste containing hydrogen sulfide, organic disulfides and mercaptans.^[8] Because of this the process is banned in many countries but in 2006, Trafigura used the process and then dumped the waste in Africa.^[9]

Pulp manufacturing for producing paper

Sodium hydroxide was also widely used in making paper. Along with sodium sulfide, NaOH 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 later 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.

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 combined with water the fat would turn into soap, which would be tainted with methanol. It is used more often than potassium hydroxide because it is cheaper and a smaller quantity is needed.

Aluminium etching

Strong bases attack aluminium. Sodium hydroxide reacts with aluminium and water to release hydrogen gas. The aluminium takes the oxygen atom from sodium hydroxide (NaOH), which in turn takes the oxygen atom from the water, and releases the two hydrogen atoms. In this reaction, sodium hydroxide acts as an agent to make the solution alkaline, which aluminium can dissolve in. This reaction can be useful in etching, removing anodizing, or converting a polished surface to a satin-like finish, but without further passivation such as anodizing or alodining the surface may become degraded, either under normal use or in severe atmospheric conditions.

Cleaning agent

Sodium hydroxide is frequently used as an industrial cleaning agent where it is often called "caustic". It is added to water, heated, and then used to clean the process equipment, storage tanks, etc. It can dissolve grease, oils, fats and protein based deposits. The sodium hydroxide solution can also be added surfactants to stabilize dissolved substances to prevent redeposition. A sodium hydroxide soak solution is used as a powerful degreaser on stainless and glass bakeware. It is also a common ingredient in oven cleaners.

A common use of sodium hydroxide is in the production of parts washer detergents. Parts washer detergents based on sodium hydroxide are some of the most aggressive parts washer cleaning chemicals. The sodium hydroxide based detergent include surfactants, rust inhibitors and defoamers. A parts washer heats water and the detergent in a closed cabinet and then sprays the heated sodium hydroxide and hot water at pressure against dirty parts for degreasing applications. Sodium hydroxide used in this manner replaced many solvent based systems in the early 1990s when trichloroethane was outlawed by the Montreal Protocol. Water and sodium hydroxide detergent based parts washers are considered to be an environmental improvement over the solvent based cleaning methods.

Leaded glass window restoration

Sodium hydroxide can be used to dissolve old "weatherproofing cement" in leaded glass windows by soaking the panels overnight in a moderately concentrated solution of sodium hydroxide.

Paint stripper

A solution of sodium hydroxide in water was traditionally used as the most common paint stripper on wooden objects. Because of its caustic nature and the fact that it can damage the wood surface raising the grain and staining the color, its use has become less common.

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 sodium hydroxide and water, which breaks chemical bonds keeping the body intact. This eventually turns the body into a coffee-like^[10][11] liquid, and the only solid that remains are bone hulls, which could be crushed between one's fingertips.^[12] Sodium hydroxide is frequently used in the process of decomposing roadkill dumped in landfills by animal disposal contractors.^[11]

Sodium hydroxide has also been used by criminals to dispose of their victims' bodies^[13].

Food preparation

Food uses of sodium hydroxide include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel coloring production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in sodium hydroxide to soften them, while pretzels and German lye rolls are glazed with a sodium hydroxide solution before baking to make them crisp. Due to the difficulty in obtaining food grade sodium hydroxide in small quantities for home use, Sodium carbonate is often used in place of sodium hydroxide^[14].

Specific foods processed with sodium hydroxide include:

• The Pinoy or Filipino kakanin called kutsinta have lye to jellify the kutsinta.
• Also the Filipino kakanin called pitsi-pitsi or pichi-pichi (pit-chi-pit-chi) also have a small amount of lye.
• 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 frying to make corn nuts or by drying and grinding to make grits. Nixtamal is similar, but uses calcium hydroxide instead of sodium hydroxide.
• 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 carbonate solution or cold sodium hydroxide solution before baking, which contributes to their unique crust.
• Most yellow coloured Chinese noodles are made with lye-water but are commonly mistaken for containing egg.

CD/DVD manufacturing

Used to clean excess photo-resist off freshly plated nickel stampers.

Domestic uses

Sodium hydroxide is used in the home as a drain cleaning agent for clearing clogged drains. It is distributed as a dry crystal or as a thick liquid gel. The chemical mechanism employed is the conversion of grease to a form of soap. Soap is water-soluble, and can be dissolved by flushing with water. This conversion occurs far more rapidly at high temperatures, so commercial drain cleaners may also contain chemicals that react with water to generate heat. Sodium hydroxide also decomposes complex molecules such as the protein that composes hair. Such drain cleaners (and their acidic versions) are highly caustic and should be handled with care.

Sodium hydroxide has been used as a relaxer to straighten hair. However, because of the high incidence and intensity of chemical burns, chemical relaxer manufacturers have now switched to other alkaline chemicals. Sodium hydroxide relaxers are still available, but they are used mostly by professionals.

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 standardised 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.

Safety

Solid sodium hydroxide or solutions of sodium hydroxide will cause chemical burns, permanent injury or scarring if it contacts unprotected human or animal tissue. It will cause blindness if it contacts with the eye. Protective equipment such as rubber gloves, safety clothing and eye protection should always be used when handling the material or its solutions.

Dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables. It also produces heat when reacted with acids. Sodium hydroxide is corrosive to glass and some metals and should be.kept away from aluminium.

See also

• HAZMAT Class 8 Corrosive Substances
• Common chemicals
• List of elemental cleaning agents

References

Bibliography

• Euro Chlor-How is chlorine made? Chlorine Online
• Greenwood, Norman N.; Earnshaw, Alan. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0080379419 
• Heaton, A. (1996) An Introduction to Industrial Chemistry, 3rd edition, New York:Blackie. ISBN 0-7514-0272-9.
• Kirk-Othmer Encyclopedia of Chemical Technology 5th edition (online, account needed), John Wiley & Sons. Accessed November 21, 2005.

External links

• International Chemical Safety Card 0360
• NIOSH Pocket Guide to Chemical Hazards
• European Chemicals Bureau
• European Union Risk Assessment Report
• Production by brine electrolysis
• Titration of acids with sodium hydroxide; freeware for data analysis, simulation of curves and pH calculation

Sodium compounds NaAlO2 · NaBH3(CN) · NaBH4 · NaBr · NaBrO4 · NaCH3COO · NaCN · NaC6H5CO2 · NaC6H4(OH)CO2 · NaCl · NaClO · NaClO2 · NaClO3 · NaClO4 · NaF · NaH · NaHCO3 · NaHSO3 · NaHSO4 · NaI · NaIO3 · NaIO4 · NaMnO4 · NaNH2 · NaNO2 · NaNO3 · NaN3 · NaOH · NaO2 · NaPO2H2 · NaReO4 · NaSCN · NaSH · NaTcO4 · NaVO3 · Na2CO3 · Na2C2O4 · Na2CrO4 · Na2Cr2O7 · Na2MnO4 · Na2MoO4 · Na2O · Na2O2 · Na2O(UO3)2 · Na2S · Na2SO3 · Na2SO4 · Na2S2O3 · Na2S2O4 · Na2S2O5 · Na2S2O6 · Na2S2O7 · Na2S2O8 · Na2Se · Na2SeO3 · Na2SeO4 · Na2SiO3 · Na2Te · Na2TeO3 · Na2Ti3O7 · Na2U2O7 · NaWO4 · Na2Zn(OH)4 · Na3N · Na3P · Na3VO4 · Na4Fe(CN)6  · Na5P3O10