Sodium Hydroxide

What is Sodium Hydroxide?

Sodium hydroxide (chemical formula NaOH), also called lye or caustic soda is a strong base that contains the sodium ion (Na⁺) and hydroxide (OH⁻) anion in its structure. The white crystalline solid and aqueous solution of sodium hydroxide is used in many industrial and cleaning products. An aqueous solution of sodium hydroxide causes bond breakage in proteins due to its high alkalinity or pH level. The uses of sodium hydroxide may include making soaps, detergents, artificial textile fibers, and paper drain cleaners, extraction of aluminum, oil refining, treatment of water, food processing, etc.

It is highly soluble in water and readily absorbs water and carbon dioxide from the air.  Generally, we use solid or a 50% sodium hydroxide solution for domestic and industrial purposes.

How to Make Sodium Hydroxide?

Sodium hydroxide is made commercially by electrolysis of sodium chloride solution using mercury cathode and carbon anode. Sodium is discharged on mercury to form an amalgam. It is treated with water to form NaOH.

At cathode: Na⁺ + e → Na (amalgam)
Na (amalgam) + H₂O → NaOH + H₂ + Hg

At anode: 2 Cl⁻ → Cl₂ + 2e

The method creates environmental pollution hazards by contamination of soil and water with mercury. Therefore, we use graphite rods for both the cathode and anode. Using such electrodes can decrease the efficiency of the cathode.

The causticizing of sodium carbonate provides an alternative method to make sodium hydroxide.
Na₂CO₃ + Ca(OH)₂ → 2 NaOH + CaCO₃

The reversible equilibrium is determined by the solubility of both Ca(OH)₂ and CaCO₃. The efficiency of causticizing increases when a dilute solution of sodium carbonate is used.

Chemistry of NaOH

The alkali metal hydroxide, NaOH is a very strong and corrosive base. It is a white crystalline solid soluble in water and alcohols like methanol and ethanol. When dissolved in water or neutralized with acid, NaOH liberates heat. It is insoluble in non-polar solvents.

Properties

At room temperature, caustic soda is a white crystalline solid that absorbs moisture from the air. Pure sodium hydroxide is melted at the temperature of 318 °C and boils at 1388 °C.

Some common properties of NaOH are given below in the table,

Chemical Properties

Reaction with Acids

Sodium hydroxide is a strong base that reacts with protic acid to form water and corresponding salts. For example, it reacts with hydrochloric acid to form water and sodium chloride.
NaOH + HCl → H₂O + NaCl

Similarly, it neutralizes sulfuric acid to form sodium sulfate and water.
2 NaOH + H₂SO₄ → Na₂SO₄ + 2 H₂O

All organic acids can react with NaOH to form soluble salts.

Reaction with Acidic Oxide

It reacts with acidic oxides such as sulfur dioxide to form sodium sulfite. Such a reaction is used in environmental chemistry to the reduction of harmful gases like SO₂ emitted by burning coal.
2 NaOH + SO₂ → Na₂SO₃ + H₂O

Reaction with Metals

Sodium hydroxide solution reacts with amphoteric metals such as aluminum, zinc, or tin to form sodium aluminate, sodium zincate, or sodium stannite, and hydrogen gas.
2 Al + 2 NaOH + 2 H₂O → 2 NaAlO₂ + 3 H₂
2Zn + 2 NaOH + 2 H₂O → 2 NaZnO₂ + 3 H₂
2 Sn + 2 NaOH + 2 H₂O → 2 NaSnO₂ + 3 H₂

Sodium aluminate is an important inorganic chemical used for the production of aluminum hydroxide. Aluminum hydroxide obtained by this reaction uses widely in water treatment, construction technology, and paper industries.

Sodium Hydroxide Solution

It dissolved in water to form an aqueous NaOH solution. The solution is used mainly as a pH buffer.

pH of Sodium Hydroxide

The pH scale ranges from 0 to 14. If the pH value is higher than 7, the solution is alkaline. When the pH value is lower than 7, the solution is acidic.

Sodium hydroxide solution has a high pH value. Therefore, it is a highly alkaline solution. The exact pH value of the NaOH solution can be measured by calculating the molarity of the solution.

Problem: Calculate the pH of a solution that contains 1 g of NaOH in 250 ml of water?

Solution: The number of moles of NaOH present in 250 of solution = 1/40 = 0.025.

Therefore, the number of moles of NaOH present in 1000 ml of solution,
= 0.025 × 4
= 0.1
It is the molarity of the NaOH.

NaOH is a strong base that ionizes completely in water. Therefore, an aqueous solution of NaOH dissociates completely to form Na⁺ and OH⁻ ions. Therefore, 0.1 mol of NaOH will dissociate to form 0.1 mol of Na⁺ and 0.1 mol OH⁻.

To calculate pH, first we apply the formula,
pOH = − log[OH⁻]
= − log [0.1]
= 1

Again we know that pH + pOH = 14
Therefore, pH of such sodium hydroxide solution,
= 14 − 1
= 13

Uses of Sodium Hydroxide

• Sodium hydroxide has wide industrial uses. A large quantity is used in the manufacture of soap via reaction with fats.
• NaOH is used in the textile industry for mercerizing cotton thread by dissolving undesirable fuzzy fibers.
• It is also used in rayon making, purification of petroleum, and manufacturing of paper.

The most individual uses of NaOH are given below,

Sodium Hydroxide in Cleaning Products

• Sodium hydroxide or caustic soda is used for manufacturing many cleaning products such as soaps and detergents.
• It is used for cleaning waste discharge pipes under sinks and drains where it can dissolve grease, oils, fats, and protein-based deposits.
• It is used with chlorine for industrial beaching and cleaning.

Sodium Hydroxide in Water Treatment

NaOH uses to remove heavy metals from water in the municipal drinking water treatment system.

It is a pH-adjusting chemical which also used for the production of sodium hypochlorite, a water disinfectant.

Sodium Hydroxide in Medicine

It is used to manufacture a variety of medicinal and pharmaceutical products such as pain relievers, anticoagulants, and cholesterol-reducing medicine.

It is used for the detection of carbon monoxide poisoning. The blood samples of such patients turned to a vermilion color when we added a few drops of NaOH solution.

Today, we use CO oximetry for the detection of carbon monoxide poisoning.

In Food Processing

It has several food processing applications. Caustic soda is used as a leavening agent, preservative agent, packaging material, alkalizing agent, etc.

• It is used as a leavening agent in bread and dough processing.
• It is used as a preservative agent to control acidity and prevent mold growth.
• NaOH is found in food packaging materials such as plastic resins that are highly resistant to water and other elements.
• It is a basic substance that has the ability to neutralize acids. Therefore, it is used to control the pH of commercial soft drinks and bottled mineral water.
• It is added to powdered milk to create cheese curds and other dairy products such as cottage cheese.

Aluminum Production

In the Bayer production process, NaOH is used for refining alumina containing ores (bauxite) to produce aluminum oxide or alumina.

Alumina is the raw material used for the production of aluminum metal via the electrolytic Hall-Héroult process. At high pH, sodium hydroxide solution may dissolve impurities like iron, zinc, and lead present in alumina.

In Fuel Cell Production

An alkaline fuel cell is a device that contains an aqueous solution of sodium hydroxide or potassium hydroxide as an electrolyte. Therefore, for making alkaline fuel cells, we use NaOH solution.

In Paper Making

In many paper-making industries, wood is treated with sodium sulfide and sodium hydroxide solution. It helps to dissolve most of the unwanted materials present in the wood. Therefore, it produces pure cellulose which is used in the paper-making process.

Sodium hydroxide or caustic soda solution is also used in the paper recycling process to separate the ink from the paper fibers.