৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

In the diaphragm cell process for caustic soda (NaOH) production:
1. The anode compartment produces chlorine gas (Cl₂) from chloride ions.
2. The cathode compartment produces hydrogen gas (H₂) and caustic soda (NaOH) from water reduction.
3. If these two compartments were not separated, chlorine could react with NaOH to form hypochlorite, reducing efficiency and causing safety hazards.

The diaphragm (usually made of asbestos or modern polymeric materials) serves these functions:
✔ It separates the anode and cathode compartments so that chlorine and caustic soda do not mix.
✔ It is porous, allowing brine (NaCl solution) to flow through to the cathode side to maintain ionic conductivity.

In the membrane cell process for caustic soda (NaOH) production:
The membrane separates the anode (Cl₂ generation) and cathode (NaOH + H₂ generation) compartments.
It allows only Na⁺ ions (cations) to pass from the anode side to the cathode side while blocking Cl⁻ ions and OH⁻ back-migration.
This selectivity prevents the formation of hypochlorite (ClO⁻) and ensures high-purity caustic soda.
The membranes are usually made from perfluorinated cation-exchange polymers such as Nafion, which have sulfonic acid groups (-SO₃H) for ionic conduction
​
​Other Options:
​
​ক) Polypropylene fibers → Used in diaphragm cells, not in membrane cells.
গ) Anion-exchange polymer → Would allow Cl⁻ passage, which is undesirable.
ঘ) Porous ceramic material → Too permeable and non-selective for modern processes.

The mercury cell (Castner-Kellner) process is being phased out primarily because:
1. Mercury is highly toxic and poses severe environmental and health risks.
2. During operation, mercury can escape into the environment through brine, air emissions, and solid wastes, contaminating ecosystems and entering the food chain.
3. International regulations like the Minamata Convention on Mercury aim to eliminate such practices.

Why not the others?

ক) High electrical energy consumption only → While mercury cells do consume more energy compared to membrane cells, this is not the main reason for their phase-out.
খ) Low product purity → Actually, mercury cells produce very pure caustic soda (50% NaOH solution, almost chloride-free).
ঘ) Poor current efficiency → Mercury cells generally have good current efficiency, so this isn’t the primary issue.

In the Solvay process (used for soda ash – Na₂CO₃ production):
Ammonia is bubbled into brine (NaCl solution) to make it ammoniacal brine.
When CO₂ is introduced, the following reaction occurs in the ammoniacal solution:
NaCl+NH₃+CO₂+H₂O⟶NaHCO₃(↓)+NH₄Cl
 
​The role of NH₃ is to convert water into a weak base medium, which allows CO₂ to form bicarbonate ions without significantly reducing pH. This promotes precipitation of NaHCO₃ (which is sparingly soluble), while NH₄Cl remains dissolved.

So, ammonia essentially helps precipitate NaHCO₃ by controlling solubility and equilibrium,

During carbonation, NH₄Cl is formed as a byproduct.

To reuse NH₃, NH₄Cl is treated with Ca(OH)₂ (milk of lime):
                  
2NH₄Cl+Ca(OH)₂⟶2NH₃↑+CaCl₂+2H₂O

The released NH₃ is returned to the brine saturation step.

Byproduct: CaCl₂ (disposed or used in de-icing).

In a mercury cell, sodium is deposited at the cathode and forms a Na–Hg amalgam.

The anode current density directly affects the rate of chlorine evolution, which in turn influences the rate of sodium deposition at the cathode. Too high or too low current density can reduce current efficiency.

Brine purity is critical because impurities (like calcium or magnesium ions) can poison the anode, reduce chlorine evolution efficiency, and lead to lower sodium amalgam formation.

While mercury flow rate and cathode overpotential are important for proper amalgam handling and hydrogen suppression, the anode conditions (current density and brine purity) are the most significant factors for overall efficiency.

Dual-ammonia (Hou’s) process improves CO₂ efficiency by using a two-step ammonia absorption.

Like the Solvay process, NH₄Cl is still formed, but the second ammonia step recovers more CO₂ and reduces ammonia loss.

Ammonia is still recycled in both processes.

Precipitation does not occur in concentrated NaOH; it is still in ammoniacal brine.

ক) Calcination of CaCO₃ – Incorrect statement:
True: It requires high thermal energy (~900°C) and is endothermic, not low-temperature/exothermic.

খ) Carbonation of ammoniacal brine – Incorrect statement:
This is a low-temperature, exothermic reaction, no significant electrical energy is required.

গ) Ammonia recovery from NH₄Cl with Ca(OH)₂ – Correct
NH₄Cl reacts with slaked lime to release NH₃ gas:
2NH₄Cl+Ca(OH)₂→2NH₃↑+CaCl₂+2H₂O
​
Requires chemical energy (via lime preparation) and moderate heat to drive the reaction.

ঘ) Filtration of NaHCO₃ – Incorrect statement:
Filtration is a mechanical process; it does not require high thermal energy.

In membrane cell caustic soda production:
The membrane is a cation-exchange polymer that selectively allows Na⁺ ions to pass while blocking Cl⁻, OH⁻, and other cations.
Hardness ions (Ca²⁺, Mg²⁺) from raw brine can:
Deposit on the membrane, reducing its efficiency.
Increase electrical resistance, raising energy consumption.
Cause mechanical damage or early failure of the membrane.
Therefore, pure brine (deionized or softened) is required to maintain long membrane life and high-purity NaOH.
​
​ক) Prevent electrode corrosion → Corrosion is mostly controlled by anode material and operating conditions, not brine purity.
খ) Increase chlorine production rate → Brine purity doesn’t significantly affect the rate; current density is the main factor.
ঘ) Reduce current density → Current density is set for efficiency and production rate, not brine purity.

In modern caustic soda (NaOH) production, the membrane cell process is preferred because:

1. Highest current efficiency (~90–98%)
The cation-exchange membrane allows only Na⁺ ions to pass to the cathode side.
This minimizes side reactions, such as formation of hypochlorite or loss of OH⁻.

2. Lowest energy consumption (~2,500–3,000 kWh/ton NaOH)
Unlike diaphragm cells (~3,000–3,500 kWh/ton) or mercury cells (~2,800–3,200 kWh/ton), the membrane process uses less electrical energy per ton.

3. High-purity NaOH (~32% without contamination)
Free of Cl⁻, Hg, or other cations because the membrane blocks them.

In the mercury cell (Castner-Kellner) process:

Cathode: Mercury acts as a liquid cathode.
Reaction: Sodium ions are reduced to metallic sodium, which immediately forms an amalgam with mercury:
Na⁺+e⁻ → Na (in Hg, amalgam)

The amalgam is then reacted with water in a separate reactor to produce NaOH and H₂ gas:
2Na(Hg) + 2H₂O → 2NaOH + H₂ + 2Hg

​Other options:
​ক) Na⁺ + H₂O → NaOH + ½ H₂ → This is the reaction of metallic Na with water, not the cathode reaction.
খ) Cl⁻ → ½ Cl₂ + e⁻ → This occurs at the anode, producing chlorine gas.
গ) 2H₂O + 2e⁻ → H₂ + 2OH⁻ → This is the cathode reaction in diaphragm or membrane cells, but not in mercury cells.

The diaphragm is porous, allowing brine (NaCl solution) to pass but separating the cathode and anode compartments.

Cathode compartment: NaOH is produced along with H₂.

Limiting factor: As the NaOH concentration rises above ~12%, OH⁻ ions tend to diffuse back through the diaphragm toward the anode, where they react with chlorine.

This reduces current efficiency and prevents further concentration increase inside the cell.
​
​খ) Membrane swelling due to high pH → Membranes are used in membrane cells, not diaphragm cells.
গ) Formation of hypochlorite side products → This can happen if NaOH and Cl₂ mix, but the main limitation is back-diffusion.
ঘ) Energy inefficiency at high concentrations → Energy loss is secondary; the primary physical limitation is ion back-diffusion.

In a membrane cell process for NaOH production:

Anode reaction (oxidation of chloride):
2Cl⁻→Cl₂+2e^-
Cathode reaction (reduction of water):
2H₂O+2e⁻→H₂+2OH⁻
Overall reaction:
2NaCl+2H₂O→2NaOH+Cl₂+H₂

This shows the simultaneous production of:
1. NaOH in the cathode compartment
2. Chlorine gas at the anode
3. Hydrogen gas at the cathode

In a membrane cell for NaOH production:

The cation-exchange membrane separates the anode (Cl₂) and cathode (NaOH + H₂) compartments.

Critical requirements:
           1. High Na⁺ conductivity – allows sodium ions to pass from the anode side to the cathode side efficiently.
           2. Chemical resistance – must withstand highly corrosive alkaline NaOH on the cathode side and chlorine on the anode side without degradation.​
           3. Selectivity – blocks Cl⁻ and OH⁻ ions to prevent side reactions and maintain high current efficiency. 

​​Other options:
​
          ​ক) High selectivity for Cl⁻ ions → Wrong; the membrane should block Cl⁻, not conduct it.
          খ) High electrical conductivity with OH⁻ → Membrane does not conduct OH⁻; it conducts Na⁺ ions only.
          ঘ) Mechanical strength to support the anode → Some strength is required, but chemical stability and ion selectivity are more critical.ent efficiency.

Membrane cell advantages over diaphragm cell:

1. High purity NaOH (~99%)
          The cation-exchange membrane blocks Cl⁻, Mg²⁺, Ca²⁺, and other impurities from reaching the cathode compartment.
          Diaphragm cells produce only ~30–32% NaOH, which still contains some NaCl.

2. Higher current efficiency (~90–98%)
          Membrane cells minimize side reactions like Cl⁻ back-diffusion.

3. Lower energy consumption per ton
          Membrane cells are more energy-efficient than diaphragm cells.
​
​Other Options:
​
          ​ক) Higher energy consumption → False; membrane cells consume less energy than diaphragm cells.
          গ) Back-diffusion of OH⁻ is enhanced → False; the membrane prevents back-diffusion, which improves efficiency.
          ঘ) Requires mercury as a cathode → False; mercury is only used in mercury cells, not membrane cells.

In a diaphragm cell:

Increasing current density means more charge passes per unit area of the electrode.
Limitation: At high current densities:
​
1.  Mass transport of Cl⁻ ions to the anode surface becomes slower than the rate of reaction.
2. This leads to reduced chlorine evolution efficiency and possibly increased side reactions (like oxygen evolution).

The diaphragm is porous, so OH⁻ can also back-diffuse, but the main limiting factor for high current density is mass transport of chloride ions.
​
​Other Options:
খ) Overheating of the diaphragm → Diaphragms are usually designed for proper cooling; not the main limitation.
গ) Membrane chemical degradation → Diaphragm cells use porous diaphragms, not polymer membranes; chemical degradation is minimal
ঘ) Formation of sodium chlorate → Chlorate forms at high temperatures or very high current densities over time, but mass transport limitation occurs first, making it the primary limitation.

In the mercury cell process:

Na-Hg amalgam is formed at the cathode:

Na⁺+e⁻→Na(amalgam)

Decomposer reaction with water produces NaOH and H₂:

2Na(Hg)+2H₂O→2NaOH+H₂↑+2Hg

Other Options:
​
                 ​খ) Chlorine partial pressure → Relevant in the anode compartment, not the decomposer.
                 গ) Cathode surface area → Affects amalgam formation but not the safety in the decomposer.
                  ঘ) Brine concentration → Affects electrolysis efficiency but not directly explosion risk during decomposer reaction.

In the membrane cell process for caustic soda production:
The cation-exchange membrane allows only Na⁺ ions to pass from the anolyte (brine) to the catholyte (where NaOH is formed).
This prevents Cl⁻ ions and OH⁻ ions from migrating across compartments.

As a result:
NaOH solution in the cathode compartment remains pure (without chloride contamination).
Current efficiency improves because unwanted side reactions are minimized.
​
​Other Options:
​ক) Incorrect: Hydroxide ions (OH⁻) do not migrate through the membrane; only Na⁺ moves. The design is to stop OH⁻ migration, not enhance it.
গ) Incorrect: Hydrogen recombination is not a function of the membrane; it's related to the cathode reaction.
ঘ) Incorrect: The membrane doesn't increase brine concentration; that depends on feed control and circulation, not the membrane.

In the mercury cell process, the formation of the sodium–mercury amalgam at the cathode depends heavily on brine purity and proper anode current density:
Brine purity: Even trace impurities like Ca²⁺, Mg²⁺, Fe, heavy metals, or organics catalyze hydrogen evolution on the mercury surface. This reduces sodium deposition and drastically lowers current efficiency.
Anode current density: If it's too high or too low, side reactions increase, chlorine back-reaction occurs, and energy losses rise—affecting overall efficiency.
​
​Other Options:
​(ক) Cathode overpotential for hydrogen evolution – This matters, but the biggest factor influencing it is actually brine purity. On its own, it's not the primary determinant of efficiency.
(খ) Chlorine solubility in the catholyte – Less relevant because in the mercury cell, NaOH forms later in a separate decomposer, not in the main cell.
(ঘ) Flow rate of mercury and cell temperature – These affect the transport and concentration of amalgam, but they are optimization factors, not the dominant determinant of current efficiency.

In a diaphragm cell:
The anode and cathode compartments are separated by a porous diaphragm, which is permeable to the brine but not fully selective like a membrane.
To avoid back-migration of caustic soda (NaOH) from the cathode compartment to the anode compartment (where it could react with chlorine, forming hypochlorite or chlorate), a positive hydrostatic pressure on the anode side is maintained.
This is achieved by keeping the brine level slightly higher in the anode compartment.
​
​Other Options:
​(ক) Incorrect – Diaphragm cells do not use ion-exchange membranes; hydration/swelling is an issue for membrane cells, not diaphragms.
(খ) Incorrect – High brine concentration affects chlorine purity only indirectly; oxygen evolution is controlled by electrode material and overpotential, not brine head.
(ঘ) Incorrect – Chlorine solubility in catholyte is a concern, but the hydrostatic head primarily addresses backflow, not gas solubility.

In diaphragm cells:
The diaphragm is porous (traditionally asbestos, now often modified polymer fibers) and its purpose is not ion-selective like a membrane.
It:
   1. Allows brine to flow from the anode compartment to the cathode compartment.
   2. Prevents rapid mixing of chlorine (from anode) with caustic soda (formed at cathode).
   3. Maintains low electrical resistance to keep energy consumption reasonable.
​
​Other Options:
​
​(খ) Incorrect – It does not significantly control chlorine solubility; that depends more on temperature and brine saturation.

(গ) Incorrect – The diaphragm is not a catalyst; hydrogen evolution occurs at the cathode.

(ঘ) Incorrect – That’s the role of a cation-exchange membrane in membrane cells, not a diaphragm.

In a mercury cell, sodium is deposited on the mercury cathode as an amalgam instead of producing hydrogen. To suppress secondary hydrogen evolution, two factors are crucial:

Mercury depth: A deeper mercury layer ensures uniform current distribution and minimizes spots where hydrogen evolution can compete.

Proper mercury flow velocity: Continuous movement of mercury helps remove freshly deposited sodium quickly, preventing its reaction with water and reducing local alkalinity, which favors hydrogen evolution.

In membrane cell technology:
The ion-exchange membrane relies on being fully hydrated to maintain ionic conductivity and selectivity (allowing Na⁺ to pass, blocking OH⁻ and Cl⁻).
When NaOH concentration exceeds ~32–35%, water activity in the catholyte drops sharply, causing membrane dehydration.
Dehydration → loss of ion-exchange sites, increase in resistance, and mechanical shrinkage, ultimately damaging the membrane and lowering efficiency.

​​Other Options:

​(ক) Hypochlorite formation due to chlorine solubility happens if chlorine and NaOH mix, but in membrane cells, chlorine and NaOH are in separate compartments, so it’s not the primary limitation.
(খ) Viscosity does rise with concentration, but pumping and ion transport are still manageable; membrane stability is the real bottleneck.
(গ) Hydrogen overpotential does increase slightly, but it’s a minor effect compared to membrane failure risk.

In the mercury process, the decomposer (also called denuder) reacts sodium–mercury amalgam with water to form NaOH and hydrogen gas:
2Na(Hg)+2H₂O→2NaOH+H₂↑+Hg

This reaction is highly exothermic.
If temperature rises uncontrollably or water is added too rapidly, it can cause:
1. Rapid hydrogen evolution, leading to an explosion hazard.
2. Local overheating, damaging equipment and increasing mercury vaporization (toxic risk).

Hence, careful control of water addition and heat removal is critical.

Membrane cells are designed with a cation-exchange membrane that prevents chloride ions from migrating into the cathode compartment, allowing the production of high-purity NaOH (low Cl⁻ content, around 32–35%).
Their design also minimizes electrical losses, so energy consumption is lower compared to mercury cells.
Mercury cells produce very pure NaOH as well, but the process consumes more energy and has significant environmental and safety concerns due to mercury handling.
Diaphragm cells use a porous diaphragm, which allows some mixing of chlorine and caustic soda, producing lower-purity NaOH. Although their energy consumption is moderate, the product typically contains NaCl and requires further purification if high purity is needed.

Electrolysis of brine directly converts NaCl into NaOH, Cl₂, and H₂ at ambient or slightly elevated temperatures, without using limestone or generating CO₂.
This is a major advantage over the lime-soda process, which involves:
1. Heating Na₂CO₃ with Ca(OH)₂ to produce NaOH.
2. Significant CO₂ release from lime (CaCO₃ → CaO + CO₂) and higher energy input due to high temperatures.

The electrolysis route also produces valuable by-products (chlorine and hydrogen) rather than just Na₂CO₃.
 Other Options:​
​
​(খ) Incorrect – Electrolysis actually produces high-purity NaOH; water consumption is moderate, but purity is higher, not lower.
(গ) Incorrect – Na₂CO₃ is not a by-product; chlorine and hydrogen are.
(ঘ) Incorrect – Electrolysis uses concentrated brine (NaCl solution) and sometimes caustic electrolyte handling, so “avoiding hazardous electrolytes entirely” is not accurate.

In a membrane cell, the cation-exchange membrane separates the anode (chlorine evolution) and cathode (NaOH and H₂ evolution) compartments.

The membrane is constantly exposed to highly corrosive chemicals: concentrated NaOH on the cathode side and chlorine/brine on the anode side.

Chemical resistance ensures that the membrane maintains its structural integrity, selectivity, and longevity under these harsh conditions.

The membrane allows Na⁺ ions to pass but blocks OH⁻ and Cl⁻; it does not transport OH⁻ to the anode, nor is its primary function related to hydrogen recombination or maintaining brine concentration.

In a diaphragm cell, the diaphragm separates the anode (chlorine evolution) and cathode (NaOH and H₂ evolution) compartments.

A positive hydrostatic head (higher brine level in the anode compartment) ensures that the flow of solution is from anode to cathode through the diaphragm.

This prevents the back-diffusion of caustic soda (OH⁻) into the anode compartment, which would otherwise react with chlorine and reduce current efficiency or create safety hazards.

Using a cation-selective diaphragm is a feature of membrane cells, not conventional diaphragm cells. Increasing current density or cooling the catholyte does not prevent backflow.

Membrane cells produce NaOH directly in the cathode compartment without forming a Na–Hg amalgam.

Mercury cells require an additional decomposer step to convert Na–Hg amalgam into NaOH, which consumes extra energy.

Diaphragm cells produce lower-concentration NaOH and require additional purification or evaporation, increasing energy consumption.

By eliminating the mercury handling and decomposer step, membrane cells reduce both energy usage and environmental hazards, making them more efficient.

Mercury cells produce ultra-pure NaOH with chloride content <0.001%, the highest among all processes.

Membrane cells produce high-purity NaOH (~32–35%) with very low chloride contamination.

Diaphragm cells produce moderate-purity NaOH (~10–12%) containing significant NaCl, which may require further purification for industrial use.

Summary Ranking of NaOH Purity:
Mercury > Membrane > Diaphragm

The lime-soda (causticization) process involves treating sodium carbonate (Na₂CO₃) with slaked lime (Ca(OH)₂) to produce NaOH:
Na_2​CO₃​+Ca(OH)_2​→2NaOH+CaCO_3​↓
Magnesium (Mg²⁺) and excess calcium (Ca²⁺) ions in the feedwater or soda can form insoluble hydroxides or double salts, which:
     1. Precipitate before reacting completely,
     2. Reduce NaOH yield, and
     3. Cause operational difficulties like scaling.

Other Options:
ক) Solubility of Na₂CO₃ in water → Na₂CO₃ is highly soluble; not a limiting factor.
খ) Particle size of Ca(OH)₂ → Affects reaction rate slightly, but not the primary yield limitation.
গ) Reaction temperature and residence time → Mildly affects kinetics, but yield is mainly limited by impurities.

1.The Solvay process is the most common industrial method for producing sodium carbonate (soda ash, Na₂CO₃).
2.The key raw materials are:
     Sodium chloride (NaCl) – common salt
     Calcium carbonate (CaCO₃) – limestone
3.The overall sequence involves:
Ammonia absorption in brine (NaCl solution).
Reaction with CO₂ (from calcined limestone, CaCO₃ → CaO + CO₂) to precipitate NaHCO₃.
Thermal decomposition of NaHCO₃ to Na₂CO₃.
                 2NaCl+CaCO₃→Na₂CO₃+CaCl₂ 

Other Options:
ক) Na₂SO₄ + CaCO₃ → Used in Leblanc process, not Solvay.
গ) NaCl + Na₂CO₃ → Na₂CO₃ is the product, not a raw material.
ঘ) Na₂CO₃ + Ca(OH)₂ → Used in lime-soda process for NaOH, not Solvay soda ash.

In the Solvay process, ammonia is used to absorb CO₂ and precipitate sodium bicarbonate (NaHCO₃), which is later calcined to produce soda ash (Na₂CO₃). However, during regeneration of ammonia using Ca(OH)₂, ammonium chloride (NH₄Cl) is produced as a by-product.
1.NH₄Cl is highly soluble in water and can cause severe water pollution and soil contamination if discharged without treatment.
2.The usual practice is to convert NH₄Cl into fertilizers like ammonium sulfate (NH₄)₂SO₄) or use recovery systems.

Other Options:
ক)NaHCO₃ → Main intermediate, not a pollutant.
খ)CO₂ → Recycled in the process, minimal emission if well-controlled.
ঘ)Na₂CO₃·H₂O → Hydrated soda ash, a product, not a pollutant.

In the lime-soda process, the main reaction is:
Na₂CO₃+Ca(OH)₂⟶2NaOH+CaCO₃↓

1. Ca(OH)₂ (lime) is sparingly soluble, so the reaction is heterogeneous.
2. If the amount of Ca(OH)₂ is just stoichiometric, some Na₂CO₃ might remain unreacted because of incomplete contact and equilibrium limitations.
3. Adding excess Ca(OH)₂ shifts the equilibrium to the right, ensuring almost all Na₂CO₃ converts to NaOH.

Other Options:
ক) Improve crystallization of NaOH → NaOH remains in solution; no crystallization occurs during the main reaction.
খ) Reduce reaction time → Excess Ca(OH)₂ might speed up the reaction slightly, but the main reason is not time, it’s completeness.
গ) Increase the solubility of Na₂CO₃ → Solubility of Na₂CO₃ is not influenced by Ca(OH)₂.

In a diaphragm cell:
1.The anode side generates Cl₂ gas, and the cathode side produces NaOH solution + H₂ gas.
2.The diaphragm is porous, allowing brine to pass but not gases.
3.As NaOH concentration rises in the catholyte, the concentration gradient for OH⁻ ions increases.
4.Beyond about 10–12% NaOH, OH⁻ back-diffuses through the diaphragm to the anode compartment.
5.There, OH⁻ reacts with Cl₂, forming hypochlorite (ClO⁻) and chlorate (ClO₃⁻):
           Cl₂+2OH⁻→Cl⁻+ClO⁻+H₂O
This reduces NaOH yield and causes contamination.

Other Options:
খ) Chlorine solubility in catholyte → Leads to contamination and safety risks, but not the main concentration limit.
গ) Membrane dehydration → Relevant for membrane cells, not diaphragm cells.
ঘ) Hydrogen overpotential → Impacts energy efficiency, not concentration limitation.

In a mercury cell process for caustic soda production:
1.The anode is usually titanium or graphite, and the cathode is a flowing layer of mercury.
2.Sodium ions are reduced at the mercury cathode to form sodium amalgam, which is later decomposed to NaOH.
The cell voltage depends on:
1. Brine concentration:
      Higher brine concentration = higher conductivity = lower resistance = lower cell voltage.
      As brine gets depleted, resistance rises, and cell voltage increases.
2. Anode-cathode distance:
     This is crucial because the mercury cathode layer is mobile and the gap affects the ohmic resistance of the electrolyte.

Greater distance → higher resistance → higher cell voltage.

Other Options:
খ) Catholyte temperature → Affects kinetics slightly, but the mercury cell operates at nearly constant temperature; not the dominant factor.
গ) Membrane ion selectivity → There is no membrane in mercury cells (applies to membrane cells).
ঘ) Mercury purity → Important for amalgam stability, but has negligible effect on cell voltage compared to brine concentration and gap.

In a membrane cell for chlor-alkali production:
1.The anode compartment contains brine (NaCl solution), where Cl₂ gas evolves.
2.The cathode compartment produces H₂ gas and NaOH solution.
3.A cation-exchange membrane (usually perfluorinated sulfonic polymer like Nafion) separates the compartments.
The membrane is selective for cations (Na⁺), so:
   1. Na⁺ ions migrate through the membrane to the cathode side.
   2. Cl⁻ ions and OH⁻ ions are blocked to prevent:
              Cl⁻ migration → which would contaminate NaOH.
              OH⁻ back-diffusion → which would cause side reactions (chlorate formation).

Other Options:
ক) Only OH⁻ ions move to the cathode → OH⁻ cannot cross the membrane; that’s the whole point.
গ) Cl⁻ ions are recombined at the cathode → Cl⁻ stays on the anode side to form Cl₂, not recombine at the cathode.
ঘ) H₂ gas is absorbed in the membrane → H₂ forms at the cathode and bubbles out; membrane does not absorb it.

In a diaphragm cell, hypochlorite (ClO⁻) forms when Cl₂ reacts with OH⁻ in the electrolyte:
 Cl₂ + 2OH⁻ ⟶ Cl⁻ + ClO⁻ + H₂O
the following conditions are critical:
1.Brine purity:
    Impurities like Fe, Mg, and Ca increase side reactions and reduce current efficiency, promoting hypochlorite formation.
2.Current density:
    If too low → less Cl₂ evolution rate relative to OH⁻ accumulation → more time for Cl₂ to dissolve and react with OH⁻.
    If too high → other issues, but generally, proper current density helps maintain efficient Cl₂ evolution and limits its contact with OH⁻.

Other Options:
ক) Anode temperature → Temperature does affect reaction kinetics, but it is not the primary control factor for hypochlorite formation.
খ) Mercury depth → Relevant to mercury cells, not diaphragm cells.
গ) Cathode overpotential → Influences energy efficiency, not directly hypochlorite formation.

In a mercury cell:
1.The cathode is liquid mercury.
2.Instead of water being reduced to H₂ at the cathode, sodium ions are reduced to sodium metal, which immediately forms a sodium-mercury amalgam.
3.This prevents direct hydrogen evolution, allowing sodium to be safely transported in the amalgam to a separate decomposition vessel where it reacts with water to produce NaOH and H₂ in a controlled manner.

Other Options:
ক) High brine concentration suppresses H₂ → Minor effect; the main mechanism is amalgam formation.
গ) Membrane selectively blocks OH⁻ → Applies to membrane cells, not mercury cells.
ঘ) Diaphragm restricts water reduction → Applies to diaphragm cells, not mercury cells.

In diaphragm cells:
1.The diaphragm is porous to allow ionic flow but prevent mixing of Cl₂ and NaOH.
2.Impurities like Ca²⁺ and Mg²⁺ in the brine:
     React with OH⁻ to form insoluble hydroxides (Ca(OH)₂, Mg(OH)₂).
     These deposit in the diaphragm pores, blocking ion flow.
      Result: Reduced current efficiency, increased cell resistance, and lower NaOH yield.

Other Options:
ক) Increase chlorine purity → Impurities do not improve Cl₂ purity; they mainly hinder the process.
গ) Promote NaOH crystallization → Unrelated; deposits mainly block pores.
ঘ) Enhance hydrogen evolution → Not significant; hydrogen evolution is mostly controlled by cathode potential.