Sodium hypochlorite is a liquid chemical compound at standard temperature and pressure (STP). This compound is a colorless industrial solution and can be identified as a greenish to yellow liquid with an odor of chlorine (Ponzano, 2007). Its molecular weight is 74.44g/mol, and the constituent elements include sodium, oxygen, and chlorine, as it has a chemical formula of NaClO. This chemical compound features an ionic and a covalent bond. The covalent bond between the chlorine and oxygen atoms combines to form the hypochlorite ion (ClO–). Since covalent bonding involves sharing electrons between two or more atoms, the chlorine atom shares one electron to bond covalently with oxygen. The hypochlorite ion is negatively charged and requires one cation ion to form a neutral compound. The hypochlorite ion forms an ionic bond with one sodium cation (Na+) to form sodium hypochlorite as the neutral compound. The hypochlorite ion is slightly negatively charged because of its status as the pole having the most electrons, while the sodium ion is slightly positively charged as it has the fewest electrons.
Sodium hypochlorite is soluble in water at STP and room temperature and pressure (RTP). At STP, its solubility in water is 29.3g/100g. The density of 5% sodium hypochlorite is 1.093g/cm3 while a 14% aqueous solution of NaClO at 200C is 1.21g/ml. Its boiling point is 180C, and it has a boiling point of 1010C. Its usage encompasses several applications such as cleaning, bleaching, and deodorizing as it supports saponification energy flows through oxidation and hydrolysis. Its health-related uses include wastewater treatment, reduction of skin damage, and root canal treatment (Chung et al., 2022). The last three years of recent information show that sodium hypochlorite production has stagnated. In contrast, its unit cost has increased from $0.60 to $0.90 per gallon. These trends have informed recent efforts to establish new cost-effective methods for manufacturing the solution. For example, brine has appeared in many studies as an alternative to sodium hydroxide (NaOH) and chlorine as the main raw materials for production. Brine is cost-effective as it supports the on-site production of sodium hypochlorite (Baydum & Sarubbo, 2023).
The production of sodium hypochlorite involves an oxidation-reduction (redox) reaction. This reaction involves a direct reaction between cold dilute NaOH and chlorine gas, as shown below. Chlorine gas is oxidized to OCl– ion and reduced to Cl– ion.
Cl2 + 2 NaOH = NaOCl + NaCl + H2O
This redox reaction is exothermic since 100.11kJ of energy must be removed from this reaction at STP. The reaction enthalpy is the product of subtracting the sum of enthalpies of all reactants from that of the products. The reactants require energy for bond breaking, while the products emit heat energy through bond formation (Nilsson & Niedderer, 2014). The enthalpy of the reaction to produce sodium hypochlorite is summarized below.
[1ΔHf(NaCl (aq)) + 1ΔHf(NaClO (aq)) + 1ΔHf(H2O (ℓ))] – [1ΔHf(Cl2 (g)) + 2ΔHf(NaOH (aq))]
[1(-407.25) + 1(-347.21) + 1(-285.83)] – [1(0) + 2(-470.09)] = -100.11 kJ.
The main products from this redox reaction include sodium chloride, sodium hypochlorite, and water. At the same time, NaOH and chlorine gas are the only reactants. The reaction does not require any catalysts. The balanced equation with all the states of participants is:
Cl2 (g) + 2 NaOH (aq) = NaOCl (aq) + NaCl (aq) + H2O (l)
The preparation of sodium hypochlorite must feature safety considerations. NaOH could cause burns and irritation because it is a corrosive reactant. It could cause harmful effects on mucous membranes, eyes, and skin. Exposure to low levels of chlorine gas may lead to the same adverse effects. However, there are no side reactions that may produce unwanted products. Another method to produce this compound would entail the reaction of sodium carbonate (Na2CO3) with chlorinated lime. The chlorinated lime combines calcium hydroxide, calcium chloride, and calcium hypochlorite.
The reaction between NaOH and Cl2 leads to several qualitative and quantitative differences when NaOH is hot and highly concentrated. On the one hand, the main qualitative differences relate to the state of the final products and reactivity. On the other hand, a change in the heat of reaction and molecular weight represents the main quantitative differences. However, these differences tend to overall between the two categories. The reaction between hot concentrated NaOH and chlorine gas does not produce NaClO as it produces sodium chlorate (v), NaClO3, as shown in the balanced chemical equation below.
3Cl2 (g) + 6 NaOH (aq) = NaClO3 (aq) + 5NaCl (aq) + 3H2O (l)
Increasing the temperature and concentration of NaOH leads to forming a different, correlated product. For example, chlorine gas is oxidized to a ClO3– ion rather than the OCl– ion, thus forming sodium chlorate (III) rather than sodium chlorate (I).
The molecular weight changes from 74.44g/mol to 106.44g/mol after using hot concentrated NaOH. Moreover, the reactivity of hot concentrated NaOH increases by more than three times since 372.3 kJ of energy must be removed from this exothermic redox reaction.
[3ΔHf(NaCl (aq)) + 1ΔHf(NaClO3 (aq)) + 3ΔHf(H2O (ℓ))] – [3ΔHf(Cl2 (g)) + 6ΔHf(NaOH (aq))]
[3(-407.25) + 1(-365.4) + 3(-285.83)] – [3(0) + 6(-470.09)] = -372.3 kJ.
The change in heat and concentration would make this reaction process extremely unsafe because concentrated sodium hydroxide exerts a stronger corrosive action to cause immediate degeneration of the tissue. For example, poor handling of hot concentrated NaOH may cause severe burns to the skin and may damage the eye’s cornea, resulting in blindness (Ahmadi & Seyedin, 2019). The increase in molecular weight reduces the solubility of the final compound in water. Even though sodium chlorate and sodium hypochlorite can be used in water treatment, the latter has several pitfalls, while the former is expensive.
Ahmadi, M., & Seyedin, S. H. (2019). Investigation of NaOH properties, production and sale mark in the world. Journal of Multidisciplinary Engineering Science and Technology (JMEST), 6(10), 10809-10813. http://www.jmest.org/wp-content/uploads/JMESTN42353137.pdf
Baydum, V., & Sarubbo, L. (2023). Feasibility of producing sodium hypochlorite for disinfection purposes using desalination brine. Biointerface Research in Applied Chemistry, 13(2), 176. https://doi.org/10.33263/BRIAC132.176
Chung, I., Ryu, H., Yoon, S., & Ha, J. C. (2022). Health effects of sodium hypochlorite: Review of published case reports. Environmental Analysis Health and Toxicology, 37(1), e2022006. https://doi.org/10.5620/eaht.2022006
Nilsson, T., & Niedderer, H. (2014). Undergraduate students’ conceptions of enthalpy, enthalpy change and related concepts. Chemistry Education Research and Practice, 15(3), 336-353. https://doi.org/10.1039/C2RP20135F
Ponzano, G. P. (2007). Sodium hypochlorite: History, properties, electrochemical production. Contributions to Nephrology, 154, 7-23. https://doi.org/10.1159/000096810
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