It is widely used in households as a disinfectant, bleaching, and cleaning agent. It appears greenish-yellow in solution. It was developed by Claude Louis Berthollet in1785. So, Is bleach an alkali? Yes, bleach is an alkali. The pH value of sodium hypochlorite ranges between 11 and 13. In an aqueous solution, it dissociates into hypochlorite anion and sodium ions. The hypochlorite ions formed to accept a proton from water thereby causing the release of OH- ions in the solution, thus, making the solution basic. The equation is given below: NaClO —-> Na+ + ClO-
Is Bleach Acidic?
Acids are substances that release protons or accept unshared pairs of electrons in their aqueous solution. They taste sour and turn blue litmus to red. The pH of all the acids is below 7 on the pH scale. So, Is bleach acidic? No, sodium hypochlorite is not acid as it has a pH value above 7. Also, it is neither a proton donor nor accepts the unshared electrons. In an aqueous solution, it turns red litmus paper to blue, which is a characteristic of alkalis. The sodium ion formed after dissociation of NaClO is neutral while the ClO- ion has an unshared pair of electrons to be given away. Therefore, bleach is not an acid.
pH of Bleach
The pH of any solution gauges its acidity or alkalinity. The values on the pH scale range from 1 to 14. Any solution for which the pH value lies between 1 and 7 is considered acidic while the solution with a pH value is between 7 and 14 is termed alkaline. In the case of NaOCl, we already know that it is a basic compound formed by two ions viz. ClO- and Na+. Therefore, the pH of NaOCl should be less than 7. Calculating the pH for basic solutions is different than acidic solutions where only a Ka value is required. Here, we will first have to calculate the Kb value of the solution through which the concentration of OH- ions is calculated. The entire procedure is as follows: For calculating the pH of 0.01 M solution of sodium hypochlorite we will first have to calculate its Kb value. We know that the Ka value for this solution is 3 X 10-8. So, Ka X Kb = 10-14 Therefore, Kb = 10-14/ 3 X 10-8 = 3.3 X 10-7 Now, calculating the concentration of hydroxide ions using the value of Kb Kb = ([ClO-] [ OH-]) / ([NaClO] – [ClO-]) As, [ClO-] = [ OH-] 3.3 X 10-7 = x^2/0.1 – x [OH-] = 1.8 X 10^-4 The pOH value for the solution is now calculated using the [OH-] value pOH = – log [OH] = 3.7 Also, we know that pOH + pH = 14 pH = 14 – 3.7 = 10.3 As the pH value is more than 7, therefore, it is clear that NaOCl is basic in nature.
Why Bleach is Alkaline?
A number of theories are given that describe why a substance behaves as a base. Three of the most important theories are given below: • Bronsted-Lowry Theory: As per this theory a substance behaves as a base if it is ready to absorb the protons given away by another molecule or specific water in an aqueous solution. • Arrhenius Theory: This theory states that a substance is known to be alkaline if it is able to produce hydroxide ions (OH-) in a solution. • Lewis theory: As per this theory a base is a substance that has unshared pair of electrons available to be donated to other molecules. Looking at sodium hyper chlorite, we know that the dissociation equation for NaOCl is given as: NaClO <====> Na+ + ClO- The Na+ ion released in this reaction is neutral and does not play any role in determining the nature of the solution. However, the hypochlorite ion acts as a proton acceptor in the aqueous solution. Hence, it behaves as an acid according to the Bronsted-Lowry theory. This reaction results in the formation of hydroxide ions, the equation is given below. ClO- + H2O <====> HClO + OH- Therefore, the molecule also fulfills the condition for behaving as acid as per the Arrhenius theory. Also, as lesser protons mean a decline in the acidity of a solution it makes the solution more alkaline. In addition, we also know that a negative charge indicates an unshared pair of electrons. As seen in the reaction above the hypochlorite ion (ClO-) had an unshared pair of electrons which it donated to the hydrogen ion of the water molecule. Hence, it is also acting as a base as per the Lewis theory.
Why Bleach is not Acidic?
The three theories discussed above also provide us with the definition of acids. These are given below: • Bronsted-Lowry Theory: As per this theory a substance behaves as an acid if it is ready to give away a proton to another molecule. • Arrhenius Theory: This theory states that a substance is known to be acidic if it is able to produce hydrogen ions (H+) in a solution. • Lewis theory: As per this theory an acid is a substance that has accepts unshared pair of electrons donated by other molecules. As none of these conditions are fulfilled by sodium hypochlorite, therefore, bleach is not an acid.
What is Bleach made of?
The most commonly used bleach is sodium hypochlorite, NaOCl. One molecule of sodium hypochlorite is made up of one atom of sodium, one atom of oxygen, and one atom of chlorine. The molecules break into sodium and hypochlorite ions as described in the above sections. Looking at its preparation methods, NaOCl is manufactured through different processes, a few of which are given below: • Chlorination of soda: In this process, chlorine is passed through sodium hydroxide resulting in oxidation as well as reduction of chlorine in the same reaction. The reaction equation is given below: Cl2 + 2 NaOH —–> NaCl + NaClO + H2O • Electrolysis of brine: The reaction process here is the same as described in the previous method except that the sodium hydroxide and chlorine used for the reaction are produced by the electrolysis of brine. • From calcium hypochlorite: In this process calcium hypochlorite is treated with sodium carbonate or washing soda. The reaction involved is as follows: Na2CO3 + Ca(OCl)2 —–> CaCO3 + 2NaOCl Another method involves the reaction of sodium chloride with ozone as per the following reaction: NaCl + O3 —–> NaClO + O2
Properties
Some properties of sodium hypochlorite are listed below: • It is a greenish-yellow solid in appearance which is present as a pentahydrate. • It is denoted by the chemical formula NaOCl. • It is an ionic compound comprising of sodium and hypochlorite ions. • It has a molecular mass of 74.442 g/mol. • It has a sweetish, chlorine-like smell. • The density of sodium hypochlorite is 1.11 g/cm3. • The melting and boiling points of NaOCl are 18°C and 101°C, respectively. • Its solubility in water is 29.3 g/100mL, at 0°C temperature. • The pKa and pKb values for NaOCl are 7.5185 and 6.4815, respectively. • Anhydrous sodium hypochlorite is highly unstable and may explode easily. However, the pentahydrate molecule is stable and can be stored in the refrigerator. • It is a very strong oxidizer and therefore quite corrosive in nature. • With metals, it may form hydrogen due to which the container may explode on heating.
Uses
Sodium perchlorate finds in use in many places. A few of its uses are given below: • The most common use of sodium hypochlorite is household bleach. A 3-8% solution of NaOCl along with 0.01-.005% NaOH is used. Sodium hydroxide is added to slow down the decomposition processes. • It is also used as a destaining agent. It is especially useful in removing mold stains, tea or coffee stains on crockery, dental stains, etc. • Due to its broad-spectrum, anti-microbial properties sodium hypochlorite is also used as a disinfectant. A 0.5% solution of NaOCl is used for disinfecting highly contaminated areas. • Dakin’s solution is a solution containing a low concentration of sodium hypochlorite along with some amount of boric acid. • It is also used for removing foul odor due to its oxidation and hydrolysis reaction. It actually makes the organic dirt water-soluble and non-volatile due to which it acts as a deodorizer. • It is used in endodontic surgeries due to its effectiveness against pathogenic microbes. The concentration of NaOCl used to dissolve necrotic tissues and bacterial species varies between 5.5 and 5.25%. • Due to its biocidal properties, the 10-15% solution of sodium hypochlorite is used for water treatment at the paper and pulp mills. • It is also used for removing cyanide wastes from water. It oxidizes toxic cyanide ions into non-toxic cyanate ions. • The nerve agent used in chemical warfare may also be neutralized using sodium hypochlorite. The Personal Protective Equipment used by the professionals is cleaned using 50% solution o NaOCl. • A dilute solution of sodium hypochlorite is also used for the treatment id eczema. • It is also used for the treatment of skin damage caused due to excess sun exposure of radiation therapy, etc. • It is also used in detergents as well as textile industries. • It is used in sanitizing food preparation equipment. • Also, it is used as a swimming pool cleaning agent. • It is used in petroleum product refining.
Conclusion
Bleach or sodium hypochlorite is a base and its pH ranges between 11 and 13. In an aqueous solution, it dissociates to form sodium and hypochlorite ions. The ClO- ions further react with water molecules resulting in the formation of hydroxide ions. These hydroxide ions are responsible for the basicity of sodium hypochlorite solution. The sodium hypochlorite molecules accept protons, donate unshared pairs of electrons and also release OH- ions in the aqueous solution. Therefore, NaOCl fits into the definition of base given by Bronsted-Lowry theory, Arrhenius theory, and Lewis theory of acids and base, respectively.
