Chlorine and Fluoride Ions in Water Treatment for the ESAT

The treatment of drinking water is a vital public health measure that involves several chemical processes. While filtration removes large particles and sediment, the addition of specific ions ensures the water is safe to drink and provides secondary health benefits. For the ESAT, you must distinguish between the biological safety provided by chlorine and the dental health benefits provided by fluoride ions.

The Purpose of Chlorine in Water Treatment

Chlorine is added to water supplies primarily as a disinfectant. Its main purpose is to kill bacteria, viruses, and other microorganisms that can cause waterborne diseases such as cholera and typhoid. When chlorine gas ($Cl_{2}$) is added to water, it undergoes a disproportionation reaction (a type of redox reaction where the same element is both oxidised and reduced) to form a mixture of acids:

$Cl_{2}(g) + H_{2}O(l) \rightleftharpoons HCl(aq) + HClO(aq)$

In this reaction, $HClO$ is known as chloric(I) acid (or hypochlorous acid). This compound is a powerful oxidising agent that can penetrate the cell walls of bacteria and destroy their internal enzymes and genetic material, effectively neutralising the threat.

1. Sterilisation: The primary goal is the destruction of pathogens.
2. Residual Protection: A small, safe amount of chlorine is often left in the water to prevent re-contamination as the water travels through the pipe network to consumers.
3. Taste and Odour: While effective, too much chlorine can give water a distinct chemical taste, which must be carefully monitored by water companies.

The Purpose of Fluoride Ions in Water Treatment

Unlike chlorine, fluoride ions ($F^{-}$) are not added to make the water 'safe' in a biological sense. Instead, they are added for their medicinal benefits in preventing dental caries, also known as tooth decay.

Fluoride ions work by interacting with the enamel on human teeth. Enamel is primarily composed of a mineral called hydroxyapatite. When fluoride ions are present in drinking water, they can replace the hydroxide ions in the mineral to form fluorapatite:

$Ca_{5}(PO_{4})_{3}(OH) + F^{-} \rightarrow Ca_{5}(PO_{4})_{3}F + OH^{-}$

Fluorapatite is significantly harder and more resistant to acid attack from bacteria in the mouth than hydroxyapatite. This chemical change makes the teeth of the population, particularly children whose teeth are still developing, much less susceptible to cavities.

Ethical and Health Considerations

The addition of these ions is often a subject of debate. While the benefits of chlorine are almost universally accepted because it prevents immediate and deadly disease outbreaks, fluoridation is sometimes controversial. Some individuals view the mandatory addition of fluoride as a form of 'mass medication', arguing that people should have the choice whether or not to consume it. However, public health authorities generally maintain that the level of fluoride added (typically around 1 part per million) is safe and provides a massive benefit to lower-income communities who may have less access to dental care.

Example: Calculating Ion Concentration

A water treatment plant adds $2.1$ kg of sodium fluoride ($NaF$) to $1,000,000$ dm³ of water. Calculate the concentration of fluoride ions in mol dm⁻³.

Step 1: Find the relative formula mass ($M_{r}$) of $NaF$. $A_{r}$ values: $Na = 23.0$, $F = 19.0$. $M_{r}(NaF) = 23.0 + 19.0 = 42.0$.

Step 2: Calculate the number of moles of $NaF$. Mass in grams = $2100$ g. $Moles = \frac{2100}{42.0} = 50$ mol.

Step 3: Determine the concentration. $Concentration = \frac{moles}{volume} = \frac{50}{1,000,000} = 5 \times 10^{-5}$ mol dm⁻³.

Exercises

Exercise 1: State the name of the active oxidising agent formed when chlorine reacts with water.

Exercise 2: Explain why fluoride is added to water even though it does not kill bacteria.

Solutions:

1. Chloric(I) acid (or hypochlorous acid), $HClO$.
2. Fluoride is added to strengthen tooth enamel and prevent tooth decay by converting hydroxyapatite into acid-resistant fluorapatite.