Question 2 of 5: Alkalinity Titration; Ozonation vs. Chlorination

Question 2: Alkalinity Titration; Ozonation vs. Chlorination (25 marks: a 15, b 10)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a. Alkalinity titration and speciation (15 marks)

Given. A 20 mL water sample is titrated with 0.02N $H_2SO_4$: 3 mL to reach the phenolphthalein endpoint (pH 8.3), and a further cumulative total of 5 mL to reach the Bromocresol Green endpoint (pH 4.5) — i.e. the standard double-titration where the phenolphthalein reading $P$ and the total reading $T$ are both measured from the start of the titration.

Find. The alkalinity indicated by each endpoint (P-alkalinity, T-alkalinity), and the additional alkalinity type(s) recoverable from the P/T relationship, all as mg/L CaCO₃.

Approach. Convert each titrant volume to mg/L as CaCO₃ with the standard normality formula, then apply the Standard-Methods P/T decision table to split total alkalinity into its hydroxide, carbonate and bicarbonate components.

1. Phenolphthalein alkalinity. The pH 8.3 endpoint marks the point where all carbonate ($CO_3^{2-}$) has converted to bicarbonate and any hydroxide has been fully neutralized — this is phenolphthalein (P) alkalinity. $$P_{alk}=\frac{P\times N\times 50{,}000}{V_s}=\frac{3\times0.02\times50{,}000}{20}=\boxed{150\ \text{mg/L as }CaCO_3}.$$
2. Total alkalinity. The pH 4.5 endpoint marks the point where essentially all bicarbonate has converted to carbonic acid/$CO_2$ — this is total (T) alkalinity. $$T_{alk}=\frac{T\times N\times 50{,}000}{V_s}=\frac{5\times0.02\times50{,}000}{20}=\boxed{250\ \text{mg/L as }CaCO_3}.$$
3. Which species table row applies. Compare $P$ to $T/2=2.5$ mL: since $P=3\ \text{mL} > T/2$, the sample falls in the "hydroxide + carbonate present, no bicarbonate" regime of the Standard Methods P/T table.
4. Other alkalinity values calculable — hydroxide and carbonate alkalinity. $$OH_{alk}=2P_{alk}-T_{alk}=2(150)-250=\boxed{50\ \text{mg/L as }CaCO_3},$$ $$CO_{3,alk}=2\left(T_{alk}-P_{alk}\right)=2(250-150)=\boxed{200\ \text{mg/L as }CaCO_3}.$$ Bicarbonate alkalinity is $T_{alk}-OH_{alk}-CO_{3,alk}=250-50-200=0$, consistent with the P>T/2 regime.

b. Ozonation vs. chlorination for disinfection (10 marks)

Ozone ($O_3$) disinfects by direct oxidative attack on microbial cell walls, enzymes and nucleic acids, exploiting its very high oxidation potential (2.07 V, second only to fluorine among practical oxidants); because it is unstable, ozone must be generated on site by passing dry oxygen or air through a high-voltage electrical discharge (corona discharge) and is then bubbled into a contact chamber through fine diffusers, where it decomposes within minutes back to oxygen, so it cannot be stored or piped like chlorine and leaves no persistent residual once it has reacted.

Advantages of ozonation over chlorination: (1) ozone is a far stronger and faster-acting oxidant/disinfectant, achieving effective inactivation of chlorine-resistant protozoan cysts such as Giardia and, especially, Cryptosporidium (which is highly resistant to chlorine at practical CT values) with much lower CT requirements; (2) ozone does not form chlorinated disinfection by-products (trihalomethanes, haloacetic acids) when it reacts with natural organic matter, avoiding that regulated by-product family entirely (though it can form its own by-product, bromate, in bromide-rich source waters).

Disadvantages of ozonation over chlorination: (1) ozone leaves no persistent residual in the distribution system, so a secondary disinfectant (typically chlorine or chloramine) is still required downstream to protect against regrowth and recontamination between the plant and the tap — ozone alone cannot fulfil this role; (2) ozonation is substantially more capital- and energy-intensive, requiring on-site generation equipment (air-prep/oxygen supply, high-voltage generators), dedicated contact chambers, and an off-gas destruction system, and it demands more operator training/monitoring than the comparatively simple chlorine feed systems, making it a materially more expensive technology to install and operate.