Chemistry Extended Essay

“How does pH affect the concentration of trihalomethanes formed in a sample of reservoir water when it is disinfected with chlorine or chloramine and to what extent do both chloramine and free chlorine decompose when exposed to ultraviolet light?”

Abstract

This investigation aims to find out how pH affects the concentration of trihalomethanes formed in a sample of reservoir water when it is disinfected with chlorine or chloramine and the extent to which both free chlorine and chloramine decompose when exposed to ultraviolet light.

Headspace gas chromatography and mass spectrometry were used to measure the concentration of the four main constituents of total trihalomethanes (TTHMs), namely chloroform, …show more content…
Free chlorine is the chlorine available to sterilise the water and is usually represented by hypochlorous acid and hypochlorite ions. When a solution containing free chlorine is exposed to ultraviolet light both hypochlorous acid and hypochlorite ions are broken down as shown below:

2OC〖l^-〗_((aq.))+u.v.→2C〖l^-〗_((aq.))+〖O_2〗_((g))

2HOCl_((aq.))+u.v.→2HCl_((aq.))+〖O_2〗_((g))

When chloramine is exposed to high-energy ultraviolet light, the Cl-N or Cl-H bonds will be broken. Then, shorter wavelength ultraviolet light (less than 230nm) will break down water molecules into hydroxyl radicals that will oxidize the chloramines rapidly. This process is known as photo-oxidation.

Apparatus and chemicals list Beakers. Measuring cylinders. N, N diethyl-1, 4 phenylenediamine sulfate (DPD) free chlorine reagent. Chlorine standard solutions. Ammonia solution. Buffer …show more content…
Concentration of chloramine solution (trial 1) / mg/L ± 0.01 Concentration of chloramine solution (trial 2) / mg/L ± 0.01 Concentration of chloramine solution (trial 3) / mg/L ± 0.01 Concentration of chloramine solution (trial 4) / mg/L ± 0.01 Concentration of chloramine solution (trial 5) / mg/L ± 0.01 Average concentration of chloramine solution / mg/L ± 0.01
1 0 1.00 1.00 1.00 1.00 1.00 1.00
2 15 0.92 0.93 0.91 0.93 0.91 0.92
3 30 0.87 0.89 0.85 0.87 0.86 0.87
4 45 0.85 0.86 0.78 0.82 0.84 0.83
5 60 0.82 0.82 0.73 0.80 0.78 0.79
6 75 0.76 0.78 0.70 0.77 0.74 0.75
7 90 0.63 0.75 0.66 0.63 0.73 0.68
8 105 0.58 0.69 0.62 0.60 0.66 0.63
9 120 0.53 0.64 0.57 0.59 0.57 0.58
10 135 0.50 0.62 0.53 0.58 0.52 0.55
11 150 0.44 0.56 0.47 0.52 0.46 0.49
12 165 0.42 0.52 0.44 0.47 0.45 0.46
13 180 0.39 0.51 0.42 0.46 0.42