Chlorine has been used for decades to disinfect drinking water. An undesirable side effect of this treatment is reaction with organic impurities to create organochlorine compounds, some of which could be toxic. Monitoring total organic halide (designated TOX) is required for many water providers. A standard procedure for TOX is to pass water through activated charcoal, which adsorbs organic compounds. Then the charcoal is combusted to liberate hydrogen halides:

$\mathbf{Organic}\mathbf{}\mathbf{halide}\mathbf{}\mathbf{}\left(\mathrm{RX}\right)\mathbf{}\stackrel{{\mathbf{o}}_{\mathbf{2}}\mathbf{/}\mathbf{800}\mathbf{°}\mathbf{C}}{\mathbf{\to }\mathbf{}}\mathbf{}\mathbf{}{\mathbf{CO}}_{\mathbf{2}}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{+}\mathbf{HX}$

HX is absorbed into aqueous solution and measured by coulometric titration with a silver anode:

${\mathbf{X}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{Ag}\left(s\right)\mathbf{\to }\mathbf{AgX}\left(s\right)\mathbf{+}{\mathbf{e}}^{\mathbf{-}}$

When 1.00 L of drinking water was analyzed, a current of 4.23 mA was required for 387 s. A blank prepared by oxidizing charcoal required 6 s at 4.23 mA. Express the TOX of the drinking water as $\mathit{\mu }$mol halogen/L. If all halogen is chlorine, express the TOX as $\mathit{\mu }\mathit{g}\mathbf{CI}\mathbf{/}\mathbf{L}$.

Step by step solution

01

Equation for number of moles of electrons

The charge q is equal to the product of current l and time t at any point in an electric circuit.

$\mathrm{q}=\mathrm{l}·\mathrm{t}$

In a reaction with a certain time t, the number of moles of electrons reacting is n.

02

Express total organic halide in one liter of drinking water

The time required for coulometric titration is $387-6=381\mathrm{s}$.

The number of moles of electrons reacted is determined as

$$\begin{gathered} \mathrm{Reacted}\mathrm{moles}=\frac{\mathrm{l}.\mathrm{t}}{\mathrm{nF}} \\ =\frac{\left(4.23\mathrm{mA}\right)\left(38\mathrm{s}\right)}{\left(96485\mathrm{C}/\mathrm{mol}\right)} \\ =16.7\mu {\mathrm{mole}}^{-} \end{gathered}$$

The strength of organohalide is $16.7\mu \mathrm{M}$since one electron is equal to one halide ion. If all the halides are chlorine, then TOX is $592\mu g\mathrm{Cl}/\mathrm{L}$

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