principles of extraction and the extraction of semivolatile organics ...

52 principles of extraction
pC
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0
1
2
3
4
5
6
[H
7
8
9
10
11
12
13
14
2,4-DB [COOH]
+ ] [OH− ]
2,4-DB [COO− pH
]
Figure 2.8. Master variable (pC–pH) diagram for 2,4-DB; pK a ¼ 4:8, CT ¼ 1 10 8 M.
To prepare a pC–pH diagram, the master variable, pH, is plotted on the
x-axis. On the y-axis, the concentration of chemical species is plotted as a
function of pH. The concentration, C, of each chemical species is expressed
as a logarithm (log C), or more often as the negative logarithm of its concentration,
that is pC (analogous to pH). The pC–pH diagram (Figure 2.8)
for a representative acidic solute, 4-(2,4-dichlorophenoxy)butanoic acid or
2,4-DB, is prepared by first determining that the pKa for this compound is
4.8. A reasonable concentration to assume for trace levels of this compound
in water is 2.5 ppm or 1 10 8 M, since the molecular weight of 2,4-DB is
249.1. Based on the molar concentration of 1 10 8 ,pC has a value of 8.
By mass balance, the total concentration at any given pH value, CT, is the
sum of all species. That is,
CT ¼½HAŠþ½A Š ð2:13Þ
for a monoprotic acid, as in the example in Figure 2.8. The diagonal line
connecting pH, pC values ð0; 0Þ with ð14; 14Þ represents the hydrogen
ion concentration, and the diagonal line connecting pH, pC values ð0; 14Þ
with ð14; 0Þ represents the hydroxide ion concentration, according to the
expression
½H þ Š½OH Š¼KW ¼ 10 14
ð2:14Þ