Organic complexing agent H 5 X can be used to inactivate toxic metal (M) in wate

Question

Organic complexing agent H5X can be used to inactivate toxic metal (M) in water. H5X needs to be completely dissociated to X5- to complex metal M (MX is formed). H5X forms the following species in water:

H5X

H4X-                pKa1=1.5

H3X2-               pKa2=3.6

H2X3-               pKa3=7.8

HX4-                pKa4=10.9

X5-                   pKa5=11.5

a.In water what is the mass balance of all species X

b.Express the fraction () for species X5- with respect to pH

c.Would metal M be complexed by our organic agent in natural water?

d.At the following pH’s what is the predominate ion of X present in natural water?

8.67

5.45

13.45   

Explanation / Answer

a) in water pH is 7 @ 25 °C

Pka 1.25 =a strong acid

Pka 3.6 =somewhat strong

Pka 7.8 =weak acid

Pka 10.9 =will not dissociate

Pka 11.5 =will not dissociate

Therefore at pH=7

H4X- will not exist. H3X2- exists. H2X3- also can exist. HX4- not formed. HX5- not formed.

(b) At lower pH values alpha value for X5- tend to be zero. At higher values of pH alpha value for X 5- will tend to be 1.

(c) Metal M will not be complexed in natural water as X5- form is not available.

(d) At pH =8.67. H2X3-

     At pH = 5.45. H3X2-

At pH = 13.45. X5-

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