Question

Ferrick chloride FeCl3 is widely use for water Treatment.

1) Recall its role and describe its action.

Durant its use, its important to control the Ph to limit the amount of iron remaining in solution. To determine the optimum Ph range, the Solubility of iron hydroxide Fe(OH) 3 (s) which is the solid species which controls the iron content un water,it will be characterized by plotting the log ( ) =f (Ph) diagram of the system define by the four equilibria of the data below.

2) Give the expression of the logarithme of the concentration of the differents species of iron as function of Ph.

3) Plot these variations in a log ( ) =f (Ph) diagram

4) Plot the variation in total iron concentration as a function of Ph , what can you conclude ?

Fe(OH)3 (3) Fe(OH)3 (5) Fe(OH)3 (8) Fe(OH)3 (3) + OH = Fe(OH)*+ OH Fe(OH)2+ + 2OH Fe3+ + 3 OH Fe(OH) log K = -16,5 log K2 = -27 log K3 = -37,2 log Ks = -4,5

Answer 1

1. Action:-

• Ferric chloride is used for enhanced phosphorous removal
• FeCl3 + H3 PO4 -----> FePO4 + 3HCl3
• It helps in reducing the alkalinity, by reducing the pH and by increasing TDS (total dissolved solids).

2. Henderson- Hasselberg equation:

pH = pKa + log [A-] / [HA]

[A-] - concentration of conjugate base

[HA] - Concentration of acid

pKa - acid dissociation constant

pH = pKa + log [OH-] / [FeOH3]