Chemical Treatment Precipitation and Reduction/Oxidation (Section 9-4)

Solubility and Equilibrium A m B n (s)  mA n+ + nB m- K sp = [A n+ ] m [B m- ] n

Example: Solubility Equation What concentration of calcium would be in solution at equilibrium with phosphate?

Activity – Team If 50.0 mg of carbonate and 50.0 mg of calcium are present in 1 L of water, what will be the final (equilibrium) concentration of calcium? (Remember that the solubility equation uses the final (equilibrium) concentration, i.e., after precipitation has occurred.)

Precipitants and Solubility Hydroxides Sulfides Carbonates Sulfates Chlorides

Activity – Team Given a saturated solution of calcium carbonate, how many moles of calcium ion will remain in solution after the addition of 3.16 x M Na 2 CO 3 at 25 o C?

Example: Precipitant Dose Simple System Given a solution with an aqueous silver concentration of 100 mg/L, determine the mass of sodium chloride that must be added to reduce the aqueous silver concentration to 1 mg/L. The total solution volume is 55 gals (208.2 L).

Example: Precipitant Dose Complexation Consider a cadmium-containing wastewater. It is to be precipitated with hydroxide to a free cadmium concentration of 1 mg/L (8.896 x M). Determine the required equilibrium pH and the total soluble-phase cadmium concentration.

Example continued Soluble Cadmium Species Cd 2+ + H 2 O  CdOH + + H + K sp = Cd H 2 O  Cd(OH) 2 + 2H + K sp = Cd H 2 O  Cd(OH) H + K sp = Cd H 2 O  Cd(OH) H + K sp = Cd(OH) 2 (s) + H +  Cd H 2 O K sp =

Coagulation/Flocculation/Settling Successful precipitation requires effective separation.

Example: Jar Test

Process

Design of Precipitation Process

Activity – Team A metal plating firm is installing a precipitation system to remove zinc. They plan to use a pH meter to control the feed of hydroxide solution to the mixing tank. What pH should the controller be set at to achieve a zinc effluent concentration of 0.80 mg/L?

Redox Reactions